1.5.3

Enthalpy Changes

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Enthalpy Changes

For a given process or reaction, we can measure the enthalpy change.

Definition of enthalpy change

Definition of enthalpy change

An enthalpy change is a measure of the heat given out or taken in during a process.
When objects are heated, they use energy to expand.
Enthalpy takes into account the energy used in the expansion.

Enthalpy vs energy changes

Enthalpy vs energy changes

We use enthalpy instead of energy because we cannot easily measure energy changes. This is because objects expand when heated.
To measure an energy change, we would have to fix the volume of the object.
Enthalpy is much easier to use as it allows for expansion.
- Enthalpy changes are instead measured under constant pressure.
- The atmosphere is at a constant pressure, so we require no extra equipment.

Endothermic vs exothermic reactions

Endothermic vs exothermic reactions

An exothermic reaction is one which gives out heat energy.
An exothermic reaction has a negative enthalpy change.
An endothermic reaction is one which takes in heat energy.
An endothermic reaction has a positive enthalpy change.

Combustion is exothermic

Combustion is exothermic

Combustion is an exothermic process as it gives out heat!
- E.g. Burning methane:
  - CH₄ + 2O₂ → CO₂ + 2H₂O
  - ΔH = −882.00kJmol^-1

Thermal decomposition is endothermic

Thermal decomposition is endothermic

Thermal decomposition is an endothermic process.
- E.g. The thermal decomposition of calcium carbonate:
  - CaCO₃ → CaO + CO₂
  - ΔH = +178.30kJmol^-1

Bond Breaking and Making

The basis of chemical reactions is a series of bond breaking and making.

Bond enthalpies

Bond enthalpies

During a reaction, some chemical bonds must be broken and made.
The energy to break or make a bond is known as the bond enthalpy.
- Energy is needed to break a bond so it is an endothermic process.
- Energy is given off when a bond is made so it is an exothermic process.
The enthalpy change of a reaction is a sum of the individual bond enthalpies being broken and made during the reaction.

The dominating term

The dominating term

During a chemical reaction, we must determine whether the reaction requires more energy to break bonds or to make bonds.
- This will give an overall reaction enthalpy that is either positive or negative.

Endothermic vs exothermic

Endothermic vs exothermic

Since bond breaking is endothermic and bond making is exothermic:
- More energy required to break bonds in a reaction will lead to an overall endothermic reaction.
- More energy released making bonds in a reaction will lead to an overall exothermic reaction.

Standard Conditions

For many measurements, we can record them under standard conditions. This is true for enthalpy changes - we call it the standard enthalpy change.

Symbols

Symbols

Enthalpy is given the symbol H.
Enthalpy changes are given the symbol ΔH.
- A negative enthalpy change (pictured) is one which gives out heat.
- A positive enthalpy change is one which takes in heat.

Standard conditions

Standard conditions

You'll hear the term 'standard conditions' a lot in chemistry. These conditions refer to:
- A pressure of 1 bar or 100 kPa.
- A temperature of 298K.
- A substance's most stable state at 298K and 1 bar pressure.
  - E.g. For water, it is liquid.
  - E.g. For carbon, it is graphite.

Standard enthalpy changes

Standard enthalpy changes

A standard enthalpy change is an enthalpy change carried out under standard conditions.
There are two you need to know;
- Standard enthalpy of combustion (Δ_cH^θ).
- Standard enthalpy of formation (Δ_fH^θ).
The superscript θ signifies that it is a standard enthalpy change.

Δ<sub>c</sub>H<sup>θ</sup>

Δ_cH^θ

The standard enthalpy change of combustion is defined as:
- The enthalpy change when one mole of a substance in its standard state burns completely in oxygen under standard conditions of 298K and 1 bar pressure.

Δ<sub>f</sub>H<sup>θ</sup>

Δ_fH^θ

The standard enthalpy change of formation is defined as:
- The enthalpy change when one mole of a substance is formed in its standard state from the pure elements in their standard states under standard conditions of 298K and 1 bar pressure.

