Enthalpy, Entropy, and Spontaneity Explained. Review of Enthalpy Change.

Enthalpy, Entropy, and Spontaneity

Explained

Review of Enthalpy Change

Review of Enthalpy Change

Enthalpy change (DH) is amount of heat released or absorbed in a reaction carried out at constant pressure.

Review of Enthalpy ChangeIn an endothermic reaction:

Review of Enthalpy ChangeIn an endothermic reaction:1. DH is + :

Review of Enthalpy ChangeIn an endothermic reaction:1. DH is + : e.g. A + B C DH = + 45

kJ


kJ2. Heat term is on the left side:



e.g. A + B + 45 kJ C



e.g. A + B + 45 kJ C3. Potential Energy Diagram looks like:



e.g. A + B + 45 kJ C3. Potential Energy Diagram looks like: PE

Reaction Proceeds

Reactants

Products




Reaction Proceeds

Reactants

Products




DH = + 45 kJ

Reaction Proceeds

Reactants

Products

Review of Enthalpy ChangeIn an exothermic reaction:

Review of Enthalpy ChangeIn an exothermic reaction:1. DH is –

Review of Enthalpy ChangeIn an exothermic reaction:1. DH is – : e.g. X + Y Z DH = – 36

kJ


kJ2. Heat term is on the right side:



e.g. X + Y Z + 36 kJ



e.g. X + Y Z + 36 kJ 3. Potential Energy Diagram looks like:



e.g. X + Y Z + 36 kJ 3. Potential Energy Diagram looks like: PE

Reaction Proceeds

Reactants

Products




Reaction Proceeds

Reactants

Products




DH = – 36 kJ

Reaction Proceeds

Reactants

Products

Minimum gravitational

potential energy

There is a natural tendency for a chemical system to reach a state of minimum enthalpy.


There is a natural tendency for the enthalpy of a chemical system to decrease.


There is a natural tendency for the enthalpy of a chemical system to decrease.

Equilibrium tends to favour a state of minimum enthalpy.


An Endothermic Reaction


PE

DH is +

Reaction Proceeds

Reactants

Products



PE

DH is +

Reaction Proceeds

Reactants

Products



Enthalpy

Reaction Proceeds

Reactants

Products


DH is +


Enthalpy

Reaction Proceeds

Reactants

Products


Reactants have Minimum Enthalpy


Enthalpy

Reaction Proceeds

Reactants

Products



In an endothermic reaction, the reactants haveMinimum enthalpy,


Enthalpy

Reaction Proceeds

Reactants

Products



In an endothermic reaction, the reactants haveMinimum enthalpy,


Enthalpy

Reaction Proceeds

Reactants

Products



In an endothermic reaction, the reactants haveMinimum enthalpy, so if no other factors areconsidered, equilibrium tends to favour the REACTANTS.


An Exothermic Reaction


Enthalpy

Reaction Proceeds


DH = – 36 kJ

Reactants

Products


Enthalpy

Reaction Proceeds


Products have Minimum Enthalpy

DH = – 36 kJ

Reactants

Products


Enthalpy

Reaction Proceeds



In an exothermic reaction, the products haveMinimum enthalpy,

DH = – 36 kJ

Reactants

Products


Enthalpy

Reaction Proceeds



In an exothermic reaction, the products haveMinimum enthalpy,

DH = – 36 kJ

Reactants

Products


Enthalpy

Reaction Proceeds



In an exothermic reaction, the products haveMinimum enthalpy, so if no other factors areconsidered, equilibrium tends to favour the PRODUCTS.

DH = – 36 kJ

Reactants

Products

Consider the following reaction:


2(g) 2(g) (g)H F 2HF H 537 kJ


2(g) 2(g) (g)H F 2HF H 537 kJ

Does the tendency toward minimum enthalpy favour the reactants, or the products?


2(g) 2(g) (g)H F 2HF H 537 kJ



2(g) 2(g) (g)H F 2HF H 537 kJ


Exothermic


2(g) 2(g) (g)H F 2HF H 537 kJ


ExothermicEnthalpy

Reaction Proceeds

Products

Reactants


2(g) 2(g) (g)H F 2HF H 537 kJ


ExothermicEnthalpy

Reaction Proceeds

Products

Reactants Products have Minimum Enthalpy


2(g) 2(g) (g)H F 2HF H 537 kJ

The tendency toward minimum enthalpy favours the products.

ExothermicEnthalpy

Reaction Proceeds

Products

Reactants Products have Minimum Enthalpy


2(g) 2(g) (g)H F 2HF H 537 kJ

Equilibrium tends to favour reactions in which enthalpy is decreasing.


