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

תגובות כימיות רבות מתקדמות בשני כיוונים, קדימה ואחורה. עם הזמן, התגובות קדימה ואחורה יתרחשו באותו קצב, והריכוז של המגיבים והמוצרים לא ישתנה עוד. זה ידוע בשם שיווי משקל כימי.
בשיווי משקל כימי, הריכוזים של כל רכיב קשורים זה לזה על ידי קבוע שיווי המשקל, K, שהוא היחס בין ריכוזי המכפלה לריכוזי המגיבים, שכל אחד מהם מועלה לחזקת המקדמים הסטויכיומטריים שלהם.
אבל איך קובעים ריכוזי שיווי משקל? שיטה אחת מודדת את עוצמת אורך הגל של האור שהמוצר בולע לפני ואחרי שהוא עובר דגימה. הפרש העוצמה נקרא ספיגה, והוא מתאים לכמות התרכובת הבולעת בדגימה.
אתם אולי זוכרים שאלקטרונים תופסים בעיקר את מצב הקרקע. כאשר הם סופגים כמות מסוימת של אנרגיה, הם נרגשים לרמת אנרגיה גבוהה יותר. אנרגיה זו מתאימה לאורך גל מסוים של אור. ניתן למצוא אורך גל זה ולמדוד בליעה באמצעות ספקטרופוטומטר, המכוון קרן אור דרך הדגימה ומודד את השינוי בעוצמתו באורכי גל אחד או יותר.
הבליעה שווה ללוג השלילי של עוצמת האור המוחלש על פני עוצמת אור האירוע. על ידי התוויית ערכי הספיגה של תמיסות מרובות עם ריכוזי מוצרים ידועים שונים, אנו רואים קשר ליניארי בין ספיגה לריכוז. זו דוגמה לחוק הבירה.
חוק באר מבוטא מתמטית על ידי משוואה זו, כאשר A הוא הבליעה, אפסילון הוא מקדם ההנחתה המולרי, קבוע המשתנה עבור כל תרכובת, l הוא אורך נתיב האור דרך הדגימה, ו- c הוא ריכוז התרכובת.
על ידי זיהוי הפונקציה הליניארית עבור תרכובת נתונה באורך גל מסוים ובאורך נתיב מסוים, ניתן להשתמש בנתוני הבליעה של תמיסה בשיווי משקל כדי לקבוע את ריכוז שיווי המשקל של המכפלה. משם, ניתן לחשב את ריכוזי שיווי המשקל של המגיבים ולפתור עבור קבוע שיווי המשקל. במעבדה זו, תוכלו להכין תמיסות של ברזל (III) isothiocyanate ולהשתמש ספקטרופוטומטר כדי לקבוע את ספיגתו בריכוזים שונים.

