Chemistry 111

Walczak

April 8, 2004

Energy and Entropy: The Driving Forces of Chemical Reactions

Hopefully Helpful Example Problems

Below are some unbalanced chemical reactions. Balance the equations and then decide whether entropy increases or decreases in the course of the reaction.

	Reaction	Entropy Change?
1	Al2O3(s) + 3 C(s)  ---> 2 Al(s) + 3 CO2(g)	Entropy INCREASES because the products contain a gas. All other species are solids.
2	N2(g) + 3 H2(g) <==> 2 NH3(g)	Entropy change is from 4 moles of gas to two moles of gas, so there is a net DECREASE in entropy.
3	Cl2O(g) + H2O(l) ---> 2 HClO(aq)	Entropy DECREASES because the product is in a liquid (aqueous) state and the reactants contain a gas.
4	4 Fe(s) + 3 O2(g) ---> 2 Fe2O3(s)	Entropy DECREASES because the reactants contain a gas and the other species are solids.

None of these reactions are ones that you are likely to have any experience with, with the possible exception of the rusting of iron (eqn 4). Therefore, you are not expected to predict the energy change or the likelihood of these reactions.

Now, consider the equilibrium endothermic reaction for the formation of ammonia:

N₂(g) + 3 H₂(g) <==> 2 NH₃(g)

If this reaction is at equilibrium in a closed container, will the amount of products or reactants increase in response to the following stresses:

Stress applied to original equilibrium mixture:	Which direction does equilibrium shift?
More N2(g) is added	Equilibrium shifts towards products because a reactant is being added.
Some NH3(g) is removed	Equilibrium shifts towards products because the product is being removed.
The reaction mixture is heated	Equilibrium shifts towards products because the reaction is endothermic.
The pressure applied to the mixture is increased	Equilibrium shifts towards products because the products contain fewer moles than the reactants.

Finally, consider the combustion of octane, the reaction that makes your car go!

2 C₈H₁₈ (g) + 25 O₂(g) ---> 16 CO₂(g) + 18 H₂O(g)

Balance this chemical reaction.

Is this reaction exothermic or endothermic? Exothermic because the energy generated is used to make the car go.

Does entropy increase or decrease as a result of this reaction going to completion? Entropy increases because there are a total of 27 moles of gas on the reactant side and 34 moles of gas on the product side.

Based on the changes in entropy and energy, do you predict this reaction to occur? Yes, both energy decreasing and entropy increasing are indicators of a reaction that goes to completion.

What is molecular weight of octane? (8 x 12 g/mole) + (18 x 1 g/mole) = 114 g/mole.

What is the molecular weight of CO2? (1 x 12 g/mole) + (2 x 16 g/mole) = 44 g/mole.

What is the molecular weight of water? (2 x 1 g/mole) + (1 x 16 g/mole) = 18 g/mole.

What is the molecular weight of O2? (2 x 16 g/mole) = 32 g/mole.

Suppose you have a 12 gallon tank of gasoline in your car. The density of gasoline is about 0.7 grams per milliliter. If you burn the entire tank of gasoline (octane), how many kilograms of carbon dioxide are generated?

How many kilograms of water are generated?

How many kilograms of oxygen are consumed?