Name: measuring reaction rates
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 53 s
Channel: JoVE

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Reaction Rate

תגובה כימית כוללת את השינוי הכימי של מגיבים לתוצרים. ככל שהתגובה מתקדמת, ריכוז המגיבים פוחת בעוד ריכוז התוצרים גדל. ניתן לשרטט את השינוי הזה בריכוזים של מגיבים ותוצרים באמצעות גרף, כפונקציה של זמן.המהירות בה התגובה מתקדמת נקראת קצב התגובה. היא מודדת את קצב היעלמות המגיבים או את קצב הופעת התוצרים ומתבטא ביחידות של מולריות לשנייה. את קצב התגובה הממוצע ניתן לחשב מתוך השינוי בריכוזים של מגיבים או תוצרים במהלך פרק זמן מסוים.ערך הריכוז המולרי מצויין בתוך הסוגריים t מייצג את משך הזמן והסימן דלתא״מסמל את את השינוי בגודל מסוים. כיוון שהמגיבים נעלמים במהלך תגובה כימית, ערך שינוי הריכוז של המגיבים תמיד שלילי. לכן, לקצב תגובה המחושב במונחים של מגיבים מתווסף סימן מינוס, כדי להפוך את הערך הכולל לחיובי.קצב התגובה אינו אחיד במשך כל התגובה. הקצב בתחילת התגובה, ב"זמן אפס"נקרא קצב התגובה הראשוני. ככל שריכוז המגיבים יורד, קצב התגובה יורד, או שהתגובה מאטה.קצב התגובה בנקודת זמן נתונה, או קצב התגובה הרגע, נמדד על ידי חישוב השיפוע של משיק שנמשך על עקומת התגובה, עבור מגיבים או תוצרים, בזמן הרלוונטי. ערך השיפוע או הקצב הרגעי בנקודת זמן ספציפית שווה לכל המגיבים והתוצרים. קצב התגובה של תגובה כימית משקף גם את המקדמים הסטויוכיומטריים הממשיים של המגיבים והתוצרים.לכן, לכל תגובה מאוזנת, שבה a מולים של A מגיבים עם b מולים של B ומייצרים c מולים של C וd מולים של D, קצב התגובה יכול להיות מבוטא ומחושב באמצעות הנוסחה הכללית:מינוס 1/a כפול השינוי בריכוז המגיב A עם הזמן, שווה למינוס 1/b כפול השינוי בריכוז המגיב B עם הזמן. ערכים אלה שווים לפלוס 1/c כפול שינוי בריכוז של תוצר C עם הזמן, שהוא זהה לפלוס 1/c כפול שינוי בריכוז של תוצר D עם הזמן. קביעת שיעורי תגובה היא מהותית ללמידת קינטיקה כימית, שעוזרת להבין כמה מהר תרופה, זרז או תגובה סינתטית יפעלו, כך שניתן יהיה לשלוט בהם בצורה טובה יותר ולטייב את פעולתם.

קצב תגובה

<div style="direction:rtl">

קצב התגובה הוא השינוי בכמות המגיב או התוצר ליחידת זמן. קצבי התגובה נקבעים לפיכך על ידי מדידת התלות בזמן של נכס כלשהו שיכול להיות קשור לכמויות המגיבים או התוצר. קצבי התגובות שצורכים או מייצרים חומרים גזים, למשל, נקבעים בצורה נוחה על ידי מדידת שינויים בנפח או בלחץ. 

הייצוג המתמטי של השינוי בריכוז המגיבים והתוצרים, לאורך זמן, הוא ביטוי הקצב של התגובה. הסוגריים מציינים ריכוזים מולריים, והסמל דלתא (Δ) מציין "שינוי ב". 
לדוגמה, ריכוז מי חמצן, H2O2, בתמיסה מימית משתנה לאט עם הזמן כשהיא מתפרקת לפי המשוואה: 
<img alt="Eq1" src="/files/ftp_upload/11373/11373_Equation_1.png" style="height: 50px;"> 

ניתן לבטא את הקצב שבו מימן על-חמצני מתפרק במונחים של קצב השינוי של ריכוזו: 
<img alt="Eq2" src="/files/ftp_upload/11373/11373_Equation_2.png" style="height: 70px;"> 

 לפיכך, [H2O2]t1 מייצג את הריכוז המולרי של מימן על-חמצני בזמן em> t1; באופן דומה, [H2O2]t2 מייצג את הריכוז המולרי של מימן על-חמצני במועד מאוחר יותר t2; ו- Δ[H2O2] מייצג את השינוי בריכוז המולרי של מימן על-חמצני במהלך מרווח הזמן Δt (כלומר, t2 - t1). מכיוון שריכוז המגיבים יורד ככל שהתגובה מתמשכת,Δ[H2O2] היא כמות שלילית. קצבי התגובה הם, לפי מוסכמה, כמויות חיוביות, ולכן השינוי השלילי הזה בריכוז מוכפל ב-−1.

