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

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.7

IPC.7.F

Reaction Rate

التفاعل الكيميائي يتضمن التحويل الكيميائي للمواد المتفاعلة إلى نواتج. عندما يجري التفاعل،يتناقص تركيز المواد المتفاعلة بينما تركيز النواتج يتزايد. الاختلاف في تركيز المواد المتفاعلة والنواتج يمكن وضعه في مخطط بياني كدالّة على الزمن.السرعة التي يجري بها التفاعل تسمى معدل التفاعل. وتقيس سرعة اختفاء المواد المتفاعلة،أو سرعة ظهور النواتج،ويُعبر عنها بوحدات تركيب جزيئي لكل ثانية. يمكن احتساب متوسط معدل التفاعل،من خلال تغير تركيزات المواد المتفاعلة أو النواتج أثناء فترة زمنية محددة.قيمة التركيز المولي يشار إليها بالأقواس،الرمز t يمثل الزمن،والرمز دلتا يمثل التغير الحاصل"بما أن المواد المتفاعلة تنضب أثناء تفاعل كيميائي،تكون قيمة التغير في التركيز،دائمًا سالبة. وعليه فمعدل التفاعل،إذا حُسب،من حيث المواد المتفاعلة يضاف إليها إشارة سالب لجعل القيمة الإجمالية موجبة. معدل التفاعل لا يبقى موحدًا طوال زمن التفاعل.المعدل عند بداية التفاعل،عند وقت الصفر يسمى معدل التفاعل الابتدائي. مع تناقص تركيز المواد المتفاعلة،ينخفض معدل التفاعل،أو يصبح التفاعل بطيئًا. معدل التفاعل عند نقطة زمنية معينة،أو معدل التفاعل الآني،يقاس بحساب انحدار خط ظل يرسم على منحنى التفاعل،الذي يمثل المواد المتفاعلة أو النواتج،في اللحظة المرغوب بقياسها.قيمة الانحدار أو المعدل الآني عند نقطة زمنية محددة،يكون متساوي لجميع المواد المتفاعلة والنواتج. معدل التفاعل لتفاعل كيميائي،يعكس أيضًا مُعامِلات التكافؤ الفعلية للمواد المتفاعلة والنواتج. لذا فأي تفاعل متكافئ،حيث يتفاعل عدد مولات من مادة مع عدد مولات من مادة لإنتاج عدد مولات من مادة وعدد مولات من مادة يمكن التعبير عن معدل التفاعل وحسابه باستخدام الصيغة العامة:سالب 1/أ ضرب التغير في تركيز المادة المتفاعلة في الزمن،مساوياًسالب 1/ب ضرب التغير في تركيز المادة المتفاعلة في الزمن.هذه القيم مساوية لموجب 1/ج ضرب التغير في تركيز الناتج في الزمن،وهي نفسها موجب 1/د ضرب التغير في تركيز الناتج في الزمن. تحديد معدلات التفاعل،أساسي في دراسة الحركية الكيميائية،ويساعد في فهم مدى سرعة عمل عقار أو محفّز،أو تفاعل صناعي من أجل تحسين التحكم فيه أكثر،وتحسين وظيفته.

