Read and answer the following question.

Experiment 6 - Rates of Reactions.docx

Student Name (Print):__________________________

Pre-lab Study Questions (Submit upon arrival for experiment; show work or explanation to the question. )

An experiment is designed to study the rates of reaction: 2 I^-(aq) + S₂O₈^2-(aq) I₂(aq) + 2 SO₄^2-(aq)

Experiment	[I-] (M)	[S2O82-] (M)	Rate (M/s)
1	0.040	0.040	1.48 10-3
2	0.080	0.040	5.79 10-3
3	0.040	0.080	2.82 10-3

(1) Determine the rate law of the reaction.

(2) Calculate the value of k.

(3) Predict the reaction rate when [I^-] = 0.080 M and [S₂O₈^2-] = 0.160 M.

Experiment 6. Determining Reaction Rates

Student Name (Print):__________________________

I. Objectives

(2) Understand rate laws

(1) Measure reaction rates and calculate reaction orders using measured rates

II. Materials

(1) 4.0 M acetone

(2) 1.0 M hydrochloric acid HCl

(3) 0.005 M I₂

III. Safety Information and Requirements

(1) Wear goggles or safety glasses;

(2) Dump waste according to instruction;

IV. Procedure and Data Collection

IV 1. Introduction

The rate at which a chemical reaction occurs depends on several factors: the nature of the reaction, the concentrations of the reactants, the temperature, the surface area (particle size) of solid reactants, and the presence of possible catalysts or inhibitors. In this lab, we study the rates of the reaction between iodine I₂ and acetone:

CH₃COCH₃ (aq) + I₂ (aq) CH₃COCH₂I (aq) + H⁺ (aq) + I^- (aq)

This reaction can be easily investigated experimentally. I₂ has a brown color, so one can readily follow changes in I₂ concentration visually and determine the reaction rate by the following equation:

..equation (1)

where [I₂]₀ is the starting molarity of I₂ in the reaction mixture and Dt is the time taken for [I₂]₀ to be completely consumed in the reaction (brown colorless).

The rate of this reaction is found to depend on the concentration of H⁺ in the solution as well as presumably on the concentrations of the two reactants. The rate law for this reaction is:

rate = k[CH₃COCH₃]^m[H⁺]ⁿ[I₂]^p ...equation (2)

where m, n, and p are the orders of the reaction with respect to acetone, I₂ and H⁺, respectively, and k is the rate constant for the reaction.

Reaction orders m, n, and p and rate constant k can be determined by varying the initial concentrations of acetone, I₂ and H⁺ and measuring the reaction rates.

IV 2. Information

Dilution is the process of decreasing the concentration of a solute in a solution, usually simply by mixing with more solvent like adding more water to a solution. Mixing different solutions together is also a dilution to the individual original solutions. To calculate the concentration in the solution mixture, one uses the dilution equation: M₁V₁ = M₂V₂ therefore,

.. equation (3)

where M₁ and V₁ are the molarity and the volume of the original solution before mixing. V₂ is the total volume of the mixture solution. M₂ is the new molarity of the solution in the mixture.

For example: 10.0 mL 4.0 M acetone, 10.0 mL 1.0 M HCl, 10.0 mL 0.0050 M I₂ and 20.0 mL H₂O are mixed together to prepare a reaction mixture. The total volume of the mixture = 10.0 + 10.0 + 10.0 + 20.0 = 50.0 mL In the mixture,

IV 3. Data Collection: Four reaction mixtures are prepared to measure reaction rates:

Mixture	4.0 M CH3COCH3	1.0 M HCl	H2O	0.0050 M I2
1	10	10	20	10
2	20	10	10	10
3	10	20	10	10
4	10	10	10	20

(1) To prepare mixture 1, measure 10.0 mL of the 4.0 M acetone solution and add it into a clean Erlenmeyer flask; then measure 10.0 mL 1 M HCl and add it to the acetone in the flask. Add 20.0 mL deionized water to the flask. In a graduated cylinder, measure 10.0 mL 0.005 M I₂ solution.

(2) When you are ready, pour the I₂ solution in the graduated cylinder to the Erlenmeyer flask and start the time immediately. Swirl the flask and stop the timer when the color of the solution becomes colorless. Record for mixture 1.

Record

Calculate the molarities of acetone CH₃COCH₃, H⁺ (i.e. HCl) and I₂ in reaction mixture 1 based on the information given in IV 2.

The molarity of acetone [CH₃COCH₃] in mixture 1:

________ M.

The molarity of H⁺ [H⁺] in mixture 1:

________ M.

The molarity of I₂ [I₂]₀ in mixture 1:

________ M.

Seconds for the solution to change color:

Dt = _________ seconds.

Rate = _______________ (equation (1) in IV 1.

(3) Repeat steps (1) (2) using the volumes listed in the Data Collection table for Mixture 2.

Record

Calculate the molarities of acetone CH₃COCH₃, H⁺ (i.e. HCl) and I₂ in reaction mixture 2 based on the information given in IV 2.

The molarity of acetone [CH₃COCH₃] in mixture 2:

________ M.

The molarity of H⁺ [H⁺] in mixture 2

________ M.

The molarity of I₂ [I₂]₀ in mixture 2

________ M.

Seconds for the solution to change color:

Dt = _________ seconds.

Rate = _______________ (equation (1) in IV 1.

(4) Repeat steps (1) (2) using the volumes listed in the Data Collection table for Mixture 3.

Record

Calculate the molarities of acetone CH₃COCH₃, H⁺ (i.e. HCl) and I₂ in reaction mixture 3 based on the information given in IV 2.

The molarity of acetone [CH₃COCH₃] in mixture 3:

________ M.

The molarity of H⁺ [H⁺] in mixture 3:

________ M.

The molarity of I₂ [I₂]₀ in mixture 3:

________ M.

Seconds for the solution to change color:

Dt = _________ seconds.

Rate = _______________ (equation (1) in IV 1.

(5) Repeat steps (1) (2) using the volumes listed in the Data Collection table for Mixture 4.

Record

Calculate the molarities of acetone CH₃COCH₃, H⁺ (i.e. HCl) and I₂ in reaction mixture 1 based on the information given in IV 2.

The molarity of acetone [CH₃COCH₃] in mixture 4:

________ M.

The molarity of H⁺ [H⁺] in mixture 4:

________ M.

The molarity of I₂ [I₂]₀ in mixture 4:

________ M.

Seconds for the solution to change color:

Dt = _________ seconds.

Rate = _______________ (equation (1) in IV 1.

IV 4. Calculation:

Transfer the data calculated in Data Collection into the following table:

Mixture	[CH3COCH3]	[H+]	[I2]	Rate
1
2
3
4

Rate = k[CH₃COCH₃]^m[H⁺]ⁿ[I₂]^p

Use data from Mixture 1 and Mixture 2, you can determine the reaction order in acetone CH₃COCH₃, m.

m = __________________.

Use data from Mixture 1 and Mixture 3, you can determine the reaction order in H⁺, n.

n = __________________.

Use data from Mixture 1 and Mixture 4, you can determine the reaction order in I₂, p.

p = __________________.

IV. Post-lab Questions

(1) Given the values of m, n, and p as determined above, calculate the rate constant k for each experiment by simply substituting those orders, the concentrations and the rate into equation (2). Take average of all four k values and report it as the value of rate constant k.

(2) A 5^th reaction mixture was made up in the following way:

10 mL CH₃COCH₃ + 20 mL HCl + 15 mL H₂O + 5 mL I₂

Predict the seconds it takes for the I₂ color to disappear at the same temperature of the above experiments.