Exothermic and Endothermic ReactionsCambridge IGCSE 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 thermal energy to the surroundings, causing the temperature of the surroundings to increase.
Define an endothermic reaction.: An endothermic reaction is a reaction that takes in thermal energy from the surroundings, causing the temperature of the surroundings to decrease.
What is enthalpy change (ΔH)?: Enthalpy change is the transfer of thermal energy during a reaction. It is negative (ΔH < 0) for exothermic reactions and positive (ΔH > 0) for endothermic reactions.
Define activation energy (Ea).: Activation energy is the minimum energy that colliding particles must have in order for a reaction to occur.
What does a reaction pathway diagram show?: A reaction pathway diagram shows the energy of reactants and products during a reaction. It displays the reactants at their initial energy level, the activation energy barrier, the products at their final energy level, and the enthalpy change (ΔH).
On a reaction pathway diagram, what indicates an exothermic reaction?: On a reaction pathway diagram, products are at a lower energy level than reactants, resulting in a negative enthalpy change (ΔH < 0), shown with an arrow pointing downwards.
On a reaction pathway diagram, what indicates an endothermic reaction?: On a reaction pathway diagram, products are at a higher energy level than reactants, resulting in a positive enthalpy change (ΔH > 0), shown with an arrow pointing upwards.
Is bond breaking endothermic or exothermic?: Bond breaking is endothermic; it requires energy input to break chemical bonds.
Is bond making endothermic or exothermic?: Bond making is exothermic; it releases energy when new chemical bonds form.
Explain how enthalpy change relates to bond breaking and bond making.: The enthalpy change of a reaction is determined by comparing energy required to break bonds in reactants with energy released when bonds form in products. If energy released (bond making) > energy required (bond breaking), the reaction is exothermic (ΔH < 0). If energy required > energy released, the reaction is endothermic (ΔH > 0).
State the equation for calculating enthalpy change using bond energies.: ΔH = (sum of bond energies of bonds broken) − (sum of bond energies of bonds formed) or ΔH = energy required to break bonds − energy released when bonds form.
What is the sign of ΔH when bond making releases more energy than bond breaking requires?: The enthalpy change (ΔH) is negative, indicating an exothermic reaction.
What is the sign of ΔH when bond breaking requires more energy than bond making releases?: The enthalpy change (ΔH) is positive, indicating an endothermic reaction.
How would you label a reaction pathway diagram for an exothermic reaction?: Label the reactants on the left at a higher energy level, the products on the right at a lower energy level, draw an activation energy barrier, label Ea (height of barrier), and label ΔH as negative (arrow pointing downwards from reactants to products).
How would you label a reaction pathway diagram for an endothermic reaction?: Label the reactants on the left at a lower energy level, the products on the right at a higher energy level, draw an activation energy barrier, label Ea (height of barrier), and label ΔH as positive (arrow pointing upwards from reactants to products).