1

Physical Chemistry

1.1

Atoms, 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

1.2

Atomic 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

1.3

Chemical 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

1.4

States 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

1.5

Chemical 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

1.6

Electrochemistry

1.6.1Electrochemical Cells 1.6.2Electrochemical Series 1.6.3Commercial Application 1.6.4Nernst Equation 1.6.5Redox

1.7

Equilibria

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

1.8

Partition Coefficient

1.9

Reaction 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

2

Inorganic Chemistry

2.1

The 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

2.2

Group 2

2.2.1Group 2 Chemistry

2.3

Group 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

2.4

Transition 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

2.5

Nitrogen & 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

3

Organic Chemistry & Analysis

3.1

Introduction to Organic Chemistry

3.1.1Naming Conventions 3.1.2Mechanism 3.1.3Isomerism 3.1.4End of Topic Test - Introduction to Organic Chem

3.2

Hydrocarbons

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

3.3

Halogen 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

3.4

Hydroxy 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

3.5

Carbonyl 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

3.6

Carboxylic Acids & Derivatives

3.6.1Properties of Carboxylic Acids 3.6.2Esters 3.6.3Acyl Chlorides 3.6.4End of Topic Test - Carboxylic Acids

3.7

Nitrogen Compounds

3.7.1Nature & Preparation of Amines 3.7.2Basicity & Nucleophilicity 3.7.3Amides 3.7.4Reactions of Amino Acids 3.7.5Amino Acids

3.8

Polymerisation

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

3.9

Analytical 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

3.10

Organic Synthesis

3.10.1Chirality & Drug Synthesis 3.10.2Organic Synthesis 3.10.3Examples of Synthetic Routes 3.10.4End of Topic Test - Organic Synthesis

Jump to other topics

1

Physical Chemistry

1.1

Atoms, 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

1.2

Atomic 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

1.3

Chemical 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

1.4

States 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

1.5

Chemical 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

1.6

Electrochemistry

1.6.1Electrochemical Cells 1.6.2Electrochemical Series 1.6.3Commercial Application 1.6.4Nernst Equation 1.6.5Redox

1.7

Equilibria

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

1.8

Partition Coefficient

1.9

Reaction 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

2

Inorganic Chemistry

2.1

The 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

2.2

Group 2

2.2.1Group 2 Chemistry

2.3

Group 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

2.4

Transition 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

2.5

Nitrogen & 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

3

Organic Chemistry & Analysis

3.1

Introduction to Organic Chemistry

3.1.1Naming Conventions 3.1.2Mechanism 3.1.3Isomerism 3.1.4End of Topic Test - Introduction to Organic Chem

3.2

Hydrocarbons

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

3.3

Halogen 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

3.4

Hydroxy 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

3.5

Carbonyl 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

3.6

Carboxylic Acids & Derivatives

3.6.1Properties of Carboxylic Acids 3.6.2Esters 3.6.3Acyl Chlorides 3.6.4End of Topic Test - Carboxylic Acids

3.7

Nitrogen Compounds

3.7.1Nature & Preparation of Amines 3.7.2Basicity & Nucleophilicity 3.7.3Amides 3.7.4Reactions of Amino Acids 3.7.5Amino Acids

3.8

Polymerisation

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

3.9

Analytical 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

3.10

Organic Synthesis

3.10.1Chirality & Drug Synthesis 3.10.2Organic Synthesis 3.10.3Examples of Synthetic Routes 3.10.4End of Topic Test - Organic Synthesis

Practice questions on Enthalpy Changes

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1
Types of enthalpy changesFill in the list
2
Which of the following is true of exothermic reactions?True / false
3
Bond breaking and making:Fill in the list
4
If the bond making terms dominate in a reaction, the reaction will be:Multiple choice
5
Standard conditions:Fill in the list

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Bond Enthalpies

Hess' Law