Exp' 23 Determination of a Rate Law p' 267

1
Exp. 23 Determination of a Rate Law

p. 267

• To determine the rate law for a chemical
  reaction.
• To utilize a graphical analysis of the
  experimental data.

2
Exp. 23 Introduction

• The rate of a chemical reaction is determined by
  a number of factors (Exp. 22)
• Nature of reactants
• Temperature
• Presence of a catalyst
• R of the reactants
• Surface area of the reactants.

3
Exp. 23 Introduction

• You can observe the rate as
• a change in of one of the reactants or
  products per unit of time
• the volume of gas produced per unit of time
• Change in color (T) per unit of time

4
Exp. 23 Introduction

• For the reaction A2 2B2 ? 2AB2
• the rate of the reaction is related to the
  initial concentration of each reactant. This is
  called the rate law for the reaction and is
  written rate kA2pB2q
• The rate constant is k which relates the rate of
  a reaction to the initial concentrations of
  reactants. It varies with temperature and is
  independent of reactant concentrations.

5
Exp. 23 Introduction

• A study of reaction kinetics involves determining
  the values of k, p, and q.

6
Exp. 23 Introduction

• In this experiment, you will determine p, q, and
  k for the following reaction
• 2I-(aq) H2O2(aq) 2H3O(aq) ? I2(aq) 4H2O(l)
• Where rate kI-pH2O2qH3Or
• The addition of a buffer controls the speed of
  the reaction, minimizing the contribution of the
  H3O.
• The rate law becomes kI-pH2O2q

7
Exp. 23 Introduction

• Determination of p, the order of the reaction
  with respect to I-
• H2O2 and H3O are maintained constant by an
  excess of H2O2 and a buffer.
• By taking the logarithmic form of the rate law
  for this reaction, we determine that log(rate)
  plogI-C (ymx b). So, a plot of log(rate)
  vs. log I- produces a line with a slope p (fig.
  p. 268)

8
Exp. 23 Introduction

• Determination of q, the order of the reaction
  with respect to H2O2
• I- and H3O are maintained constant by an excess
  of KI and a buffer.
• A second plot, of log(rate) vs. logH2O2
  produces a straight line with a slope equal to q
  (see the figure on p. 268).

9
Exp. 23 Procedural Notes

• A.1. You will be using 150 mm test tubes.
  Clean your glassware, but do not dry with paper
  towels.
• A.2. Follow Table 23.1
• A.4. Rapidly add solution B (H2O2) to the
  solution in test tube A. Start timing. Watch
  for the sudden appearance of deep blue color.
  Stop timer. Be patient, this may take as long as
  3 minutes.

10
Exp. 23 Report Sheet

• Questions 1, 3, 5