5.1 Exothermic & Endothermic Reactions (Cambridge (CIE) O Level Chemistry): Flashcards
Exam code: 5070
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Define an exothermic reaction.
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Define an exothermic reaction.
An exothermic reaction is one that transfers thermal energy to the surroundings, so the temperature of the surroundings increases.
True or False?
In an exothermic reaction, the temperature of the surroundings increases.
True.
Thermal energy is transferred from the chemical system to the surroundings in an exothermic reaction, so the temperature of the surroundings rises.
In an exothermic reaction, thermal energy is transferred ______ the surroundings. In an endothermic reaction, it is taken ______ the surroundings.
In an exothermic reaction, thermal energy is transferred to the surroundings. In an endothermic reaction, it is taken from the surroundings.
Define an endothermic reaction.
An endothermic reaction is one that takes in thermal energy from the surroundings, so the temperature of the surroundings decreases.
True or False?
In an endothermic reaction, the temperature of the reaction mixture increases because energy is absorbed.
False.
In an endothermic reaction, thermal energy is taken from the surroundings, including the mixture itself, so the mixture gets colder.
Why does the temperature of the surroundings decrease during an endothermic reaction?
In an endothermic reaction, thermal energy is taken in from the surroundings to supply the chemical system. As energy leaves the surroundings, their temperature falls.
A sporting injury cold pack uses an ______ reaction, causing the temperature of the pack to ______.
A sporting injury cold pack uses an endothermic reaction, causing the temperature of the pack to decrease.
True or False?
Combustion and neutralisation are both examples of exothermic reactions.
True.
Both processes transfer thermal energy to the surroundings, causing a temperature increase.
Define activation energy (Ea).
Activation energy (Ea) is the minimum energy that colliding particles must have to react.
True or False?
The enthalpy change (ΔH) is positive for an exothermic reaction.
False.
ΔH is negative for exothermic reactions (thermal energy is transferred to the surroundings) and positive for endothermic reactions.
For an endothermic reaction, ΔH is ______. For an exothermic reaction, ΔH is ______.
For an endothermic reaction, ΔH is positive. For an exothermic reaction, ΔH is negative.
True or False?
On a reaction pathway diagram, the initial rise in energy from the reactants represents the activation energy (Ea).
True.
The initial rise shows the minimum energy that colliding particles must have to react. The taller this rise, the higher the activation energy.
Why do some reactions require more energy to start than others?
Different reactions have different activation energies because different bonds must be broken. Reactions requiring stronger bonds to be broken have a higher Ea and need more energy to get started.
On a reaction pathway diagram, four features must be labelled: reactants, products, the ______ energy (Ea) and the ______ change (ΔH).
On a reaction pathway diagram, four features must be labelled: reactants, products, the activation energy (Ea) and the enthalpy change (ΔH).
In a reaction pathway diagram for an exothermic reaction, where are the products positioned relative to the reactants?
The products are at a lower energy level than the reactants. This shows that thermal energy has been transferred to the surroundings, giving a negative ΔH.
True or False?
Activation energy (Ea) is the maximum energy that colliding particles must have in order to react.
False.
Activation energy is the minimum energy that particles must have to react. Particles with less than this energy collide but do not react.
True or False?
Breaking chemical bonds is always an endothermic process.
True.
Breaking bonds always requires energy to be taken in from the surroundings. Bond making is always the opposite: it is exothermic, releasing energy to the surroundings.
What is meant by bond energy?
Bond energy is the amount of energy required to break a specific chemical bond. It is also equal to the energy released when that same bond is formed.
The enthalpy change of a reaction is calculated as:
ΔH = energy ______ (bonds broken) − energy ______ (bonds formed).
The enthalpy change of a reaction is calculated as:
ΔH = energy in (bonds broken) − energy out (bonds formed).
Explain, in terms of bond energies, what determines whether a reaction is exothermic or endothermic overall.
Compare the total energy absorbed breaking bonds in the reactants with the total energy released forming bonds in the products. If energy released > energy absorbed, the reaction is exothermic (ΔH negative). If energy absorbed > energy released, it is endothermic (ΔH positive).
True or False?
If the enthalpy change (ΔH) of a reaction is positive, more energy was absorbed breaking bonds than was released forming bonds.
True.
A positive ΔH means energy in > energy out. More energy was needed to break bonds in the reactants than was released forming bonds in the products, making the reaction endothermic.
Describe the method for calculating the enthalpy change of a reaction using bond energies.
Write the balanced equation and identify all bonds by drawing the displayed formula. Calculate energy in (bonds broken) and energy out (bonds formed), then apply ΔH = energy in − energy out.
Bond ______ is always exothermic.
Bond ______ is always endothermic.
Bond making is always exothermic.
Bond breaking is always endothermic.
True or False?
Energy is needed to form new chemical bonds during a reaction.
False.
Energy is released when new bonds form: bond making is always exothermic. Saying energy is 'needed' or 'used' to make bonds is a common error that examiners penalise.
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