AP Chem FRQ: Magnesium in hydrochloric acid

Long free-response question, 10 pointsAbout 23 minutes · 10 points

Data for Experiment 1: magnesium reacting with excess hydrochloric acid, hydrogen collected over water
Quantity	Value
Mass of Mg ribbon	0.0360 g
Volume of gas collected	36.4 mL
Temperature of water and gas	22.0 °C
Barometric pressure	755.0 torr
Vapor pressure of water at 22.0 °C	19.8 torr

A student studies the reaction of magnesium metal with hydrochloric acid, represented by the equation below. Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) In Experiment 1 the student drops a strip of magnesium ribbon into excess HCl(aq) and collects the H₂(g) produced over water in an inverted graduated tube. The data are in the table. In Experiment 2 the student adds 0.243 g of Mg ribbon to 100.0 mL of 1.00 M HCl(aq) in a polystyrene cup calorimeter. The temperature of the solution rises from 21.4 °C to 32.0 °C. Assume the solution has a mass of 100.0 g and a specific heat capacity of 4.18 J/(g·°C), and that the calorimeter absorbs no heat.

(A) Write the balanced net ionic equation for the reaction of Mg(s) with HCl(aq). 1 point

(B) For Experiment 1: (i) Calculate the partial pressure of H₂(g) in the tube, in atm. (ii) Calculate the number of moles of H₂(g) collected. 2 points

(D) A second student repeats Experiment 1 but forgets to account for the water vapor in the tube. Would that student's calculated percent yield be greater than, less than, or equal to the value in part C? Justify your answer. 1 point

(E) For Experiment 2: (i) Identify the limiting reactant. Support your answer with a calculation. (ii) Calculate the magnitude of the heat, in J, absorbed by the solution. (iii) Calculate ΔH of the reaction in kJ per mole of Mg, including its sign. 3 points

(F) The student repeats Experiment 2 using 0.243 g of powdered magnesium instead of ribbon. (i) Predict how the initial rate of the reaction will compare with the rate using ribbon. Justify your prediction in terms of collisions between particles. (ii) Predict whether the temperature change will be greater than, less than, or equal to 10.6 °C. Justify your prediction. 2 points