Chemistry 12

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Chemistry 12

Unit 1 - Reaction Kinetics

Chemistry 12 Worksheet 1-3 - Reaction Mechanisms
1. It is known that compounds called chlorofluorocarbons (C.F.C.s) (eg. CFCl3) will break up in the presence of ultraviolet radiation, such as found in the upper atmosphere, forming single chlorine atoms: CFCl3  CFCl2 + Cl The Cl atoms then react with Ozone (O3) as outlined in the following mechanism. Step 1: Step 2: Cl + O3  ClO + O2 ClO + O  Cl + O2 (single "O" atoms occur naturally in the atmosphere.)

a) Write the equation for the overall reaction. (Using steps 1 and 2) _________________________________________________________ b) What is the catalyst in this reaction? c) Identify an intermediate in this reaction _________________________________ _______________________________

d) Explain how a small amount of chlorofluorocarbons can destroy a large amount of ozone. _________________________________________________________ _________________________________________________________ e) What breaks the bond in the CFCl3 and releases the free Cl atom? _________________________________________________________ 2. Given the following mechanism, answer the questions below: Step 1: Step 2: O3 + NO  NO2 + O2 (slow) NO2 + O  NO + O2 (fast)

a) Give the equation for the overall reaction. _________________________________________________________________ b) What could the catalyst be in this mechanism? c) What is an intermediate in this mechanism? ________________________ ______________________________

Worksheet 1-3 - Reaction Mechanisms

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Chemistry 12

Unit 1 - Reaction Kinetics

d) Given that the uncatalyzed overall reaction is a slow exothermic reaction, draw a potential energy graph which shows the possible shape of the curve for the uncatalyzed reaction. On the same graph, show a possible curve for the catalyzed reaction.

Progress of Reaction
3. Consider the following mechanism: Step 1: Step 2: H2O2 + I-  H2O + IO(slow)

H2O2 + IO-  H2O + O2 + I- (fast)

a) Give the equation for the overall reaction. __________________________________________________________________ b) What acts as a catalyst in this mechanism? _________________________________ ____________________________

c) What acts as an intermediate in this mechanism? 4.

What is meant by the rate determining step in a reaction mechanism? _______________ ____________________________________________________________________

5.

What is meant by a reaction mechanism?

____________________________________

____________________________________________________________________

Worksheet 1-3 - Reaction Mechanisms

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Chemistry 12

Unit 1 - Reaction Kinetics

6.

How are reaction mechanisms determined?

___________________________________

____________________________________________________________________

7.

Given the following Potential Energy Diagram for a 3 step reaction, answer the questions

below it: a) Which arrow indicates the activation energy for the first step of the reverse reaction? ________ b) Which arrow indicates the activation energy for the first step of the forward reaction? ________ c) Which arrow indicates the activation energy for the second step of the forward reaction? ________ d) Which arrow indicates the enthalpy change (ΔH) or "enthalpy change" for the overall forward reaction? ________

e) Which arrow indicates the enthalpy change (ΔH) or "enthalpy change" for the overall reverse reaction? Worksheet 1-3 - Reaction Mechanisms ________ Page 3

Chemistry 12

Unit 1 - Reaction Kinetics

f) Which arrow indicates the activation energy for the overall forward reaction? ________ g) Which step would be the rate determining step in the forward reaction? ________

h) In a dashed line or another colour sketch a possible curve that would represent the route for the uncatalyzed overall reaction. Label this on the graph. 8. Given the reaction: 4HBr + O2  2H2O + 2Br2 a) Would you...
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