Exothermic and endothermic reactions - CIE iGCSE Chemistry Notes - New Syllabus
Exothermic and endothermic reactions for iGCSE Chemistry Notes
Core Syllabus
- State that an exothermic reaction transfers thermal energy to the surroundings leading to an increase in the temperature of the surroundings
- State that an endothermic reaction takes in thermal energy from the surroundings leading to a decrease in the temperature of the surroundings
- Interpret reaction pathway diagrams showing exothermic and endothermic reactions
Supplement Syllabus
- State that the transfer of thermal energy during a reaction is called the enthalpy change, $\Delta H$, of the reaction. $\Delta H$ is negative for exothermic reactions and positive for endothermic reactions
- Define activation energy, $E_a$, as the minimum energy that colliding particles must have to react
- Draw and label reaction pathway diagrams for exothermic and endothermic reactions using information provided, to include:
(a) reactants
(b) products
(c) enthalpy change of the reaction, $\Delta H$
(d) activation energy, $E_a$ - State that bond breaking is an endothermic process and bond making is an exothermic process and explain the enthalpy change of a reaction in terms of bond breaking and bond making
- Calculate the enthalpy change of a reaction using bond energies
Exothermic and Endothermic Reactions
Exothermic and Endothermic Reactions
Exothermic Reaction:
An exothermic reaction is a chemical reaction that transfers thermal energy to the surroundings. This energy transfer results in an increase in the temperature of the surroundings, which can often be detected by touch or measured with a thermometer.
Features:
- Heat is released into the surroundings
- The surroundings become warmer
- The temperature of the surroundings increases
- Energy is usually released in the form of heat and/or light
Examples of Exothermic Reactions:
- Combustion (burning fuels):
\( \text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} \) - Neutralisation reactions (acid + base):
\( \text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O} \) - Respiration in living organisms:
\( \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} \) - Displacement reactions (e.g. zinc + copper(II) sulfate)
Example
Define an exothermic reaction and give one everyday example.
▶️Answer/Explanation
An exothermic reaction is a chemical reaction that releases thermal energy to the surroundings, increasing their temperature.
Example: Combustion of natural gas in a stove.
Endothermic Reaction
An endothermic reaction is a chemical reaction that takes in thermal energy from the surroundings. This energy absorption leads to a decrease in the temperature of the surroundings, which can be felt as cooling or measured using a thermometer.
Features:
- Heat is absorbed from the surroundings
- The surroundings become cooler
- The temperature of the surroundings decreases
- Energy is used to break bonds or allow the reaction to proceed
Examples of Endothermic Reactions:
- Thermal decomposition of calcium carbonate:
\( \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \) - Photosynthesis:
\( 6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \) - Dissolving certain salts in water (e.g. potassium chloride)
- Cooking or baking (e.g. absorbing heat to initiate chemical changes in food)
Example
Define an endothermic reaction and state one chemical process that is endothermic.
▶️Answer/Explanation
An endothermic reaction is a chemical reaction that absorbs thermal energy from the surroundings, leading to a decrease in temperature.
Example: Thermal decomposition of calcium carbonate.
Enthalpy Change (ΔH)
Enthalpy Change (ΔH)
The enthalpy change of a chemical reaction, represented by the symbol \( \Delta H \), is the amount of thermal energy transferred during a chemical reaction at constant pressure.
Facts:
- \( \Delta H \) tells us whether a reaction is exothermic or endothermic
- It is measured in kilojoules per mole (kJ/mol)
Sign of \( \Delta H \):
- Exothermic Reaction: Thermal energy is released
→ \( \Delta H \) is negative
→ Temperature of surroundings increases - Endothermic Reaction: Thermal energy is absorbed
→ \( \Delta H \) is positive
→ Temperature of surroundings decreases
Examples:
- Combustion of methane (exothermic):
\( \text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} \quad \Delta H = -890 \ \text{kJ/mol} \) - Photosynthesis (endothermic):
\( 6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \quad \Delta H = +2801 \ \text{kJ/mol} \)
Example
State the value and sign of \( \Delta H \) for an exothermic reaction and explain what it means.
▶️Answer/Explanation
For an exothermic reaction, \( \Delta H \) is negative. This means that thermal energy is released into the surroundings, increasing the temperature.
Activation Energy (Ea)
Activation Energy (Ea)
Activation energy is the minimum amount of energy that reacting particles must have for a reaction to occur.
It is represented by the symbol \( E_a \) and is measured in kilojoules per mole (kJ/mol).
Main Concepts:
- Particles must collide with enough energy to break existing bonds and form new ones.
- If the energy of the collision is less than the activation energy, no reaction will happen.
- The activation energy is shown as a “hump” or peak in reaction pathway diagrams.
Energy Profile and Activation Energy:
- The higher the activation energy, the slower the reaction (fewer successful collisions).
- Catalysts work by providing an alternative pathway with lower activation energy, increasing the rate of reaction.
Visual Guide (for diagrams):
- On an energy profile diagram, draw a curve from reactants to products.
- Label the peak of the curve as activation energy (Ea).
- For catalysed reactions, show a second lower curve representing reduced Ea.
Why is Ea important?
- It helps explain why some reactions happen quickly and others slowly.
- It also explains how heating or using a catalyst can help start a reaction.
Example
Define activation energy and explain its role in chemical reactions.
▶️Answer/Explanation
Activation energy is the minimum amount of energy needed for reacting particles to collide successfully and form products. If particles do not have enough energy (less than Ea), they bounce off each other and do not react.
