Introduction to Thermochemistry

Part 1: Thermochemistry and Heat

When a chemical reaction happens, energy is transferred to or from the surroundings and often there is a temperature change. For example, when a fire burns it transfers heat energy to the surroundings. Objects near a fire become warmer and the temperature rise can be measured with a thermometer. Thermochemistry is the branch of chemistry that studies this energy change. Heat is energy. Heat is the total amount of energy possessed by the molecules in a piece of matter. Heat is often

measured in units of joules (J) or kilojoules (kJ). When heat energy goes into a substance, one of two things can happen:

Diagram A Diagram B

1. In diagram A, what happens to the temperature of the substance when heat energy is added? ___________________

2. Would the temperature change in diagram A (ΔT = T2 – T1) be positive or negative? Why? _____________________________________________________________________________________________________________

3. Imagine that the dots in the diagrams represent particles. Describe what happens to the particles when heat is added in diagram A. ________________________________________________________________________________

4. Describe what happens to the particles when heat is removed in diagram A. __________________________________________________________________________________________________________________________

5. In diagram B, how should the temperature of the gas compare to the temperature of the liquid? ___________________________________________________________________________________________

6. Would the temperature change in diagram B (ΔT = T2 – T1) be positive or negative? Why? _____________________________________________________________________________________________________________

7. Describe what happens to the particles when heat is added in diagram B? _____________________________________________________________________________________________________________________________

Name: ____________________ Number: Pd: _______ Date: _________________

8. Describe what happens to the particles when heat is removed in diagram B? __________________________________________________________________________________________________________________________

9. When heat energy is added to diagram A, the substance remains a liquid. When heat energy is added in diagram B, the substance becomes a gas. Explain what causes this difference. _____________________________________________________________________________________________________________________________________

10.Using the ideas you just learned, determine which of the following has more heat energy and why: a swimming pool of cold water or a pot of boiling water. __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Part 2: Endothermic vs. Exothermic reactions

Exothermic reactions are chemical reactions that transfer energy to the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to get hotter. The temperature increase can be detected using a thermometer. An example of an exothermic reaction is burning (combustion).

Endothermic reactions are reactions that take in energy from the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to get colder. The temperature decrease can be detected using a thermometer. An example of an endothermic reaction is melting of an ice cube.

Exothermic and endothermic reactions are used extensively in everyday life and in industry. Airbags, a safety device in modern cars, utilize an exothermic reaction. Cold packs used to treat muscular injury involve an endothermic reaction.

11.Label the following diagrams as either exothermic or endothermic and explain why.

Type of Reaction:

Explanation:

12.Fill in the blanks to accurately describe each type of reaction.

a. In an exothermic reaction, energy is _________________ a substance. This causes the particles to move

(added to/removed from)_________________ and the temperature of the substance to _________________. (faster/slower) (increase/decrease)

b. In an endothermic reaction, energy is __________________ a substance. This causes the particles to move (added to/removed from)_________________ and temperature of the substance to _________________. (faster/slower) (increase/decrease)

13.Decide whether each of the following reactions is exothermic or endothermic. Underline the key word(s) in each statement that helped you decide.

a. When two chemicals mix, their temperature rises. ____________________________

b. A solid burns brightly and releases heat, light, and sound. _______________________________

c. When two chemicals are mixed, their temperature drops. _____________________________

d. Two chemicals will only react if you heat them continuously. _____________________________

e. Plants take in light energy for photosynthesis. _________________________________f. Liquid water cools into snowflakes in the clouds. ___________________________________