Name: absorbance spectra and beer-lambert analysis
Uploaded: 2020-03-26T07:22:26.000Z
Duration: 2 min 53 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.7

IPC.7.E

Beer's Law

De nombreuses réactions chimiques progressent dans deux directions, en avant et en arrière. Au fil du temps, les réactions directe et inverse se produiront au même rythme et la concentration des réactifs et des produits ne changera plus. C’est ce qu’on appelle l’équilibre chimique.
À l’équilibre chimique, les concentrations de chaque composant sont liées les unes aux autres par la constante d’équilibre, K, qui est le rapport entre les concentrations de produits et les concentrations de réactifs, chacune portée à la puissance de ses coefficients stœchiométriques.
Mais comment déterminer les concentrations à l’équilibre ? Une méthode mesure l’intensité d’une longueur d’onde de lumière que le produit absorbe avant et après son passage dans un échantillon. La différence d’intensité est appelée absorbance et correspond à la quantité de composé absorbant dans l’échantillon.
Vous vous souvenez peut-être que les électrons occupent principalement l’état fondamental. Lorsqu’ils absorbent une certaine quantité d’énergie, ils sont excités à un niveau d’énergie plus élevé. Cette énergie correspond à une longueur d’onde spécifique de la lumière. Vous pouvez trouver cette longueur d’onde et mesurer l’absorbance à l’aide d’un spectrophotomètre, qui dirige un faisceau de lumière à travers l’échantillon et mesure le changement d’intensité à une ou plusieurs longueurs d’onde.
L’absorbance est égale au logarithme négatif de l’intensité de la lumière atténuée sur l’intensité de la lumière incidente. En traçant les valeurs d’absorbance de plusieurs solutions avec différentes concentrations de produit connues, nous observons une relation linéaire entre l’absorbance et la concentration. C'est un exemple de la loi de la bière.
La loi de Beer est exprimée mathématiquement par cette équation, où A est l'absorbance, epsilon est le coefficient d'atténuation molaire, une constante qui varie pour chaque composé, l est la longueur du trajet de la lumière à travers l'échantillon et c est la concentration du composé.
En identifiant la fonction linéaire d'un composé donné à une longueur d'onde et à une longueur de trajet spécifiques, vous pouvez utiliser les données d'absorbance d'une solution à l'équilibre pour déterminer la concentration à l'équilibre du produit. À partir de là, vous pouvez calculer les concentrations d’équilibre des réactifs et calculer la constante d’équilibre. Dans ce laboratoire, vous préparerez des solutions d’isothiocyanate de fer(III) et utiliserez un spectrophotomètre pour déterminer son absorbance à diverses concentrations.

