### Richard A. Paselk

General Chemistry

Spring 2011

Exercise: Thermal Chemistry

# Thermal Chemistry

## Calorimeter Problems

Heat = q = mCT; C = the heat capacity of a substance

1. How much heat will it take to raise the temperature of a 15.0 g gold bracelet from 16.1 °C to 49.3 °C? Assume a constant heat capacity of 25.4 J °C-1mol-1 over this temperature range.

2. A 0.95 kg cast iron pan is heated from room temperature (20.0 °C) to 125.0 °C on a gas stove. Assuming the heat capacity of the pan remains constant over this temperature range, calculate the quantity of heat absorbed by the pan ( heat capacity of Fe = 25.1 J °C-1mol-1).

3. What is the heat capacity of a calorimeter that contains 125.0 g of water, if it took 40.0 kJ to raise the temperature of the calorimeter and the water 14.00 °C? (The heat capacity of water is 4.184 Jg-1°C-1)

4. A 75.0 g bar of copper metal at 128.2 °C is dropped into a liter of cool water in an insulated container where its temperature drops to 24.1 °C, in equilibrium with the water. Assuming all of the heat goes to heat up the water (the container has a heat capacity of zero), what was the initial temperature of the water? (The molar heat capacity of copper is 24.4 Jmol-1°C-1, the heat capacity of water is 4.184 Jg-1°C-1.)

5. A 0.828 g sample of methanol is completely combusted in a bomb calorimeter with excess oxygen. The calorimeter contains 1.35 kg of water and the instrument has a heat capacity of 1.06 kJ °C-1 without water. Combustion of the methanol increases the temperature of the calorimeter from 23.10 to 25.90°C. Find the molar heat of combustion of methanol (heat released in burning one mole of methanol).

Section 6.2 in your textbook—you should be able to do the examples and exercises in the assigned problems listed on the Schedule.

## Hess's Law & Enthalpy of Formation Problems

1. Find the value of H for the reaction below at 25°C:

NH3(g) + HCl(g) NH4Cl(s)

2. Calculate the value of H for the reaction below at 25°C:

Na2O(s) + H2O(l) 2 NaOH(s)

3. Find the molar heat of combustion of methanol (CH3OH) in oxygen to give water and carbon dioxide.