1.4.5

Hess' Law

Test yourself on Hess' Law

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Hess' Law

Hess' Law provides a useful way of calculating enthalpy changes.

Hess' Law

Hess' Law

The enthalpy change of a reaction is independent of the pathway.
- This means that the enthalpy change of the reaction is the same for if the reaction is direct or indirect.

What does this mean?

What does this mean?

Hess' Law is useful for obtaining enthalpy changes that aren't directly observable.
These are often obscure reactions.
With the help of some easy reactions that involve some of these compounds, we can derive the enthalpy changes we're interested in.

The break down

The break down

This means that the following will all have the same enthalpy change:
- A → B → C → D → E
- A → C → E
- A → E
Overall, they all just convert A to E and so the overall enthalpy change will be identical for all of them.

Hess Cycles

We can use Hess' Law to derive Hess Cycles which will help us with calculations. We can create cycles using enthalpies of formation or combustion.

Hess cycles

Hess cycles

Above shows how we can convert the principle of Hess' Law into an actual equation.
We can set up an equation using the pathways:
- ΔH₁ + ΔH₂ = ΔH₃
We can then subtitute in the known values and rearrange to find the unknown values.

Using enthalpies of formation

Using enthalpies of formation

The first type of Hess cycle is those that use enthalpies of formation to calculate an overall reaction's enthalpy change.
- We set up a cycle, as shown, which includes two enthalpies of formation arrows to create the cycle.

Using enthalpies of formation

Using enthalpies of formation

To do this we must know all the enthalpies of formation for the reactants and the products.
- The enthalpy of formation for any element is zero.

Using enthalpies of combustion

Using enthalpies of combustion

We can use a similar method to set up a cycle using enthalpies of combustion.
- Below ΔH₂ and ΔH₃ represent enthalpies of combustion.
  - This can be spotted by the combustion products: CO₂ + H₂O

1

Physical Chemistry

1.1

Atomic 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

1.2

Amount 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 1.2.11Diagnostic Misconceptions - Moles

1.3

Bonding

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 1.3.13Diagnostic Misconceptions - Ions 1.3.14Diagnostic Misconceptions - Ionic & Covalent 1.3.15Diagnostic Misconceptions - Phase Change 1.3.16Diagnostic Misconceptions - Boiling 1.3.17Diagnostic Misconceptions - Polar Bonds

1.4

Energetics

1.4.1Enthalpy Changes 1.4.2Bond Breaking and Making 1.4.3Enthalpy Changes: Standard Conditions 1.4.4Calorimetry 1.4.5Hess' Law 1.4.6Bond Enthalpies 1.4.7End of Topic Test - Energetics 1.4.8Diagnostic Misconceptions - Calorimetry

1.5

Kinetics

1.5.1Collision Theory 1.5.2Maxwell-Boltzmann Distribution 1.5.3Increasing Rates 1.5.4End of Topic Test - Kinetics

1.6

Equilibria

1.6.1Le Chatelier 1.6.2Equilibrium Constants 1.6.3A-A* (AO3/4) - Energetics, Kinetics, Equilibria 1.6.4End of Topic Test - Equilibria 1.6.5Diagnostic Misconceptions - Solids

1.7

Redox

1.7.1Redox 1.7.2End of Topic Test - Redox

2

Physical Chemistry 2 (A2 Only)

2.1

Thermodynamics (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

2.2

Rate 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

2.3

The Equilibrium Constant Kp (A2 Only)

2.3.1Properties of Kp (A2 Only)

2.4

Electrochemical 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 2.4.6Diagnostic Misconceptions - Salt Bridges 2.4.7Diagnostic Misconceptions - Electrode, Electrolyte

2.5

Acids & 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 2.5.11Diagnostic Misconceptions - Ammonia is an Alkali 2.5.12Diagnostic Misconceptions - Water's Neutrality 2.5.13Diagnostic Misconceptions - Concentrate & Strength

