Name: tracking reaction kinetics with light and pressure
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 3 min 9 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.7

IPC.7.F

Measuring Reaction Rates

ניתן ללמוד קצב של תגובה על ידי קביעת השינוי בריכוזים של מגיבים או תוצרים כפונקציה של זמן. שינויים בריכוז ניתן למדוד על ידי טכניקות ניסיוניות כמו מדידת קיטוב האור, ספקטרוסקופיה או מדידות לחץ. מדידת קיטוב האור עושה שימוש באור מקוטב מישור עם שדה חשמלי המכוון לאורך מישור אחד בלבד.היא מודדת את היכולת של תרכובות לסובב אור מקוטב, שתלויה במבנה המולקולרי של התרכובת הנחקרת. חשבו על ההידרוליזה של סוכרוז, שיוצרת גלוקוז ופרוקטוז. מד קיטוב האור(פולרימטר)משמש למדידת זווית הסיבוב של אור מקוטב מישור דרך תמיסת הסוכרוז המגיבה.סוכרוז גורם לסיבוב עם כיוון השעון, ואילו גלוקוז ופרוקטוז מניעים סיבוב נגד כיוון השעון. על ידי מדידת הזווית של סיבוב האור במרווחי זמן מוגדרים, נינתן לחשב את הריכוזים היחסיים של סוכרוז, גלוקוז או פרוקטוז ולקבוע את קצב התגובה. קצבי תגובה יכולים גם להימדד באמצעות שיטות ספקטרופוטומטריות, תוך ניצול היכולת של מגיבים או התוצרים לספוג אור באורכי גל יחודיים.ככל שהריכוז של החומר הרלוונטי גבוה יותר, כך ספיגת האור תהיה עוצמתית יותר. לדוגמה, גז מימן חסר צבע מגיב עם אדי יוד סגולים ליצירת יוד מימן חסר צבע. אדי יוד סופגים אור באזור הצהוב-ירוק ומחזירים אור סגול.במהלך התגובה, ספקטרופוטומטר מודד את כמות האור הנספג על ידי הדוגמא ומנתח את האור המועבר. כך, ככל שהתגובה מתקדמת, הירידה בריכוז אדי יוד נצפית על ידי הירידה של ספיגת האור הצהוב-ירוק. בעזרת חוק בר-למברט, ניתן לחשב את עוצמת האור הנספגת בנקודות זמן שונות ולקשר אותה לשינויים בריכוז.לחלופין, אם אחד מהמגיבים או התוצרים הם גז, משתמשים במדידות לחץ לקביעת קצב התגובה על ידי מעקב אחר שינויי לחץ. לדוגמה, במהלך פירוק מי חמצן, קצב התגובה נלמד באמצעות מנומטר המתעד את הלחץ של גז החמצן המשתחרר. ככל שהתגובה מתקדמת ועוד גז חמצן מתפתח, הלחץ עולה.באמצעות משוואת הגזים האידיאלים, ערכי לחץ המתועדים בנקודות זמן שונות מומרים לריכוזים. שינוי הריכוז כפונקציה של זמן משמש לקביעת קצב התגובה. לתגובות ממושכות, ניתן לקחת דוגמאות או דוגמיות מתערובת התגובה במרווחי זמן קבועים.לאחר מכן, הריכוזים היחסיים נמדדים באמצעות טכניקות מיכשוריות כמו כרומטוגרפיית גז, ספקטרומטריית מסה או טיטרציה, לחישוב קצבי תגובות.

