Name: battery power from redox reactions
Uploaded: 2020-03-26T07:22:26.000Z
Duration: 3 min 31 s
Channel: JoVE

תגובה אלקטרוכימית היא תהליך שבו אלקטרונים מועברים בין שני חומרים, כאשר אחד מאבד אלקטרונים והשני זוכה בהם. כאשר חומר מאבד אלקטרונים, אנו אומרים שהוא עבר חמצון או שהוא עבר חמצון. כאשר הוא צובר אלקטרונים, אנו אומרים שהוא עבר הפחתה או הצטמצם. כדי לעזור לכם להבחין בין השניים, זכרו את הביטוי 'אסדת נפט', שמשמעותו חמצון הוא הפסד, הפחתה היא רווח. כמה תגובות אלקטרוכימיות מתרחשות באופן ספונטני, אשר אנו יכולים לחקור בתא גלווני.
התא הגלווני מורכב מארבעה חלקים עיקריים. ראשית, ישנן שתי אלקטרודות מתכת, שכל אחת מהן שקועה בתמיסת אלקטרוליטים אחרת. התמיסות מורכבות מהיונים הדרושים לתגובות החצי, תגובת החמצון או ההפחתה, המתרחשות בכל אלקטרודה. התמיסות מחוברות באמצעות גשר מלח, המאפשר זרימת יונים ושומר על ניטרליות המטען של התמיסות. האלקטרודות מחוברות באמצעות מעגל חיצוני, דרכו עוברים האלקטרונים במהלך התגובה.
האנודה היא האלקטרודה שמוותרת על אלקטרונים בתגובה או מחומצנת. הקתודה היא האלקטרודה שצוברת אלקטרונים ומצטמצמת. כדי לעזור לכם להבחין בין שתי התוויות הללו, זכרו את הביטוי 'חתול אדום', מכיוון שהפחתה מתרחשת בקתודה.
אז איך נדע איזו מתכת פועלת כאנודה ואיזו פועלת כקתודה? אנו יכולים להשתמש בפוטנציאל האלקטרודות הסטנדרטי של המתכת, שהוא מדד לנטייתה של המתכת לאבד אלקטרונים. פוטנציאל אלקטרודה שלילי יותר פירושו שלמתכת יש נטייה גבוהה יותר לאבד אלקטרונים. אלקטרונים תמיד זורמים מהמתכת עם הפוטנציאל הסטנדרטי הנמוך יותר למתכת עם הפוטנציאל הסטנדרטי הגדול יותר.
בתא גלווני, אלקטרונים זורמים מהאנודה אל הקתודה, ולכן האנודה היא המתכת בעלת פוטנציאל התקן הנמוך יותר, והקתודה היא המתכת בעלת פוטנציאל התקן הגבוה יותר. כאשר האלקטרונים נעים בין שתי האלקטרודות, נמדד פוטנציאל חשמלי. גודל המתח המופק שווה לפוטנציאל הסטנדרטי של הקתודה פחות הפוטנציאל הסטנדרטי של האנודה.
לכן, אם משתמשים בשתי מתכות בעלות פוטנציאל אלקטרודה סטנדרטי דומה בתא הגלווני, כמו עופרת ובדיל, למשל, גודל המתח שיופק יהיה נמוך. אם למתכות יש פוטנציאלים סטנדרטיים שונים, כמו כסף ומגנזיום, המתח שיופק יהיה גבוה.
במעבדה זו תבנו תא גלווני באמצעות שתי אלקטרודות מתכת ותמדדו את הפוטנציאל המיוצר כאשר מתרחשות תגובות החמצון והחיזור. לאחר מכן תזהה אלקטרודות מתכת לא ידועות באמצעות פוטנציאלים סטנדרטיים ידועים ותקבע את גודל המתח המיוצר.

