Define an exothermic reaction .

An exothermic reaction transfers heat energy to the surroundings, causing the temperature of the surroundings to increase. Examples include combustion and neutralisation.

A student feels a test tube get warm during a reaction. Is the reaction exothermic or endothermic?

The reaction is exothermic : the temperature of the surroundings increases because heat energy is transferred out of the system.

In an exothermic reaction, the temperature of the surroundings ________ . In an endothermic reaction, the temperature of the surroundings ________ .

In an exothermic reaction, the temperature of the surroundings increases . In an endothermic reaction, the temperature of the surroundings decreases .

Define an endothermic reaction .

An endothermic reaction takes in heat energy from the surroundings, causing the temperature of the surroundings to decrease. Examples include thermal decomposition and the reaction between citric acid and sodium hydrogencarbonate.

True or False? Neutralisation reactions are always exothermic.

True. Neutralisation always releases heat energy to the surroundings, so the temperature of the solution always increases.

When ammonium nitrate dissolves in water, the temperature first drops to a minimum, then slowly rises back to room temperature. Explain both observations.

The temperature decreases because dissolving ammonium nitrate is endothermic : heat energy is taken in from the surroundings. After dissolving is complete, the temperature rises because energy transfers to the solution from the surroundings until it returns to room temperature.

What is the role of a polystyrene cup in calorimetry experiments?

A polystyrene cup acts as an insulator, minimising heat loss to the surroundings and giving a more accurate measurement of the temperature change in the reaction.

When writing a method for a calorimetry experiment measuring temperature changes, what is the most common omission that prevents a valid outcome?

Failing to record the starting temperature of the solution before adding the reactant. Without this, the temperature change cannot be calculated.

In a calorimetry experiment using acid and a solid, stating the ________ or ________ of acid and the ________ of solid is required to gain credit for control variables.

In a calorimetry experiment using acid and a solid, stating the volume or concentration of acid and the mass of solid is required to gain credit for control variables.

True or False? "Measure the temperature until it is constant" is an acceptable description of how to record the final temperature in a calorimetry experiment.

False. "Until constant" is wrong as the temperature may cool back to room temperature. You should "record the highest temperature reached " (or "until the temperature stops increasing").

A student writes "I would repeat the experiment" as an improvement to increase the reliability of their results. Why is this insufficient, and what should they write instead?

"Repeat" alone is not enough for an exam answer. It should say: "repeat the experiment and calculate the mean, ignoring any anomalous results ." Without stating that the mean is calculated, the answer is not creditworthy.

In a calorimetry experiment, why does the temperature fall after the peak on a temperature–volume graph?

After the peak, all the limiting reactant has been used up. Further reagent added is now in excess : it dilutes the solution without contributing to further reaction, so the temperature decreases.

Define activation energy ( E a ).

Activation energy ( E a ) is the minimum energy that colliding particles must have for a reaction to occur. All reactions have an activation energy because the bonds in the reactant molecules must first be broken.

On a reaction profile, how can you tell whether a reaction is exothermic or endothermic from the position of the product line?

In an exothermic reaction, the products line is below the reactants line (energy is released, ΔH is negative). In an endothermic reaction, the products line is above the reactants line (energy is taken in, ΔH is positive).

On a reaction profile, the activation energy ( E a ) arrow must start at the ________ line and go up to the ________ of the curve.

On a reaction profile, the activation energy ( E a ) arrow must start at the reactants line and go up to the peak of the curve.

True or False? A reaction profile is complete as long as it shows a reactants line and an activation energy peak, even without a products line.

False. A reaction profile is incomplete without a horizontal products line . Failing to include it is the most common reason students lose marks when drawing profiles.

How does a catalyst lower the activation energy?

A catalyst provides a different reaction pathway that has a lower activation energy . Students who only write "lowers the activation energy" without mentioning the alternative pathway do not gain full marks in an exam.

Two reaction profiles are shown on one graph: one catalysed, one uncatalysed. The catalysed curve has a lower peak. How does the graph show that the catalyst does not change the overall energy change?

Both reaction profiles start at the same energy level (same reactants energy) and end at the same energy level (same products energy). The overall energy change (Δ H ) depends only on the difference between reactants and products, which the catalyst does not alter.

