Define activation energy .

Activation energy ( E a ) is the minimum energy that colliding particles must possess for a reaction to occur.

A successful collision requires particles to collide in the correct .......... and with sufficient .......... to overcome the activation energy.

A successful collision requires particles to collide in the correct orientation and with sufficient energy to overcome the activation energy.

True or False? An unsuccessful collision always occurs because particles lack sufficient energy.

False. An unsuccessful collision can occur either because the particles lack sufficient energy OR because they collide in the wrong orientation.

What is collision frequency and how does increasing concentration affect it?

Collision frequency is the number of collisions between particles per unit time. Increasing concentration places more particles in the same volume, leading to more frequent and more successful collisions.

What is a successful (effective) collision ?

A successful (effective) collision is one in which particles collide in the correct orientation and with energy equal to or greater than the activation energy , resulting in a chemical reaction.

Why does increasing pressure increase the rate of a gas-phase reaction?

Increasing pressure reduces the volume available to the gas particles. The same number of particles occupy a smaller space, increasing collision frequency and therefore the rate of reaction.

In an endothermic reaction, the activation energy is relatively .......... compared to an exothermic reaction because the .......... are lower in energy than the products.

In an endothermic reaction, the activation energy is relatively larger compared to an exothermic reaction because the reactants are lower in energy than the products.

True or False? Reducing particle size increases collision frequency because it increases the surface area exposed to reactants.

True. Breaking a solid into smaller pieces increases surface area , exposing more particles and increasing the frequency of successful collisions.

State two ways in which the rate of a reaction can be measured experimentally.

The rate of reaction can be measured by monitoring the loss of mass of a reactant (e.g. a gas escaping) or the volume of gas produced over time. Other valid methods include timing a colour change or the formation of a precipitate.

Define collision theory .

Collision theory states that a chemical reaction occurs when particles collide with the correct orientation and with energy equal to or greater than the activation energy of the reaction.

What is the rate of reaction ?

The rate of reaction is a measure of how quickly reactants are consumed or products are formed over time. It can be expressed as the change in concentration, mass or volume per unit time.

Why is mass loss used to measure the rate of some reactions?

When a gaseous product escapes from the reaction vessel, the mass of the system decreases over time. Plotting mass loss against time gives a measure of how quickly the reactant is consumed.

In the mass loss method, the gas must have a sufficiently high .......... for the change in mass to be detectable on a balance. Carbon dioxide ( M r = 44.0) is suitable, but hydrogen ( M r = 2.0) is not .

In the mass loss method, the gas must have a sufficiently high M r for the change in mass to be detectable on a balance. Carbon dioxide ( M r = 44.0) is suitable, but hydrogen ( M r = 2.0) is not .

True or False? Collecting gas by displacement of water is only valid if the gas is not water-soluble.

True. If a gas is water-soluble it dissolves into the water rather than displacing it, so the volume collected would not accurately represent the amount of gas produced.

What is quenching in the context of measuring reaction rates?

Quenching is the deliberate stopping of a reaction at a specific time so that a sample can be removed and analysed (e.g. by titration ) without the reaction continuing to affect the result.

What is the initial rate of reaction ?

The initial rate of reaction is the rate at time = 0, found by drawing a tangent to the concentration–time (or volume–time) curve at the origin and calculating its gradient .

The rate of reaction is calculated from the gradient of a tangent to a curve: rate = .......... ÷ change in time .

The rate of reaction is calculated from the gradient of a tangent to a curve: rate = change in concentration (or volume or mass) ÷ change in time .

True or False? The disappearing cross experiment generates a large number of data points for a rate–concentration graph.

False. The disappearing cross experiment typically generates one data point — the time taken for the cross to disappear — which is a limitation of this method.

How does the gas syringe method allow the rate of reaction to be measured over time?

As a gaseous product forms, it enters the gas syringe and the volume is recorded at regular time intervals. Plotting volume of gas against time produces a curve whose gradient gives the rate at any point.

What is a Maxwell-Boltzmann distribution ?

A Maxwell-Boltzmann distribution is a graph showing the distribution of kinetic energies among particles in a gas sample at a given temperature. Most particles have intermediate energies; very few have very low or very high energies.

How does the Maxwell-Boltzmann distribution curve change when temperature is increased?