Bond Breaking and Bond Making
Bond Breaking and Bond Making
Chemical reactions involve the breaking of bonds in the reactants and the formation of new bonds in the products.
- Bond breaking requires energy – it is an endothermic process.
- Bond making releases energy – it is an exothermic process.
Overall Enthalpy Change (\( \Delta H \)):
- The overall energy change in a reaction depends on the balance between energy taken in to break bonds and energy released when new bonds are formed.
- \( \Delta H = \text{Energy used to break bonds} – \text{Energy released when bonds are made} \)
- If more energy is released than absorbed → \( \Delta H \) is negative → reaction is exothermic.
- If more energy is absorbed than released → \( \Delta H \) is positive → reaction is endothermic.
Explanation in terms of particles:
- In order to react, particles must break the bonds holding them together.
- This requires energy input (endothermic).
- New bonds then form between atoms in the products, releasing energy (exothermic).
Example
Explain why combustion reactions are exothermic in terms of bond breaking and bond forming.
▶️Answer/Explanation
In a combustion reaction, strong bonds in oxygen and the fuel are broken (endothermic), and stronger bonds form in the products (e.g., \( \text{CO}_2 \), \( \text{H}_2\text{O} \)) releasing a lot of energy. Since more energy is released in bond formation than is used to break bonds, the reaction is exothermic.
Drawing and Labelling Reaction Pathway Diagrams
Drawing and Labelling Reaction Pathway Diagrams
Reaction pathway diagrams (also called energy profile diagrams) show how the energy of the system changes during a chemical reaction. These diagrams help us understand whether a reaction is exothermic or endothermic and how much energy is involved.
Parts of a Reaction Pathway Diagram:
- Reactants
- The starting energy level
- The starting substances, with a certain amount of stored chemical energy.
- Products
- The final energy level
- The substances formed after the reaction, with more or less energy than the reactants.
- Activation energy (Ea) – The minimum energy required for the reaction to start (shown as a peak).
- Enthalpy change (ΔH) – the overall energy released or absorbed
Labels to include in your diagram:
- Y-axis: Energy
- X-axis: Progress of reaction
- Activation energy (\( E_a \)) – arrow from reactants to peak
- Enthalpy change (\( \Delta H \)) – arrow between reactants and products
- Clearly mark Reactants and Products
Optional: Show a second, lower peak for a catalysed reaction (lower Ea).
(a) Exothermic Reaction:
- The energy of the products is lower than that of the reactants
- The curve starts high, rises to the activation energy peak, then drops below the starting level
- This means energy has been released to the surroundings
- Enthalpy change \( (\Delta H) \) is negative
Diagram:
- Reactants high → Products lower
- Label: Activation energy (Ea) is the peak
- Label: \( \Delta H \) is negative (arrow going down)
(b) Endothermic Reaction:
- The energy of the products is higher than that of the reactants
- The curve starts low, rises to the activation energy peak, and ends higher than where it started
- This means energy has been absorbed from the surroundings
- Enthalpy change \( (\Delta H) \) is positive
Diagram:
- Reactants low → Products higher
- Label: Activation energy (Ea) is the peak
- Label: \( \Delta H \) is positive (arrow going up)
Example
Sketch and label a reaction pathway diagram for an exothermic reaction, showing \( \Delta H \) and \( E_a \).
▶️Answer/Explanation
The diagram should show:
A curve that rises from the energy level of the reactants to a peak (activation energy)
Then falls to a lower energy level for the products
Label the height from reactants to peak as \( E_a \)
Label the difference between reactants and products as \( \Delta H \), pointing down
Reactants at the start, products at the end
Example
Explain how you can tell if a reaction is exothermic or endothermic from a reaction pathway diagram.
▶️Answer/Explanation
If the products are at a lower energy level than the reactants, the reaction is exothermic because energy is released. If the products are at a higher energy level, the reaction is endothermic because energy is absorbed.
Calculating the Enthalpy Change of a Reaction Using Bond Energies
Calculating the Enthalpy Change of a Reaction Using Bond Energies
The enthalpy change (\( \Delta H \)) of a reaction can be calculated by comparing the total energy required to break the bonds in the reactants and the total energy released when new bonds form in the products.
Bond energy:
The energy required to break one mole of a particular type of bond in the gaseous state (measured in kJ/mol).
Formula to use:
\( \Delta H = \text{Total energy to break bonds} – \text{Total energy released when forming bonds} \)
Steps to calculate:
- Write the balanced chemical equation.
- Draw the displayed formulae of all reactants and products (to see which bonds are broken and formed).
- Use bond energies (provided in a table) to calculate the total energy for:
- Bonds broken (endothermic – add energy)
- Bonds formed (exothermic – release energy)
- Subtract:
\( \Delta H = \text{Bonds broken} – \text{Bonds formed} \)
Important:
- If \( \Delta H \) is negative → the reaction is exothermic.
- If \( \Delta H \) is positive → the reaction is endothermic.
Example
Calculate the enthalpy change for the reaction:
\( \text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl} \)
Given bond energies (in kJ/mol):
H-H = 436, Cl-Cl = 242, H-Cl = 431
▶️Answer/Explanation
Bonds broken:
1 × H-H = 436 kJ
1 × Cl-Cl = 242 kJ
Total = 436 + 242 = 678 kJ
Bonds formed:
2 × H-Cl = 2 × 431 = 862 kJ
\( \Delta H = 678 – 862 = -184 \, \text{kJ/mol} \)
The reaction is exothermic because \( \Delta H \) is negative.