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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:1. DH is + : e.g. A + B C DH = + 45
kJ2. Heat term is on the left side:
Review of Enthalpy ChangeIn an endothermic reaction:1. DH is + : e.g. A + B C DH = + 45
kJ2. Heat term is on the left side:
e.g. A + B + 45 kJ C
Review of Enthalpy ChangeIn an endothermic reaction:1. DH is + : e.g. A + B C DH = + 45
kJ2. Heat term is on the left side:
e.g. A + B + 45 kJ C3. Potential Energy Diagram looks like:
Review of Enthalpy ChangeIn an endothermic reaction:1. DH is + : e.g. A + B C DH = + 45
kJ2. Heat term is on the left side:
e.g. A + B + 45 kJ C3. Potential Energy Diagram looks like: PE
Reaction Proceeds
Reactants
Products
Review of Enthalpy ChangeIn an endothermic reaction:1. DH is + : e.g. A + B C DH = + 45
kJ2. Heat term is on the left side:
e.g. A + B + 45 kJ C3. Potential Energy Diagram looks like: PE
Reaction Proceeds
Reactants
Products
Review of Enthalpy ChangeIn an endothermic reaction:1. DH is + : e.g. A + B C DH = + 45
kJ2. Heat term is on the left side:
e.g. A + B + 45 kJ C3. Potential Energy Diagram looks like: PE
DH = + 45 kJ
Reaction Proceeds
Reactants
Products
Review of Enthalpy ChangeIn an exothermic reaction:1. DH is – : e.g. X + Y Z DH = – 36
kJ2. Heat term is on the right side:
Review of Enthalpy ChangeIn an exothermic reaction:1. DH is – : e.g. X + Y Z DH = – 36
kJ2. Heat term is on the right side:
e.g. X + Y Z + 36 kJ
Review of Enthalpy ChangeIn an exothermic reaction:1. DH is – : e.g. X + Y Z DH = – 36
kJ2. Heat term is on the right side:
e.g. X + Y Z + 36 kJ 3. Potential Energy Diagram looks like:
Review of Enthalpy ChangeIn an exothermic reaction:1. DH is – : e.g. X + Y Z DH = – 36
kJ2. 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 ChangeIn an exothermic reaction:1. DH is – : e.g. X + Y Z DH = – 36
kJ2. 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 ChangeIn an exothermic reaction:1. DH is – : e.g. X + Y Z DH = – 36
kJ2. Heat term is on the right side:
e.g. X + Y Z + 36 kJ 3. Potential Energy Diagram looks like: PE
DH = – 36 kJ
Reaction Proceeds
Reactants
Products
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.
Equilibrium tends to favour a state of minimum enthalpy.
PE
DH is +
Reaction Proceeds
Reactants
Products
An Endothermic Reaction
Equilibrium tends to favour a state of minimum enthalpy.
PE
DH 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
DH 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 haveMinimum 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 haveMinimum 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 haveMinimum enthalpy, so if no other factors areconsidered, equilibrium tends to favour the REACTANTS.
Equilibrium tends to favour a state of minimum enthalpy.
Enthalpy
Reaction Proceeds
An Exothermic Reaction
DH = – 36 kJ
Reactants
Products
Equilibrium tends to favour a state of minimum enthalpy.
Enthalpy
Reaction Proceeds
An Exothermic Reaction
Products have Minimum Enthalpy
DH = – 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 haveMinimum enthalpy,
DH = – 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 haveMinimum enthalpy,
DH = – 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 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
Does the tendency toward minimum enthalpy favour the reactants, or the products?
Consider the following reaction:
2(g) 2(g) (g)H F 2HF H 537 kJ
Does the tendency toward minimum enthalpy favour the reactants, or the products?
Consider the following reaction:
2(g) 2(g) (g)H F 2HF H 537 kJ
Does the tendency toward minimum enthalpy favour the reactants, or the products?
Exothermic
Consider the following reaction:
2(g) 2(g) (g)H F 2HF H 537 kJ
Does the tendency toward minimum enthalpy favour the reactants, or the products?
ExothermicEnthalpy
Reaction Proceeds
Products
Reactants
Consider the following reaction:
2(g) 2(g) (g)H F 2HF H 537 kJ
Does the tendency toward minimum enthalpy favour the reactants, or the products?
ExothermicEnthalpy
Reaction Proceeds
Products
Reactants Products have Minimum Enthalpy
Consider the following reaction:
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
Consider the following reaction:
2(g) 2(g) (g)H F 2HF H 537 kJ
Equilibrium tends to favour reactions in which enthalpy is decreasing.
Consider the following reaction:
2(g) 2(g) (g)H F 2HF H 537 kJ
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:
2(g) 2(g) (g)H F 2HF H 537 kJ
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:
(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?
Consider the following reaction:
(s) 2(g) 2 4(g)2C 2H 52.3 kJ C H
The heat term is on the left side of the equation,
Consider the following reaction:
(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
Consider the following reaction:
(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
Enthalpy
Reactants
Products
Consider the following reaction:
(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
Enthalpy
Reactants
Products
Reactants have
Minimum Enthalpy
Consider the following reaction:
(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
Enthalpy
Reactants
Products
Reactants have
Minimum Enthalpy
In this reaction, the tendency toward minimum enthalpy favours the reactants.
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.
Increasing Entropy
Less gas particles in reactants More gas particles in products
Increasing Entropy
More gas particles in reactants Less gas particles in products
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 (Minimum H )• Maximum Entropy (Maximum S )
help determine what will actually happen when reactants are mixed together.
Minimum Enthalpy favours Reactants
Maximum Entropy favours Reactants
No reaction will occur when reactants are mixed.
Reactants 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
Maximum Entropy favours Reactants
The reaction will reach a state of equilibrium when reactants are mixed.
Reactants Products
Minimum Enthalpy favours Reactants
Maximum Entropy favours Products
The reaction will reach a state of equilibrium when reactants are mixed.
MinimumEnthalpyFavours
MaximumEntropyFavours Result Spontaneity
Reactants Reactants No Reaction Non-spontaneous
MinimumEnthalpyFavours
MaximumEntropyFavours Result Spontaneity
Reactants Reactants No Reaction Non-spontaneous
Reactants Products Equilibrium
MinimumEnthalpyFavours
MaximumEntropyFavours Result Spontaneity
Reactants Reactants No Reaction Non-spontaneous
Reactants Products Equilibrium Spontaneous
MinimumEnthalpyFavours
MaximumEntropyFavours Result Spontaneity
Reactants Reactants No Reaction Non-spontaneous
Reactants Products Equilibrium Spontaneous
Products Reactants Equilibrium
MinimumEnthalpyFavours
MaximumEntropyFavours Result Spontaneity
Reactants Reactants No Reaction Non-spontaneous
Reactants Products Equilibrium Spontaneous
Products Reactants Equilibrium Spontaneous
MinimumEnthalpyFavours
MaximumEntropyFavours Result Spontaneity
Reactants Reactants No Reaction Non-spontaneous
Reactants Products Equilibrium Spontaneous
Products Reactants Equilibrium Spontaneous
Products Products Completion