At higher temperature, the curve flattens and the peak shifts to the right . A greater proportion of molecules have energy greater than the activation energy , increasing the rate of reaction.

When temperature increases, the Maxwell-Boltzmann distribution curve .......... and its peak shifts to the .......... , so a greater proportion of molecules exceed the activation energy .

When temperature increases, the Maxwell-Boltzmann distribution curve flattens and its peak shifts to the right , so a greater proportion of molecules exceed the activation energy .

True or False? Increasing temperature increases the rate of reaction solely by increasing collision frequency.

False. Increasing temperature increases both collision frequency and the proportion of molecules with energy greater than the activation energy . The increase in the proportion exceeding E a has the greater effect on rate.

Catalysis is the process by which the rate of a chemical reaction is increased by adding a catalyst , which provides an alternative reaction pathway with a lower activation energy and is not consumed in the reaction.

How does a catalyst affect the Maxwell-Boltzmann distribution curve?

A catalyst does not change the shape of the Maxwell-Boltzmann distribution curve. It lowers the activation energy , so a greater proportion of molecules already have sufficient energy for an effective collision , increasing the reaction rate.

A catalyst increases the rate of reaction by providing an alternative .......... with a .......... activation energy, without being consumed in the reaction.

A catalyst increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy, without being consumed in the reaction.

True or False? A catalyst shifts the Maxwell-Boltzmann distribution curve to the right.

False. A catalyst does not alter the distribution curve. It lowers the activation energy ( E a ), increasing the proportion of molecules that already have sufficient energy to react.

What is the key difference between a homogeneous and a heterogeneous catalyst?

A homogeneous catalyst is in the same phase as the reactants (e.g. both in solution). A heterogeneous catalyst is in a different phase from the reactants (e.g. a solid catalyst with gaseous reactants).

Why does only a small proportion of molecules react at any given temperature?

According to the Maxwell-Boltzmann distribution , only a small proportion of molecules have energy greater than or equal to the activation energy ( E a ). Only these molecules can undergo effective collisions and react.

What is a heterogeneous catalyst ?

A heterogeneous catalyst is one that is in a different phase from the reactants; for example, a solid iron catalyst used with gaseous reactants in the Haber Process.

What is the role of the iron catalyst in the Haber Process?

The iron catalyst provides a surface onto which nitrogen and hydrogen molecules can adsorb , weakening the bonds within N 2 and H 2 . This lowers the activation energy and allows ammonia (NH 3 ) to form and desorb from the surface.

In the Haber Process, the temperature used is approximately .......... °C and the pressure is approximately .......... MPa.

In the Haber Process, the temperature used is approximately 450 °C and the pressure is approximately 20 MPa.

True or False? A higher temperature is used in the Haber Process to maximise the yield of ammonia.

False. A higher temperature favours the reverse (endothermic) reaction, reducing the yield of ammonia. The temperature of 450 °C is a compromise between a reasonable yield and an acceptable reaction rate.

State two environmental or economic benefits of using a catalyst in an industrial process.

Although higher pressure increases the yield of ammonia (favouring the side with fewer moles of gas), very high pressures require expensive, specialist equipment and pose significant safety risks. 20 MPa is a compromise between yield, cost and safety.

In heterogeneous catalysis, reactant molecules first .......... onto the catalyst surface, react to form products, then .......... from the surface.

In heterogeneous catalysis, reactant molecules first adsorb onto the catalyst surface, react to form products, then desorb from the surface.

True or False? Impurities in reactant gases can reduce the effectiveness of a heterogeneous catalyst.

True. Impurities can adsorb onto the catalyst surface and block active sites , preventing reactant molecules from adsorbing and reducing the catalyst's effectiveness.

State two environmental or economic benefits of using a catalyst in an industrial process.

A catalyst allows the reaction to proceed at lower temperatures and pressures , reducing energy costs and CO 2 emissions from burning fossil fuels. It can also enable reactions with better atom economy and reduced waste.

What is a homogeneous catalyst ?

A homogeneous catalyst is one that is in the same phase as the reactants; for example, a catalyst dissolved in the same solution as the reacting species.