מדידת קצב תגובה

<div style="direction:rtl">פולארימטריה מוצאת יישום בקינטיקה כימית למדידת הריכוז וקינטיקה של התגובה של חומרים פעילים אופטית במהלך תגובה כימית. לחומרים פעילים אופטית יש את היכולת לסובב את מישור הקיטוב של אור מקוטב ליניארי העובר דרכם - תכונה הנקראת סיבוב אופטי. פעילות אופטית מיוחסת למבנה המולקולרי של חומרים. אור מונוכרומטי רגיל אינו מקוטב ובעל תנודות של השדה החשמלי בכל המישורים האפשריים בניצב לכיוון התפשטותו. כאשר אור לא מקוטב עובר דרך מקטב, אור מקוטב ליניארי השומר על תנודות במישור אחד יוצא החוצה. מכשיר פולארימטר קובע את כיוון הקיטוב של האור או את הסיבוב שנוצר על ידי חומר פעיל אופטית. בפולארימטר, האור המקוטב במישור מוכנס לצינור המכיל את התמיסה המגיבה, וניתן לעקוב אחר התגובה מבלי להפריע למערכת. אם המדגם מכיל חומרים לא פעילים אופטית, לא יהיה שינוי בכיוון מישור האור המקוטב. האור יהיה גלוי באותה עוצמה על מסך הנתח, וקריאה של זווית הסיבוב (ɑ) תקרא אפס מעלות. עם זאת, הנוכחות של תרכובות פעילות אופטית בדגימה המגיבה גורמת לסיבוב של מישור האור המקוטב שעובר דרכו. האור שיוצא החוצה יהיה פחות בהיר. יהיה צורך לסובב את הציר של מכשיר הנתח בכיוון השעון (סיבוב ימינה) או נגד כיוון השעון (סיבוב שמאלה) כדי לראות את הבהירות המקסימלית. הכיוון שאליו צריך לסובב את המנתח תלוי באופי התרכובת הקיימת. הסיבוב האופטי הנמדד הוא פרופורציונלי לריכוז החומרים הפעילים אופטית המצויים בדגימה. על ידי ניתוח של מדידת זווית הסיבוב בנקודות זמן שונות, ניתן לקבוע את הריכוזים של התרכובות הפעילות אופטית כפונקציה של זמן. ספקטרומטריה שיטות ניסוי אופטיות כמו ספקטרומטריה משמשות לעתים קרובות גם כדי לנטר תגובות כימיות ולאבטח מידע כמותי על קינטיקה של התגובה. באמצעות ספקטרומטריה, האור של אורך גל מסוים גורם לעבור דרך דגימה מגיבה. המולקולות או התרכובות (או מגיב או תוצר) בתוך הדגימה עשויות לספוג מעט אור תוך העברת הכמות הנותרת, הנמדדת על ידי גלאי. כמות האור הנספגת תלויה בריכוז התרכובת או המולקולה המעניינת. לדוגמה, ככל שהריכוז של תרכובת גבוה יותר, הספיגה שלה גדולה יותר. מהספיגה, המכשיר יוכל לקבוע את ריכוז התרכובת המעניינת. בדגימה מגיבה, הספיגה הנמדדת במרווחים תקופתיים ומחשבת את ריכוזי המגיב או התוצר כפונקציה של זמן. מדידות לחץ עבור תגובות המערבות חומרים בשלב גז, קינטיקה של התגובה מלווה בכימות השינויים במספר מולות הגזים כפונקציה של השינויים בלחץ. ניתן לחבר את הגדרות הניסוי של תגובת שלב הגז למנומטר שיכול למדוד את הלחץ של מגיב גזי או תוצר. ככל שהתגובה מתקדמת, הלחץ של המגיבים יורד, ו(או) לחץ התוצרים עולה. ניתן למדוד זאת על ידי המנומטר כפונקציה של זמן. על ידי שימוש בחוק הגז האידיאלי - ריכוז הגז פרופורציונלי ללחץ החלקי שלו - ניתן לחשב את קצב התגובה הכימית.</div>

פולרימטריה מוצאת יישום בקינטיקה כימית למדידת קינטיקה של ריכוז ותגובה של חומרים פעילים אופטית במהלך ...

Education

JoVE Core

Chemistry

כימיה

אחד

Chemical Kinetics

קינטיקה כימית

KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

tracking reaction kinetics with light and pressure

Reaction kinetics can be tracked with polarimetry, spectrometry, and pressure measurements. These methods let scientists follow a reaction without disturbing the sample. They also help determine how reactant or product concentrations change over time.

Polarimetry is used for optically active substances. These substances rotate the plane of polarized light, which is called optical rotation. The effect comes from molecular structure. Normal monochromatic light is unpolarized, but after it passes through a polarizer, it becomes linearly polarized with vibrations in one plane.

In a polarimeter, this plane-polarized light is sent through a tube of reacting solution. The instrument then shows how much the light has been rotated. If the sample contains optically inactive substances, the light keeps the same orientation, the analyzer screen stays equally bright, and the rotation angle is zero degrees. If optically active compounds are present, the light becomes dimmer and the analyzer must be turned clockwise or counter-clockwise to restore maximum brightness. Clockwise rotation is dextrorotatory, and counter-clockwise rotation is levorotatory.

The amount of optical rotation is proportional to the concentration of the optically active substance in the sample. By taking rotation readings at different times, the concentration can be followed as the reaction proceeds. Spectrometry gives similar kinetic information by passing light of a specific wavelength through a reacting sample. Some of the light is absorbed by the reactant or product, and a detector measures the remaining light. Higher concentration gives higher absorbance, so repeated absorbance measurements can be used to find concentration over time.

