Free Energy (A2 Only)

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

1.1Atomic Structure

1.1.1Fundamental Particles

1.1.2Isotopes & Mass Number

1.1.3Mass Spectrometry

1.1.4Electron Shells, Sub-Shells & Orbitals

1.1.5Electron Configuration

1.1.6Ionisation Energy

1.1.7Factors Affecting Ionisation Energies

1.1.8Trends of Ionisation

1.1.9Specific Impacts on Ionisation Energies

1.1.10End of Topic Test - Atomic Structure

1.1.11A-A* (AO3/4) - Atomic Structure

1.2Amount of Substance

1.2.1Relative Masses

1.2.2The Mole

1.2.3The Ideal Gas Equation

1.2.4Empirical & Molecular Formulae

1.2.5Balanced Equations

1.2.6Percentage Yield

1.2.7A-A* (AO3/4) - Percentage Yield

1.2.8Atom Economy

1.2.9End of Topic Test - Amount of Substance

1.2.10A-A* (AO3/4) - Substances & Yield

1.3Bonding

1.3.1Ionic Bonding

1.3.2Covalent & Dative Bonding

1.3.3Carbon Structures

1.3.4Metallic Bonding

1.3.5Physical Properties

1.3.6Shapes of Molecules

1.3.7Polarity

1.3.8Intermolecular Forces

1.3.9Intermolecular Forces 2

1.3.10End of Topic Test - Bonding

1.3.11Exam-Style Question - Shape of Molecules

1.3.12A-A* (AO3/4) - Bonding

1.4Energetics

1.4.1Enthalpy Changes

1.4.2Calorimetry

1.4.3Hess' Law

1.4.4Bond Enthalpies

1.4.5End of Topic Test - Energetics

1.5Kinetics

1.5.1Collision Theory

1.5.2Maxwell-Boltzmann Distribution

1.5.3Increasing Rates

1.5.4End of Topic Test - Kinetics

1.6Equilibria

1.6.1Le Chatelier

1.6.2Equilibrium Constants

1.6.3A-A* (AO3/4) - Energetics, Kinetics, Equilibria

1.6.4End of Topic Test - Equilibria

1.7Redox

1.7.1Redox

1.7.2End of Topic Test - Redox

2Physical Chemistry 2 (A2 Only)

2.1Thermodynamics (A2 Only)

2.1.1Born-Haber Cycles (A2 Only)

2.1.2Born-Haber Cycle Calculations (A2 Only)

2.1.3Entropy (A2 Only)

2.1.4Free Energy (A2 Only)

2.1.5End of Topic Test - Thermodynamics

2.1.6A-A* (AO3/4) - Thermodynamics

2.2Rate Equations (A2 Only)

2.2.1Rate Equations (A2 Only)

2.2.2Rate Equations 2 (A2 Only)

2.2.3The Arrhenius Equation (A2 Only)

2.2.4Experimental Methods for Rates (A2 Only)

2.2.5End of Topic Test - Rate Equations

2.2.6Exam-Style Question - Rates

2.2.7A-A* (AO3/4) - Rate Equations

2.3The Equilibrium Constant Kp (A2 Only)

2.3.1Properties of Kp (A2 Only)

2.4Electrochemical Cells (A2 Only)

2.4.1Electrochemical Cells (A2 Only)

2.4.2Electrochemical Series (A2 Only)

2.4.3Commercial Application (A2 Only)

2.4.4End of Topic Test - Kp & Electrochemistry

2.4.5A-A* (AO3/4) - Electrochemical Cells

2.5Acids & Bases (A2 Only)

2.5.1Brønsted-Lowry Acids & Bases (A2 Only)

2.5.2pH (A2 Only)

2.5.3The Ionic Product of Water (A2 Only)

2.5.4Weak Acids & Bases (A2 Only)

2.5.5pH Curves & Titrations (A2 Only)

2.5.6pH Curves & Titrations 2 (A2 Only)

2.5.7Buffer Solutions (A2 Only)

2.5.8End of Topic Test - Acids & Bases

2.5.9Exam-Style Question - Weak Acids

2.5.10A-A* (AO3/4) - Acids & Bases

3Inorganic Chemistry

3.1Periodicity & Trends

3.1.1The Periodic Table

3.1.2Trends in the Periodic Table

3.1.3End of Topic Test - Periodicity & Trends

3.2Group 2

3.2.1Group 2 Chemistry

3.2.2End of Topic Test - Group 2

3.3Group 7

3.3.1Introduction to Halogens

3.3.2Reactions of Halogens

3.3.3Chlorine & Chlorate(I)

3.3.4End of Topic Test - Group 7

3.3.5Exam-Style Question - Group 7

3.3.6A-A* (AO3/4) - Periodicity, Group 2 & 7

4Inorganic Chemistry 2 (A2 Only)

4.1Period 3 (A2 Only)

