Free Energy

Test yourself

Free Energy

Free energy is a simple way to determine if a reaction is feasible. Free energy includes both entropy and enthalpy.

Feasibility

For a reaction to happen, the total entropy of everything involved must increase.
- This includes its molar entropies, but also the entropy change of the air when it is heated.
- Instead of calculating the entropies of everything, we can define the Gibbs free energy change as ΔG = ΔH - TΔS.
- There’s some complex maths, but it boils down to: If ΔG is negative, the overall entropy increases and a reaction will happen.
- The reaction is said to be feasible.

Temperature dependance

If a reaction has a negative ΔH and a positive ΔS it will always be feasible.
- But some reactions are endothermic, or lose entropy.
- These reactions show a temperature dependence of feasibility.
- We shall explore this on the next few slides.

Endothermic reactions

If a reaction has a positive ΔH, it will only be feasible if the ΔS term is positive and larger than it. You can see a graph above of ΔG vs T for positive ΔH, and positive ΔS.

Reactions with a negative entropy change

Entropy always increases. For a reaction to happen if the standard molar entropies of the reactants decreases, it must heat it's surroundings and cause the entropy of the surroundings to increase.
- So the reaction must be exothermic.
- Above is a graph of ΔG vs T for negative ΔH and ΔS.

Free Energy Calculations

You can calculate the temperature at which a reaction becomes feasible.

Feasibility

For a reaction to happen, ΔG must be negative.
We can calculate the temperature at which a reaction switches from unfeasible to feasible by setting ΔG equal to zero.
To find this temperature, we must rearrange the equation for Gibbs free energy:
- ΔG = ΔH - TΔS becomes 0 = ΔH - TΔS
- So, T = $\frac{\Delta H}{\Delta S}$

Example - bismuth extraction

A step in the extraction of bismuth from it's ore is the reduction of bismuth hydroxide (Bi(OH)₃) by hydrogen according to the equation:
- 2(Bi(OH)₃) + 3H₂ → 2Bi + 6H₂O
If the entropy change is: +400JK^-1mol^-1 and the enthalpy change is 50kJmol^-1, what temperature does it become feasible at?
- See next slide for solution.

Solution

First, make sure your entropy change and enthalpy change both use Joules.
- 50kJmol^-1 = 50,000Jmol^-1
Insert this value into the rearranged Gibbs free energy equation to find the temperature at which this reaction is feasible:
- T = 50,000 ÷ 400 = 125K
- So, the reaction is feasible at temperatures above 125K.