A LevelChemistryEdexcelFlashcards1. Physical ChemistryKinetics I

Kinetics I (Edexcel A Level Chemistry): Flashcards

Exam code: 9CHO

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Cards in this collection (38)

Define activation energy.
Activation energy (E_a) is the minimum energy that colliding particles must possess for a reaction to occur.
A successful collision requires particles to collide in the correct .......... and with sufficient .......... to overcome the activation energy.
A successful collision requires particles to collide in the correct orientation and with sufficient energy to overcome the activation energy.
True or False?
An unsuccessful collision always occurs because particles lack sufficient energy.
False.
An unsuccessful collision can occur either because the particles lack sufficient energy OR because they collide in the wrong orientation.
What is collision frequency and how does increasing concentration affect it?
Collision frequency is the number of collisions between particles per unit time. Increasing concentration places more particles in the same volume, leading to more frequent and more successful collisions.
What is a successful (effective) collision?
A successful (effective) collision is one in which particles collide in the correct orientation and with energy equal to or greater than the activation energy, resulting in a chemical reaction.
Why does increasing pressure increase the rate of a gas-phase reaction?
Increasing pressure reduces the volume available to the gas particles. The same number of particles occupy a smaller space, increasing collision frequency and therefore the rate of reaction.
In an endothermic reaction, the activation energy is relatively .......... compared to an exothermic reaction because the .......... are lower in energy than the products.
In an endothermic reaction, the activation energy is relatively larger compared to an exothermic reaction because the reactants are lower in energy than the products.
True or False?
Reducing particle size increases collision frequency because it increases the surface area exposed to reactants.
True.
Breaking a solid into smaller pieces increases surface area, exposing more particles and increasing the frequency of successful collisions.
State two ways in which the rate of a reaction can be measured experimentally.
The rate of reaction can be measured by monitoring the loss of mass of a reactant (e.g. a gas escaping) or the volume of gas produced over time. Other valid methods include timing a colour change or the formation of a precipitate.
Define collision theory.
Collision theory states that a chemical reaction occurs when particles collide with the correct orientation and with energy equal to or greater than the activation energy of the reaction.
What is the rate of reaction?
The rate of reaction is a measure of how quickly reactants are consumed or products are formed over time. It can be expressed as the change in concentration, mass or volume per unit time.
Why is mass loss used to measure the rate of some reactions?
When a gaseous product escapes from the reaction vessel, the mass of the system decreases over time. Plotting mass loss against time gives a measure of how quickly the reactant is consumed.
In the mass loss method, the gas must have a sufficiently high .......... for the change in mass to be detectable on a balance. Carbon dioxide (M_r = 44.0) is suitable, but hydrogen (M_r = 2.0) is not.
In the mass loss method, the gas must have a sufficiently high M_r for the change in mass to be detectable on a balance. Carbon dioxide (M_r = 44.0) is suitable, but hydrogen (M_r = 2.0) is not.
True or False?
Collecting gas by displacement of water is only valid if the gas is not water-soluble.
True.
If a gas is water-soluble it dissolves into the water rather than displacing it, so the volume collected would not accurately represent the amount of gas produced.
What is quenching in the context of measuring reaction rates?
Quenching is the deliberate stopping of a reaction at a specific time so that a sample can be removed and analysed (e.g. by titration) without the reaction continuing to affect the result.
What is the initial rate of reaction?
The initial rate of reaction is the rate at time = 0, found by drawing a tangent to the concentration–time (or volume–time) curve at the origin and calculating its gradient.
The rate of reaction is calculated from the gradient of a tangent to a curve: rate = .......... ÷ change in time.
The rate of reaction is calculated from the gradient of a tangent to a curve: rate = change in concentration (or volume or mass) ÷ change in time.
True or False?
The disappearing cross experiment generates a large number of data points for a rate–concentration graph.
False.
The disappearing cross experiment typically generates one data point — the time taken for the cross to disappear — which is a limitation of this method.
How does the gas syringe method allow the rate of reaction to be measured over time?
As a gaseous product forms, it enters the gas syringe and the volume is recorded at regular time intervals. Plotting volume of gas against time produces a curve whose gradient gives the rate at any point.
What is a Maxwell-Boltzmann distribution?
