THERMO CHEMISTRYCHAPTER 11 – STATE CHANGE AND BOMB CALORIMETERS

Heating a substance in the same state requires energy in the for of Q=mcΔt

However, changing the state requires energy as well, in the form ΔH=nΔf/vHm

Fusion Melting and Boiling Condensation

Molar enthalpy of fusion of ice:

6.02kJ/mol =ΔfusHm

Meaning it takes 6.02kJ to melt 1 mole of ice at o oC into liquid water at 0 oC

Molar enthalpy of vaporization of water:

40.7kJ/mol = ΔvapHm

Meaning it requires 40.7kJ to turn 1mol of liquid water at 100oC to steam at 100oC

Use the chart on the previous page to answer the following questions:

1. Find the energy required to melt 255g of ice at 0 oC into water at 0

oC

2. Find the energy released when 36.0g of steam 100 oC condenses to water at 100 oC

3. Find the energy required to heat 125g of ice from -30.0 oC to water at 50.0 oC

4. Find the Energy released when 15g of steam at 150 oC condenses to water at 25 oC

5. Find the energy required to heat 1.5kg of ice at -50.0 oC to steam at 125 oC

Bomb Calorimeter: used to measure the enthalpy of combustion for hydrocarbons (or many foods)

Calorimeter, an instrument for measuring the amount of heat released or absorbed in physical and chemical processes. Calorimeters are of many types. They are used to determine the heat content, specific heat, latent heat, and other thermal properties of substances. Calorimeter readings are usually taken in calories or British thermal units (Btu).

The bomb calorimeter is a common type used to determine the heat of combustion of such substances as fuel and food. The substance to be tested is burned in a metal chamber that is placed in a well-insulated vessel of water. Heat generated by the burning is transferred to the water. The increase in temperature of

the water and the measured weights of both the substance and the water are used to determine the heat released by the substance.

http://science.howstuffworks.com/dictionary/physics-terms/calorimeter-info.htm

Calculations:

With bomb calorimeters, often heat capacity C (kJ/oC) is used rather than

specific heat capacity c (J/goC)

Questions:

1. A bomb calorimeter with a heat capactiy of C=480.6J/oC heats up 12.0 oC due to a sample of propane being burned.

a. What is the total energy produced?

b. What is the mass of propane burned? (ΔcHm = 2220kJ/mol)

2. A bomb calorimeter with a heat capacity of 420J/oC burns a 0.500g sample of naphthalene (C10H8), heating it from 20.0oC to 28.4oC. Based on this information, what is the molar heat of combustion of naphthalene in kJ/mol?

3. A bomb calorimeter containing 650g of water (c=4.19J/goC) with a heat capacity of 420J/oC burns a 0.500g sample of naphthalene (C10H8), heating it from 20.0oC to 26.4oC. Based on this information, what is the molar heat of combustion of naphthalene in kJ/mol?

https://www.youtube.com/watch?v=3VkWpycr5xA