תאים גלוונים

Galvanic Cells

אלקטרוכימיה
אלקטרוכימיה היא ענף בכימיה החוקר את הקשר בין אנרגיה חשמלית לשינוי כימי. תגובות כימיות אלה כרוכות בתנועה של אלקטרונים ממין אחד למשנהו. תנועה זו יוצרת זרם, או שהיא מונעת על ידי זרם מיושם.
תגובת המפתח באלקטרוכימיה היא תגובת חמצון-חיזור, או תגובת חמצון-חיזור. תגובת החיזור מורכבת משתי חצאי תגובות; תגובת החמצון, שבה חומר מאבד אלקטרונים, ותגובת החיזור, שבה חומר צובר אלקטרונים. תגובה כימית זו גורמת לכל חומר לשנות את מצב החמצון שלו.
האטום או המולקולה שמאבדים אלקטרונים, או מחומצנים, הם הגורם המחזר. האטום או המולקולה שמקבלים את האלקטרונים, או מופחתים, הם הגורם המחמצן. אחת השיטות לזכור את היחסים האלה היא באמצעות הביטוי 'אסדת נפט', שמשמעותו 'oxidation is losing – reduction is gaining'.
<h4>תאים אלקטרוכימיים</h4>
תא אלקטרוכימי הוא מכשיר המייצר זרם חשמלי מהאנרגיה המשתחררת על ידי תגובת חיזור. תא אלקטרוכימי מורכב מתא תגובה או תאים ושתי אלקטרודות מוליכות: אנודה וקתודה. האנודה והקתודה מחוברות חשמלית, ותא התגובה מלא באלקטרוליט. בין שני התאים נמצא גשר מלח, המשלים את המעגל ומאפשר ליונים לעבור בין אלקטרודות.
הפחתה מתרחשת בקתודה, בעוד חמצון מתרחש באנודה. ניתן לזכור זאת בקלות באמצעות "חתול אדום", כלומר יניקה אדומהמתרחשת בהוד החתול.
ישנם שני סוגים של תאים אלקטרוכימיים. אחד מהם הוא התא האלקטרוליטי, המשתמש באנרגיה חשמלית כדי להניע תגובה לא ספונטנית. בסוג זה של תא, אנרגיה חשמלית מסופקת ממקור כוח חיצוני. הסוג השני של התא הוא התא הגלווני, המשתמש בתגובה ספונטנית כדי לייצר אנרגיה חשמלית. סוללות הן דוגמה לתאים גלווניים.
<h4>תאים גלוונים</h4>
תאים גלווניים, הידועים גם בשם תאים וולטאיים, מורכבים משני חצאי תאים. כל חצי תא מכיל אלקטרודת מתכת השקועה באלקטרוליט. מעגל חיצוני מחבר בין שתי האלקטרודות, וגשר מלח מחבר בין שתי תמיסות האלקטרוליטים. האלקטרונים זורמים מהאנודה אל הקתודה. חצי תגובת החמצון מתרחשת באנודה, ואילו חצי תגובת החמצון מתרחשת בקתודה. לדוגמה, בתא גלווני בין נחושת למגנזיום, מתרחשת חצי התגובה הבאה בקתודה:
Cu2+ + 2e- → Cu
וחצי התגובה הבאה מתרחשת באנודה:
Mg → Mg2+ + 2e-
כאשר אלקטרונים הולכים לאיבוד במהלך חמצון באנודה, הם עוברים דרך המעגל החיצוני כדי להפחית את הקתודה וליצור זרם.
כאשר האנודה מחומצנת, ריכוז הקטיונים עולה באלקטרוליט. באופן דומה, ככל שהקתודה מופחתת, ריכוז האניונים עולה באלקטרוליט. כדי לשמור על ניטרליות חשמלית, היונים נעים על פני גשר המלח. כאשר קטיונים נוצרים באנודה, אניונים נעים מהתמיסה בצד האנודה באמצעות גשר המלח. בצד הקתודה נוצרים אניונים, מה שגורם לקטונים לנוע מגשר המלח לתמיסה בצד הקתודה. חשוב לזכור שהאלקטרונים עוברים דרך חוטי המעגל החיצוני, והיונים זורמים דרך גשר המלח והתמיסות.
<h4>פוטנציאל הפחתה</h4>
למתכות מסוימות יש נטייה גדולה יותר מאחרות לאבד אלקטרונים. לכן, עוצמת הזרם החשמלי המיוצר על ידי תא גלווני תלויה בסוגי אלקטרודות המתכת. פוטנציאל האלקטרודה הסטנדרטי (Eo) של חומר הוא מדד לנטייה של חומר לאבד אלקטרונים. אם נעשה שימוש בשתי מתכות בעלות פוטנציאל אלקטרודה כמעט שווה, אזי גודל הזרם המיוצר יהיה קטן. אם משתמשים בשתי מתכות עם פוטנציאלי אלקטרודה שונים מאוד, אז גודל הזרם יהיה גדול. ככל שפוטנציאל ההפחתה גדול יותר, כך גדל הסיכוי שהמתכת תצומצם ותשמש כגורם מחמצן.
אם נחזור לתא גלווני נחושת-מגנזיום, פוטנציאל האלקטרודה הסטנדרטי של נחושת הוא 0.337 וולט, בעוד פוטנציאל האלקטרודה הסטנדרטי של מגנזיום הוא -2.370 וולט. בדוגמה זו, נחושת היא הקתודה ומגנזיום היא האנודה.
בעת ביצוע ניסוי תאים גלווני, הפרש הפוטנציאל בין שתי האלקטרודות מנוטר באמצעות מולטימטר. המתח הנמדד שווה להפרש הפוטנציאל בין שתי חצאי התגובה.
ΔEº =Eº cathode -אנודה Eº
פוטנציאל האלקטרודה הסטנדרטי מניח ששני חצאי התאים נמצאים בתנאים סטנדרטיים של 1 M, 1 bar ו- 298.15 K (25 ° C). המתח תלוי בריכוז תמיסות האלקטרוליטים, אותו ניתן לקבוע באמצעות משוואת נרנסט.
<img src="/CDNSource/lm/labs/52/52_Concepts_4.jpg">
כאן, E מתאים להפרש הפוטנציאלים או למתח הנמדד, Eo הוא פוטנציאל ההפחתה הסטנדרטי, R הוא קבוע הגז האוניברסלי (8.314462618 J/mol·K), T הוא הטמפרטורה הנמדדת בקלבין, F הוא קבוע פאראדיי (96,485.33212 C⋅mol-1), n הוא מספר האלקטרונים המועברים בתגובה, ו- Qh הוא מנת התגובה. מנת התגובה היא המקבילה האלקטרוכימית של קבוע שיווי המשקל.
<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.</li>
	<li style="font-size:17px">Harris, D.C. (2015). אנליזה כימית כמותית. מהדורה שישית. ניו יורק, ניו יורק: W.H. Freeman and Company.</li>
</ol>

