Enthalpy, Entropy, and Spontaneity Explained

Review of Enthalpy Change

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

Review of Enthalpy Change In an endothermic reaction:

Review of Enthalpy Change In an endothermic reaction:  H is + :

Review of Enthalpy Change In an endothermic reaction:  H is + : e.g. A + B  C  H = + 45 kJ

Review of Enthalpy Change In an endothermic reaction:  H is + : e.g. A + B  C  H = + 45 kJ 2.Heat term is on the left side:

Review of Enthalpy Change In an endothermic reaction:  H is + : e.g. A + B  C  H = + 45 kJ 2.Heat term is on the left side: e.g. A + B + 45 kJ  C

Review of Enthalpy Change In an endothermic reaction:  H is + : e.g. A + B  C  H = + 45 kJ 2.Heat term is on the left side: e.g. A + B + 45 kJ  C 3.Potential Energy Diagram looks like:

Review of Enthalpy Change In an endothermic reaction:  H is + : e.g. A + B  C  H = + 45 kJ 2.Heat term is on the left side: e.g. A + B + 45 kJ  C 3.Potential Energy Diagram looks like: PE Reaction Proceeds Reactants Products

Review of Enthalpy Change In an endothermic reaction:  H is + : e.g. A + B  C  H = + 45 kJ 2.Heat term is on the left side: e.g. A + B + 45 kJ  C 3.Potential Energy Diagram looks like: PE Reaction Proceeds Reactants Products

Review of Enthalpy Change In an endothermic reaction:  H is + : e.g. A + B  C  H = + 45 kJ 2.Heat term is on the left side: e.g. A + B + 45 kJ  C 3.Potential Energy Diagram looks like: PE  H = + 45 kJ Reaction Proceeds Reactants Products

Review of Enthalpy Change In an exothermic reaction:

Review of Enthalpy Change In an exothermic reaction:  H is –

Review of Enthalpy Change In an exothermic reaction:  H is – : e.g. X + Y  Z  H = – 36 kJ

Review of Enthalpy Change In an exothermic reaction:  H is – : e.g. X + Y  Z  H = – 36 kJ 2.Heat term is on the right side:

Review of Enthalpy Change In an exothermic reaction:  H is – : e.g. X + Y  Z  H = – 36 kJ 2.Heat term is on the right side: e.g. X + Y  Z + 36 kJ

Review of Enthalpy Change In an exothermic reaction:  H is – : e.g. X + Y  Z  H = – 36 kJ 2.Heat term is on the right side: e.g. X + Y  Z + 36 kJ 3.Potential Energy Diagram looks like:

Review of Enthalpy Change In an exothermic reaction:  H is – : e.g. X + Y  Z  H = – 36 kJ 2.Heat term is on the right side: e.g. X + Y  Z + 36 kJ 3.Potential Energy Diagram looks like: PE Reaction Proceeds Reactants Products

Review of Enthalpy Change In an exothermic reaction:  H is – : e.g. X + Y  Z  H = – 36 kJ 2.Heat term is on the right side: e.g. X + Y  Z + 36 kJ 3.Potential Energy Diagram looks like: PE Reaction Proceeds Reactants Products

Review of Enthalpy Change In an exothermic reaction:  H is – : e.g. X + Y  Z  H = – 36 kJ 2.Heat term is on the right side: e.g. X + Y  Z + 36 kJ 3.Potential Energy Diagram looks like: PE  H = – 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 a chemical system to reach a state of minimum enthalpy. 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

Equilibrium tends to favour a state of minimum enthalpy. PE  H is + Reaction Proceeds Reactants Products An Endothermic Reaction

Equilibrium tends to favour a state of minimum enthalpy. PE  H is + Reaction Proceeds Reactants Products An Endothermic Reaction

Equilibrium tends to favour a state of minimum enthalpy. Enthalpy Reaction Proceeds Reactants Products An Endothermic Reaction  H is +