A Maxwell-Boltzmann distribution is a graph showing the distribution of kinetic energies among particles in a gas sample at a given temperature. Most particles have intermediate energies; very few have very low or very high energies.
How does the Maxwell-Boltzmann distribution curve change when temperature is increased?
At higher temperature, the curve flattens and the peak shifts to the right. A greater proportion of molecules have energy greater than the activation energy, increasing the rate of reaction.
When temperature increases, the Maxwell-Boltzmann distribution curve .......... and its peak shifts to the .......... , so a greater proportion of molecules exceed the activation energy.
When temperature increases, the Maxwell-Boltzmann distribution curve flattens and its peak shifts to the right, so a greater proportion of molecules exceed the activation energy.
True or False?
Increasing temperature increases the rate of reaction solely by increasing collision frequency.
False.
Increasing temperature increases both collision frequency and the proportion of molecules with energy greater than the activation energy. The increase in the proportion exceeding E_a has the greater effect on rate.
Define catalysis.
Catalysis is the process by which the rate of a chemical reaction is increased by adding a catalyst, which provides an alternative reaction pathway with a lower activation energy and is not consumed in the reaction.
How does a catalyst affect the Maxwell-Boltzmann distribution curve?
A catalyst does not change the shape of the Maxwell-Boltzmann distribution curve. It lowers the activation energy, so a greater proportion of molecules already have sufficient energy for an effective collision, increasing the reaction rate.
A catalyst increases the rate of reaction by providing an alternative .......... with a .......... activation energy, without being consumed in the reaction.
A catalyst increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy, without being consumed in the reaction.
True or False?
A catalyst shifts the Maxwell-Boltzmann distribution curve to the right.
False.
A catalyst does not alter the distribution curve. It lowers the activation energy (E_a), increasing the proportion of molecules that already have sufficient energy to react.
What is the key difference between a homogeneous and a heterogeneous catalyst?
A homogeneous catalyst is in the same phase as the reactants (e.g. both in solution). A heterogeneous catalyst is in a different phase from the reactants (e.g. a solid catalyst with gaseous reactants).
Why does only a small proportion of molecules react at any given temperature?
According to the Maxwell-Boltzmann distribution, only a small proportion of molecules have energy greater than or equal to the activation energy (E_a). Only these molecules can undergo effective collisions and react.
What is a heterogeneous catalyst?
A heterogeneous catalyst is one that is in a different phase from the reactants; for example, a solid iron catalyst used with gaseous reactants in the Haber Process.
What is the role of the iron catalyst in the Haber Process?
The iron catalyst provides a surface onto which nitrogen and hydrogen molecules can adsorb, weakening the bonds within N₂ and H₂. This lowers the activation energy and allows ammonia (NH₃) to form and desorb from the surface.
In the Haber Process, the temperature used is approximately .......... °C and the pressure is approximately .......... MPa.
In the Haber Process, the temperature used is approximately 450 °C and the pressure is approximately 20 MPa.
True or False?
A higher temperature is used in the Haber Process to maximise the yield of ammonia.
False.
A higher temperature favours the reverse (endothermic) reaction, reducing the yield of ammonia. The temperature of 450 °C is a compromise between a reasonable yield and an acceptable reaction rate.
State two environmental or economic benefits of using a catalyst in an industrial process.
Although higher pressure increases the yield of ammonia (favouring the side with fewer moles of gas), very high pressures require expensive, specialist equipment and pose significant safety risks. 20 MPa is a compromise between yield, cost and safety.
In heterogeneous catalysis, reactant molecules first .......... onto the catalyst surface, react to form products, then .......... from the surface.
In heterogeneous catalysis, reactant molecules first adsorb onto the catalyst surface, react to form products, then desorb from the surface.
True or False?
Impurities in reactant gases can reduce the effectiveness of a heterogeneous catalyst.
True.
Impurities can adsorb onto the catalyst surface and block active sites, preventing reactant molecules from adsorbing and reducing the catalyst's effectiveness.
State two environmental or economic benefits of using a catalyst in an industrial process.
A catalyst allows the reaction to proceed at lower temperatures and pressures, reducing energy costs and CO₂ emissions from burning fossil fuels. It can also enable reactions with better atom economy and reduced waste.
What is a homogeneous catalyst?
A homogeneous catalyst is one that is in the same phase as the reactants; for example, a catalyst dissolved in the same solution as the reacting species.

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