Pressure measurements are useful for gas-phase reactions. A manometer can be connected to the reaction setup to monitor the pressure of a gaseous reactant or product. As the reaction goes on, reactant pressure may decrease and product pressure may increase. Using the ideal gas law, the concentration of a gas is proportional to its partial pressure, so reaction rate can be calculated from the pressure change.

Tracking Reaction Kinetics with Light and Pressure

tracking-reaction-kinetics-with-light-and-pressure

reaction rate

Measuring reaction rates shows how fast reactants are used up or products are formed over time. The rate of reaction is the change in the amount of a reactant or product per unit time. Because of that, scientists measure how a property changes as the reaction proceeds and connect that change to reactant or product amounts. For reactions that make or use a gas, the rate can be followed by changes in volume or pressure. A rate expression is the mathematical way to write how concentration changes...

measuring reaction rates

Reaction kinetics can be tracked with polarimetry, spectrometry, and pressure measurements. These methods let scientists follow a reaction without disturbing the sample. They also help determine how reactant or product concentrations change over time. Polarimetry is used for optically active substances. These substances rotate the plane of polarized light, which is called optical rotation. The effect comes from molecular structure. Normal monochromatic light is unpolarized, but after it passes...

concentration and rate law

Reaction Order from Concentration

Reaction order from concentration shows how reactant amounts affect reaction rate. Rate laws, also called differential rate laws or rate equations, are mathematical expressions that connect reaction rate with the concentrations of reactants. For a generic reaction a A + b B → products, the rate law can be written using the reactant concentrations [A] and [B] and a rate constant k. In a rate law, k is the rate constant for one specific reaction at one specific temperature. The exponents m and n...

determining order of reaction

Finding Rate Law from Initial Rates

Rate laws connect the speed of a chemical reaction to the concentration of its reactants. The rate constant, k, and the reaction orders are not taken from the chemical equation. They are found experimentally by watching how the rate changes when the reactant concentrations change. One common way to do this is the method of initial rates. In this method, several trials are run with different starting concentrations. The measured rates are then compared to determine the reaction orders and,...

the integrated rate law: the dependence of concentration on time

the-integrated-rate-law-the-dependence-of-concentration-on-time

Reading Reaction Progress with Integrated Laws

Integrated rate laws connect reaction concentration to time. They help show how much reactant or product is present after a certain period. They can also help estimate how long a reaction must run to reach a given extent. A common use is finding how long radioactive material must be stored before its radioactivity drops to a safe level. These equations come from the differential rate law after it is integrated with respect to time. For a first-order reaction, where rate = k[A], the integrated...

half-life of a reaction

Reaction Half-Life in Kinetics

Reaction half-life is the time needed for half of a reactant to be consumed. After each half-life, half of the remaining reactant is used up. A hydrogen peroxide decomposition example shows this pattern clearly. The concentration drops from 1.000 M to 0.500 M in the first 6.00 hours, then to 0.250 M in the next 6.00 hours, and then to 0.125 M in the third period. Each later 6.00-hour interval removes half of what is left. For first-order reactions, the half-life does not depend on the reactant...

temperature dependence on reaction rate

Collision Theory and Reaction Speed

Collision theory explains why reaction speed depends on how particles collide. Atoms, molecules, or ions must come close together before they can react. The theory says the reaction rate depends on the collision rate, the orientation of the colliding species, and whether the collision has enough energy for bonds to change. A gas-phase reaction between carbon monoxide and oxygen shows why orientation matters. The molecules can collide in more than one way. If the oxygen side of carbon monoxide...

arrhenius plots

Determining Activation Energy from Temperature

The Arrhenius equation links activation energy and the rate constant, k, for chemical reactions. It is written as k = Ae−Ea/RT. In this equation, R is the ideal gas constant with a value of 8.314 J/mol·K, T is the temperature on the kelvin scale, Ea is the activation energy in J/mole, e is the constant 2.7183, and A is the frequency factor. The frequency factor depends on collision frequency and the orientation of reacting molecules. Experimental kinetic data can be used to find the activation...

reaction mechanisms

Elementary Steps in Reaction Mechanisms

Chemical reactions can happen in steps, and a balanced equation does not show the full path at the molecular level. A reaction mechanism, also called a reaction path, gives the step-by-step process that explains how reactants become products. The decomposition of ozone is one example of a reaction that can be described by a sequence of steps. Each step in a mechanism is an elementary reaction. These elementary reactions happen in order, and when they are added together, they give the balanced...