אלקטרוכימיה
אלקטרוכימיה היא ענף בכימיה החוקר את הקשר בין אנרגיה חשמלית לשינוי כימי. כימיקלים אלה ...

Education

JoVE Lab Manual

Chemistry

כימיה

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 species that loses electrons is the reducing agent, and the species that gains electrons is the oxidizing agent. A common memory aid is “oil-rig,” which means oxidation is losing and reduction is gaining.

An electrochemical cell is a device that turns the energy from a redox reaction into electric current. It includes reaction chamber or chambers and two conductive electrodes, an anode and a cathode. The electrodes are connected electrically, and the chamber contains an electrolyte. A salt bridge sits between the two chambers. It completes the circuit and lets ions move between the electrodes. Reduction occurs at the cathode, and oxidation occurs at the anode. Students often remember this with “red cat,” which means reduction occurs at the cathode. The two main kinds of electrochemical cells are electrolytic cells and galvanic cells. An electrolytic cell uses external electrical energy to drive a nonspontaneous reaction. A galvanic cell uses a spontaneous reaction to generate electrical energy. Batteries are examples of galvanic cells.

Galvanic cells, also called voltaic cells, have two half-cells. Each half-cell contains a metal electrode immersed in an electrolyte. An external circuit connects the electrodes, and a salt bridge connects the two electrolyte solutions. Electrons flow from the anode to the cathode. Oxidation happens at the anode, and reduction happens at the cathode. In a copper-magnesium galvanic cell, copper is reduced at the cathode through the half-reaction Cu2+ + 2e- → Cu. Magnesium is oxidized at the anode through the half-reaction Mg → Mg2+ + 2e-. As electrons move through the external circuit, they reduce the cathode and generate current. As the anode is oxidized, the concentration of cations increases in that electrolyte. As the cathode is reduced, the concentration of anions increases in that electrolyte.

The salt bridge keeps the cell electrically neutral. When cations are created at the anode, anions move from the anode side through the salt bridge. On the cathode side, anions are created, so cations move from the salt bridge into that solution. It is important to separate electron flow from ion flow. Electrons travel through the external wires. Ions move through the salt bridge and the solutions.

Some metals lose electrons more easily than others. For that reason, the current produced by a galvanic cell depends on the metals used for the electrodes. The standard electrode potential, E°, measures this tendency. If two metals have nearly equal electrode potentials, the current is small. If their electrode potentials are very different, the current is larger. A larger reduction potential means a metal is more likely to be reduced and act as the oxidizing agent. In the copper-magnesium cell, copper has a standard electrode potential of 0.337 volts, and magnesium has a standard electrode potential of -2.370 volts. That is why copper is the cathode and magnesium is the anode.

In a galvanic cell experiment, a multimeter is used to measure the potential difference between the two electrodes. The measured voltage equals the difference between the two half-reactions, written as ΔE° = E° cathode - E° anode. Standard electrode potential assumes standard conditions of 1 M, 1 bar, and 298.15 K, or 25 °C. The voltage also depends on electrolyte concentration. That dependence can be found with the Nernst equation. In the equation, E is the measured potential difference, E° is the standard reduction potential, R is the universal gas constant, T is temperature in Kelvin, F is Faraday’s constant, n is the number of electrons transferred, and Qh is the reaction quotient. The reaction quotient is the electrochemical equivalent of the equilibrium constant.

Battery Power from Redox Reactions

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 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...

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

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.

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

Learn how galvanic cells use redox reactions, salt bridges, and electrode potentials to generate current and measure voltage in batteries.

Battery Power from Redox Reactions | JoVE K12

התכוננו ללמד את תאים גלוונים עם מדריך מדריכים זה באורך 6 דקות. כולל שלבי הכנה מרכזיים עבור מעבדת Chemistry ב-JoVE Lab Manual.

הכנת מדריך עבור תאים גלוונים ב-Chemistry - 3.9K+ צפיות | JoVE Lab Manual

עקבו אחר הנהלים של תאים גלוונים שלב אחר שלב בסרטון פרוטוקול סטודנטים זה באורך 14 דקות. חלק מאוסף Chemistry ב-JoVE Lab Manual.

פרוטוקול סטודנטים עבור תאים גלוונים ב-Chemistry - 26K+ צפיות | JoVE Lab Manual

Battery Power from Redox Reactions

Description

Transcript