1Physical Chemistry

1.1Atoms, Molecules & Stoichiometry

1.1.1Relative Masses

1.1.2The Mole

1.1.3Mass Spectrometry

1.1.4Empirical & Molecular Formulae

1.1.5Balanced Equations

1.1.6Molar Volume

1.1.7End of Topic Test - Amount of Substance

1.2Atomic Structure

1.2.1Fundamental Particles

1.2.2Isotopes & Mass Number

1.2.3Electron Shells, Sub-Shells & Orbitals

1.2.4Electron Configuration

1.2.5Ionisation Energy

1.2.6Factors Affecting Ionisation Energies

1.2.7Trends of Ionisation

1.2.8Specific Impacts on Ionisation Energies

1.2.9Electron Affinity

1.2.10End of Topic Test - Atomic Structure

1.2.11A-A* (AO2/3) - Atomic Structure

1.3Chemical Bonding

1.3.1Ionic Bonding

1.3.2Covalent & Dative Bonding

1.3.3Shapes of Molecules

1.3.4Intermolecular Forces

1.3.5Intermolecular Forces 2

1.3.6Electronegativity

1.3.7Bond Length, Bond Energy, & Bond Polarity

1.3.8Metallic Bonding

1.3.9Physical Properties

1.3.10End of Topic Test - Bonding

1.3.11A-A* (AO2/3) - Bonding

1.4States of Matter

1.4.1Simple Covalent Molecules

1.4.2The Ideal Gas Equation

1.4.3States of Matter

1.4.4Particle Diagrams

1.4.5The Liquid State

1.4.6Giant Ionic Lattices

1.4.7Giant Metallic Lattices

1.4.8Giant Covalent Structures

1.4.9Graphene & Fullerene

1.4.10Recycling

1.5Chemical Energetics

1.5.1Calorimetry

1.5.2Bond Enthalpies

1.5.3Enthalpy Changes

1.5.4Hess' Law

1.5.5Born-Haber Cycles

1.5.6Born-Haber Cycle Calculations

1.5.7Entropy

1.5.8Free Energy

1.5.9End of Topic Test - Energetics

1.6Electrochemistry

1.6.1Electrochemical Cells

1.6.2Electrochemical Series

1.6.3Commercial Application

1.6.4Nernst Equation

1.6.5Redox

1.7Equilibria

1.7.1Dynamic Equilibrium & Le Chatelier

1.7.2Kc

1.7.3Kp

1.7.4pH

1.7.5The Ionic Product of Water

1.7.6Weak Acids & Bases

1.7.7Introduction to Solubility Equilibria

1.7.8Solubility Equilibria Calculations

1.7.9Free Energy of Dissolution

1.7.10pH and Solubility

1.7.11Common-Ion Effect

1.7.12End of Topic Test - Kp & Electrochemistry

1.7.13A-A* (AO2/3) - Electrochemical Cells

1.8Partition Coefficient

1.8.1Kpc

1.9Reaction Kinetics

1.9.1Collision Theory

1.9.2Orders, Rate Constants & Equations

1.9.3Rate Graphs

1.9.4Rate Determining Step

1.9.5Maxwell-Boltzmann Distribution

1.9.6Catalysts

1.9.7Homogeneous Catalysts

1.9.8Heterogeneous Catalysts

1.9.9End of Topic Test - Kinetics

1.9.10End of Topic Test - Rate Equations

1.9.11A-A* (AO2/3) - Rate Equations

2Inorganic Chemistry

2.1The Periodic Table

2.1.1The Periodic Table

2.1.2Trends in the Periodic Table

2.1.3End of Topic Test - Periodicity & Trends

2.1.4Ceramics

2.1.5Period 3 Elements & Oxides

2.1.6Period 3 Oxides

2.1.7End of Topic Test - Period 3

2.2Group 2

2.2.1Group 2 Chemistry

2.3Group 17

2.3.1Introduction to Halogens

2.3.2Reactions of Halogens

2.3.3Chlorine & Chlorate(I)

2.3.4End of Topic Test - Group 17

2.3.5A-A* (AO2/3) - Periodicity, Group 2 & 17

2.4Transition Metals

2.4.1General Properties

2.4.2Titrations

2.4.3Shapes of Complex Ions

2.4.4Variable Oxidation States

2.4.5Ligands

2.4.6Colours of Ions

2.4.7Stereoisomerism of Complex Ions

2.4.8Cisplatin

2.4.9Ligand Substitutions & Kstab

2.4.10End of Topic Test - Transition Metals

2.4.11A-A* (AO2/3) - Transition Metals

2.5Nitrogen & Sulfur

2.5.1Nitrogen & Ammonia

2.5.2Formulated vs Manure Fertilisers

2.5.3Nitrogen Containing Fertilisers

2.5.4Fertilizers & Eutrophication

2.5.5Common Atmospheric Pollutants & Their Properties

2.5.6Catalytic Converters

2.5.7Atmospheric Oxides

3Organic Chemistry & Analysis

3.1Introduction to Organic Chemistry

3.1.1Naming Conventions

3.1.2Mechanism

3.1.3Isomerism

3.1.4End of Topic Test - Introduction to Organic Chem

3.2Hydrocarbons

3.2.1Fractional Distillation

3.2.2Cracking

3.2.3Combustion

3.2.4Chlorination

3.2.5End of Topic Test - Alkanes

3.2.6Introduction to Alkenes

3.2.7Reactions of Alkenes

3.2.8Polymerisation Reactions

3.2.9End of Topic Test - Alkenes

3.2.10Arenes

3.2.11Evidence for Structure of Arenes

3.2.12Reactions of Benzene

3.2.13End of Topic Test -Arenes

3.3Halogen Derivatives

3.3.1Properties of Haloalkanes

3.3.2Substitution Reactions of Haloalkanes

3.3.3Environmental Impacts of Haloalkanes

3.3.4End of Topic Test - Halogenoalkanes

3.4Hydroxy Compounds

3.4.1Types of Alcohol

3.4.2Oxidation 1

3.4.3Oxidation 2

3.4.4Elimination

3.4.5Acids & Esters

3.4.6Acyl Chlorides & Acylation

3.4.7Phenols & Their Reactions

3.4.8End of Topic Test - Alcohols

3.5Carbonyl Compounds

3.5.1Carbonyls

3.5.2Hydrogen Bonding & Carbonyls

3.5.3Aldehydes & Ketones

3.5.4Tri-Iodomethane Reaction

3.5.5End of Topic Test -Carbonyls

3.6Carboxylic Acids & Derivatives

3.6.1Properties of Carboxylic Acids

3.6.2Esters

3.6.3Acyl Chlorides

3.6.4End of Topic Test - Carboxylic Acids

3.7Nitrogen Compounds

3.7.1Nature & Preparation of Amines

3.7.2Basicity & Nucleophilicity

3.7.3Amides

3.7.4Reactions of Amino Acids

3.7.5Amino Acids

3.8Polymerisation

3.8.1Properties of Addition Polymers

3.8.2Condensation Polymerisation

3.8.3Effects of Side-Chains on Polymers

3.8.4Hydrolysis of Ester & Amide Groups

3.8.5Proteins

3.8.6DNA

3.8.7Adhesive & Conducting Polymers

3.8.8End of Topic Test - Amines & Proteins

3.9Analytical Techniques

3.9.1Chromatography

3.9.2High-Performance Liquid Chromatography

3.9.3Gas Chromatography

3.9.4IR Spectroscopy

3.9.5Uses of IR Spectroscopy

3.9.6Mass Spectrometry

3.9.7Mass Spectrometry Analysis

3.9.8Nuclear Magnetic Resonance

3.9.9Carbon-13 NMR

3.9.10Proton NMR I

3.9.11Proton NMR II

3.9.12End of Topic Test - Analytical Techniques

3.9.13A-A* (AO2/3) - Analytical Techniques

3.10Organic Synthesis

3.10.1Chirality & Drug Synthesis

3.10.2Organic Synthesis

3.10.3Examples of Synthetic Routes

3.10.4End of Topic Test - Organic Synthesis

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