loi de la bière

Absorbance et fluorescence
Lorsque la lumière frappe une substance, elle est soit absorbée, soit transmise, soit réfléchie. En règle générale, une substance interagit avec une gamme de longueurs d’onde de la lumière, chacune interagissant différemment avec les molécules ou les atomes. Une substance peut absorber une gamme spécifique de longueurs d’onde, réfléchir une autre gamme de longueurs d’onde et transmettre les autres longueurs d’onde de la lumière.
Lorsqu’une molécule absorbe de la lumière, l’énergie est utilisée de quatre manières différentes : (1) la traduction, qui amène la molécule à modifier sa vitesse moléculaire ; (2) les vibrations, qui font changer rapidement la distance entre les molécules ; (3) la rotation, qui fait tourner les atomes autour des liaisons d’une molécule ; et (4) l’excitation des électrons, qui provoque la transition des électrons vers des niveaux d’énergie plus élevés.
<h4>Niveaux d’énergie</h4>
En 1913, Niels Bohr a proposé un modèle pour l’atome d’hydrogène dans lequel les électrons se déplacent autour du noyau sur des orbites fixes et circulaires, appelées états stationnaires. L’énergie associée à chaque orbite, ou état stationnaire, n’existe qu’à des énergies fixes et discrètes. Ce n’est que lorsqu’un électron se déplace vers une autre orbite que de l’énergie est absorbée ou émise. L’électron n’est jamais entre deux états. Ce changement ne se produit que si l’énergie absorbée ou émise est égale à la différence entre les deux états d’énergie.
Dans le modèle de Bohr, le nombre quantique n représente l’énergie de l’électron. Lorsqu’un électron occupe l’état d’énergie le plus bas possible, on dit qu’il occupe l’état fondamental, qui est n = 1. Lorsqu’un électron absorbe un photon, dont l’énergie est égale à la différence entre le premier et le deuxième état, l’électron devient excité et passe de l’état fondamental à l’état excité, où n = 2. Si l’énergie du photon est égale à la différence entre le premier et le troisième état, l’électron passe au troisième état, ou n = 3, et ainsi de suite.
Les électrons peuvent revenir spontanément à l’état fondamental ou à tout autre état inférieur et excité. Lorsque cela se produit, l’énergie excédentaire obtenue grâce à l’excitation est libérée sous la forme d’un photon émis. L’énergie du photon est égale à la différence entre les deux états d’énergie et correspond à différentes longueurs d’onde de la lumière.
<h4>Spectres d’absorption et d’émission</h4>
Bien que la plupart des substances absorbent ou émettent le maximum de lumière à une longueur d’onde, elles ont également tendance à absorber ou à émettre de la lumière dans une gamme de longueurs d’onde. Cette gamme de longueurs d’onde s’appelle un spectre. L’énergie de la lumière absorbée est quantifiée et visualisée à l’aide d’un spectre d’absorption, tandis que l’énergie de la lumière émise est quantifiée et visualisée à l’aide d’un spectre d’émission.
Les spectres d’absorption et d’émission sont mesurés à l’aide d’un spectrophotomètre, qui est un appareil qui transmet la lumière à travers un échantillon, puis mesure à la fois la longueur d’onde et l’intensité de la lumière qui le traverse. À l’intérieur du spectrophotomètre se trouve soit un réseau de diffraction, soit un prisme, qui sépare la lumière entrante dans les longueurs d’onde de ses composantes. Les différentes longueurs d’onde sont ensuite transmises à travers l’échantillon, et l’intensité est enregistrée sur un détecteur CCD (dispositif à couplage de charge linéaire). Le CCD est un circuit intégré gravé sur une surface en silicium qui forme des éléments sensibles à la lumière appelés pixels. Le CCD collecte et trie la lumière diffractée et la relit à une longueur d’onde d’absorption.
Lors de la mesure de l’absorbance d’un échantillon, le soluté est généralement dissous dans un solvant et placé dans un récipient appelé cuvette. Ensuite, l’échantillon est placé à l’intérieur du spectrophotomètre, et l’intensité de la lumière transmise est mesurée en même temps que les longueurs d’onde de la lumière pour obtenir un spectre d’absorbance. Comme prévu, l’intensité de la lumière transmise est plus faible que lorsqu’il n’y a pas d’échantillon présent à l’intérieur du spectrophotomètre.
En effet, la lumière transmise est absorbée par l’échantillon, la cuvette et le solvant. Avant de mesurer des échantillons, le spectrophotomètre doit être étalonné à l’aide d’un « blanc ». Une ébauche est une cuvette qui ne contient que le solvant utilisé pour dissoudre le soluté. Le spectrophotomètre est étalonné de manière à ce que l’absorbance totale due à la cuvette et au solvant soit soustraite de l’absorbance mesurée de l’échantillon. Cela nous permet d’enregistrer l’absorbance qui n’est attribuée qu’à l’espèce d’intérêt.
L’absorbance est souvent mesurée à une longueur d’onde, la longueur d’onde d’absorbance maximale. Cependant, l’absorption peut également être mesurée à une gamme de longueurs d’onde pour acquérir le spectre d’absorption. Pour cela, l’échantillon est exposé à une gamme de longueurs d’onde de lumière incidente, et l’absorption est enregistrée à chaque longueur d’onde. Si l’échantillon émet de la lumière, le spectre d’émission est mesuré de la même manière, sauf que la longueur d’onde incidente est fixée à la longueur d’onde de l’absorbance maximale. L’instrument mesure ensuite l’intensité de la lumière émise sur une gamme de longueurs d’onde.
<h4>Loi Beer-Lambert</h4>
L’absorbance d’un échantillon à la longueur d’onde de l’absorbance maximale fournit des informations sur l’échantillon, à savoir sa concentration. La loi de Beer-Lambert est une équation qui relie la transmittance à la concentration de l’échantillon. La transmittance, ou intensité de la lumière transmise, est la fraction de la lumière d’origine qui traverse l’échantillon, I, divisée par l’intensité de la lumière incidente, I0.
<img src="/CDNSource/lm/labs/49/49_Concepts_1.jpg" text-align center><img src="/CDNSource/lm/labs/49/49_Concepts_2.jpg" align-texte centre><img src="/CDNSource/lm/labs/49/49_Concepts_3.jpg" font-size>Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Chimie et réactivité chimique. Belmont, Californie : Brooks/Cole, Cengage Learning.
	<li style="font-size :17px">Silderberg, M.S. (2009). Chimie : la nature moléculaire de la matière et du changement. Boston, MA : McGraw Hill, Boston.</li>
	<li style="font-size :17px">Harris, D.C. (2015). Analyse chimique quantitative. New York, NY : W.H. Freeman et compagnie.</li>