חוק הבירה

ספיגה ופלואורסצנטיות
כאשר אור פוגע בחומר, הוא נספג, מועבר או מוחזר. בדרך כלל, חומר מקיים אינטראקציה עם טווח של אורכי גל של אור, כאשר כל אחד מהם מתקשר עם המולקולות או האטומים באופן שונה. חומר עשוי לבלוע טווח מסוים של אורכי גל, להחזיר טווח אחר של אורכי גל, ולהעביר את אורכי הגל האחרים של האור.
כאשר מולקולה בולעת אור, האנרגיה משמשת בארבע דרכים שונות: (1) תרגום, הגורם למולקולה לשנות את מהירותה המולקולרית; (2) רטט, הגורם למרחק בין המולקולות להשתנות במהירות; (3) סיבוב, הגורם לאטומים להסתובב סביב הקשרים במולקולה; ו-(4) התרגשות אלקטרונים, הגורמת לאלקטרונים לעבור לרמות אנרגיה גבוהות יותר.
<h4>רמות אנרגיה</h4>
בשנת 1913 הציע נילס בוהר מודל לאטום המימן שבו אלקטרונים נעים סביב הגרעין במסלולים קבועים ומעגליים, הנקראים מצבים נייחים. האנרגיה המשויכת לכל מסלול, או מצב נייח, קיימת רק באנרגיות קבועות ובדידות. רק כאשר אלקטרון נע למסלול אחר נספגת או נפלטת אנרגיה. האלקטרון לעולם אינו נמצא בין מצבים. שינוי זה מתרחש רק אם האנרגיה הנבלעת או הנפלטת שווה להפרש בין שני מצבי האנרגיה.
במודל של בוהר, המספר הקוונטי n מייצג את האנרגיה של האלקטרון. כאשר אלקטרון תופס את מצב האנרגיה הנמוך ביותר האפשרי, הוא אמור לתפוס את מצב הקרקע, שהוא n = 1. כאשר אלקטרון בולע פוטון, שהאנרגיה שלו שווה להפרש בין המצב הראשון והשני, האלקטרון נעשה נרגש ועובר ממצב הקרקע למצב המעורר, כאשר n = 2. אם האנרגיה של הפוטון שווה להפרש בין המצב הראשון למצב השלישי, האלקטרון עובר למצב השלישי, או n = 3, וכן הלאה.
אלקטרונים יכולים לחזור באופן ספונטני למצב הקרקע או לכל מצב נמוך ומעורר אחר. כאשר זה קורה, האנרגיה העודפת שהושגה מעירור משתחררת בצורה של פוטון שנפלט. האנרגיה של הפוטון שווה להפרש בין שני מצבי האנרגיה ומתאימה לאורכי גל שונים של אור.
<h4>ספקטרום בליעה ופליטה</h4>
בעוד שרוב החומרים בולעים או פולטים את כמות האור המרבית באורך גל אחד, הם גם נוטים לבלוע או לפלוט אור בטווח של אורכי גל. טווח זה של אורכי גל נקרא ספקטרום. האנרגיה של האור הנבלע מכומתת ומדמיינת באמצעות ספקטרום בליעה, בעוד האנרגיה של האור הנפלט מכומתת ומומחישה באמצעות ספקטרום פליטה.
ספקטרום בליעה ופליטה נמדד באמצעות ספקטרופוטומטר, שהוא מכשיר המעביר אור דרך דגימה ולאחר מכן מודד הן את אורך הגל והן את עוצמת האור העובר דרכה. בתוך הספקטרופוטומטר נמצא סורג עקיפה או מנסרה, המפרידה את האור הנכנס לאורכי הגל המרכיבים אותו. אורכי הגל השונים מועברים לאחר מכן דרך הדגימה, והעוצמה נרשמת בגלאי מכשיר צימוד מטען ליניארי (CCD). CCD הוא מעגל משולב החרוט על משטח סיליקון היוצר אלמנטים רגישים לאור הנקראים פיקסלים. ה-CCD אוסף וממיין את האור המפוזר וקורא אותו בחזרה באורך גל בליעה.
בעת מדידת ספיגת דגימה, המומס מומס בדרך כלל בממס ומונח במיכל המכונה קובטה. לאחר מכן, הדגימה ממוקמת בתוך הספקטרופוטומטר, ועוצמת האור המשודר נמדדת יחד עם אורכי הגל של האור כדי לקבל ספקטרום בליעה. כצפוי, עוצמת האור המשודר נמוכה יותר מאשר כאשר אין דגימה בתוך הספקטרופוטומטר.
הסיבה לכך היא שהאור המועבר נבלע על ידי הדגימה, הקובט והממס. לפני מדידת הדגימות, יש לכייל את הספקטרופוטומטר עם 'ריק'. ריק הוא קובטה המכילה רק את הממס המשמש להמסת המומס. הספקטרופוטומטר מכויל כך שהספיגה הכוללת של הקובטה והממס מופחתת מהספיגה הנמדדת של הדגימה. זה מאפשר לנו לרשום את הספיגה המיוחסת רק למינים המעניינים.
הבליעה נמדדת לרוב באורך גל אחד, אורך גל הבליעה המרבי. עם זאת, ניתן למדוד את הבליעה גם בטווח של אורכי גל כדי לרכוש את ספקטרום הבליעה. לשם כך, הדגימה נחשפת לטווח אורכי גל של אור אירוע, והבליעה נרשמת בכל אורך גל. אם הדגימה פולטת אור, ספקטרום הפליטה נמדד באופן דומה, אלא שאורך גל האירוע קבוע באורך הגל של בליעה מרבית. לאחר מכן המכשיר מודד את עוצמת האור הנפלט על פני טווח של אורכי גל.
<h4>חוק באר-למברט</h4>
בליעת דגימה באורך גל של בליעה מקסימלית מספקת מידע על הדגימה, כלומר על ריכוזה. חוק באר-למברט הוא משוואה המתייחסת בין העברה לריכוז דגימה. ההעברה, או עוצמת האור המשודר, היא שבריר האור המקורי העובר דרך הדגימה, I, חלקי עוצמת אור האירוע, I0.
