1.4.1
Enthalpy Changes
Enthalpy Changes
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
- E.g. Burning methane:
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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
- E.g. The thermal decomposition of calcium carbonate:
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
1.3Bonding
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
1.5Kinetics
1.6Equilibria
2Physical Chemistry 2 (A2 Only)
2.1Thermodynamics (A2 Only)
2.2Rate Equations (A2 Only)
2.3The Equilibrium Constant Kp (A2 Only)
2.4Electrochemical Cells (A2 Only)
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
4Inorganic Chemistry 2 (A2 Only)
4.1Period 3 (A2 Only)
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)
5Organic Chemistry 1
5.1Introduction
5.2Alkanes
5.3Halogenoalkanes
5.4Alkenes
5.5Alcohols
5.6Organic Analysis
5.7A-A* (AO3/4) - Organic 1
6Organic Chemistry 2 (A2 Only)
6.1Optical Isomerism (A2 Only)
6.2Aldehydes & Ketones (A2 Only)
6.3Carboxylic Acids & Esters (A2 Only)
6.4Aromatic Chemistry (A2 Only)
6.5Amines (A2 Only)
6.6Polymers (A2 Only)
6.7Biological Organic (A2 Only)
6.8Organic Synthesis (A2 Only)
6.9NMR Spectroscopy (A2 Only)
6.10Chromatography (A2 Only)
6.11A-A* (AO3/4) - Organic 2
Jump to other topics
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
1.3Bonding
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
1.5Kinetics
1.6Equilibria
2Physical Chemistry 2 (A2 Only)
2.1Thermodynamics (A2 Only)
2.2Rate Equations (A2 Only)
2.3The Equilibrium Constant Kp (A2 Only)
2.4Electrochemical Cells (A2 Only)
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
4Inorganic Chemistry 2 (A2 Only)
4.1Period 3 (A2 Only)
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)
5Organic Chemistry 1
5.1Introduction
5.2Alkanes
5.3Halogenoalkanes
5.4Alkenes
5.5Alcohols
5.6Organic Analysis
5.7A-A* (AO3/4) - Organic 1
6Organic Chemistry 2 (A2 Only)
6.1Optical Isomerism (A2 Only)
6.2Aldehydes & Ketones (A2 Only)
6.3Carboxylic Acids & Esters (A2 Only)
6.4Aromatic Chemistry (A2 Only)
6.5Amines (A2 Only)
6.6Polymers (A2 Only)
6.7Biological Organic (A2 Only)
6.8Organic Synthesis (A2 Only)
6.9NMR Spectroscopy (A2 Only)
6.10Chromatography (A2 Only)
6.11A-A* (AO3/4) - Organic 2

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