Define activation energy ( E a )

Activation energy ( E a ) is the minimum energy required for a collision between reactant particles to result in a chemical reaction .

True or False? A collision between two reactant particles will always lead to a reaction if they collide with the correct orientation.

False. Particles must also possess a minimum energy equal to the activation energy ( E a ) — correct orientation alone is not sufficient for an effective collision .

What two conditions must be met for a collision between reactant particles to be effective?

Particles must collide with the correct orientation and possess a minimum energy equal to the activation energy ( E a ).

Increasing the .......... of a solution increases the number of particles per unit volume, which increases .......... frequency and therefore increases the .......... of reaction.

Increasing the concentration of a solution increases the number of particles per unit volume, which increases collision frequency and therefore increases the rate of reaction.

How does increasing pressure affect the rate of a reaction involving gases?

Increasing pressure forces gas molecules into a smaller volume, increasing collision frequency and therefore increasing the rate of reaction.

True or False? Doubling the concentration of a solution doubles the number of particles per unit volume and doubles the frequency of effective collisions.

True. Doubling concentration doubles the number of particles per unit volume, leading to double the collision frequency and therefore an increased rate of reaction.

Define rate of reaction

A rate of reaction is the change in concentration of a reactant or product per unit time. Units: mol dm -3 s -1 (or mol dm -3 min -1 ).

The rate of reaction is calculated as the change in .......... of a reactant or product, divided by the .......... taken.

The rate of reaction is calculated as the change in concentration of a reactant or product, divided by the time taken.

How is the instantaneous rate of reaction found from a concentration–time graph?

Draw a tangent to the curve at the required time point, then calculate the gradient of that tangent. The steeper the gradient, the greater the rate of reaction at that point.

A catalyst is a substance that increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy , and is not consumed in the process.

How does a catalyst increase the rate of a reaction?

A catalyst provides an alternative pathway with a lower activation energy ( E a ), so a greater proportion of molecules have sufficient energy for an effective collision.

True or False? A catalyst is consumed during a chemical reaction.

False. A catalyst is not consumed — it may be involved in intermediate steps but is regenerated at the end of the reaction, appearing chemically unchanged.

A .......... catalyst is in the same phase as the reactants, whereas a .......... catalyst is in a different phase to the reactants.

A homogeneous catalyst is in the same phase as the reactants, whereas a heterogeneous catalyst is in a different phase to the reactants.

Give two industrial benefits of using a catalyst in a chemical process.

Lower temperatures can be used, reducing energy costs. Lower pressures can be used, reducing CO 2 emissions from burning fossil fuels.

True or False? Adding a catalyst to an equilibrium mixture shifts the position of equilibrium to the right.

False. A catalyst speeds up the rate of both the forward and reverse reactions equally — it only allows equilibrium to be reached faster and has no effect on the equilibrium position.

Define Maxwell-Boltzmann distribution curve

A Maxwell-Boltzmann distribution curve is a graph showing the distribution of energies among particles in a sample at a given temperature , with the area under the curve representing the total number of particles.

What does the area under a Maxwell-Boltzmann curve to the right of E a represent?

It represents the fraction of molecules that possess sufficient energy to undergo an effective collision and cause a chemical reaction.

True or False? Increasing the temperature causes the Maxwell-Boltzmann curve to flatten and its peak to shift to the right.

True. At higher temperatures, particles have more kinetic energy , so the curve flattens and the peak shifts right — a greater proportion of molecules exceed the activation energy .

Adding a catalyst does not change the .......... distribution curve — instead, it lowers the .......... threshold, increasing the proportion of molecules with sufficient energy to react.

Adding a catalyst does not change the Maxwell-Boltzmann distribution curve — instead, it lowers the activation energy threshold, increasing the proportion of molecules with sufficient energy to react.

Why does the total area under a Maxwell-Boltzmann curve remain constant when temperature changes?

The total area represents the total number of particles , which does not change — only the distribution of energies shifts with temperature.

True or False? The Maxwell-Boltzmann distribution curve must start at the origin and must never touch the x-axis at high energies.

True. No molecules have zero energy (curve starts at the origin ) and there is no upper energy limit — a very small number of molecules can always have very high energy, so the curve never reaches the x-axis .

