WebLecture: How Concentrations Affect Rates

Kotz and Triechel, Chemistry and Chemical Reactivity Chapter 14: Sections 3-4.

• 14.3 The reaction rate can be expressed as the change in concentration over time.
  • The rate equation or rate law is given for each reactant separately: different reactants can have different rates. For reactant A of a reaction, rate δ[A]/δt = k[A], where k is the experimentally determined rate constant. For a general reaction aA + bB → xX, the rate is k[A]^m[B]ⁿ. The rate constant and exponents m and n must be experimentally determined.
  • The order of a reaction for a particular reactant is the exponent, such as m above; the overall reaction order is the sum of exponents of the reactants m + n + ... + z.
  • Units of the rate constant k vary with order: first (time^-1), second (L/mole*time), zeroth (mole/liter*time).
  • The rate equation can be determined by examining the rate of change at different levels of concentration for each reactant.
    • If the rate is independent of any concentration change, the reaction is zeroth order for that reactant.
    • If the rate doubles when the concentration of a reactant doubles, the reaction is first order for that reactant.
    • If the rate quadruples as the concentration doubles (that is, it varies as the square of the change), then the reaction is second order.
    Once the rate equation is known, it can be used to determine the rate of a reaction overall.
• 14.4 Rate laws can be expressed as integrated rate equations, using calculus to integrate the rate laws. These are often graphed to show the relationship between the integrated expression and time as linear.

  Order	Rate Law	Integrated Equation
  Zero	-ΔR/Δt = k[R]0	[R]0 - [R]t = kt
  First	-ΔR/Δt = k[R]	ln [R]t/[R]0 = -kt
  Second	-ΔR/Δt = k[R]2	1/[R]t - 1/[R]0 = kt

  We can do a qick graphical analysis as well. If we make three graphs, and plot [R], ln[R], and 1/[R] against time, the graph which gives us a straight line tells us what the reaction order is for the reactant R.

Videos for Chapter 14: The Rates of Chemical Reactions

Review the Videos at Thinkwell Video Lessons.

• Under "Chemical Kinetics"
  • Orders of Reactions
    • First-Order Reactions
    • Second-Order Reactions
    • A Kinetics Problem

Homework problems: See your Moodle assignment!

AP #11 GUIDED INQUIRY — Titration Methods — Phase I

Design an experiment that uses titration methods to determine the concentration of an unknown solution. You will need to analyze data from a oxidation-reduction (redox) reaction and determine solution concentrations for at least two different solutions.

Identify the equipment and materials you will need, the solutions you will analyze, and outline the steps you will follow to collect data.

Resources:

• AP2009 11 Determination of appropriate indicators for various acid-base titrations pH determination
• APGIE Investigation 8 Redox Titrations: How Can We Determine the Actual Percentage of H2O2 in a Drugstore Bottle of Hydrogen Peroxide?
• HSCKM V-1 Determine the Effect of Concentration on pH and the pH Range of Indicators
• HCSKM V-2 Determine the Molarity of Vinegar by Titration
• IGHCE Lab 11.1 Concentration effects on pH
• IGHCE Lab 11.2 pH in salt solutions

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