Loi de la bière

Absorbance et fluorescence
Lorsque la lumière frappe une substance, elle est soit absorbée, soit transmise, soit réfléchie. En règle générale, une substance interagit avec une gamme ...

Education

Enseignement

JoVE Lab Manual

Chemistry

Chimie

absorbance spectra and beer-lambert analysis

Absorbance spectra and the Beer-Lambert Law show how light can be used to study molecules in solution. When light hits a substance, it may be absorbed, transmitted, or reflected. A substance often interacts with a range of wavelengths, and each wavelength can affect the molecules or atoms in a different way.

When a molecule absorbs light, that energy can be used for translation, vibration, rotation, or electron excitement. Electron excitement is especially important in spectroscopy because it moves an electron to a higher energy level. In Niels Bohr’s model, electrons in the hydrogen atom move in fixed circular orbits called stationary states, and each state has a discrete energy. An electron can move only when it absorbs or emits energy equal to the difference between two states.

The lowest energy state is the ground state, or n = 1. If a photon has the right energy, the electron can move to a higher state such as n = 2 or n = 3. When the electron returns to a lower state, it releases the extra energy as an emitted photon. The photon energy matches the energy gap between the two states and corresponds to specific wavelengths of light.

Most substances absorb or emit most strongly at one wavelength, but they usually interact with a range of wavelengths. That range is called a spectrum. An absorption spectrum shows how much light a sample absorbs at different wavelengths, and an emission spectrum shows the light a sample gives off. These spectra are measured with a spectrophotometer, which sends light through a sample and records the wavelength and intensity of the light that passes through.

Inside the spectrophotometer, a diffraction grating or a prism separates incoming light into its component wavelengths. The light then passes through the sample, and a linear charge-coupled device, or CCD, detector records the intensity. A CCD is an integrated circuit on silicon that contains light-sensitive pixels. It collects and sorts the diffracted light and reads it back at an absorption wavelength.

For absorbance measurements, the solute is usually dissolved in a solvent and placed in a cuvette, which is the container used in the instrument. The sample is then placed in the spectrophotometer, and the intensity of transmitted light is measured along with wavelength to produce an absorbance spectrum. The transmitted light is lower than when no sample is present because light is absorbed by the sample, the cuvette, and the solvent. Before samples are measured, the spectrophotometer is calibrated with a blank, which is a cuvette that contains only the solvent. This step removes the absorbance from the cuvette and solvent so the reading reflects only the species of interest.

Absorbance is often measured at the wavelength of maximum absorbance, but it can also be recorded across a range of wavelengths. For emission spectra, the incident wavelength is fixed at the wavelength of maximum absorbance, and the instrument measures emitted light over a range of wavelengths. The Beer-Lambert Law then connects absorbance to sample concentration. Transmittance is the fraction of light that passes through the sample, I divided by I0, and absorbance is related to the negative log of transmittance.

A second form of the Beer-Lambert Law shows that absorbance is directly proportional to concentration when wavelength and pathlength stay constant. The molar attenuation coefficient, ε, describes how strongly a species absorbs light at a given wavelength. The pathlength, l, is the distance the light travels through the sample, which is the width of the cuvette. Standard cuvettes have a 1 cm pathlength. This linear relationship makes it possible to find the concentration of an unknown sample from its absorbance.

To do that, a standard curve is made with known concentrations of the solute. The absorbance at λ max is measured for each standard, and the results are plotted as concentration versus absorbance. The line that appears matches the Beer-Lambert equation, and its slope equals the product of the pathlength and the molar attenuation coefficient. Once that line is known, the concentration of an unknown sample can be calculated from its absorbance.