2(g) 2(g) (g)H F 2HF H 537 kJ


Exothermic

Enthalpy

Reaction Proceeds

Products

Reactants

As an exothermic reaction proceeds in the forward direction, the enthalpy is decreasing.


2(g) 2(g) (g)H F 2HF H 537 kJ


Exothermic

Enthalpy

Reaction Proceeds

Products

Reactants

As an exothermic reaction proceeds in the forward direction, the enthalpy is decreasing. So this is a favourable change


(s) 2(g) 2 4(g)2C 2H 52.3 kJ C H

Does the tendency toward minimum enthalpy tend to favour the reactants, or the products?


(s) 2(g) 2 4(g)2C 2H 52.3 kJ C H

The heat term is on the left side of the equation,


(s) 2(g) 2 4(g)2C 2H 52.3 kJ C H

The heat term is on the left side of the equation, so the reaction is endothermic


(s) 2(g) 2 4(g)2C 2H 52.3 kJ C H


Enthalpy

Reactants

Products


(s) 2(g) 2 4(g)2C 2H 52.3 kJ C H


Enthalpy

Reactants

Products

Reactants have

Minimum Enthalpy


(s) 2(g) 2 4(g)2C 2H 52.3 kJ C H


Enthalpy

Reactants

Products

Reactants have

Minimum Enthalpy

In this reaction, the tendency toward minimum enthalpy favours the reactants.

The other factor that affects equilibrium is entropy.

Entropy means disorder,

Entropy means disorder, or randomness.

Solids are very ordered,

Solids are very ordered, so they have low entropy


Liquids are less ordered,


Liquids are less ordered, so they have more entropy than solids



–

+

Aqueous solutions are mixtures,



–

+

Aqueous solutions are mixtures, so they have more disorder (entropy) than pure solids or liquids.



–

+


Gases are in rapid random motion,



–

+


Gases are in rapid random motion, so They have the most entropy.

–

+

Solids < Liquids < Aqueous Solutions < Gases

Increasing Entropy

Less gas particles More gas particles

Less gas particles More gas particles

Increasing Entropy

Increasing Entropy

Less gas particles in reactants More gas particles in products

Increasing Entropy

Less gas particles in reactants More gas particles in products

Increasing Entropy

More gas particles in reactants Less gas particles in products

PCl5(g) Cl2(g) + PCl3(g)


1 mol of gas


1 mol of gas

2 mol of gas


1 mol of gas

2 mol of gas

Increasing Entropy

CO(g) + 3H2(g) CH4(g) + H2O(g)

CO(g) + 3H2(g) CH4(g) + H2O(g)

4 mol of gas

CO(g) + 3H2(g) CH4(g) + H2O(g)

4 mol of gas

2 mol of gas

CO(g) + 3H2(g) CH4(g) + H2O(g)

4 mol of gas

2 mol of gas

Increasing Entropy

CO(g) + 3H2(g) CH4(g) + H2O(g)

4 mol of gas

2 mol of gas

Decreasing Entropy

There is a natural tendency for a system to reach a state of minimum enthalpy.

There is a natural tendency for a system to reach a state of minimum enthalpy.

There is a natural tendency for a system to reach a state of maximum entropy.

Both tendencies:

Both tendencies:

• Minimum Enthalpy

Both tendencies:

• Minimum Enthalpy (Minimum H )

Both tendencies:

• Minimum Enthalpy (Minimum H )• Maximum Entropy

Both tendencies:

• Minimum Enthalpy (Minimum H )• Maximum Entropy (Maximum S )

Both tendencies:

• Minimum Enthalpy (Minimum H )• Maximum Entropy (Maximum S )

help determine what will actually happen when reactants are mixed together.

Here are the possibilities:

Reactants Products

Minimum Enthalpy favours Reactants


Maximum Entropy favours Reactants

Reactants Products



No reaction will occur when reactants are mixed.

Reactants Products

Minimum Enthalpy favours Products

Reactants Products


Reactants Products

Maximum Entropy favours Products


Reactants Products


The reaction will go to completion when reactants are mixed.

Reactants Products


Reactants Products



Reactants Products



The reaction will reach a state of equilibrium when reactants are mixed.

Reactants Products


Reactants Products



Reactants Products



The reaction will reach a state of equilibrium when reactants are mixed.

MinimumEnthalpyFavours

MaximumEntropyFavours Result Spontaneity



Reactants Reactants No Reaction



Reactants Reactants No Reaction Non-spontaneous




Reactants Products Equilibrium




Reactants Products Equilibrium Spontaneous





Products Reactants Equilibrium





Products Reactants Equilibrium Spontaneous






Products Products Completion






Products Products Completion Spontaneous

Enthalpy, Entropy, and Spontaneity Explained. Review of Enthalpy Change.

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