Exp 12:  "Spontaneity" of Reactions

Introduction:

The Gibbs energy change, ΔG, 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:  ΔG = ΔH - TΔS, one may predict the spontaneity of a particular reaction. ΔG can also be calculated using the relationship ΔG = Σ ΔGf(products) - Σ ΔGf(reactants).

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.

  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) + H2O(l) ----> Ca2+(aq) + OH-(aq) + H2(g)

Procedure:
Place the following amounts separately in each of 5 small test tubes: a 3 cm piece of magnesium ribbon, a piece of copper wire, 1 mL (1 dropperful) 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 (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. Be sure you note vigor of any reactions which occur as well as temperature changes.

Results and Discussion:

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 ΔG?

How do the results of each reaction based on ΔG values correlate with what you have already learned about predicting reactions based on the activity series or on solubility rules?

Conclusions
Compare your results to you hypothesis.

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) + H1+(aq) ----> Sn2+(aq) + H2(g)

  2. Predict the spontaneity of the following reactions at 800 oC, rather than 25 oC.

      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?

Gwen Sibert
Roanoke Valley Governor's School
gsibert@rvgs.k12.va.us
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