Beer’s Law can also be used for reactions at equilibrium. If absorbance is measured at the λ max of a specific product or reactant, the equilibrium concentration of that species can be found. The concentrations of the other reactants and products can then be determined, and the equilibrium constant K eq can be solved.

Absorbance Spectra and Beer-Lambert Analysis

absorbance-spectra-and-beer-lambert-analysis

lab techniques

Measuring with Accuracy in the Lab

Measuring with accuracy in the lab starts with careful records, the right tools, and correct reading of data. Scientists use lab notebooks to record how an experiment was done. These notes include the amounts of materials used and each step of the procedure. Without that record, an experiment cannot be repeated in the same way. Balances and glassware are used to measure solids and liquids for experiments. The precision of a measurement depends on the equipment used. Digital balances show the...

scientific measurement and lab skills

Graphs, Curve Fit, and Standard Deviation

Two-dimensional graphs help students compare an independent variable with a dependent variable in science data. In a chemistry example, temperature is the independent variable and volume is the dependent variable. After data are collected, the relationship between the two measurements must be quantified so it can be evaluated and compared with other relationships. A graph can also show when no relationship exists between the two variables. To make a two-dimensional graph, each data point needs...

stoichiometry, product yield, and limiting reactants

stoichiometry-product-yield-and-limiting-reactants

Balancing Equations and Reactant Limits

Chemical equations show how reactants change into products during a reaction. Stoichiometry is the study of the relative amounts of reactants and products. It is based on the Law of Conservation of Mass, which says matter is neither created nor destroyed. Because atoms are only rearranged, the number and identity of atoms must match on both sides of a reaction. That is why a chemical equation must be balanced. A balanced equation has the same number of atoms for each element in the reactants...

redox reactions

Oxidation States in Redox Reactions

Oxidation states help identify redox reactions. In a redox reaction, oxidation is the loss of one or more electrons, and reduction is the gain of one or more electrons. These reactions are often tracked by changes in oxidation state, also called oxidation number. Each atom in a molecule has an oxidation state. This number shows how oxidized an atom is compared with its free elemental form. It is written as the charge an atom would have if every bond were purely ionic. In that model, bonding...

ideal gas law

Gas Laws Behind the Ideal Gas Equation

The ideal gas equation brings together the main gas laws used in chemistry. Gases have molecules that are far apart, so their behavior is described with four measurable variables: pressure, volume, number of moles, and temperature. The ideal gas law links these variables with the gas constant, R, which equals 8.314 J·K−1 mol−1. Boyle’s law describes the link between pressure and volume. When temperature and the number of moles stay constant, pressure and volume change in opposite directions.

acid and base concentrations

Acids, Bases, and pH in Titration

Acids, bases, and pH explain how acidic or basic a solution is. An Arrhenius acid releases hydrogen ions when it dissolves in water, while an Arrhenius base releases hydroxide ions. These reactions also create conjugate partners: an acid forms a conjugate base after losing hydrogen, and a base forms a conjugate acid after losing hydroxyl. Every acid has a conjugate base, and every base has a conjugate acid. pH measures the amount of hydrogen ions in a solution. The pH scale runs from 0 to 14...

buffers

Buffer pH and the Common Ion Effect

Buffer solutions help keep pH steady when acids or bases are added. In water, an Arrhenius acid (HA) dissociates into hydrogen ions, H+, and a conjugate base, A-. More hydrogen ions make a solution more acidic and lower its pH. An Arrhenius base (BOH) dissociates into a conjugate acid, B+, and hydroxide ions, OH-. Strong acids and strong bases usually change pH a lot because they react completely in water and raise the amount of H+ or OH-. Buffers reduce those pH changes. A buffer usually...

enthalpy of reaction

Reaction Heat, Hess’s Law, and Calorimetry

Reaction heat is explained through enthalpy, Hess’s law, and calorimetry. These ideas show how heat moves during chemical reactions and how scientists measure it. Thermodynamics is the study of heat energy and other forms of energy, such as work. Its laws are used across science, from biology to physics. The first law says energy is conserved in an isolated system. Energy can change form, such as thermal energy becoming work, but it cannot be created or destroyed. The second law of...

solubility

Solubility in Water and Other Solvents

Solubility describes how much of a solute can dissolve in a specific solvent. It is usually reported as solute mass per solvent volume or as solute mass per solvent mass. For example, sodium chloride has a solubility of 36 g per 100 mL of water at room temperature. If solubility is given by solvent mass, that mass must be converted to volume for later calculations. Temperature changes solubility. Sodium carbonate dissolves at about 7 g per 100 mL of water at 0 °C, 22 g per 100 mL at room...