<img src="/CDNSource/lm/labs/49/49_Concepts_1.jpg" alt="משוואת שידור \(T = \frac{I}{I_0}\), דיאגרמה לניתוח צפיפות אופטית." data-altupdatedat="1754907201000">
חוק באר-למברט קובע כי הבליעה האופטית, A, של מין בתמיסה קשורה ליומן השלילי של ההעברה.
<img src="/CDNSource/lm/labs/49/49_Concepts_2.jpg" alt="נוסחת ספיגה A=-log(T), משוואה לניתוח ספיגה אופטית במחקרי ספקטרוסקופיה." data-altupdatedat="1754907201000">
גרסה חלופית של חוק באר-למברט קובעת כי הבליעה האופטית, A, של מין בתמיסה פרופורציונלית באופן ליניארי לריכוז, c, של אותו מין כאשר אורך הגל, λ ואורך המסלול, l, מוחזקים קבועים.
<img src="/CDNSource/lm/labs/49/49_Concepts_3.jpg" alt="משוואת חוק באר-למברט, A=εcl, מושג ספיגה, ניתוח נתונים ספקטרוסקופיים, דיאגרמת פיזיקה." data-altupdatedat="1754907201000">
מקדם ההנחתה המולרי, ε, הוא מדד לעוצמת בליעת האור באורך גל נתון. ככל שמקדם ההנחתה המולרי גדול יותר, כך הספיגה גדולה יותר. אורך המסלול, l, הוא המרחק שהאור עובר דרך הדגימה, שהוא רוחב הקובט. לקוביטות סטנדרטיות יש אורך מסלול של 1 ס"מ.
קשר ליניארי זה בין ספיגה לריכוז הוא כלי רב עוצמה המשמש לקביעת ריכוז של דגימה לא ידועה בהתבסס על ספיגתה. כדי לעשות זאת, עקומה סטנדרטית נוצרת באמצעות שיפוע של ריכוזים ידועים של המומס. הבליעה בשיא אורך גל הבליעה, λmax, נמדדת עבור כל ריכוז.
על ידי התוויית ריכוז לעומת ספיגה, נצפה קשר ליניארי המתאים למשוואת באר-למברט. שיפוע קו זה שווה למכפלה של אורך המסלול ומקדם ההנחתה המולרי. באמצעות פונקציה ליניארית מחושבת זו, אם הספיגה של המדגם הלא ידוע ידועה, ניתן לקבוע בקלות את הריכוז.
אם הדגימה המנותחת היא תגובה בשיווי משקל, ניתן להשתמש בחוק באר כדי לקבוע את ריכוז שיווי המשקל של מוצר או מגיב אם הספיגה נמדדת ב λמקסימום ספציפי לאותו מוצר או מגיב. ברגע שהריכוז ידוע, ניתן לקבוע את ריכוזי שיווי המשקל של המגיבים והתוצרים הנותרים ולאחר מכן לפתור עבור קבוע שיווי המשקל Keq.
<h4>הפניות</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). כימיה ותגובתיות כימית. בלמונט, קליפורניה: Brooks/Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silderberg, M.S. (2009). כימיה: הטבע המולקולרי של חומר ושינוי. בוסטון, מסצ'וסטס: מקגרו היל, בוסטון.</li>
	<li style="font-size:17px">Harris, D.C. (2015). אנליזה כימית כמותית. ניו יורק, ניו יורק: W.H. Freeman and Company.</li>
</ol>

ספיגה ופלואורסצנטיות
כאשר אור פוגע בחומר, הוא נספג, מועבר או מוחזר. בדרך כלל, חומר מקיים אינטראקציה עם טווח ...

Education

JoVE Lab Manual

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כימיה

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

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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

Comprehensive, curriculum-focused videos for both instructors and students in introductory chemistry lab courses.

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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

התכוננו ללמד את חוק הבירה עם מדריך מדריכים זה באורך 7 דקות. כולל שלבי הכנה מרכזיים עבור מעבדת Chemistry ב-JoVE Lab Manual.

הכנת מדריך עבור חוק הבירה ב-Chemistry - 4.7K+ צפיות | JoVE Lab Manual

עקבו אחר הנהלים של חוק הבירה שלב אחר שלב בסרטון פרוטוקול סטודנטים זה באורך 17 דקות. חלק מאוסף Chemistry ב-JoVE Lab Manual.

פרוטוקול סטודנטים עבור חוק הבירה ב-Chemistry - 40K+ צפיות | 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 ...

Phase Transitions: Melting and Freezing

Heating a crystalline solid increases the average energy of its atoms, molecules, or ions, and the solid gets hotter. At some point, the added energy ...

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

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