معدّل التفاعل

معدل التفاعل هو التغير في كمية المادة المتفاعلة أو المنتج لكل وحدة زمنية. لذلك يتم تحديد معدلات التفاعل عن طريق قياس الاعتماد الزمني لبعض الخصائص التي يمكن أن تكون مرتبطة بكميات المادة المتفاعلة أو المنتج. معدلات التفاعلات التي استهلاك أو إنتاج المواد الغازية ، على سبيل المثال، يتم تحديدها بشكل ملائم عن طريق قياس التغيرات في الحجم أو الضغط. التمثيل الرياضي للتغير في تركيز المواد المتفاعلة والنواتج، بمرور الوقت، هو التعبير المعدل للتفاعل. تشير الأقواس إلى التركيزات المولية ، ويشير رمز دلتا (Δ) إلى “التغيير في.” على سبيل المثال، تركيز بيروكسيد الهيدروجين، H2O2 ، في محلول مائي يتغير ببطء بمرور الوقت حيث يتحلل وفقًا للمعادلة:  <img alt="Eq1" src="/files/ftp_upload/11373/11373_Equation_1.png" style="height: 50px;" /> يمكن التعبير عن معدل تحلل بيروكسيد الهيدروجين من حيث معدل تغير تركيزه:  <img alt="Eq2" src="/files/ftp_upload/11373/11373_Equation_2.png" style="height: 70px;" /> وهكذا ، فإن[H2O2]t1 يمثل التركيز المولي لبيروكسيد الهيدروجين في وقت آخر t2؛ وبالمثل، يمثل [H 2 O 2 ] t 2 التركيز المولي لـ بيروكسيد الهيدروجين في وقت لاحق t 2 ؛ و Δ[H2O2] يمثل التغير في التركيز المولي لبيروكسيد الهيدروجين خلال الفترة الزمنيةΔt (أي t2 − t1). نظرًا لأن تركيز المادة المتفاعلة ينخفض ​​مع استمرار التفاعل، فإن Δ[H2O2] هي كمية سالبة. معدلات التفاعل هي، حسب الاصطلاح، كميات موجبة، ولذا فإن هذا التغيير السلبي في التركيز مضروب في −1.متوسط ​​معدل التفاعل ومعدل التفاعل الفوري تختلف معدلات التفاعل مع الوقت وتنخفض مع استمرار التفاعل. يمكن حساب متوسط ​​معدل التفاعل على مدى فترة زمنية باستخدام التركيزات في بداية ونهاية هذه الفترة، والتي يتغير خلالها معدل التفاعل. في أي وقت محدد، يُعرف المعدل الذي يسير فيه التفاعل باسم معدله اللحظي. المعدل اللحظي للتفاعل عند “الوقت صفر,” عندما يبدأ التفاعل، يكون معدله الأولي. يمكن تحديد معدل التفاعل الفوري بإحدى الطريقتين. إذا سمحت الظروف التجريبية بقياس تغيرات التركيز على فترات زمنية قصيرة ، فإن حساب المعدلات المتوسطة يوفر تقديرات تقريبية جيدة بشكل معقول للمعدلات الآنية. بدلاً من ذلك ، يمكن استخدام إجراء رسومي. على سبيل المثال ، في مثال تحلل بيروكسيد الهيدروجين ، من خلال رسم تركيز بيروكسيد الهيدروجين مع الوقت ، يمكن حساب معدل التحلل الفوري لـ H2O2 في أي وقت ‘t’ من منحدر الظل المرسوم على المنحنى في ذلك الوقت.  <img alt="Image1" src="/files/ftp_upload/11373/11373_Image_1.png" style="height: 300px;" /> يوضح هذا الرسم البياني مخططًا للتركيز مقابل الوقت لحل 1.000 M من H2O2. المعدل في أي وقت يساوي سالب ميل خط مماس للمنحنى في ذلك الوقت. يتم عرض الظل في t = 0 ساعة (“المعدل الأولي”) وعند t = 12 ساعة (“المعدل الفوري” عند 12 ساعة).المعدلات النسبية لردود الفعل يمكن التعبير عن معدل التفاعل على أنه التغيير في تركيز أي مادة متفاعلة أو منتج. بالنسبة إلى أي تفاعل معين ، ترتبط جميع تعبيرات المعدل هذه ببعضها البعض وفقًا لقياسات العناصر المتفاعلة. معدل التفاعل العامaA ⟶ bB يمكن التعبير عن B من حيث الانخفاض في تركيز A أو زيادة تركيز B. وترتبط هاتان التعبيرات المعدلان بقياسات العناصر المتكافئة للتفاعل ، حيث:  <img alt="Eq3" src="/files/ftp_upload/11373/11373_Equation_3.png" style="height: 70px;" /> لاحظ أنه تم تضمين علامة سالبة كعامل لحساب الإشارات المعاكسة لتغييرات المقدار (كمية المادة المتفاعلة تتناقص أثناء زيادة كمية المنتج). هذا النص مقتبس من <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>

معدل التفاعل هو التغير في كمية المادة المتفاعلة أو المنتج لكل وحدة زمنية. لذلك يتم تحديد معدلات التفاعل عن طريق قياس الوقت ...

Education

JoVE Core

Chemistry

الكيمياء

واحد

Chemical Kinetics

الحركية الكيميائية

KEY TERMS AND CONCEPTS

المصطلحات والمفاهيم الأساسية

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

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

Louisiana Student Standards for Science

Matter And Its Interactions

HS-PS1-4

HS-PS1-6

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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

Arizona Science Standards

HS.P1

HS.P1U1.3

new-jersey-student-learning-standards-for-science

New Jersey Student Learning Standards for Science

Matter and Its Interactions

HS-PS1-5

Alabama Course of Study Science

Matter and Its Interactions: Energy

9-12.C.3.3.d

Next Generation Science Standards

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.D

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Measuring Reaction Rates

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