metal flame emission

Flame Test Metal Color Clues

Flame testing uses a metal sample and a flame to identify a substance by its color. Each element gives off a characteristic light emission in the flame, so the color can help distinguish one element from another. This same idea appears in fireworks, where different metal components create different colors. The color comes from energy absorbed by the metallic component. When a species absorbs energy, that energy can go into translation, vibration, rotation, or electron excitement. Electron...

balmer series

Hydrogen Emission Lines

Hydrogen emission lines show how electrons move between fixed energy levels in the Bohr model. Niels Bohr proposed this model for the hydrogen atom in 1913. In this model, an electron can only occupy certain orbits, and it cannot exist between them. These energy states are labeled with the quantum number n. The ground state is n = 1. Higher values, such as n = 2, 3, 4, and so on, are excited states. An electron changes state only by absorbing or emitting energy equal to the difference between...

beer's law

Absorbance spectra and the Beer-Lambert Law show how light can be used to study molecules in solution. When light hits a substance, it may be absorbed, transmitted, or reflected. A substance often interacts with a range of wavelengths, and each wavelength can affect the molecules or atoms in a different way. When a molecule absorbs light, that energy can be used for translation, vibration, rotation, or electron excitement. Electron excitement is especially important in spectroscopy because it...

concentration dependence

Reaction Order and Rate Constants

Chemical kinetics describes the speed of a chemical reaction. The rate depends on the reaction mechanism, the complexity of the reaction, and the number of reactants involved. Reactant concentration also affects how fast a reaction happens. The rate law is the experimental relationship that connects these factors to reaction rate. In the rate law, each reactant contributes through a reaction order. The general form is r = k[A]^m[B]^n for aA + bB + ... → cC. Here, r is the reaction rate, k is...

temperature dependence

Reaction Rate, Arrhenius, and Catalysts

Chemical kinetics explains how fast a reaction happens and why that speed changes. The reaction rate is the speed of a chemical reaction. It is defined as the change in concentration of a reactant or product over time. Reaction speed depends on several factors, including the concentration of the reactants and the temperature of the reaction. Each reactant affects the rate in a specific way. That relationship is described by the rate law. The rate law is an equation that connects the...

galvanic cells

Battery Power from Redox Reactions

Electrochemistry studies how chemical change and electrical energy are linked. In this branch of chemistry, electrons move from one species to another. That movement can either produce current or be driven by an applied current. The main reaction in electrochemistry is the oxidation-reduction, or redox, reaction. A redox reaction has two half-reactions. Oxidation is the loss of electrons, and reduction is the gain of electrons. These changes also shift the oxidation state of each substance. The...

electrolytic cells

Electroplating with Electrolytic Cells

Electrochemistry links electrical energy with chemical change, and electrolytic cells show how electricity can drive a nonspontaneous reaction. In these reactions, electrons move from one species to another. The key chemistry is a redox reaction, or oxidation-reduction reaction, which happens in two half-reactions. Oxidation is the loss of electrons, and reduction is the gain of electrons. The substance that loses electrons is the reducing agent, and the substance that gains electrons is the...

proper lab notebook keeping

Recording a Reproducible Lab Experiment

Accurate lab notebook records are essential in science and organic chemistry labs. Without clear notes, descriptions, quantities, and observations, an experiment cannot be repeated or checked. Good record keeping supports reproducibility, which means other people can follow the same work and get similar results. A laboratory notebook is useful in school labs, research settings, and industry. It can also become a legal record when an idea, process, substance, or technique is patented and...

basic organic chemistry techniques

Organic Lab Glassware and Safe Handling

Organic chemistry labs use specific glassware, careful weighing steps, and controlled heating to keep reactions safe and accurate. Beakers and Erlenmeyer flasks are often used for mixing or holding solvents, but they are not meant for measuring volume unless only an approximate amount is needed. Volumetric glassware is used when a precise volume matters. This group includes graduated cylinders, volumetric pipettes, and volumetric flasks. A graduated cylinder is commonly used in the organic lab...