What are the energy rules for bond breaking and bond making ?

Bond breaking is always endothermic : energy must be taken in from the surroundings. Bond making is always exothermic : energy is released to the surroundings. The overall reaction type depends on which process requires more energy.

In a bond energy calculation, energy in (bonds broken) = 678 kJ, energy out (bonds made) = 862 kJ. What is the energy change, and what does the sign mean?

Energy change = 678 − 862 = −184 kJ . The negative sign means more energy was released than absorbed, so the reaction is exothermic .

In the reaction N 2 + 3H 2 → 2NH 3 , the energy out must account for the bonds in ________ molecules of NH 3 . Each NH 3 has ________ N–H bonds, so you calculate ________ × the N–H bond energy.

In the reaction N 2 + 3H 2 → 2NH 3 , the energy out must account for the bonds in 2 molecules of NH 3 . Each NH 3 has 3 N–H bonds, so you calculate 6 × the N–H bond energy.

True or False? When calculating bond energies for 2H 2 + O 2 → 2H 2 O, you should use the O–O single bond energy value for the oxygen molecule.

False. Molecular oxygen (O 2 ) contains a double bond (O=O), not a single bond. Using the O–O single bond value is a common exam error that loses marks.

In a bond energy calculation, a reaction releases 93 kJ mol⁻¹. The total energy input from bonds broken is 945 kJ mol⁻¹ and the total energy output from bonds made is 3 X . Calculate the value of X .

Δ H = energy in − energy out −93 = 945 − 3 X 3 X = 945 + 93 = 1038 X = 346 kJ mol⁻¹ The most common error is subtracting Δ H from energy in without accounting for the negative sign, giving 3 X = 945 − 93 = 852.

Why does showing clear working matter in a bond energy calculation, even if you make an arithmetic error?

Mark schemes award error carried forward marks: a wrong value from step 1 does not lose marks in step 2, provided working is visible. Students who keep working to a minimum lose marks even when their method is correct.

GCSEScienceAQACombined Science: TrilogyChemistryFlashcardsEnergy ChangesExothermic & Endothermic Reactions

Exothermic & Endothermic Reactions (AQA GCSE Combined Science: Trilogy: Chemistry): Flashcards

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FrontEnergy Transfer in Reactions
Define an exothermic reaction.

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Cards in this collection (24)

