Reaction Kinetics Written Response August 1999

January 2000

April 2000

June 2000

August 2000

January 2001

April 2001

June 2001 1. The mass of a burning candle is monitored to determine the rate of combustion of paraffin. An accepted reaction for the combustion of paraffin is:

2C28H58(s) + 85O2(g) → 56CO2(g) + 58H2O(g) The following data is observed: a) Calculate the average rate of consumption of paraffin in g/min for the time interval 12.0 to 24.0 minutes. b) Calculate the rate of CO2 production in mol/min for the time interval 12.0 to 24.0 minutes. August 2001 1. Consider the following proposed reaction mechanism:

Step 1 Fe3+ + H2O2 → FeH2O23+

Step 2 FeH2O23+ → FeOH3+ + HO

Step 3 HO + H2O2 → H2O + HO2 Step 4 FeOH3+ + HO2 → Fe3+ + H2O + O2

a) Write the overall reaction b) Define the term catalyst and identify a catalyst in the above mechanism.

Time (min)

Mass of Candle (g)

0.0 25.6 6.0 25.1 12.0 24.5 18.0 23.9 24.0 23.4 30.0 22.8

January 2002 1. Consider the following reaction:

C12H22O11(s) → 11H2O(g) + 12C(s) The rate of decomposition of C12H22O11 is 0.75mol/min. What mass of C is produced in 10.0 seconds? 2. Define the term activation energy. April 2002 1. Consider the reaction:

C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(g) At certain conditions, 0.15 mol CO2 is produced in 2.0 minutes. What is the rate of consumption of C2H4 in g/s? 2. Define the term reaction mechanism.

June 2002 1. Consider the reaction:

2Al(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2(g) A 10.0g sample of Al reacts completely in excess HCl in 300.0s. What is the rate of production of H2 in mol/s ? 2. Using collision theory, give two reactions why reactions occur more rapidly at a higher temperature. August 2002 1. Consider the reactions:

2H2O(l) → 2H2(g) + O2(g) The rate of production of O2 is 1.2 × 10-2 mol/s. How many seconds will it take to compose 100.0g H2O? 2. Define the term catalyst.

January 2003 1. Using the axes below, sketch a PE diagram for the reacting system where: 2. Consider the following reaction mechanism:

Step 1 2NO → N2O2 Step 2 N2O2 + H2 → N2O + H2O Step 3 N2O + H2 → N2 + H2O

a) Determine the overall reaction. Overall Reaction: _________________________________________________ b) Identify a reaction intermediate. ________________________________________________________________

April 2003 1. Consider the following reaction:

3Cu(s) + 8HNO3(aq) → 3Cu(NO3)2(aq) + 2NO(g) + 4H2O(l) A piece of copper is added to a nitric acid solution in an open beaker, allowing the NO(g) to escape. The following data was obtained:

Time (min)

Mass of Beaker and Contents (g)

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

200.00 197.50 195.45 193.55 191.70 189.90 188.15 186.45 184.80

a) Calculate the reaction rate for the time period 2.0 to 6.0 min. b) Calculate the mass of copper consumed in the first 5 minutes. 2. Using collision theory, explain why reactions between two solutions occur more rapidly than reactions between two solids.

1. Consider the following reaction in an open flask:

CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g) A 155.0g sample of CaCO3(s) is placed in the flask and HCl(aq) is added. The reaction consumed HCl(aq) at an average rate of 0.200 mol/min for 10.0 min. What mass of CaCO3(s) remains? 2. a) Write the equation for Step 3 in the following reaction mechanism.

Step 1 2NO → N2O2 Step 2 N2O2 + H2 → N2O + H2O Step 3 ?

Overall Reaction 2NO + 2H2 → N2 + 2H2O b) Identify a reaction intermediate in the above mechanism.

August 2003 1. Consider the following reaction:

Mg(s) + 2HBr(aq) → MgBr2(aq) + H2(g) + energy In terms of collision theory, describe how each of the factors below would influence the reaction rate. a) Increasing the concentration of HBr : _____________________________________ _____________________________________________________________________ b) Decreasing the temperature : ____________________________________________ _____________________________________________________________________ c) Increasing the surface area of Mg : ________________________________________ _____________________________________________________________________ 2. Consider the following reaction mechanism:

Step 1 NO(g) + O2(g) → NO3(g) slow

Step 2 NO3(g) + NO(g) → 2NO2(g)

The overall reaction is exothermic. Sketch a PE diagram on the axes below to describe the energy changes that occur as the reaction takes place.

January 2004 1. The release of O2(g) resulting from the decomposition of bleach was measured in two different experiments. Data was collected and the following graph was drawn:

a) Calculate the average rate of reaction for each experiment. b) Identify a variable from Experiment 1 and how it was changed to produce the different reaction rate for Experiment 2. Explain using collision theory.

Solutions: August 1999

January 2000 Question 1

Question 2

April 2000 Question 1

Question 2

June 2000

August 2000

January 2001

April 2001

June 2001 1. a) b) August 2001 1. a) 2H2O2 → 2H2O + O2 b) A species which speeds up a reaction by providing a lower energy pathway. Catalyst : Fe3+ January 2002 1. 2. Activation energy is the minimum amount of energy required to form the activated complex from the reactants.

April 2002 1. 2. A reaction mechanism is a series of steps that result in the overall reaction. June 2002 1. 2. There is a greater fraction of collisions with sufficient energy. There are more frequent collisions.

August 2002 1. 2. A catalyst is a substance that increases the rate of a chemical reaction and may be recovered at the end of the reaction. January 2003 1. 2. a) Overall reaction : 2NO + 2H2 → N2 + 2H2O b) N2O2 OR N2O

April 2003 1. a) b) 2. Particles must be able to collide to react. Only the particles on the surface of a solid are available for reaction. In a solution, all particles are available. June 2003 1. 2. a) Step 3 : N2O + H2 → N2 + H2O b) Either N2O2 OR N2O

August 2003 1. a) Greater concentration and more collisions. Therefore more successful collisions and a greater rate. b) Fewer collisions with sufficient energy to overcome PE barrier. Therefore, a lower rate. c) Increased surface area leads to more collisions and more successful collisions. therefore, a higher rate. 2. January 2004 1. a)

b)

OR Concentration of reactants

was decreased. Fewer collisions.

Variable/Change Explanation

Temperature is decreased. Lower fraction of effective collisions.