C5 - Exothermic And Endothermic Reactions
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35 Terms
What is an exothermic reaction?
A reaction that transfers energy to the surroundings
What happens to the temperature of the surroundings in an exothermic reaction?
It increases (surroundings get hotter)
Give 3 examples of exothermic reactions
Combustion, oxidation reactions, and neutralisation
What is an endothermic reaction?
A reaction that takes in energy from the surroundings
What happens to the temperature of the surroundings in an endothermic reaction?
It decreases (surroundings get colder)
Give an example of an endothermic reaction
Thermal decomposition
How do scientists represent energy changes in reactions?
By drawing energy profile diagrams
In an exothermic energy profile, which has more energy: reactants or products?
Reactants have more energy than products
Why do products have less energy than reactants in exothermic reactions?
Because energy is transferred to the surroundings
In an endothermic energy profile, which has more energy: reactants or products?
Products have more energy than reactants
Why do products have more energy than reactants in endothermic reactions?
Because energy is taken in from the surroundings
What does the difference in energy between reactants and products represent?
The overall energy change of the reaction
How is energy change shown on an energy profile diagram?
As the vertical difference between the reactants and the products
What is the activation energy?
The minimum amount of energy particles need to react
Why is activation energy needed?
To break bonds in the reactants and start the reaction
How is activation energy shown on an energy profile diagram?
From the energy level of the reactants to the peak of the curve
Is activation energy needed in both exothermic and endothermic reactions?
Yes, in all chemical reactions
Where is the peak on an energy profile diagram?
The highest point between the reactants and products
What happens at the peak of an energy profile?
The reaction has reached its transition state; bonds are breaking/forming
What are two everyday uses of exothermic reactions?
Hand warmers and self-heating cans (e.g. for food or drinks)
What happens to energy in an exothermic reaction?
It is released to the surroundings
What happens to energy in an endothermic reaction?
It is absorbed from the surroundings
What would you feel if you touched a container where an exothermic reaction is occurring?
It would feel hot
What would you feel if you touched a container where an endothermic reaction is occurring?
It would feel cold
What is meant by "energy is conserved in chemical reactions"?
The total amount of energy remains the same; it is transferred, not created or destroyed
What does the size of the activation energy affect?
The rate of reaction - higher activation energy means a slower reaction
Why might a reaction with a high activation energy not happen easily?
Because particles often don't have enough energy to react when they collide
What is the symbol for energy change?
ΔH (delta H)
What is ΔH for an exothermic reaction?
Negative, because energy is released
What is ΔH for an endothermic reaction?
Positive, because energy is taken in
How do you recognise an exothermic energy profile diagram?
The line ends lower than it starts (products lower than reactants)
How do you recognise an endothermic energy profile diagram?
The line ends higher than it starts (products higher than reactants)
What section of the graph shows the energy released or absorbed?
The vertical gap between the reactants and the products
In an exam, what must you label on an energy profile diagram?
Reactants, products, activation energy (Ea), and energy change (ΔH)
Why is understanding energy profiles important?
It helps predict if reactions will happen and how much energy will be required or released