Time for dessert!
When you are baking something, you heat the oven to the temperature indicated in the recipe. Then you mix all the ingredients, put them in the proper baking dish, and place them in the oven for a specified amount of time. If you had mixed the ingredients and left them out at room temperature, not much would change. The materials need to be heated to a given temperature for a set time in order for the ingredients to react with one another and produce a delicious final product.
The free energy change of a reaction can be calculated using the following expression:
Sample Problem: Gibbs Free Energy
Methane gas reacts with water vapor to produce a mixture of carbon monoxide and hydrogen according to the balanced equation below.
Step 1: List the known values and plan the problem.
- ΔH∘=206.1 kJ/mol
- ΔS∘=215 J/K⋅mol=0.215 kJ/K⋅mol
- T=25∘C=298 K
Prior to substitution into the Gibbs free energy equation, the entropy change is converted to kJ/K • mol and the temperature to Kelvins.
Step 2: Solve.
Step 3: Think about your result.
The unfavorable driving force of increasing enthalpy outweighed the favorable increase in entropy. The reaction will be spontaneous only at some elevated temperature.
- Calculations of free energy changes are described.
- What would happen to ΔH if you forgot to change the units for ΔS to kJ/K • mol?
- What are standard conditions for enthalpy and entropy changes?
- At what temperature would the reaction become spontaneous?
Use the resource below to answer the questions that follow.
- Why is ΔH negative in this example?
- What would happen if you forgot to change the sign of the TΔS value in the first calculation?
- What indicates that the reaction is spontaneous?