rate-determining steps

Reaction Bottlenecks in Chemical Kinetics

Reaction bottlenecks help explain rate-determining steps in multistep reaction mechanisms. In a mechanism with several elementary steps, one step is often much slower than the others. That slowest step is called the rate-limiting step, or rate-determining step. A reaction cannot move faster than its slowest step, so this step controls the overall reaction rate. The idea is like traffic on a four-lane freeway with one lane closed. Cars can only move as fast as the bottleneck allows. In the same...

catalysis

Catalyst Action in Reaction Rates

Catalysts change the rate of a chemical reaction by giving it a faster path. A catalyst increases reaction rate without being consumed during the process. Its effect is often explained with reaction mechanisms and energy diagrams. For an endothermic reaction, the reactants and products start and end at the same energies in both cases. The uncatalyzed path is a one-step mechanism with one transition state. The catalyzed path follows a different mechanism with two transition states and a lower...

enzymes

Enzyme Action and Inhibition

Enzymes are biological catalysts that speed up many reactions in living organisms. They help control cellular metabolism by lowering the activation energy needed for biochemical reactions. When activation energy drops, reaction rates can increase. Enzyme problems can also lead to serious disease. A genetic abnormality that causes G6PD, or glucose-6-phosphate dehydrogenase, deficiency can disrupt the pathway that supplies NADPH to cells. In red blood cells, this can reduce glutathione and...

key terms and concepts

See how polarimetry, spectrometry, and pressure measurements monitor reaction kinetics by tracking optical rotation, absorbance, and gas pressure changes.

Tracking Reaction Kinetics with Light and Pressure | JoVE K12

chemistry

JoVE Core Chemistry brings general chemistry concepts to life using concise and easy-to-understand animated video lessons. Plus, in the scientist-in-action videos, we demonstrate classical and original research experiments performed in today's laboratories worldwide.

north-carolina-k-12-science-standard-course-of-study

North Carolina K-12 Science Standard Course of Study

9-12

Matter and its Interactions

PS.Chm.5.1

Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

3.2.9-12.D

3.2.9-12.E

ohios-learning-standards-and-model-curriculum

Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

Virginia  Standards of Learning

HIgh School

CH.7

CH.7.e

Minnesota Academic Standards in Science

3.2.1 9C.3.1.1.2

Alabama Course of Study Science

Matter and Its Interactions: Energy

9-12.C.3.3.d

Next Generation Science Standards

Matter and Its Interactions

HS-PS1-4

HS-PS1-5

Florida Department of Education

SC.912.P.12

SC.912.P.12.12

new-york-state-science-learning-standards

New York State Science Learning Standards

HS. Chemical Reactions

HS-PS1-5.

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IPC.7.C

Emission Spectra

When solids, liquids, or condensed gases are heated sufficiently, they radiate some of the excess energy as light. Photons produced in this manner have a ...

Metal Flame Emission

Metal Flame Emission
Flame testing is an analytical technique where a sample is placed in a flame, and the characteristic flame color is used to identify ...

Atomic Orbitals

An atomic orbital represents the three-dimensional regions in an atom where an electron has the highest probability to reside. The radial distribution ...

The Pauli Exclusion Principle

The arrangement of electrons in the orbitals of an atom is called its electron configuration. We describe an electron configuration with a symbol that ...

The Energies of Atomic Orbitals

In an atom, the negatively charged electrons are attracted to the positively charged nucleus. In a multielectron atom, electron-electron repulsions are ...

Explain how electrons can transition from a high energy level to a low energy state, emitting photons at different frequencies for different energy transitions.

IPC.7.D

The de Broglie Wavelength

In the macroscopic world, objects that are large enough to be seen by the naked eye follow the rules of classical physics. A billiard ball moving on a ...

The Uncertainty Principle

Werner Heisenberg considered the limits of how accurately one can measure properties of an electron or other microscopic particles. He determined that ...

The Quantum-Mechanical Model of an Atom

Shortly after de Broglie published his ideas that the electron in a hydrogen atom could be better thought of as being a circular standing wave instead of ...

Quantum Numbers

It is said that the energy of an electron in an atom is quantized; that is, it can be equal only to certain specific values and can jump from one energy ...

Balmer Series

The Bohr Model
Niels Bohr proposed a model for the hydrogen atom in 1913 that described discrete energy states are associated with a fixed electron orbit ...

Explain how atomic energy levels and emission spectra present evidence for the wave particle duality.

IPC.7.E

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

Tracking Reaction Kinetics with Light and Pressure

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