3

Inorganic Chemistry

3.1

Periodicity & Trends

3.1.1The Periodic Table 3.1.2Trends in the Periodic Table 3.1.3End of Topic Test - Periodicity & Trends 3.1.4Diagnostic Misconceptions - Atomic Radius

3.2

Group 2

3.2.1Group 2 Chemistry 3.2.2End of Topic Test - Group 2

3.3

Group 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

4

Inorganic Chemistry 2 (A2 Only)

4.1

Period 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

4.2

Transition 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

4.3

Reactions 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

5

Organic Chemistry 1

5.1

Introduction

5.1.1Naming Conventions 5.1.2Mechanism 5.1.3Isomerism 5.1.4End of Topic Test - Introduction to Organic Chem

5.2

Alkanes

5.2.1Fractional Distillation 5.2.2Cracking 5.2.3Combustion 5.2.4Chlorination 5.2.5End of Topic Test - Alkanes

5.3

Halogenoalkanes

5.3.1Substitution Reactions 5.3.2Elimination Reactions 5.3.3Ozone Depletion 5.3.4End of Topic Test - Halogenoalkanes

5.4

Alkenes

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

5.5

Alcohols

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

5.6

Organic Analysis

5.6.1Reactions 5.6.2Reactions 2 5.6.3Mass Spectrometry 5.6.4Infrared Spectroscopy 5.6.5End of Topic Test - Organic Analysis

5.7

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

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

6

Organic Chemistry 2 (A2 Only)

6.1

Optical Isomerism (A2 Only)

6.1.1Optical Isomerism (A2 Only)6.1.2End of Topic Test - Optical Isomerism

6.2

Aldehydes & Ketones (A2 Only)

6.2.1Aldehydes & Ketones (A2 Only)6.2.2End of Topic Test - Aldehydes & Ketones

6.3

Carboxylic Acids & Esters (A2 Only)

6.3.1Acids & Esters (A2 Only)6.3.2Acylation (A2 Only)6.3.3End of Topic Test - Carboxylic Acids & Esters

6.4

Aromatic Chemistry (A2 Only)

6.4.1Structure & Reactivity (A2 Only)6.4.2Reactions of Benzene (A2 Only)6.4.3End of Topic Test - Aromatic Chemistry

6.5

Amines (A2 Only)

6.5.1Nature & Preparation of Amines (A2 Only)6.5.2Basicity & Nucleophilicity (A2 Only)6.5.3End of Topic Test - Amines

6.6

Polymers (A2 Only)

6.6.1Condensation Polymers (A2 Only)6.6.2Biodegradability (A2 Only)

6.7

Biological 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

6.8

Organic 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

6.9

NMR 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

6.10

Chromatography (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

6.11

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

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

Jump to other topics

1

Physical Chemistry

1.1

Atomic 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

1.2

Amount 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 1.2.11Diagnostic Misconceptions - Moles

1.3

Bonding

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 1.3.13Diagnostic Misconceptions - Ions 1.3.14Diagnostic Misconceptions - Ionic & Covalent 1.3.15Diagnostic Misconceptions - Phase Change 1.3.16Diagnostic Misconceptions - Boiling 1.3.17Diagnostic Misconceptions - Polar Bonds

1.4

Energetics

1.4.1Enthalpy Changes 1.4.2Bond Breaking and Making 1.4.3Enthalpy Changes: Standard Conditions 1.4.4Calorimetry 1.4.5Hess' Law 1.4.6Bond Enthalpies 1.4.7End of Topic Test - Energetics 1.4.8Diagnostic Misconceptions - Calorimetry

1.5

Kinetics

1.5.1Collision Theory 1.5.2Maxwell-Boltzmann Distribution 1.5.3Increasing Rates 1.5.4End of Topic Test - Kinetics

