Exothermic & Endothermic Reactions (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award): Chemistry): Flashcards
Exam code: 0654 & 0973
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Define an exothermic reaction.
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Define an exothermic reaction.
An exothermic reaction is one in which thermal energy is transferred from the system (reacting chemicals) to the surroundings, causing the temperature of the surroundings to rise. The ΔH value is negative.
True or False?
In an endothermic reaction, the temperature of the surroundings increases.
False.
In an endothermic reaction, thermal energy is absorbed from the surroundings into the reacting system, so the temperature of the surroundings decreases.
In an exothermic reaction, the ΔH value is ______ and thermal energy is ______ to the surroundings.
In an exothermic reaction, the ΔH value is negative and thermal energy is released to the surroundings.
Give one example of an exothermic reaction and one example of an endothermic reaction.
Exothermic
Endothermic
Combustion
Thermal decomposition
Neutralisation
Melting / boiling
Metal + water (e.g. sodium)
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How does the direction of energy transfer differ between exothermic and endothermic reactions?
In an exothermic reaction, energy transfers from the system to the surroundings, so the surroundings get warmer.
In an endothermic reaction, energy transfers from the surroundings into the system, so the surroundings get cooler.
Cold packs used for sports injuries are based on ______ reactions, which ______ heat from the surroundings.
Cold packs used for sports injuries are based on endothermic reactions, which absorb heat from the surroundings.
True or False?
Neutralisation is classified as an endothermic reaction.
False.
Neutralisation is an exothermic reaction. Energy is released to the surroundings, causing the temperature of the reaction mixture to rise.
Define activation energy (Ea).
Activation energy (Ea) is the minimum amount of energy that colliding particles must possess for a reaction to occur.
On a reaction pathway diagram, it is represented as the energy difference between the reactants and the highest point on the curve.
What does the enthalpy change (ΔH) of a reaction represent?
ΔH represents the transfer of thermal energy during a chemical reaction. It is negative for exothermic reactions (energy released to surroundings) and positive for endothermic reactions (energy absorbed from surroundings).
In a reaction pathway diagram, the ______ is shown on the x-axis and ______ is shown on the y-axis.
In a reaction pathway diagram, the progress of the reaction is shown on the x-axis and energy is shown on the y-axis.
True or False?
In an exothermic reaction, the products are at a higher energy level than the reactants on a reaction pathway diagram.
False.
In an exothermic reaction, energy is released to the surroundings so the products are at a lower energy level than the reactants. The ΔH value is negative.
How is Ea shown on a reaction pathway diagram?
Ea is shown on a reaction pathway diagram as the energy difference between the energy level of the reactants and the highest point on the curve (the transition state). It is the same for both exothermic and endothermic reactions.
For an endothermic reaction, ΔH is ______ and the energy of the products is ______ than the energy of the reactants.
For an endothermic reaction, ΔH is positive and the energy of the products is higher than the energy of the reactants.
Why do some reactions require higher temperatures than others to get started?
Some reactions have a higher Ea than others, meaning the reacting particles need more energy to collide successfully and form products. A higher temperature increases the average kinetic energy of particles, making it more likely they have enough energy to overcome the Ea.
True or False?
Bond breaking is an exothermic process.
False.
Bond breaking is endothermic: energy must be absorbed from the surroundings to break chemical bonds.
Bond forming is exothermic, releasing energy to the surroundings.
What determines whether a reaction is overall exothermic or endothermic?
The overall energy change is determined by the difference between the energy absorbed to break bonds in the reactants and the energy released when new bonds form in the products.
If more energy is released than absorbed the reaction is exothermic.
If more energy is absorbed than released the reaction is endothermic.
Bond breaking requires energy to be ______ from the surroundings, so it is an ______ process.
Bond breaking requires energy to be absorbed from the surroundings, so it is an endothermic process.
How does the ΔH value indicate whether a reaction is exothermic or endothermic in terms of bond energies?
If more energy is released forming new bonds than is absorbed breaking old bonds, ΔH is negative and the reaction is exothermic.
If more energy is absorbed breaking bonds than is released forming new bonds, ΔH is positive and the reaction is endothermic.
In an exothermic reaction, the energy released when new bonds are ______ is greater than the energy needed to ______ the bonds in the reactants.
In an exothermic reaction, the energy released when new bonds are formed is greater than the energy needed to break the bonds in the reactants.
True or False?
Bond forming releases energy to the surroundings.
True.
Bond forming is always exothermic: energy is transferred to the surroundings as new chemical bonds are made, causing the temperature of the surroundings to rise.
Why does an endothermic reaction have a positive ΔH value in terms of bond energies?
An endothermic reaction has a positive ΔH value because more energy is absorbed breaking bonds in the reactants than is released forming bonds in the products. The products therefore have more energy than the reactants.
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