Enthalpy Changes

Test yourself

Enthalpy Changes

For a given process or reaction, we can measure the 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

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

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 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 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

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

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

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

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

You'll hear the term 'standard conditions' a lot in chemistry. These conditions refer to:
- A pressure of 1 bar.
- 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

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.

Illustrative background for Δ<sub>c</sub>H<sup>θ</sup>

Illustrative background for Δ<sub>c</sub>H<sup>θ</sup> ?? "content

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.

Illustrative background for Δ<sub>f</sub>H<sup>θ</sup>

Illustrative background for Δ<sub>f</sub>H<sup>θ</sup> ?? "content

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.

1Principles of Science I

1.1Structure & Bonding

1.1.1Atomic Model

1.1.2Electron Shells, Sub-Shells & Orbitals

1.1.3Ionic Bonding

1.1.4Representing Ionic Bonds

1.1.5Covalent Bonding

1.1.6Representing Covalent Bonds

1.1.7Metallic Bonding

1.1.8Intermolecular Forces

1.1.9Intermolecular Forces 2

1.1.10End of Topic Test - Bonding

1.1.11Relative Masses

1.1.12The Mole

1.1.13Molar Calculations

1.1.14Molar Calculations 2

1.1.15Empirical & Molecular Formulae

1.1.16Balanced Equations

1.1.17Percentage Yield

1.1.18End of Topic Test - Amount of Substance

1.2Properties of Substances

1.2.1The Periodic Table

1.2.2Ionisation Energy

1.2.3Factors Affecting Ionisation Energies

1.2.4Trends of Ionisation

1.2.5Trends in the Periodic Table

1.2.6Polarity

1.2.7Metals & Non-Metals

1.2.8Alkali Metals

1.2.9Alkaline Earth Metals

1.2.10Reactivity of Alkaline Earth Metals

1.2.11Redox

1.2.12Transition Metals

1.2.13Redox Reactions of Transition Metals

1.3Cell Structure & Function

1.3.1Introduction to Cells

1.3.2Prokaryotes

1.3.3Organelles of Eukaryotic Cells

1.3.4Organelles of Eukaryotic Cells 2

1.3.5Organelles in Plant Cells

1.3.6Microscopy

1.3.7End of Topic Test - Cell Structure

1.4Cell Specialisation

1.4.1Cell Specialisation

1.4.2Blood Cells

1.4.3Plant Cells

1.4.4Gametes

1.5Tissue Structure & Function

1.5.1Human Gas Exchange

1.5.2Blood Vessels

1.5.3Atherosclerosis

1.5.4Skeletal Muscle

1.5.5Slow & Fast Twitch Fibres

1.5.6Neurones

1.5.7Speed of Transmission

1.5.8Action Potentials

1.5.9End of Topic Test - Neurones & Action Potentials

1.5.10Synapses

1.5.11Types of Synapse

1.5.12Medical Application

1.5.13End of Topic Test - Synapses

1.5.14Chemical Brain Imbalances

1.5.15Effect of Drugs on the Brain

1.6Working with Waves

1.6.1Progressive Waves

1.6.2Wave Speed & Phase Difference

1.6.3Longitudinal & Transverse Waves

1.6.4Stationary Waves

1.6.5Stationary Waves 2

1.6.6Applications of Stationary Waves

1.6.7Interference

1.6.8Diffraction

1.6.9Diffraction Gratings

1.6.10Application of Diffraction Gratings

1.7Waves in Communication

1.7.1Refraction at a Plane Surface

1.7.2Internal Reflection & Fibre Optics

1.7.3Applications of Total Internal Reflection

1.7.4Properties

1.7.5Gamma, X-Ray & UV Light

1.7.6Visible Light

1.7.7Infrared, Microwave & Radio Waves

1.7.8Intensity

1.7.9Digital & Analogue Signals

1.7.10Communication Channels

2Practical Scientific Procedures and Techniques

2.1Titration

2.1.1pH Curves & Titrations

2.1.2pH Curves & Titrations 2

2.1.3Buffer Solutions

2.2Colorimetry

2.2.1Measuring Rates

2.2.2Beer-Lambert Law

2.3Chromatography

2.3.1Overview of Chromatography

