Topics
Spontaneity and Entropy
Gibb's Free Energy

Reaction Spontaneity

a) Review

As seen in the first lesson, reactions occur due to two tendencies or drives.

a) The drive towards less energy.

b) The drive towards greater randomness or entropy.

We have studied the drive toward less energy and energy changes in reaction quite extensively in lesson 2- 5. Let's now look at the change in entropy that can accompany a reaction.

b) Entropy increase (Randomness)

Entropy increases when any of the following events occur in a reaction.

1) Change in state from solid to liquid.
2) Change in state from liquid to gas.
3) Change in state from solid to gas .
4) Formation of a mixture.
5) Increase in the volume of a system of gases.(increased number of gas particles from reactant to product).

Example: Evaporation of water Examine the following reaction, for the evaporation of water. Why does this process occur spontaneously? H2O (l) H2O (g) Answer:The evaporation of water requires energy, but the movement from a liquid to a gas state causes an increase in entropy. This is why water spontaneously evaporates.

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c) Activities: Predicting entropy change

i) Check your understanding entropy increase by attempting the following.

Check your understanding Using the 5 points above about entropy increase, predict if entropy will increase in the following reaction. 2 NO (g) + O2 (g) 2 NO2 (g) click the check mark for the answer

 

ii) Complete the following activity on general entropy change

Predicting general entropy change click on the notepad and predict if entropy will increase or decrease in the reactions given

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d) Calculating entropy change:

The exact amount of Entropy change (S) can be calculated in a very similar way to calculations of enthalpy changes.

S

=

Entropy's of products

__

Entropy's of reactants

The entropy of each substance in a reaction is the product of the standard entropy (taken from a table) and the number of moles of substance in the balanced equation.(S⁰ x Balance) Standard entropy can be found in the following table.

Example: Calculate the entropy change for the following reaction 3H2 (g) + CO (g) CH4 (g) + H2O (g) Answer: 1) Obtain values of entropy from the table.   2) Calculate entropy of each substance by multiplying the balance by the standard entropy.   3) Plug in the values in to the summation formula and calculate total entropy change.

 

 

 

 

 

 

 

 

 

 

 

Calculating entropy changes Calculate if entropy increases (+ S) or decreases (- S) in the following reactions using the summation formula.

e) Reaction Spontaneity

Both entropy and enthalpy play a role in whether a reaction will be spontaneous. The balance of these two factors is described in a quantity we call Gibb's Free Energy. Calculation of spontaneity and Gibb's Free Energy are performed in the next section

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