Thermodynamics

Group Work.5

- A Zen Moment

In silence contemplate the candle flame for 2 minutes.

During the first minute, allow your mind to go where it wants.

During the second minute, become aware—think—about what the flame is.

Topic: Spontaneity: what makes reactions Go?

The goal for this group work is to make some hypotheses, do some experiments, make observations and draw conclusions about which factors influence whether a reaction happens, that is, it is spontaneous.

Hypothesis 1: Heats of reaction, enthalpy DH, determines the spontaneity of reactions.

Heat-producing—exothermic—reactions go. Heat-using—endothermic—reactions do not go.

Now do a reaction that can either confirm or refute this hypothesis.

A. Pour both vials of white solids into the beaker. Place the parafilm over the beaker then insert the stirring rod through the hole in the film. Stir the solids vigorously, noting all changes your sense can detect.

The two white solids are barium hydroxide octahydrate: Ba(OH)2.8H2O and ammonium nitrate: NH4NO3

B. The white solid product is Ba(NO3)2. Balance the overall reaction, using your senses to confirm/detect the presence of at least one other product.

Ba(OH)2^.8H2O(s) + 2NH4NO3 <=> Ba(NO3)2(s) + 2NH3(g) + 10H2O(l)

C. How do your observations of this barium hydroxide reaction fit with hypothesis 1? Is it time for a new hypothesis? Do not turn the page until you have discussed and come to a group agreement on this question.

The reaction was spontaneous and endothermic (absorbs heat).

Yes – a new hypothesis is in order.

Hypothesis 2: The amount of disorder in a reaction—entropy DS,—determines the spontaneity of reactions.

Disorder-producing—exotropic—reactions go. Exotropic reactions have +DS,

Order-making—endotropic—reactions do not go. Endotropic reactions have -DS,

To gain some intuition about how entropy—disorder—can vary among compounds and their states, study the data below. What conclusions can you draw?

Molecule entropy, DS_f, J/K mol

H2O (g) 188.8

H2O (l) 69.9

I2 (s) 116.1

I2 (g) 260.7

I- (aq) 111.3

Br2 (l) 52

Br2 (g) 245.5

Br- (aq) 82.4

S₈ (s) 32.6

H2S (g) 205.8

H2S (aq) 21

SO2 (g) 248.2

SO3 (g) 256.8

SO4 ^2- (aq) 20.1

C, graphite(s) 5.74

CO - (g) 197.7

CO2 - (g) 213.7

The order of magnitudes of S values is gases > liquids > solids.

Gases have most disorder and solids have the least disorder.

D. Identify which component in your equation (in step B) makes the largest contribution to DS.

Liquid water and gaseous ammonia

E. How does the result of your observations in D fit hypothesis 2?

The results are in agreement with hypothesis 2, the spontaneous reaction favored the direction of increased entropy.

Try another test reaction.

F. Ask your instructor to place a drop of concentrated HCl and a drop of concentrated ammonium hydroxide side by side in a covered beaker/flask. Observe what happens then write a balanced equation for the reaction, including the phases of the reactants and products.

Predict the qualitative change entropy DS, (+ vs - vs 0 ) for this reaction.

HCl(g) NH₃(g) <=> NH₄Cl(s) DS < 0

The spontaneous reaction occurs in the gas phase (the liquids do not mix) to produce a white solid that results in a decrease in entropy.

G. How does your prediction fit with hypothesis 2?

It does not fit….

H. Do we need another hypothesis (and can you suggest one)?

Yes, how about: DG = DH - TDS

Which combines enthalpy and entropy to give a quantity, called the free energy that predicts the spontaneity of a reaction.

Negative => spontaneous Positive => Not spontaneous

So as we found above both quantities are important.