23 Questions
What is the main result of combustion of fuels?
Release of heat energy
What is the use of hydrogen as a fuel?
It can be burnt to produce energy
In a fuel cell, what is the outcome of the oxidation of hydrogen?
Production of water
What is the process involved in the combustion of fuels?
Reaction with oxygen
What is the purpose of a fuel cell?
To generate electricity
What is the result of the reaction of a fuel in a fuel cell?
Production of a potential difference
What is the characteristic of an exothermic reaction in terms of energy transfer?
An exothermic reaction transfers energy to the surroundings.
Give an example of an everyday exothermic reaction.
Self-heating cans (e.g. for coffee) or hand warmers.
What is the effect of an endothermic reaction on the temperature of the surroundings?
The temperature of the surroundings decreases.
Provide an example of an endothermic reaction.
Thermal decomposition or the reaction of citric acid and sodium hydrogencarbonate.
What is a common application of endothermic reactions?
Some sports injury packs.
What is the key difference between exothermic and endothermic reactions?
Exothermic reactions transfer energy to the surroundings, while endothermic reactions absorb energy from the surroundings.
What is the first step in calculating the energy of a reaction using bond energies?
Adding together all the bond energies for all the bonds in the reactants to get the 'energy in'.
What does a negative energy change indicate in a reaction?
An exothermic reaction.
How do you calculate the energy change of a reaction using bond energies?
By subtracting the energy out from the energy in (energy in - energy out).
What is the purpose of calculating the 'energy in' and 'energy out' in a reaction?
To determine the energy change of the reaction.
What are the two quantities that are compared to calculate the energy change of a reaction?
Energy in and energy out.
Describe the energy changes involved in bond breaking and bond forming during a chemical reaction.
Bond breaking requires energy to be taken in, making it an endothermic process, whereas bond forming releases energy, making it an exothermic process.
What is the significance of the 'buses take many routes' analogy in the context of chemical reactions?
The analogy suggests that breaking bonds requires energy to be taken in, whereas forming bonds releases energy, illustrating the endothermic and exothermic processes.
How do reaction profiles help in understanding the energy changes involved in a chemical reaction?
Reaction profiles show the relative energies of reactants and products, the activation energy, and the overall energy change of a reaction, allowing us to identify whether the reaction is endothermic or exothermic.
What is the key feature that distinguishes an endothermic reaction from an exothermic reaction on an energy level diagram?
The key feature is whether the reactants or products have more energy, with reactants having more energy in an endothermic reaction and products having more energy in an exothermic reaction.
What is the role of the activation energy (Ea) in a chemical reaction, as depicted on an energy level diagram?
The activation energy is the energy required to start a reaction, and it is labeled as 'Ea' on the diagram.
Draw an energy level diagram for an exothermic reaction and explain how it differs from an endothermic reaction.
An exothermic reaction diagram shows the products having lower energy than the reactants, with the arrow pointing downwards, indicating a release of energy. In contrast, an endothermic reaction diagram shows the products having higher energy than the reactants, with the arrow pointing upwards, indicating an absorption of energy.
Study Notes
Chemical Energetics
Energy Transfer
- Burning fuels (combustion) releases heat energy by reacting fuels with oxygen.
- Hydrogen can be used as a fuel, burnt to produce energy.
Use of Hydrogen as a Fuel
- Hydrogen reacts with oxygen in a fuel cell to generate electricity.
- Oxidization of hydrogen within the fuel cell produces a potential difference.
- The overall reaction in a hydrogen fuel cell involves the oxidation of hydrogen to produce water.
Energetics of a Reaction
Exothermic and Endothermic Reactions
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An exothermic reaction is one that transfers energy to the surroundings, increasing the temperature of the surroundings.
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Examples of exothermic reactions include combustion, many oxidation reactions, and neutralization.
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Everyday examples of exothermic reactions include self-heating cans and hand warmers.
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An endothermic reaction is one that takes in energy from the surroundings, decreasing the temperature of the surroundings.
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Examples of endothermic reactions include thermal decomposition and the reaction of citric acid and sodium hydrogencarbonate.
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Some sports injury packs are based on endothermic reactions.
Calculating Energy of a Reaction
- Calculate the energy of a reaction using bond energies:
- Add together all bond energies for reactants (energy in).
- Add together all bond energies for products (energy out).
- Calculate the energy change: energy in - energy out.
- If energy out > energy in, the energy change will be negative, showing an exothermic reaction.
- If energy out < energy in, the energy change will be positive, showing an endothermic reaction.
Bond Breaking and Bond Forming
- Bond breaking is an endothermic process, requiring energy to break bonds in reactants.
- Bond forming is an exothermic process, releasing energy when bonds are formed in products.
Reaction Profiles
- Reaction profiles can be used to show the relative energies of reactants and products, the activation energy, and the overall energy change of a reaction.
- Arrows on energy level diagrams show the activation energy (energy required to start a reaction - labeled as 'Ea' on diagram).
- Exothermic reactions have products with lower energy than reactants, while endothermic reactions have products with higher energy than reactants.
Energy Level Diagrams
- Energy level diagrams can be used to illustrate exothermic and endothermic reactions.
- Diagrams show the relative energies of reactants and products, the activation energy, and the overall energy change of a reaction.
Test your knowledge on chemical energetics, combustion reactions, and the use of fuels as energy sources. Covers the release of heat energy by burning fuels and the potential of hydrogen as a fuel. Ideal for IGCSE Chemistry students.
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