A LevelChemistryOCRFlashcards3. Periodic Table & EnergyReaction Rates

Reaction Rates (OCR A Level Chemistry A): Flashcards

Exam code: H432

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Define activation energy (E_a)

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Cards in this collection (21)

Define activation energy (E_a)
Activation energy (E_a) is the minimum energy required for a collision between reactant particles to result in a chemical reaction.
True or False?
A collision between two reactant particles will always lead to a reaction if they collide with the correct orientation.
False.
Particles must also possess a minimum energy equal to the activation energy (E_a) — correct orientation alone is not sufficient for an effective collision.
What two conditions must be met for a collision between reactant particles to be effective?
Particles must collide with the correct orientation and possess a minimum energy equal to the activation energy (E_a).
Increasing the .......... of a solution increases the number of particles per unit volume, which increases .......... frequency and therefore increases the .......... of reaction.
Increasing the concentration of a solution increases the number of particles per unit volume, which increases collision frequency and therefore increases the rate of reaction.
How does increasing pressure affect the rate of a reaction involving gases?
Increasing pressure forces gas molecules into a smaller volume, increasing collision frequency and therefore increasing the rate of reaction.
True or False?
Doubling the concentration of a solution doubles the number of particles per unit volume and doubles the frequency of effective collisions.
True.
Doubling concentration doubles the number of particles per unit volume, leading to double the collision frequency and therefore an increased rate of reaction.
Define rate of reaction
A rate of reaction is the change in concentration of a reactant or product per unit time. Units: mol dm^-3 s^-1 (or mol dm^-3 min^-1).
The rate of reaction is calculated as the change in .......... of a reactant or product, divided by the .......... taken.
The rate of reaction is calculated as the change in concentration of a reactant or product, divided by the time taken.
How is the instantaneous rate of reaction found from a concentration–time graph?
Draw a tangent to the curve at the required time point, then calculate the gradient of that tangent. The steeper the gradient, the greater the rate of reaction at that point.
Define catalyst
A catalyst is a substance that increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy, and is not consumed in the process.
How does a catalyst increase the rate of a reaction?
A catalyst provides an alternative pathway with a lower activation energy (E_a), so a greater proportion of molecules have sufficient energy for an effective collision.
True or False?
A catalyst is consumed during a chemical reaction.
False.
A catalyst is not consumed — it may be involved in intermediate steps but is regenerated at the end of the reaction, appearing chemically unchanged.
A .......... catalyst is in the same phase as the reactants, whereas a .......... catalyst is in a different phase to the reactants.
A homogeneous catalyst is in the same phase as the reactants, whereas a heterogeneous catalyst is in a different phase to the reactants.
Give two industrial benefits of using a catalyst in a chemical process.
1. Lower temperatures can be used, reducing energy costs.
2. Lower pressures can be used, reducing CO₂ emissions from burning fossil fuels.
True or False?
Adding a catalyst to an equilibrium mixture shifts the position of equilibrium to the right.
False.
A catalyst speeds up the rate of both the forward and reverse reactions equally — it only allows equilibrium to be reached faster and has no effect on the equilibrium position.
Define Maxwell-Boltzmann distribution curve
A Maxwell-Boltzmann distribution curve is a graph showing the distribution of energies among particles in a sample at a given temperature, with the area under the curve representing the total number of particles.
What does the area under a Maxwell-Boltzmann curve to the right of E_a represent?
It represents the fraction of molecules that possess sufficient energy to undergo an effective collision and cause a chemical reaction.
True or False?
Increasing the temperature causes the Maxwell-Boltzmann curve to flatten and its peak to shift to the right.
True.
At higher temperatures, particles have more kinetic energy, so the curve flattens and the peak shifts right — a greater proportion of molecules exceed the activation energy.
Adding a catalyst does not change the .......... distribution curve — instead, it lowers the .......... threshold, increasing the proportion of molecules with sufficient energy to react.
Adding a catalyst does not change the Maxwell-Boltzmann distribution curve — instead, it lowers the activation energy threshold, increasing the proportion of molecules with sufficient energy to react.
Why does the total area under a Maxwell-Boltzmann curve remain constant when temperature changes?
The total area represents the total number of particles, which does not change — only the distribution of energies shifts with temperature.
True or False?
The Maxwell-Boltzmann distribution curve must start at the origin and must never touch the x-axis at high energies.
True.
No molecules have zero energy (curve starts at the origin) and there is no upper energy limit — a very small number of molecules can always have very high energy, so the curve never reaches the x-axis.

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