6.3.1 Constant Pressure Calorimetry

Question 1

Constant Pressure Calorimetry

Answer 1

• Heat is measured through a process called calorimetry.
• Constant pressure calorimetry is useful for finding heat capacities or for determining the enthalpy change of reactions.

Question 2

note

Answer 2

• Heat is measured through a process called calorimetry.
• Calorimetry relies on the fact that the heat lost by the system flows into the surroundings. Therefore, the magnitude of the energy change is the same for the system and the surroundings, but the sign of the heat flow is opposite. This is expressed mathematically as q sys = –q surr .
• Calorimetry can be performed at constant pressure or at constant volume.
• Constant pressure calorimetry involves a simple experimental setup where the temperature change of a known amount of water is measured. A constant pressure calorimeter is not airtight, and is therefore open to atmospheric pressure, which is essentially constant.
• The heat evolved by a system at constant pressure is equal to the enthalpy change of that system. Therefore, constant pressure calorimetry can be used to determine the enthalpy change of reactions.
• Constant pressure calorimetry is also useful for finding heat capacities. In this example, a known mass of aluminum was heated to a known temperature, and then placed in a constant pressure calorimeter. The change in temperature of the water (surroundings) was used to calculate the molar heat capacity of the aluminum.
• In this experiment, the moles of water (nH 2 O), molar heat capacity of water (c p (H 2 O)), change in temperature of the water ( T H2O ), change in temperature of the aluminum ( T Al ), and moles of aluminum (n Al ) are all known. The heat (q surr ) gained by the surroundings (water) can be used to determine the heat (q sys ) lost by the aluminum. This relationship can be used to determine the molar heat capacity of aluminum.

Question 3

Which unit is used to measure heat?

Answer 3

Joule

Question 4

When performing an experiment measuring heat changes, how does the heat of the system relate to the heat of the surroundings?

Answer 4

qsys = −qsurr

Question 5

When a heated piece of metal is placed in a coffee cup calorimeter, what happens to the temperature of the water (which is at room temperature)?

Answer 5

The temperature of the water increases.

Question 6

What is coffee-cup calorimetry?

Answer 6

Coffee-cup calorimetry is a process for measuring heat of a reaction at constant pressure.

Question 7

A friend wants you to calculate the molar heat capacity of a given metal. He gives you the initial and final temperatures of the water in the calorimeter, the initial temperature of the metal, and the mass of the metal. You know that the molar heat capacity of water is 75.4 J / mol. What other information do you need before you can calculate the molar heat capacity of the metal?

Answer 7

The amount of water

Question 8

Which of the following is true about molar heat capacity?

Answer 8

It is always a positive value.

Question 9

Which of the following can be used as a constant-pressure calorimeter?

Answer 9

A thermos

Question 10

Which of the following is true regarding the heat of the system and the heat of the surroundings?

Answer 10

The heat released by the system is equal to the heat gained by the surroundings.

Question 11

A calorimeter contains 75.0 g of water at an initial temperature of 25.2 °C. 151.28 g of copper metal at a temperature of 95.5 °C was placed in the calorimeter. The equilibrium temperature was 36.2 °C. The molar heat capacity of water is 75.4 J / mol °C. Determine the molar heat capacity of the copper.

Answer 11

24.5 J / mol °C

Question 12

What are the two basic types of heat measurements that are made experimentally using calorimetry?

Answer 12

heat at constant pressure or heat at constant volume