melting points

How Impurities Change Melting Points

Melting point helps identify an organic compound by showing the temperature where a solid turns into a liquid at 1 atmosphere of pressure. It is a physical property, like solubility, density, color, and electronegativity. Because the exact start of melting is hard to measure, the result is reported as a range. The lower end is when the first drops of liquid appear. The upper end is when the last solid has melted. Scientists compare this range with literature values to help identify a compound. <br...

boiling points

Boiling Point and Vapor Pressure

Boiling point and vapor pressure are key physical properties of a liquid. For a pure compound, the boiling point can help identify the substance. In experiments, the boiling point is usually recorded as a range because exact values are hard to measure and may differ by a few degrees from the literature value. Vapor pressure explains why a liquid boils. In a sealed container, some molecules at the liquid surface gain enough energy to escape into the gas phase. At the same time, some gas...

recrystallization

Purifying Crystals by Solubility

Purifying crystals by solubility is a key organic chemistry technique. Recrystallization uses differences in solubility to separate a desired solid product from starting materials, solvent, and impurities. It is one of the three standard purification methods, along with distillation and extraction. Solubility is the maximum amount of a substance that can dissolve in a fixed volume of solvent at a given temperature. Different solutes dissolve in different solvents, and their behavior can vary...

extraction

Separating Mixtures with Solvent Extraction

Solvent extraction is a common way to separate and isolate a target compound in organic chemistry. In this process, a solute moves from one phase to another so it can be separated from unreacted starting materials or impurities. Extraction is also useful when a product is dissolved in a reaction solvent that is hard to remove by evaporation, such as a solvent with a high boiling point. There are three main types of extraction. In solid-liquid extraction, the solute moves from a solid phase...

simple distillation

Separating Liquids by Boiling Point

Separating liquids by boiling point is the goal of distillation. A miscible mixture contains liquids that mix evenly, and it can be heated so that the more volatile liquid evaporates first. Volatility is a liquid’s tendency to vaporize, and boiling begins when bubbles form in the liquid phase. In a closed container, liquid and vapor can reach equilibrium, which means the rate of vaporization equals the rate of condensation. The pressure from the vapor above the liquid is called vapor pressure.

steam distillation

Steam Distillation and Low-Boiling Separation

Steam distillation is a separation method for temperature-sensitive organic compounds. It works best when the organic compound is immiscible in water, meaning the two liquids do not mix. The method helps remove the organic molecule from a non-volatile contaminant without using the higher temperatures needed for many other separations. In steam distillation, the immiscible mixture is heated until both water and the volatile organic compound distill together. The vapor rises to a condenser,...

thin-layer chromatography

TLC Separation by Polarity

Thin-layer chromatography (TLC) separates compounds in a mixture by polarity. It uses the same basic parts as other chromatography methods: a stationary phase, a mobile phase, and the solute. In TLC, the stationary phase is a thin coating on a plate instead of material packed into a column. The stationary phase in most TLC systems is polar silica gel, a form of silicon dioxide. Its surface has OH groups that can form hydrogen bonds. The sample is first spotted on a pencil start line with a...

column chromatography

Separating Mixtures by Polarity

Column chromatography separates compounds in an organic chemistry lab by using differences in physical properties. It is especially useful for microscale separations when less than 10 grams of material is available for analysis. The method can separate a mixture based on solubility, polarity, hydrophobicity, size, and charge. Any chromatography system has three main parts: the stationary phase, the mobile phase, and the sample mixture. In column chromatography, the stationary phase is usually...

hydrolysis of an ester

Soap Formation from Triglycerides

Triglycerides are lipids made of a glycerol backbone linked to three fatty acids. They are common in vegetable oils and animal fats. Plant lipids are the basis of vegetable oils, while animal lipids are used as lard or general sources of fats. A fatty acid has a long carbon chain, usually between 12 and 20 carbons, with a weak organic acid at one end. Some fatty acids contain double bonds. These are unsaturated fats. The double bond usually gives the chain a bent, cis shape. As a result,...

synthesis of luminol

Luminol Chemiluminescence in Forensics

Luminol chemiluminescence shows how a chemical reaction can produce light and help with blood detection. Chemical reactions may be endothermic or exothermic, depending on whether they absorb or release heat as reactants become products. The enthalpy of a reaction is the energy difference between reactants and products, or ΔH. A positive ΔH means the reaction is endothermic. A negative ΔH means it is exothermic. Many reactions do not happen in a single step. They can pass through intermediates...

