Humboldt State University ® Department of Chemistry

Richard A. Paselk

	General Chemistry	Spring 2011
Exercise: Thermal Chemistry	© R. Paselk 2008
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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).

Answers 1-5

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:

NH_3(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.

Answers 1-3

In addition to these exercises you should familiarize yourself with the text materials referenced below.

• Sections 6.3-6.4, in your textbook—you should be able to do the examples and exercises in the assigned problems listed on the Schedule.

Go to Reflection Questions

© R A Paselk

Last modified 2 March 2011