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Question

A  sample of powdered zinc is added to  of a  aqueous solution of hydrobromic acid in a calorimeter. The total heat capacity of the calorimeter and solution is  . The observed increase in temperature is  at a constant pressure of  . Using these data, calculate the standard enthalpy of reaction.

ΔH∘rxn= ___kJ/mol

Answer 1

q = heat = heat capacity x ∆T

q = 448 J/ºC x 21.1ºC = 9408 J .. this is for 2.50 g Zn reacting with 100.0 ml of 2.00 M HBr.

We need to now find the heat (q) for 1 mole of Zn reacting with 2 moles HBr, as written in the equation.

2.50 g Zn x 1 mol Zn/65.39 g = 0.03823 moles Zn

0.1 L x 2 mol/L = 0.2 mol HBr so HBr is in excess and Zn is limiting.

q = ∆Hrxn = 9408 J/0.03823 moles Zn = 246,089 J/mol = -246 kJ/mol (3 significant figures)