AP®ChemistryCollege BoardStudy GuidesUnit 5: KineticsReaction RatesMeasuring Reaction Rate

Measuring Reaction Rate (College Board AP® Chemistry)

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Written by: Martín

Reviewed by: Stewart Hird

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

  • The rate of reaction is always a positive value

  • The formula for the rate of reaction in terms of the concentration of product is shown below:

rate space of space reaction left parenthesis M space straight s to the power of negative 1 end exponent right parenthesis space equals space 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

  • If the rate of reaction is calculated using the change in concentration of reactants, the formula must have a negative sign

    • This occurs because the change in concentration of reactants is negative

begin mathsize 14px style rate space of space reaction left parenthesis M space straight s to the power of negative 1 end exponent right parenthesis space equals space 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 end style

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 formula that must be used and replace the values

Since A is a reactant, the equation that must be include the concentration of reactants

rate space of space reaction left parenthesis M space straight s to the power of negative 1 end exponent right parenthesis space equals space 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

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

rate space of space reaction left parenthesis M space straight s to the power of negative 1 end exponent right parenthesis space equals space fraction numerator Final space concentration space minus space Initial space concentration over denominator time space end fraction

rate space of space reaction left parenthesis M space straight s to the power of negative 1 end exponent right parenthesis space equals space fraction numerator 0.6 italic space M space minus space 2.5 space M over denominator 20 space straight s end fraction

begin mathsize 14px style rate space of space reaction left parenthesis M space straight s to the power of negative 1 end exponent right parenthesis space equals space minus 0.095 space M space straight s to the power of negative 1 end exponent end style

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

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

gradient space equals space fraction numerator 24 space mL space over denominator 40 space straight s end fraction equals space 0.60 space mL space straight s to the power of negative 1 end exponent

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

    Author: Stewart Hird

    Expertise: Chemistry Lead

    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.