4.1.1Period 3 Elements & Oxides (A2 Only)

4.1.2Period 3 Oxides (A2 Only)

4.1.3End of Topic Test - Period 3

4.2Transition Metals (A2 Only)

4.2.1General Properties (A2 Only)

4.2.2Substitution Reactions (A2 Only)

4.2.3Shapes of Complex Ions (A2 Only)

4.2.4Colours of Ions (A2 Only)

4.2.5Variable Oxidation States (A2 Only)

4.2.6Titrations (A2 Only)

4.2.7Homogeneous Catalysts (A2 Only)

4.2.8Heterogeneous Catalysts (A2 Only)

4.2.9End of Topic Test - Transition Metals

4.2.10A-A* (AO3/4) - Transition Metals

4.3Reactions of Ions in Aqueous Solutions (A2 Only)

4.3.1Acidity (A2 Only)

4.3.2Identification (A2 Only)

4.3.3End of Topic Test - Reactions of Ions in Solution

5Organic Chemistry 1

5.1Introduction

5.1.1Naming Conventions

5.1.2Mechanism

5.1.3Isomerism

5.1.4End of Topic Test - Introduction to Organic Chem

5.2Alkanes

5.2.1Fractional Distillation

5.2.2Cracking

5.2.3Combustion

5.2.4Chlorination

5.2.5End of Topic Test - Alkanes

5.3Halogenoalkanes

5.3.1Substitution Reactions

5.3.2Elimination Reactions

5.3.3Ozone Depletion

5.3.4End of Topic Test - Halogenoalkanes

5.4Alkenes

5.4.1Introduction to Alkenes

5.4.2Reactions of Alkenes

5.4.3Polymerisation Reactions

5.4.4Properties of Polymers

5.4.5End of Topic Test - Alkenes

5.5Alcohols

5.5.1Production of Alcohols

5.5.2Oxidation of Alcohols

5.5.3Oxidation of Alcohols - 2

5.5.4Elimination Reactions

5.5.5End of Topic Test - Alcohols

5.5.6Exam-Style Question - Alcohols

5.6Organic Analysis

5.6.1Reactions

5.6.2Reactions 2

5.6.3Mass Spectrometry

5.6.4Infrared Spectroscopy

5.6.5End of Topic Test - Organic Analysis

5.7A-A* (AO3/4) - Organic 1

5.7.1A-A* (AO3/4) - Organic 1

6Organic Chemistry 2 (A2 Only)

6.1Optical Isomerism (A2 Only)

6.1.1Optical Isomerism (A2 Only)

6.1.2End of Topic Test - Optical Isomerism

6.2Aldehydes & Ketones (A2 Only)

6.2.1Aldehydes & Ketones (A2 Only)

6.2.2End of Topic Test - Aldehydes & Ketones

6.3Carboxylic Acids & Esters (A2 Only)

6.3.1Acids & Esters (A2 Only)

6.3.2Acylation (A2 Only)

6.3.3End of Topic Test - Carboxylic Acids & Esters

6.4Aromatic Chemistry (A2 Only)

6.4.1Structure & Reactivity (A2 Only)

6.4.2Reactions of Benzene (A2 Only)

6.4.3End of Topic Test - Aromatic Chemistry

6.5Amines (A2 Only)

6.5.1Nature & Preparation of Amines (A2 Only)

6.5.2Basicity & Nucleophilicity (A2 Only)

6.5.3End of Topic Test - Amines

6.6Polymers (A2 Only)

6.6.1Condensation Polymers (A2 Only)

6.6.2Biodegradability (A2 Only)

6.7Biological Organic (A2 Only)

6.7.1Amino Acids (A2 Only)

6.7.2Proteins (A2 Only)

6.7.3DNA (A2 Only)

6.7.4Anti-Cancer Drugs (A2 Only)

6.7.5End of Topic Test - Biological Organic

6.8Organic Synthesis (A2 Only)

6.8.1Organic Synthesis (A2 Only)

6.8.2Examples of Synthetic Routes (A2 Only)

6.8.3End of Topic Test - Polymers & Organic Synthesis

6.9NMR Spectroscopy (A2 Only)

6.9.1Understanding NMR (A2 Only)

6.9.2Carbon-13 NMR (A2 Only)

6.9.3Hydrogen-1 NMR (A2 Only)

6.9.4Hydrogen-1 NMR 2 (A2 Only)

6.9.5End of Topic Test - NMR

6.10Chromatography (A2 Only)

6.10.1Overview of Chromatography (A2 Only)

6.10.2Thin-Layer Chromatography (A2 Only)

6.10.3Column Chromatography (A2 Only)

6.10.4Gas Chromatography (A2 Only)

6.10.5End of Topic Test - Chromatography

6.11A-A* (AO3/4) - Organic 2

6.11.1A-A* (AO3/4) - Organic 2

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