AP®ChemistryCollege BoardStudy GuidesUnit 5: KineticsReaction RatesStoichiometry & Reaction Rate

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

Martín

Written by: Martín

Reviewed by: Stewart Hird

Updated on

Stoichiometry & Reaction Rate

  • Let’s consider the general reaction,

 aA space plus space bB space rightwards arrow space cC space plus space dD

  • The rate of reaction must be independent of the chemical substance used to calculate the rate

  • There is only one positive value for the rate of reaction

  • The stoichiometry of the reaction can be used to establish general equations for the rate of reaction

    • The rate depends on the coefficient for each substance in the chemical equation

    • The negative sign is added to the reactants since the rate is a positive value

    • The brackets represent molarity

 begin mathsize 14px style Rate space equals space minus sign 1 over a fraction numerator increment open square brackets A close square brackets over denominator increment t end fraction equals minus sign 1 over b fraction numerator increment open square brackets B close square brackets over denominator increment t end fraction equals 1 over c fraction numerator increment open square brackets C close square brackets over denominator increment t end fraction equals 1 over d fraction numerator increment open square brackets D close square brackets over denominator increment t end fraction end style

Worked Example

What is the rate of appearance of NO2, if the rate of decomposition of N2O5 in the chemical reaction is 5.0 ✕ 10-8 M/s?

2N2O5 → 4NO2 + O2

Answer:

  • Step 1: Identify the rate given by the statement. Since the rate of decomposition is calculated using a reactant, the rate must have a negative sign

fraction numerator increment open square brackets straight N subscript 2 straight O subscript 5 close square brackets over denominator increment straight t end fraction space equals space minus 5.0 space cross times 10 to the power of negative 8 end exponent space straight M space straight s to the power of negative 1 to the power of blank end exponent

  • Step 2: Set up an equation that links the rate of decomposition of N2O5 and the rate of appearance of NO2

 negative 1 half fraction numerator increment open square brackets straight N subscript 2 straight O subscript 5 close square brackets over denominator increment straight t end fraction space equals 1 fourth space fraction numerator increment open square brackets NO subscript 2 close square brackets over denominator increment straight t end fraction space

  • Step 3: Replace the values given and solve for the rate of appearance of NO2

 negative 1 half fraction numerator increment stretchy left square bracket straight N subscript 2 straight O subscript 5 stretchy right square bracket over denominator increment straight t end fraction space equals 1 fourth space fraction numerator increment stretchy left square bracket NO subscript 2 stretchy right square bracket over denominator increment straight t end fraction space

negative 1 half stretchy left parenthesis 5.0 space cross times 10 to the power of negative 8 end exponent space straight M space straight s to the power of negative 1 end exponent stretchy right parenthesis space equals 1 fourth space fraction numerator increment stretchy left square bracket NO subscript 2 stretchy right square bracket over denominator increment straight t end fraction space

negative 1 half stretchy left parenthesis 5.0 space cross times 10 to the power of negative 8 end exponent space straight M space straight s to the power of negative 1 end exponent stretchy right parenthesis cross times 4 space equals space fraction numerator increment stretchy left square bracket NO subscript 2 stretchy right square bracket over denominator increment straight t end fraction space

space fraction numerator increment stretchy left square bracket NO subscript 2 stretchy right square bracket over denominator increment straight t end fraction space equals 1.0 space cross times 10 to the power of negative 7 end exponent space straight M space straight s to the power of negative 1 end exponent

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Author
Reviewer
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 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.

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