Exothermic and Endothermic ReactionsAQA GCSE Chemistry: Flashcards

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Define an exothermic reaction.

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Define an exothermic reaction.: An exothermic reaction is a reaction that transfers energy to the surroundings, causing the temperature of the surroundings to rise. The enthalpy change (ΔH) is negative.
Define an endothermic reaction.: An endothermic reaction is a reaction that takes energy from the surroundings, causing the temperature of the surroundings to decrease. The enthalpy change (ΔH) is positive.
What is the sign of ΔH in an exothermic reaction?: Negative (ΔH is negative). Energy is released to the surroundings.
What is the sign of ΔH in an endothermic reaction?: Positive (ΔH is positive). Energy is absorbed from the surroundings.
Name three examples of exothermic reactions.: Combustion reactions, neutralisation reactions, and oxidation reactions. (Any three of these examples are acceptable.)
Name two examples of endothermic reactions.: Thermal decomposition reactions and the reaction of citric acid with sodium hydrogen carbonate.
What happens to the temperature of the surroundings during an exothermic reaction?: The temperature of the surroundings rises, as energy is transferred to the surroundings.
What happens to the temperature of the surroundings during an endothermic reaction?: The temperature of the surroundings decreases, as energy is taken from the surroundings.
Define activation energy.: Activation energy is the minimum amount of energy required for a reaction to occur. It is the energy needed to break bonds in the reactants.
How is activation energy shown on an energy level diagram (reaction profile)?: Activation energy is shown as the difference in energy between the reactants and the peak (transition state) of the reaction pathway.
On a reaction profile for an exothermic reaction, how does the energy level of products compare to reactants?: The products have a lower energy level than the reactants. The difference represents the energy transferred to the surroundings.
On a reaction profile for an endothermic reaction, how does the energy level of products compare to reactants?: The products have a higher energy level than the reactants. The difference represents the energy taken from the surroundings.
What does the peak on an energy level diagram represent?: The peak represents the transition state or activated complex, where the reactants have absorbed enough energy (activation energy) to break bonds and form products.
Explain why activation energy must be supplied even for exothermic reactions.: Activation energy is required to break the bonds in reactant molecules. Even though energy is released overall, the reactants must first reach the transition state before products can form and energy can be released.
How would you interpret an energy level diagram that shows products at a significantly lower energy level than reactants, with a high activation energy peak?: This represents an exothermic reaction with a high activation energy. The reaction releases a large amount of energy to the surroundings, but requires considerable energy input to start the reaction.