2009 AP® CHEMISTRY FREE-RESPONSE QUESTIONS

CH4(g) + 2 Cl2(g) → CH2Cl2(g) + 2 HCl(g)

3. Methane gas reacts with chlorine gas to form dichloromethane and hydrogen chloride, as represented by the equation above.

(a) A 25.0 g sample of methane gas is placed in a reaction vessel containing 2.58 mol of Cl2(g).

(i) Identify the limiting reactant when the methane and chlorine gases are combined. Justify your answer

with a calculation.

(ii) Calculate the total number of moles of CH2Cl2(g) in the container after the limiting reactant has been totally consumed.

Initiating most reactions involving chlorine gas involves breaking the Cl–Cl bond, which has a bond energy of 242 kJ mol-1.

(b) Calculate the amount of energy, in joules, needed to break a single Cl–Cl bond.

(c) Calculate the longest wavelength of light, in meters, that can supply the energy per photon necessary to break the Cl–Cl bond.

The following mechanism has been proposed for the reaction of methane gas with chlorine gas. All species are

in the gas phase.Step 1 Cl2 2 Cl fast equilibrium

Step 2 CH4 + Cl → CH3 + HCl slow

Step 3 CH3 + Cl2 → CH3Cl + Cl fast

Step 4 CH3Cl + Cl → CH2Cl2 + H fastStep 5 H + Cl → HCl fast

(d) In the mechanism, is CH3Cl a catalyst, or is it an intermediate? Justify your answer.

(e) Identify the order of the reaction with respect to each of the following according to the mechanism. In each

case, justify your answer. (i) CH4(g)

(ii) Cl2(g)

2009 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Fm B)2 H2O2(aq) → 2 H2O(l) + O2(g)

3. The mass of an aqueous solution of H2O2 is 6.951 g. The H2O2 in the solution decomposes completely according to the reaction represented above. The O2(g) produced is collected in an inverted graduated tube over water at 23.4°C and has a volume of 182.4 mL when the water levels inside and outside of the tube are the same. The atmospheric pressure in the lab is 762.6 torr, and the equilibrium vapor pressure of water at 23.4°C is 21.6 torr.

(a) Calculate the partial pressure, in torr, of O2(g) in the gas-collection tube.

(b) Calculate the number of moles of O2

(g) produced in the reaction.

(c) Calculate the mass, in grams, of H2O2 that decomposed.

(d) Calculate the percent of H2O2 , by mass, in the original 6.951 g aqueous sample.

(e) Write the oxidation number of the oxygen atoms in H2O2

and the oxidation number of the oxygen atoms in O2 in the appropriate cells in the table below.

Substance Oxidation Number of Oxygen Atoms

H2O2

O2

(f) Write the balanced oxidation half-reaction for the reaction.