CHEMISTRY

The Gibbs energy change, Delta 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 for 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: Delta G = Delta H - T Delta S , one may predict the spontaneity of a particular reaction. Delta G can also be calculated using the relationship Delta G = Sum of Delta G_f(products) - Sum of Delta G_f(reactants).

1. Mg_(s) + 2H⁺_(aq) ----> Mg²⁺_(aq) + H_2(g)

2. Cu_(s) 2H⁺_(aq) ----> Cu²⁺_(aq) + H_2(g)

3. Ca²⁺_(aq) + SO₄^2-_(aq) ----> CaSO_4(s)

4. 2Na⁺_(aq) + SO₄^2-_(aq) ----> Na₂SO_4(s)

5. Ca_(s) + H₂O_(l) ----> Ca²⁺_(aq) + OH^-_(aq) + H_2(g)

Add 1 mL of 1 M HCl to each of the first 2 test tubes, observing for a period of five (5) minutes. Then add 1 mL of 0.5 M K₂SO₄ to each of the next 2 test tubes, again observing for five (5) minutes. Lastly, add 2 mL of distilled water to the fifth test tube and observe for five minutes.

Report the spontaneity of each of the reactions you carried out. Can you say definitely that for those reactions that seemed to be nonspontaneous that they will never happen?

Do you see any relationship between the vigor of those reactions that were spontaneous and the size of Delta G?

Compare your results to you hypothesis.

1. Predict the spontaneity of the following reactions.

Br^-_(aq) + Cl_2(g) ----> Cl^-_(aq) + Br_2(l)

Br^-_(aq) + I_2(s) ----> I^-_(aq) + Br_2(l)

Cu_(s) + Fe²⁺_(aq) ----> Cu²⁺_(aq) + Fe_(s)

Sn_(s) + H^1-_(aq) ----> Sn²⁺_(aq) + H_2(g)

2H₂O_(l) ----> 2H_2(g) + O_2(g)

2NH_3(g) ----> N_2(g) + 3H_2(g)

3. What is the lowest temperature (at a constant pressure) at which ammonia will decompose into nitrogen and hydrogen?