1.6

Equilibria

1.6.1Le Chatelier 1.6.2Equilibrium Constants 1.6.3A-A* (AO3/4) - Energetics, Kinetics, Equilibria 1.6.4End of Topic Test - Equilibria 1.6.5Diagnostic Misconceptions - Solids

1.7

Redox

1.7.1Redox 1.7.2End of Topic Test - Redox

2

Physical Chemistry 2 (A2 Only)

2.1

Thermodynamics (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

2.2

Rate 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

2.3

The Equilibrium Constant Kp (A2 Only)

2.3.1Properties of Kp (A2 Only)

2.4

Electrochemical 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 2.4.6Diagnostic Misconceptions - Salt Bridges 2.4.7Diagnostic Misconceptions - Electrode, Electrolyte

2.5

Acids & 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 2.5.11Diagnostic Misconceptions - Ammonia is an Alkali 2.5.12Diagnostic Misconceptions - Water's Neutrality 2.5.13Diagnostic Misconceptions - Concentrate & Strength

3

Inorganic Chemistry

3.1

Periodicity & Trends

3.1.1The Periodic Table 3.1.2Trends in the Periodic Table 3.1.3End of Topic Test - Periodicity & Trends 3.1.4Diagnostic Misconceptions - Atomic Radius

3.2

Group 2

3.2.1Group 2 Chemistry 3.2.2End of Topic Test - Group 2

3.3

Group 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

4

Inorganic Chemistry 2 (A2 Only)

4.1

Period 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

4.2

Transition 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

4.3

Reactions 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

5

Organic Chemistry 1

5.1

Introduction

5.1.1Naming Conventions 5.1.2Mechanism 5.1.3Isomerism 5.1.4End of Topic Test - Introduction to Organic Chem

5.2

Alkanes

5.2.1Fractional Distillation 5.2.2Cracking 5.2.3Combustion 5.2.4Chlorination 5.2.5End of Topic Test - Alkanes

5.3

Halogenoalkanes

5.3.1Substitution Reactions 5.3.2Elimination Reactions 5.3.3Ozone Depletion 5.3.4End of Topic Test - Halogenoalkanes

5.4

Alkenes

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

5.5

Alcohols

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

5.6

Organic Analysis

5.6.1Reactions 5.6.2Reactions 2 5.6.3Mass Spectrometry 5.6.4Infrared Spectroscopy 5.6.5End of Topic Test - Organic Analysis

5.7

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

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

6

Organic Chemistry 2 (A2 Only)

6.1

Optical Isomerism (A2 Only)

6.1.1Optical Isomerism (A2 Only)6.1.2End of Topic Test - Optical Isomerism

6.2

Aldehydes & Ketones (A2 Only)

6.2.1Aldehydes & Ketones (A2 Only)6.2.2End of Topic Test - Aldehydes & Ketones

6.3

Carboxylic Acids & Esters (A2 Only)

6.3.1Acids & Esters (A2 Only)6.3.2Acylation (A2 Only)6.3.3End of Topic Test - Carboxylic Acids & Esters

6.4

Aromatic Chemistry (A2 Only)

6.4.1Structure & Reactivity (A2 Only)6.4.2Reactions of Benzene (A2 Only)6.4.3End of Topic Test - Aromatic Chemistry

6.5

Amines (A2 Only)

6.5.1Nature & Preparation of Amines (A2 Only)6.5.2Basicity & Nucleophilicity (A2 Only)6.5.3End of Topic Test - Amines

6.6

Polymers (A2 Only)

6.6.1Condensation Polymers (A2 Only)6.6.2Biodegradability (A2 Only)

6.7

Biological 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

6.8

Organic 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

6.9

NMR 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

6.10

Chromatography (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

6.11

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

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

Practice questions on Hess' Law

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1
If the products were CO + H<sub>2</sub>O, what would the reactants be for the enthalpy of formation equation?Multiple choice
2
Which of the following does NOT have an enthalpy of formation of zero?Multiple choice

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Calorimetry

Bond Enthalpies