AP®ChemistryCollege BoardStudy GuidesUnit 5: KineticsReaction RatesMeasuring Reaction Rate

Measuring Reaction Rate (College Board AP® Chemistry): Study Guide

Martín

Written by: Martín

Reviewed by: Stewart Hird

Updated on

Measuring Reaction Rate

  • In chemistry, it is important to determine how quick products are formed during a chemical reaction

  • The rate of reaction is the speed at which a chemical reaction takes place

  • The rate of reaction can be expressed as the change in concentration of a reactant or product per unit of time

    • The most common units for rate of reactions are M s-1 or mol L-1 s-1

How to calculate the rate of reaction

  • By convention, the rate of reaction is always a positive value

  • How you calculate it depends on whether you are measuring a product or a reactant

1. Using the Concentration of a Product

  • As a reaction proceeds, the concentration of a product increases

  • The formula for the rate of reaction in terms of a product is:

rate of reaction = fraction numerator change space in space concentration space of space products space left parenthesis M right parenthesis over denominator time space left parenthesis straight s right parenthesis end fraction

  • The change in concentration is calculated using:

change in concentration = (final concentration - initial concentration)

  • This means that the formula for the rate of reaction in terms of a product can also be written as:

rate of reaction = fraction numerator final space concentration space of space products space left parenthesis M right parenthesis minus initial space concentration space of space products space open parentheses M close parentheses over denominator time space left parenthesis straight s right parenthesis end fraction

2. Using the Concentration of a Reactant

  • As a reaction proceeds, the concentration of a reactant decreases

    • This means the "change in concentration" (final - initial) will be a negative number

  • To ensure the final rate value is positive, the formula must have a negative sign at the front

  • The formula for the rate of reaction in terms of a reactant is:

rate of reaction = - fraction numerator change space in space concentration space of space reactants space left parenthesis M right parenthesis over denominator time space left parenthesis straight s right parenthesis end fraction

  • The change in concentration is calculated using:

change in concentration = (final concentration - initial concentration)

  • This means that the formula for the rate of reaction in terms of a reactant can also be written as:

rate of reaction = - fraction numerator final space concentration space of space reactants space left parenthesis M right parenthesis minus initial space concentration space of space reactants space open parentheses M close parentheses over denominator time space left parenthesis straight s right parenthesis end fraction

Worked Example

Calculate the rate for the reaction

A → B

If the concentration of A has decreased from 2.5 M to 0.6 M in 20 seconds

Answer:

  • Step 1: Identify the correct formula

    • Since A is a reactant, we must use the formula that includes the negative sign:

rate of reaction = - fraction numerator final space concentration space of space reactants space left parenthesis M right parenthesis minus initial space concentration space of space reactants space open parentheses M close parentheses over denominator time space left parenthesis straight s right parenthesis end fraction

  • Step 2: Replace the values in the formula and calculate

rate of reaction = fraction numerator 0.6 italic space M minus 2.5 space M over denominator 20 space straight s end fraction

rate of reaction = 0.095 M s-1

Using graphs to calculate instantaneous rate

  • Experimental data from reactions can be used to determine the rate of reaction graphically

  • Two different graphs can be constructed: concentration of reactants vs time, and concentration of products vs time

  • The steeper the gradient, the quicker the rate of reaction

  • To find the instantaneous rate of reaction at any point in the graph, draw a tangent to the curve, and calculate the gradient of the tangent

    • The gradient of of a line can be calculated using the equation below

gradient space equals space fraction numerator increment straight y over denominator increment straight x end fraction

  • If the data used corresponds to the graph: concentration of reactants vs time, the sign must be changed

    • This mathematical manipulation must be done because the reaction rate is always a positive value

  • If the data corresponds to the graph: concentration of products vs time, there is no need for a mathematical manipulation

Worked Example

Iodine and methanoic acid react in aqueous solution.

I2 (aq) + HCOOH (aq) → 2I− (aq) + 2H+ (aq) + CO2 (g)

The rate of reaction can be found by measuring the volume of carbon dioxide produced per unit time and plotting a graph as shown:

rate-of-reaction-worked-example-calculating-rate

Calculate the rate of reaction in mL t-1 at 20 seconds

Answer:

  • Step 1: Draw a tangent to the curve at 20 seconds

rate-of-reaction-worked-example-calculating-rate-answer

  • Step 2: Complete the triangle and use values of x and y to calculate the gradient. The result of the gradient calculation is the rate of reaction

gradient = fraction numerator increment straight y over denominator increment straight x end fraction

gradient = fraction numerator 27 space mL minus 3 space mL over denominator 40 space straight s minus 0 space straight s end fraction

gradient = fraction numerator 24 space mL space over denominator 40 space straight s end fraction

gradient = 0.60 mL s-1

Examiner Tips and Tricks

When drawing the tangent to a curve, you must:

  • Make the triangle as large as possible

  • Intersect with grid lines if they are given

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Martín

Author: Martín

Expertise: Chemistry Content Creator

Martín, a dedicated chemistry teacher and tutor, excels in guiding students through IB, AP, and IGCSE Chemistry. As an IB Chemistry student, he came from hands-on preparation, focusing on practical exam techniques and rigorous practice. While at Universidad San Francisco de Quito, his academic journey sparked a passion for computational and physical chemistry. Martín specializes in chemistry, and he knows that SaveMyExams is the right place if he wants to have a positive impact all around the world.

Stewart Hird

Reviewer: Stewart Hird

Expertise: Chemistry Content Creator

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.