קצב תגובה ממוצע וקצב תגובה מיידי 

שיעורי התגובה משתנים עם הזמן ויורדים ככל שהתגובה מתמשכת. ניתן לחשב קצב תגובה ממוצע על פני מרווח זמן תוך שימוש בריכוזים בתחילת ובסוף תקופה זו, שבאמצעותם קצב התגובה משתנה. בכל זמן ספציפי, הקצב שבו התגובה מתרחשת ידוע כקצב המיידי שלה. הקצב המיידי של תגובה ב"זמן אפס", כאשר התגובה מתחילה, הוא הקצב ההתחלתי שלה. 

ניתן לקבוע את הקצב המיידי של תגובה באחת משתי הדרכים. אם תנאי הניסוי מאפשרים מדידת שינויי ריכוז על פני מרווחי זמן קצרים, אז חישוב הקצבים הממוצעים מספקים קירובים טובים למדי של קצבים מיידיים. לחלופין, ניתן להשתמש בהליך גרפי. לדוגמה, בדוגמה של פירוק מימן על-חמצני, על ידי שרטוט ריכוז מימן על-חמצני מול זמן, ניתן לחשב את קצב הפירוק המיידי שלH2O2 בכל זמן 't' מהשיפוע של משיק שנמשך לעקומה באותו זמן.

<img alt=" תמונה 1" src="/files/ftp_upload/11373/11373_Image_1.png" style="height: 300px;"> 

גרף זה מציג גרף של ריכוז מול זמן עבור תמיסה של 1,000 M של H2O2. הקצב בכל זמן שווה לשלילי של השיפוע של קו המשיק לעקומה באותו זמן. המשיקים מוצגים ב-t = 0 h ("קצב התחלתי") וב-t = 12 h ("קצב מיידי" ב-12 שעות). 

קצבי תגובה יחסיים 

קצב התגובה יכול להתבטא כשינוי בריכוז של כל מגיב או תוצר. עבור כל תגובה נתונה, ביטויי הקצב הללו קשורים כולם זה לזה בהתאם לסטוכיומטריית התגובה. ניתן לבטא את קצב התגובה הכללית aA ⟶ bB במונחים של הירידה בריכוז של A או העלייה בריכוז של B. שני ביטויי הקצב הללו קשורים בסטוכיומטריה של התגובה, כאשר: 
<img alt="Eq3" src="/files/ftp_upload/11373/11373_Equation_3.png" style="height: 70px;"> 

שימו לב שסימן שלילי נכלל כגורם כדי להסביר את הסימנים ההפוכים של שני השינויים בכמות (כמות המגיבים פוחתת בעוד שכמות התוצר עולה). 

טקסט זה מותאם מ<a href="https://openstax.org/books/chemistry-2e/pages/12-introduction">Openstax, Chemistry 2e, Chapter 12 Introduction</a> and <a href="https://openstax.org/books/chemistry-2e/pages/12-1-chemical-reaction-rates#12163">Openstax, Chemistry 2e, Section 12.1: Chemical Reaction Rates.</a></div>

קצב התגובה הוא השינוי בכמות המגיב או המוצר ליחידת זמן. קצבי התגובה נקבעים אפוא על ידי מדידת הזמן ...

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measuring reaction rates

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 over time. In this expression, brackets mean molar concentration, and the delta symbol means change in amount.

Hydrogen peroxide provides a clear example. As H2O2 decomposes in water, its concentration slowly decreases with time. The reaction rate can be written as the rate of change of the H2O2 concentration. If the concentration at time t1 is compared with the concentration at a later time t2, the change in concentration over the time interval Δt is found.

Because the reactant concentration goes down, Δ[H2O2] is negative. Reaction rates are reported as positive values, so the negative sign is reversed by multiplying by -1. This keeps the reaction rate positive even when the reactant amount is falling.

Reaction rates are not constant. They usually change as the reaction continues, so an average reaction rate is calculated over a time interval using the starting and ending concentrations. The rate at one exact moment is the instantaneous rate, and the instantaneous rate at time zero is the initial rate.

Instantaneous rate can be estimated in two ways. If concentration changes can be measured over very short time intervals, the average rate gives a good approximation. A graph can also be used. When concentration is plotted against time, the instantaneous rate at a given time equals the negative slope of a tangent line to the curve. In the hydrogen peroxide example, tangents can be drawn at t = 0 h for the initial rate and at t = 12 h for the instantaneous rate at 12 h.

The rate of a reaction can be written using either a reactant or a product. For a general reaction aA → bB, the decrease in A and the increase in B are linked by the reaction stoichiometry. The two rate expressions are related by that stoichiometry, and a negative sign is included to account for the reactant decreasing while the product increases.

Measuring Reaction Rates

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...

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...

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

Learn how reaction rates are measured using concentration, volume, pressure, average rate, and instantaneous rate with the hydrogen peroxide example.

Measuring Reaction Rates | JoVE K12

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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.

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Phase Transitions: Sublimation and Deposition

Some solids can transition directly into the gaseous state, bypassing the liquid state, via a process known as sublimation. At room temperature and ...

Melting Points

Melting Points in Organic Chemistry
The melting point of a compound is the temperature at which the solid phase transitions into the liquid phase at a ...

Boiling Points

Source: Lara Al Hariri at the University of Massachusetts Amherst, MA, USA

	Preparation of the LaboratoryExpand
	Here, we show the laboratory preparation ...

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

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 ...

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:

Measuring Reaction Rates

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