esterification

Ester Formation and Everyday Fragrances

Esters are organic compounds with a carbonyl group attached to an oxygen that connects to another alkyl or aryl group. Their general formula is RCOOR'. They are often made by reacting a carboxylic acid with an alcohol in an esterification reaction. In this process, the hydroxyl group of the acid is replaced by an alkyl or aryl group. Many simple esters have familiar fruit and flower smells because their structures are small and less complex. Formic acid and methanol can combine to form methyl...

identification of unknown aldehydes and ketones

identification-of-unknown-aldehydes-and-ketones

Carbonyl Tests for Aldehydes and Ketones

Aldehydes and ketones are carbonyl compounds, which means they contain a C=O functional group. A ketone has two alkyl or aryl groups attached to the carbonyl carbon. Acetone is the simplest ketone, with two methyl groups. An aldehyde is similar, but it has at least one hydrogen attached to the carbonyl carbon. Formaldehyde is the simplest aldehyde, and acetaldehyde has one hydrogen and one methyl group. The carbonyl carbon in both aldehydes and ketones is electrophilic because the oxygen atom...

uv-vis spectroscopy of dyes

How Dyes Absorb Visible Light

Dyes absorb visible light because their molecular structure lets them interact with specific wavelengths. When light hits a substance, some light is absorbed and the rest is reflected or transmitted. The color we see depends on the wavelength that is reflected. A blue substance reflects blue light, around 430 to 480 nm, and absorbs the complementary orange region, around 590 to 630 nm. Compounds that do not absorb in the visible range appear colorless to the human eye. Light also has energy and...

identifying alcohols

Alcohol Tests for Primary, Secondary, and Tertiary

Alcohol tests can help sort alcohols by structure and reactivity. Alcohols are organic molecules with a hydroxyl group attached to an alkyl or aryl group, written as ROH. If the carbon with the hydroxyl group has one R group, the alcohol is primary. If it has two R groups, it is secondary. If it has three R groups, it is tertiary. Alcohols can also be aromatic when they contain a benzene ring. Phenol is the simplest aromatic alcohol. To identify an alcohol, chemists can use its chemical...

chemistry

Des vidéos complètes, axées sur le programme de cours pour les travaux pratiques de Chimie, adaptées à la fois aux professeurs et aux étudiants.

https://cloudfront.jove.com/files/banners/scicat/labbio.jpg

Explore how spectrophotometers measure absorbance spectra, how blank calibration works, and how Beer-Lambert Law is used to find concentration and equilibrium values.

Absorbance Spectra and Beer-Lambert Analysis | JoVE K12

Louisiana Student Standards for Science

motion-and-stability-forces-and-interactions

Motion And Stability: Forces And Interactions

HS-PS2-6

Préparez l'enseignement de Loi de la bière avec ce guide instructeur de 7 minutes. Couvre les principales étapes de préparation pour le laboratoire Chemistry sur JoVE Lab Manual.

Préparation de l'instructeur pour Loi de la bière dans Chemistry - 4.7K+ vues | JoVE Lab Manual

Suivez les procédures étape par étape de Loi de la bière dans cette vidéo de protocole étudiant de 17 minutes. Fait partie de la collection Chemistry sur JoVE Lab Manual.

Protocole étudiant de Loi de la bière dans Chemistry - 40K+ vues | JoVE Lab Manual

The Atomic Theory of Matter

The earliest recorded discussion of the basic structure of matter comes from ancient Greek philosophers. Leucippus and Democritus argued that all matter ...

Subatomic Particles

Dalton was only partially correct about the particles that make up matter. All matter is composed of atoms, and atoms are composed of three smaller ...

Elements: Chemical Symbols and Isotopes

A chemical symbol is an abbreviation used to indicate an element or an atom of an element. For example, the symbol for mercury is Hg. The same symbol is ...

Ions and Ionic Charges

In ordinary chemical reactions, the nucleus &mdash; which contains the protons and neutrons of each atom and thus identifies the element &mdash; remains ...

The Periodic Table

As early chemists discovered more elements, they realized that various elements could be grouped by their similar chemical behaviors. One such grouping ...

Atomic Mass

Atoms &mdash; and the protons, neutrons, and electrons that compose them &mdash; are extremely small. For example, a carbon atom weighs less than 2 ...

Molar Mass

The identity of a substance is defined not only by the types of atoms or ions it contains but by the quantity of each type of atom or ion. For example, ...