2.3.2Thin-Layer Chromatography

2.3.3Column Chromatography

2.3.4Gas Chromatography

3Science Investigation Skills

3.1Scientific Processes

3.1.1Aims, Hypotheses & Sampling

3.1.2Pilot Studies & Design

3.1.3Ethics

3.1.4Variables & Control

3.1.5Demand Characteristics & Investigator Effects

3.1.6Features of Science

3.1.7Scientific Report

3.1.8Scientific Report 2

3.2Data Handling & Analysis

3.2.1Types of Data

3.2.2Descriptive Statistics

3.2.3Presentation & Display of Data

3.3Enzymes in Action

3.3.1Overview of Protein Structure

3.3.2Enzymes

3.3.3Factors Affecting Enzyme Activity

3.3.4Enzyme-Controlled Reactions

3.4Diffusion

3.4.1Diffusion

3.5Plants & Their Environment

3.5.1Factors Affecting Plant Growth

3.5.2Measuring Number & Distribution of Species

3.6Energy Content in Fuels

3.6.1Fractions

3.6.2Burning Hydrocarbons

3.6.3Combustion

3.6.4Energy

3.6.5Energy at Home

3.6.6Calorimetry

3.7Electrical Circuits

3.7.1Circuit Diagrams & Components

3.7.2Diodes, LDRs & Thermistors

3.7.3Resistance

3.7.4Electrical Work

3.7.5Ohm's Law

3.7.6Domestic Uses of Electricity

3.7.7Fuses

4Principles of Science II

4.1Extracting Elements

4.1.1Atom Economy

4.2Relating Properties to use of Substances

4.2.1Group 2 Chemistry

4.2.2End of Topic Test - Group 2

4.2.3Electrolysis

4.2.4Electrolysis Examples

4.2.5Extracting Metals

4.2.6Transition Metals

4.2.7Variable Oxidation States

4.2.8Heterogeneous Catalysts

4.2.9Homogeneous Catalysts

4.3Organic Chemistry

4.3.1Naming Compounds

4.3.2Functional Groups

4.3.3Isomerism

4.3.4Hydrocarbons

4.3.5Structure of Alkanes

4.3.6Boiling Points of Alkanes

4.3.7Alkenes

4.3.8Curly Arrows & Mechanisms

4.3.9Cracking

4.3.10Combustion

4.3.11Free Radical Substitution of Alkanes

4.3.12Electrophilic Addition of Alkenes

4.4Energy Changes in Industry

4.4.1The Kelvin Scale

4.4.2Enthalpy Changes

4.4.3Calorimetry

4.4.4Bond Enthalpies

4.4.5Hess' Law

4.5The Circulatory System

4.5.1The Circulatory System

4.5.2Blood Vessels

4.5.3Blood Transfusion & the ABO Rhesus System

4.5.4The Heart

4.5.5The Cardiac Cycle

4.5.6Cardiac Output

4.5.7Coordination of Heart Action

4.5.8Heart Dissection

4.5.9Controlling Heart Rate

4.5.10Electrocardiograms

4.5.11Cardiovascular Disease

4.5.12Investigating Heart Rates

4.6Ventilation & Gas Exchange

4.6.1Human Gas Exchange

4.6.2Ventilation

4.6.3Measuring Gas Exchange

4.6.4Controlling Ventilation Rate

4.6.5Lung Disease

4.6.6Lung Disease Data

4.7Urinary System

4.7.1Kidneys & Control of Water Balance

4.7.2Kidneys & Osmoregulation

4.7.3Kidneys & Blood Pressure

4.7.4Urine

4.7.5Kidney Failure & Urinalysis

4.7.6Dialysis

4.7.7Transplants

4.8Cell Transport

4.8.1Cell Membrane Structure

4.8.2Diffusion

4.8.3Osmosis

4.8.4Active Transport

4.9Thermal Physics

4.9.1Power & Efficiency

4.9.2Work & Energy

4.9.3Conservation of Energy

4.9.4Pressure

4.9.5First Law of Thermodynamics

4.9.6Second Law of Thermodynamics

4.9.7Heat Engines, Heat Pumps & Refrigerators

4.9.8Non-Flow Processes

4.9.9p-V Diagrams

4.9.10Ideal Gases

4.9.11Ideal Gases 2

4.9.12Thermal Energy Transfer

4.9.13Thermal Energy Transfer Experiments

4.10Materials

4.10.1Density

4.10.2Stress & Strain

4.10.3Properties of Materials

4.10.4Elasticity

4.10.5Plasticity

4.10.6Energy in Materials

4.10.7Young Modulus

4.11Fluids

4.11.1Fluid Flow

4.11.2Viscosity

4.11.3Density

4.11.4Continuity Equation

4.11.5Bernoulli's Equation

4.11.6Non-Newtonian Fluids

5Contemporary Issues in Science

5.1Contemporary Issues in Science

5.1.1Sustainability

5.1.2Genetic Engineering

5.1.3Uses of Genetic Modification

5.1.4Stem Cell Therapy

5.1.5Cloning

5.1.6Cloning 2

5.1.7Nanotechnology

5.1.8Ecosystems

5.2Analysing Scientific Information

5.2.1Validity

5.2.2Peer Review

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