Spontaneity of Reactions
Introduction:
The Gibbs energy change, ∆G, is the maximum amount of energy, in the form of work that can be extracted from a reaction system to perform tasks, such as lifting a mass or powering an electric motor. It depends on both the enthalpy and entropy changes of the reaction. Why some reactions occur spontaneously, on their own, and others require a continuous input of energy from an external source can be explained in terms of Gibbs energy.
By using the Gibbs-Helmholtz Equation: ∆G = ∆H - T∆S, one may predict the spontaneity of a particular reaction. ∆G can also be calculated when at a temperature of 25oC using the relationship
∆G = ∑∆Gf(products) - ∑∆Gf(reactants).
Proposal:
In this experiment, you will predict the spontaneity of the five (5) reactions represented in equations 1-5 below. Then in the laboratory, you will attempt to carry out these same reactions to see if indeed, the predictions are true. Prior to doing the experiment calculate the change in free energy for each of the 5 reactions.
1. Mg(s) + 2H+(aq) Mg2+(aq) + H2(g)
2. Cu(s) + 2H+(aq) Cu2+(aq) + H2(g)
3. Ca2+(aq) + SO42-(aq) CaSO4(s)
4. 2Na+(aq) + SO42-(aq) Na2SO4(s)
5. Ca(s) + 2H2O(l) Ca2+(aq) + 2OH-(aq) + H2(g)
Procedures:
Place the following amounts separately in each of 5 test tubes: a 3 cm piece of magnesium ribbon, a piece of copper wire, 1 mL (20 drops) 0.5 M Ca(NO3)2, 1 mL 0.5 M NaCl, and 1 small piece of calcium metal.
Add 1 mL of 1 M HCl to each of the first 2 test tubes, observing for a period of five minutes. Then add 1 mL of 0.5 M K2SO4 to each of the next two test tubes, again observing for five minutes. Lastly, add 2 mL of distilled water to the fifth test tube and observe for five minutes. Be sure you note vigor of any reactions which occurs as well as temperature changes.
Results and Discussion:
Report the spontaneity of each of the reactions you carried out in the lab. Can you say definitely that for those reactions that seemed to be nonspontaneous that they will never happen? Explain.
Do you see any relationship between the vigor of these reactions that were spontaneous and the size of ∆G?
How do the results of each reaction based on ∆G values correlate with you already learned about predicting reactions based on the activity series or on solubility rules?
Conclusions:
Compare your results to the predictions based upon the reactions listed in the proposal.
Application Problems:
1. Predict the spontaneity of the following reactions.
Br -(aq) + Cl2(g) Cl –(aq) + Br2(l)
Br -(aq) + I2(s) I –(aq) + Br2(l)
Cu(s) + Fe2+(aq) Cu2+(aq) + Fe(s)
Sn(s) + 2H1+(aq) Sn2+(aq) + H2(g)
Compare the prediction based on ∆G with what you would expect to happen using the activity
seriesof Halogens and activity series of the metals.
2. Predict the spontaneity of the following reactions at 800 0C, rather than 25 0C.
2H2O(l) 2H2(g) + O2(g)
2NH3(g) N2(g) + 3H2(g)
3. What is the lowest temperature (at a constant pressure) at which ammonia will decompose into
nitrogen and hydrogen?