Energetics (OCR GCSE Combined Science A (Gateway): Chemistry): Flashcards
Exam code: J250
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Define exothermic reaction.
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Define exothermic reaction.
An exothermic reaction is one that transfers energy to the surroundings, causing the temperature of the surroundings to increase.
Define endothermic reaction.
An endothermic reaction is one that takes in energy from the surroundings, causing the temperature of the surroundings to decrease.
True or False?
Combustion, oxidation and neutralisation are all examples of exothermic reactions.
True.
All three release energy to the surroundings, so the temperature of the surroundings increases.
Why does the temperature of the surroundings decrease during an endothermic reaction?
In an endothermic reaction, energy is taken in from the surroundings. This transfer of energy away from the surroundings causes their temperature to decrease.
In an exothermic reaction, energy is transferred .......... the surroundings, so the surroundings .......... in temperature.
In an exothermic reaction, energy is transferred to the surroundings, so the surroundings increase in temperature.
Define activation energy.
Activation energy is the minimum amount of energy required for a collision between particles to result in a chemical reaction.
On a reaction profile, what does the initial rise in energy represent?
The initial rise represents the activation energy — the minimum energy needed for the reaction to begin.
True or False?
On a reaction profile for an endothermic reaction, the products are at a higher energy than the reactants.
True.
In an endothermic reaction, energy is absorbed from the surroundings. The products have more energy than the reactants, and the temperature of the surroundings decreases during the reaction.
How does the energy of products compare to reactants in an exothermic reaction, and what does this mean for the overall energy change?
The products have lower energy than the reactants, so the overall energy change is negative — energy has been transferred to the surroundings.
The .......... is the minimum energy needed for particles to react, and it is shown on a reaction profile as the initial .......... in energy before the reaction proceeds.
The activation energy is the minimum energy needed for particles to react, and it is shown on a reaction profile as the initial rise in energy before the reaction proceeds.
Is bond breaking an exothermic or endothermic process, and why?
(Higher Tier Only)
Bond breaking is endothermic because energy must be absorbed from the surroundings to break the chemical bonds.
True or False?
Bond forming releases energy to the surroundings.
(Higher Tier Only)
True.
In an exothermic reaction, more energy is released when new bonds form in the products than is absorbed when bonds break in the reactants. The overall energy change is negative (energy is released to the surroundings).
What is a bond energy?
(Higher Tier Only)
A bond energy is the mean amount of energy required to break a particular chemical bond, or the mean energy released when that bond is formed.
Energy change = energy .......... − energy .......... .
(Higher Tier Only)
Energy change = energy taken in − energy given out.
What does a positive energy change tell you about a reaction?
(Higher Tier Only)
A positive energy change means the reaction is endothermic — more energy was absorbed to break bonds than was released when new bonds formed.
How do you calculate the overall energy change for a reaction using bond energies?
(Higher Tier Only)
The overall energy change of a reaction is calculated from bond energies: energy in (bonds broken) − energy out (bonds formed). If the result is negative, the reaction is exothermic; if positive, it is endothermic.
True or False?
If more energy is released when bonds form than is absorbed when bonds break, the reaction is endothermic.
(Higher Tier Only)
False.
If more energy is released than absorbed, the reaction is exothermic and the overall energy change is negative.
In an exothermic reaction, more energy is .......... when bonds form than is .......... when bonds break.
(Higher Tier Only)
In an exothermic reaction, more energy is released when bonds form than is absorbed when bonds break.
Why is it useful to draw a displayed formula equation before doing a bond energy calculation?
(Higher Tier Only)
A displayed formula shows all the atoms and bonds in a molecule. To calculate bond energy changes, count the bonds broken in the reactants and the bonds formed in the products using the displayed formula.
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