Define an exothermic reaction.
An exothermic reaction transfers heat energy to the surroundings, causing the temperature of the surroundings to increase. Examples include combustion and neutralisation.
A student feels a test tube get warm during a reaction. Is the reaction exothermic or endothermic?
The reaction is exothermic: the temperature of the surroundings increases because heat energy is transferred out of the system.
In an exothermic reaction, the temperature of the surroundings ________.
In an endothermic reaction, the temperature of the surroundings ________.
In an exothermic reaction, the temperature of the surroundings increases.
In an endothermic reaction, the temperature of the surroundings decreases.
Define an endothermic reaction.
An endothermic reaction takes in heat energy from the surroundings, causing the temperature of the surroundings to decrease. Examples include thermal decomposition and the reaction between citric acid and sodium hydrogencarbonate.
True or False?
Neutralisation reactions are always exothermic.
True.
Neutralisation always releases heat energy to the surroundings, so the temperature of the solution always increases.
When ammonium nitrate dissolves in water, the temperature first drops to a minimum, then slowly rises back to room temperature. Explain both observations.
The temperature decreases because dissolving ammonium nitrate is endothermic: heat energy is taken in from the surroundings.
After dissolving is complete, the temperature rises because energy transfers to the solution from the surroundings until it returns to room temperature.
What is the role of a polystyrene cup in calorimetry experiments?
A polystyrene cup acts as an insulator, minimising heat loss to the surroundings and giving a more accurate measurement of the temperature change in the reaction.
When writing a method for a calorimetry experiment measuring temperature changes, what is the most common omission that prevents a valid outcome?
Failing to record the starting temperature of the solution before adding the reactant. Without this, the temperature change cannot be calculated.
In a calorimetry experiment using acid and a solid, stating the ________ or ________ of acid and the ________ of solid is required to gain credit for control variables.
In a calorimetry experiment using acid and a solid, stating the volume or concentration of acid and the mass of solid is required to gain credit for control variables.
True or False?
"Measure the temperature until it is constant" is an acceptable description of how to record the final temperature in a calorimetry experiment.
False.
"Until constant" is wrong as the temperature may cool back to room temperature. You should "record the highest temperature reached" (or "until the temperature stops increasing").
A student writes "I would repeat the experiment" as an improvement to increase the reliability of their results. Why is this insufficient, and what should they write instead?
"Repeat" alone is not enough for an exam answer.
It should say: "repeat the experiment and calculate the mean, ignoring any anomalous results." Without stating that the mean is calculated, the answer is not creditworthy.
In a calorimetry experiment, why does the temperature fall after the peak on a temperature–volume graph?
After the peak, all the limiting reactant has been used up. Further reagent added is now in excess: it dilutes the solution without contributing to further reaction, so the temperature decreases.
Define activation energy (E_a).
Activation energy (E_a) is the minimum energy that colliding particles must have for a reaction to occur. All reactions have an activation energy because the bonds in the reactant molecules must first be broken.
On a reaction profile, how can you tell whether a reaction is exothermic or endothermic from the position of the product line?
In an exothermic reaction, the products line is below the reactants line (energy is released, ΔH is negative).
In an endothermic reaction, the products line is above the reactants line (energy is taken in, ΔH is positive).
On a reaction profile, the activation energy (E_a) arrow must start at the ________ line and go up to the ________ of the curve.
On a reaction profile, the activation energy (E_a) arrow must start at the reactants line and go up to the peak of the curve.
True or False?
A reaction profile is complete as long as it shows a reactants line and an activation energy peak, even without a products line.
False.
A reaction profile is incomplete without a horizontal products line. Failing to include it is the most common reason students lose marks when drawing profiles.
How does a catalyst lower the activation energy?
A catalyst provides a different reaction pathway that has a lower activation energy.
Students who only write "lowers the activation energy" without mentioning the alternative pathway do not gain full marks in an exam.
Two reaction profiles are shown on one graph: one catalysed, one uncatalysed. The catalysed curve has a lower peak.
How does the graph show that the catalyst does not change the overall energy change?
Both reaction profiles start at the same energy level (same reactants energy) and end at the same energy level (same products energy).
The overall energy change (ΔH) depends only on the difference between reactants and products, which the catalyst does not alter.
What are the energy rules for bond breaking and bond making?
Bond breaking is always endothermic: energy must be taken in from the surroundings.
Bond making is always exothermic: energy is released to the surroundings.
The overall reaction type depends on which process requires more energy.
In a bond energy calculation, energy in (bonds broken) = 678 kJ, energy out (bonds made) = 862 kJ.
What is the energy change, and what does the sign mean?
Energy change = 678 − 862 = −184 kJ.
The negative sign means more energy was released than absorbed, so the reaction is exothermic.
In the reaction N₂ + 3H₂ → 2NH₃, the energy out must account for the bonds in ________ molecules of NH₃.
Each NH₃ has ________N–H bonds, so you calculate ________ × the N–H bond energy.
In the reaction N₂ + 3H₂ → 2NH₃, the energy out must account for the bonds in 2 molecules of NH₃.
Each NH₃ has 3 N–H bonds, so you calculate 6 × the N–H bond energy.
True or False?
When calculating bond energies for 2H₂ + O₂ → 2H₂O, you should use the O–O single bond energy value for the oxygen molecule.
False.
Molecular oxygen (O₂) contains a double bond (O=O), not a single bond. Using the O–O single bond value is a common exam error that loses marks.
In a bond energy calculation, a reaction releases 93 kJ mol⁻¹. The total energy input from bonds broken is 945 kJ mol⁻¹ and the total energy output from bonds made is 3X.
Calculate the value of X.
ΔH = energy in − energy out
−93 = 945 − 3X
3X = 945 + 93 = 1038
X = 346 kJ mol⁻¹
The most common error is subtracting ΔH from energy in without accounting for the negative sign, giving 3X = 945 − 93 = 852.
Why does showing clear working matter in a bond energy calculation, even if you make an arithmetic error?
Mark schemes award error carried forward marks: a wrong value from step 1 does not lose marks in step 2, provided working is visible. Students who keep working to a minimum lose marks even when their method is correct.

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