Equilibrium tends to favour a state of minimum enthalpy. Enthalpy Reaction Proceeds Reactants Products An Endothermic Reaction Reactants have Minimum Enthalpy

Equilibrium tends to favour a state of minimum enthalpy. Enthalpy Reaction Proceeds Reactants Products An Endothermic Reaction Reactants have Minimum Enthalpy In an endothermic reaction, the reactants have Minimum enthalpy,

Equilibrium tends to favour a state of minimum enthalpy. Enthalpy Reaction Proceeds Reactants Products An Endothermic Reaction Reactants have Minimum Enthalpy In an endothermic reaction, the reactants have Minimum enthalpy,

Equilibrium tends to favour a state of minimum enthalpy. Enthalpy Reaction Proceeds Reactants Products An Endothermic Reaction Reactants have Minimum Enthalpy In an endothermic reaction, the reactants have Minimum enthalpy, so if no other factors are considered, equilibrium tends to favour the REACTANTS.

Equilibrium tends to favour a state of minimum enthalpy. An Exothermic Reaction

Equilibrium tends to favour a state of minimum enthalpy. Enthalpy Reaction Proceeds An Exothermic Reaction  H = – 36 kJ Reactants Products

Equilibrium tends to favour a state of minimum enthalpy. Enthalpy Reaction Proceeds An Exothermic Reaction Products have Minimum Enthalpy  H = – 36 kJ Reactants Products

Equilibrium tends to favour a state of minimum enthalpy. Enthalpy Reaction Proceeds An Exothermic Reaction Products have Minimum Enthalpy In an exothermic reaction, the products have Minimum enthalpy,  H = – 36 kJ Reactants Products

Equilibrium tends to favour a state of minimum enthalpy. Enthalpy Reaction Proceeds An Exothermic Reaction Products have Minimum Enthalpy In an exothermic reaction, the products have Minimum enthalpy,  H = – 36 kJ Reactants Products

Equilibrium tends to favour a state of minimum enthalpy. Enthalpy Reaction Proceeds An Exothermic Reaction Products have Minimum Enthalpy In an exothermic reaction, the products have Minimum enthalpy, so if no other factors are considered, equilibrium tends to favour the PRODUCTS.  H = – 36 kJ Reactants Products

Consider the following reaction:

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

Consider the following reaction: Does the tendency toward minimum enthalpy favour the reactants, or the products?

Consider the following reaction: Does the tendency toward minimum enthalpy favour the reactants, or the products? Exothermic

Consider the following reaction: Does the tendency toward minimum enthalpy favour the reactants, or the products? Exothermic Enthalpy Reaction Proceeds Products Reactants

Consider the following reaction: Does the tendency toward minimum enthalpy favour the reactants, or the products? Exothermic Enthalpy Reaction Proceeds Products Reactants Products have Minimum Enthalpy

Consider the following reaction: The tendency toward minimum enthalpy favours the products. Exothermic Enthalpy Reaction Proceeds Products Reactants Products have Minimum Enthalpy

Consider the following reaction: Equilibrium tends to favour reactions in which enthalpy is decreasing.

Consider the following reaction: Equilibrium tends to favour reactions in which enthalpy is decreasing. Exothermic Enthalpy Reaction Proceeds Products Reactants As an exothermic reaction proceeds in the forward direction, the enthalpy is decreasing.

Consider the following reaction: Equilibrium tends to favour reactions in which enthalpy is decreasing. 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

Consider the following reaction: Does the tendency toward minimum enthalpy tend to favour the reactants, or the products?

Consider the following reaction: The heat term is on the left side of the equation,

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

Consider the following reaction: The heat term is on the left side of the equation, so the reaction is endothermic Enthalpy Reactants Products

Consider the following reaction: The heat term is on the left side of the equation, so the reaction is endothermic Enthalpy Reactants Products Reactants have Minimum Enthalpy

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

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

Solids are very ordered, so they have low entropy Liquids are less ordered, so they have more entropy than solids – + Aqueous solutions are mixtures,

Solids are very ordered, so they have low entropy Liquids are less ordered, so they have more entropy than solids – + Aqueous solutions are mixtures, so they have more disorder (entropy) than pure solids or liquids.