Periodic Classification of the Elements

The periodic table arranges atoms based on increasing atomic number so that elements with the same chemical properties recur periodically. When their ...

Types of Chemical Bonds

Chemical bonding theories were pioneered by American chemist Gilbert&#xA0;N. Lewis. He developed a model called the Lewis model to explain the type and ...

Ionic Bonding and Electron Transfer

Ions are atoms or molecules bearing an electrical charge. A cation (a positive ion) forms when a neutral atom loses one or more electrons from its valence ...

The Born-Haber Cycle

Lattice Energy&#xA0;

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a ...

trends-in-lattice-energy-ion-size-and-charge

Trends in Lattice Energy: Ion Size and Charge

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a compound is a measure ...

Covalent Bonding and Lewis Structures

Compared to ionic bonds, which results from the transfer of electrons between metallic and nonmetallic atoms, covalent bonds result from the mutual ...

Electronegativity

Whether a bond is nonpolar or polar covalent is determined by a property of the bonding atoms called electronegativity.&#xA0;

Electronegativity values of ...

bond-polarity-dipole-moment-and-percent-ionic-character

Bond Polarity, Dipole Moment, and Percent Ionic Character

Bond Polarity

The absolute value of the difference in electronegativity (&Delta;EN) of two bonded atoms provides a rough measure of the polarity to be ...

lewis-structures-of-molecular-compounds-and-polyatomic-ions

Lewis Structures of Molecular Compounds and Polyatomic Ions

To draw Lewis structures for complicated molecules and molecular ions, it is helpful to follow a step-by-step procedure as outlined:


	Determine the ...

Bond Energies and Bond Lengths

Stable molecules exist because covalent bonds hold the atoms together. The strength of a covalent bond is measured by the energy required to break it, ...

Bonding in Metals

Metallic bonds are formed between two metal atoms. A simplified model to describe metallic bonding has been developed by Paul Dr&#xFC;de called the ...

Model basic atomic structure and relate an element's atomic structure to its bonding, reactivity, and placement on the Periodic Table.

IPC.7.A

physical-and-chemical-properties-of-matter

Physical and Chemical Properties of Matter

The characteristics that enable us to distinguish one substance from another are called properties.

Physical Properties of Matter

A physical property is ...

atomic-radii-and-effective-nuclear-charge

Atomic Radii and Effective Nuclear Charge

The elements in groups of the periodic table exhibit similar chemical behavior. This similarity occurs because the members of a group have the same number ...

Ionic Radii

Ionic radius is the measure used to describe the size of an ion. A cation always has fewer electrons and the same number of protons as the parent atom; it ...

Ionization Energy

The amount of energy required to remove the most loosely bound electron from a gaseous atom in its ground state is called its first ionization energy ...

Electron Affinity

The electron affinity (EA) is the energy change for adding an electron to a gaseous atom to form an anion (negative ion).



This process can be either ...

Alkali Metals

Group 1 elements are soft and shiny metallic solids. They are malleable, ductile, and good conductors of heat and electricity. The melting points of the ...

Halogens

Group 17 elements, known as halogens, are nonmetals. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine a solid. ...

Noble Gases


The elements in group 18 are noble gases (helium, neon, argon, krypton, xenon, and radon). They earned the name &ldquo;noble&rdquo; because they were ...

Use patterns within the Periodic Table to predict the relative physical and chemical properties of elements.

IPC.7.B

Classifying Matter by State

Chemistry is the study of matter and the changes it undergoes. Matter is anything that has mass and occupies space. Matter is all around us; the air, ...

Classifying Matter by Composition

Matter: Pure Substances and Mixtures
According to its composition, the matter can be classified into two broad categories &mdash; pure substances and ...

Phase Transitions

Whether solid, liquid, or gas, a substance&#39;s state depends on the order and arrangement of its particles (atoms, molecules, or ions). Particles in the ...

phase-transitions-vaporization-and-condensation

Phase Transitions: Vaporization and Condensation

The physical form of a substance changes on changing its temperature. For example, raising the temperature of a liquid causes the liquid to vaporize ...

Explain how physical and chemical properties of substances are related to their usage in everyday life such as in sunscreen, cookware, industrial applications, and fuels.

IPC.7.C

Science concepts. The student knows that relationships exist between the structure and properties of matter. The student is expected to:

Absorbance Spectra and Beer-Lambert Analysis

Description

Transcript

Tags

More videos by Standards