Chemistry

Thermochemistry Answer Key

Assignments 1- 8

Page 1.

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Answers to Energy Assignment 1

1. Indicate whether the following scenarios illustrate matter possessing mainly potenial energy, or kinetic

energy.

a) There are 30 liters of fuel in the tank of a stationary automobile.

Ð'*Potential energy (chemical energy in fact)

b) A large rock is perched at the edge of a 40m cliff.

Ð'*Potential energy (of the gravitational type)

c) Energy is supplied to a beaker of water raising its temperature from 20oC to 40oC

Ð'*The warming water is increasing its kinetic energy content

d) The average speed of a molecule of a gas at room temperature is about 1000 m/s.

Ð'*Kinetic energy is possessed by moving molecules

e) Air in an automobile tire exerts pressure on the walls of the tire.

Ð'*Kinetic energy of the moving molecules in the tire volume. Some argue that if the tire could

"blow", it could also be considered potential energy. That's acceptable also.

f) A container of dynamite sits on a storage room floor

Ð'*Chemical potential energy

g) Millions of gallons of water is held behind a hydroelectric power dam.

Ð'*Gravitational potential energy

h) Thirty grams of ice at 0oC are melted to become liquid water at 0oC.

Ð'*The ice experiences an increase in its potential energy content as it melts to become liquid water.

There is no change in kinetic energy content because there is no increase in temperature.

2. Indicate whether the following scenarios illustrate the First law, or Second law of Thermodynamics (or both).

Explain.

a) A toboggan which possesses 100 units of energy at the top of a hill converts 85 units to

kinetic energy and 15 units to heat as it slides to the bottom of the hill.

Ð'*First Law - all forms of energy accounted for, none lost or gained

b) Hot iron is plunged into a bucket of cold water which warms up.

Ð'*Second Law - heat goes from hot iron to cold water

c) An open window in December cools the house down very quickly.

Ð'*Second Law - heat transfers from hot house to cold outside air

d) Burning wood converts chemical energy to heat, light and sound.

Ð'*Both Laws

First Law- energy is being converted to many different forms and their total should equal the total energy the wood possesed in the first place.

Second Law- heat is transferred from the burning wood to the cool air

e) Chemical energy is converted to electrical energy by a battery.

Ð'*First Law (mainly) This is a simple conversion from one form to another but in reality, a small amount of the energy is converted to heat

Energy Assignment 2 - Answers

1. The chart

Scenario System

(syst) Surroundings

(surr) Exothermic or

Endothermic Direction of

Energy Flow Energy

Conversion

* a) dissolving KBr water endothermic surroundings to system Ek Surr to Ep system

b) burning CH4 air in furnace exothermic system to surroundings Ep syst to Ek surr

c) dissociating H2SO4 water component of solution exothermic system to surroundings Ep syst to Ek surr

d) combustion of candle wax surrounding air exothermic system to surroundings Ep syst to Ek surr

e) melting ice water in glass endothermic surroundings to system Ek Surr to Ep syst

f) condensing water air in cloud exothermic system to surroundings Ep syst to Ek surr

g) your metabolic reactions air around you exothermic system to surroundings Ep syst to Ek surr

h) the chemical reaction immediate surroundings endothermic surroundings to system Ek Surr to Ep syst

yours

2. The equation for the fermentation of glucose to alcohol and carbon dioxide is

C6H12O6(s) -------> 2C2H5OH(l) + 2CO2(g)

a) The enthalpy change for the reaction is -68.1 kJ. Is the reaction exothermic or endothermic?

Ans: Its exothermic as you can tell from the negative sign on the energy term.

b) Is energy absorbed or released as the reaction proceeds? Explain how you know this.

Ans: Released. All exothermic systems release energy to the surroundings.

3. The last step in the commercial production of sulfuric acid is

SO3(g) + H2O(l) -----> H2SO4(aq) DH = -250 kJ