Solids are very ordered, so they have low entropy Liquids are less ordered, so they have more entropy than solids – + Aqueous solutions are mixtures, so they have more disorder (entropy) than pure solids or liquids. Gases are in rapid random motion,

Solids are very ordered, so they have low entropy Liquids are less ordered, so they have more entropy than solids – + Aqueous solutions are mixtures, so they have more disorder (entropy) than pure solids or liquids. 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

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

PCl 5(g)  Cl 2(g) + PCl 3(g)

1 mol of gas

PCl 5(g)  Cl 2(g) + PCl 3(g) 1 mol of gas 2 mol of gas

PCl 5(g)  Cl 2(g) + PCl 3(g) 1 mol of gas 2 mol of gas Increasing Entropy

CO (g) + 3H 2(g)  CH 4(g) + H 2 O (g)

4 mol of gas

CO (g) + 3H 2(g)  CH 4(g) + H 2 O (g) 4 mol of gas 2 mol of gas

CO (g) + 3H 2(g)  CH 4(g) + H 2 O (g) 4 mol of gas 2 mol of gas Increasing Entropy

CO (g) + 3H 2(g)  CH 4(g) + H 2 O (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 maximum entropy.

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

Minimum Enthalpy favours Reactants Maximum Entropy favours Reactants No reaction will occur when reactants are mixed. Reactants  Products

Minimum Enthalpy favours Products Reactants  Products

Minimum Enthalpy favours Products Reactants  Products Maximum Entropy favours Products

Minimum Enthalpy favours Products Reactants  Products Maximum Entropy favours Products The reaction will go to completion when reactants are mixed.

Reactants Products Minimum Enthalpy favours Products

Reactants Products Minimum Enthalpy favours Products Maximum Entropy favours Reactants

Reactants Products Minimum Enthalpy favours Products Maximum Entropy favours Reactants The reaction will reach a state of equilibrium when reactants are mixed.

Reactants Products Minimum Enthalpy favours Reactants

Reactants Products Minimum Enthalpy favours Reactants Maximum Entropy favours Products

Reactants Products Minimum Enthalpy favours Reactants Maximum Entropy favours Products The reaction will reach a state of equilibrium when reactants are mixed.

Minimum Enthalpy Favours Maximum Entropy FavoursResultSpontaneity

Minimum Enthalpy Favours Maximum Entropy FavoursResultSpontaneity Reactants No Reaction

Minimum Enthalpy Favours Maximum Entropy FavoursResultSpontaneity Reactants No ReactionNon-spontaneous

Minimum Enthalpy Favours Maximum Entropy FavoursResultSpontaneity Reactants No ReactionNon-spontaneous ReactantsProductsEquilibrium

Minimum Enthalpy Favours Maximum Entropy FavoursResultSpontaneity Reactants No ReactionNon-spontaneous ReactantsProductsEquilibriumSpontaneous

Minimum Enthalpy Favours Maximum Entropy FavoursResultSpontaneity Reactants No ReactionNon-spontaneous ReactantsProductsEquilibriumSpontaneous ProductsReactantsEquilibrium

Minimum Enthalpy Favours Maximum Entropy FavoursResultSpontaneity Reactants No ReactionNon-spontaneous ReactantsProductsEquilibriumSpontaneous ProductsReactantsEquilibriumSpontaneous

Minimum Enthalpy Favours Maximum Entropy FavoursResultSpontaneity Reactants No ReactionNon-spontaneous ReactantsProductsEquilibriumSpontaneous ProductsReactantsEquilibriumSpontaneous Products Completion

Minimum Enthalpy Favours Maximum Entropy FavoursResultSpontaneity Reactants No ReactionNon-spontaneous ReactantsProductsEquilibriumSpontaneous ProductsReactantsEquilibriumSpontaneous Products CompletionSpontaneous