The Rate and Extent of Chemical Change

What is the rate of a chemical reaction?

The change in the quantity of a reactant used or product formed per unit of time.

How is the rate of a reaction calculated?

Rate of reaction = quantity of reactant used or product formed ÷ time.

What quantities can be used to measure the rate of reaction?

Mass in grams, volume in cm³ or, at Higher Tier, amount in moles.

What are the units for rate of reaction?

g/s, cm³/s or, at Higher Tier, mol/s.

HIGHER TIER: How can rate of reaction be expressed using moles?

Rate of reaction = moles of reactant used or product formed ÷ time.

What does a high rate of reaction mean?

A large quantity of reactant is used or product is formed in a short period of time.

What does a low rate of reaction mean?

A small quantity of reactant is used or product is formed in a given period of time.

How can the rate of a reaction be measured by monitoring mass loss?

Place the reaction flask on a balance → start the reaction → record the mass at regular time intervals → calculate the decrease in mass over time → plot mass against time.

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

A gaseous product is produced and escapes from the reaction flask → the mass of the apparatus and contents decreases → the decrease in mass can be measured over time.

Why is mass loss unsuitable for reactions producing hydrogen gas?

Hydrogen has a very low mass → the mass loss may be too small to measure accurately.

How can the rate of a reaction be measured by monitoring gas volume?

Connect a gas syringe to the reaction flask → measure the volume of gas produced at regular time intervals → plot gas volume against time.

Why is a gas syringe useful for measuring reaction rate?

It directly measures the volume of gas produced → measurements can be taken at different times → the change in gas volume over time gives the reaction rate.

How can the rate of a reaction be measured by the disappearance of a cross?

Place the reaction flask over a marked cross → mix the reactants → measure the time taken for the cloudy mixture to become opaque enough to hide the cross.

What type of reaction is suitable for the disappearing-cross method?

A reaction that produces a precipitate or cloudy mixture that gradually obscures the cross.

Why does the disappearing-cross method measure reaction rate?

The product makes the solution increasingly cloudy → the cross becomes harder to see → the time taken for the cross to disappear gives an indication of how quickly the product forms.

What does a shorter time for the cross to disappear indicate?

A faster reaction because the cloudy product forms more quickly.

What does a longer time for the cross to disappear indicate?

A slower reaction because the cloudy product forms more slowly.

What graph can be plotted when measuring mass loss?

Mass against time.

What graph can be plotted when measuring gas production?

Volume of gas against time.

What does the gradient of a quantity-time graph represent?

The rate of reaction.

What does a steep gradient on a reaction-rate graph indicate?

A fast rate of reaction.

What does a shallow gradient on a reaction-rate graph indicate?

A slow rate of reaction.

What does a horizontal section of a product-volume graph indicate?

The reaction has stopped because no more product is being formed.

What does a horizontal section of a mass-loss graph indicate?

The reaction has stopped because the mass is no longer decreasing.

Why does the gradient of a reaction graph usually decrease as the reaction proceeds?

Reactants are gradually used up → their concentration decreases → collisions become less frequent → the rate decreases.

Why is the initial rate of a reaction usually the fastest?

The concentration of reactants is greatest at the start → collisions occur most frequently → the reaction has its highest rate.

HIGHER TIER: How can the rate of reaction at a specific time be found from a graph?

Draw a tangent to the curve at the required time → calculate the gradient of the tangent → the gradient represents the rate of reaction at that instant.

HIGHER TIER: What is a tangent to a curve?

A straight line that touches the curve at one point and represents the gradient of the curve at that point.

HIGHER TIER: How is the gradient of a tangent calculated?

Gradient = change in y-value ÷ change in x-value.

HIGHER TIER: What does a steeper tangent mean?

A greater gradient → a faster rate of reaction at that particular time.

What are the five main factors that affect the rate of a chemical reaction?

Concentration of reactants, pressure of reacting gases, surface area of solid reactants, temperature and presence of a catalyst.

How does increasing concentration affect the rate of reaction?

Increasing concentration increases the rate of reaction.

Why does increasing concentration increase the rate of reaction?

There are more reactant particles in the same volume → collisions occur more frequently → there are more successful collisions per second → the rate increases.

What happens to the rate when the concentration of a reactant is decreased?

The rate decreases.

Why does decreasing concentration decrease the rate of reaction?

There are fewer reactant particles in the same volume → collisions occur less frequently → fewer successful collisions occur per second → the rate decreases.

How does increasing the pressure of reacting gases affect the rate?

Increasing pressure increases the rate of reaction.

Why does increasing gas pressure increase the rate of reaction?

The gas particles are compressed into a smaller volume → there are more particles in the same volume → collisions occur more frequently → the rate increases.

How does decreasing the volume of a gas affect reaction rate?

Decreasing the volume increases pressure → particles are closer together → collisions occur more frequently → the rate increases.

How does increasing the volume of a reacting gas affect the rate?

Increasing volume decreases pressure → particles are further apart → collisions occur less frequently → the rate decreases.

Why is increasing gas pressure similar to increasing concentration?

Both increase the number of particles in a given volume → collision frequency increases → the rate of reaction increases.

How does increasing the surface area of a solid affect the rate?

Increasing surface area increases the rate of reaction.

Why does increasing surface area increase the rate of reaction?

Smaller pieces expose a greater surface area to the other reactant → collisions occur more frequently → more successful collisions occur per second → the rate increases.

Why does a powder react faster than a large lump of the same solid?

The powder has a much greater surface area-to-volume ratio → more particles are exposed → collisions occur more frequently → the reaction is faster.

What happens when a solid is broken into smaller pieces?

Its total surface area increases → more particles are exposed for collisions → the rate of reaction increases.

What happens to the surface area-to-volume ratio when a solid is divided into smaller pieces?

The surface area-to-volume ratio increases.

Why does a larger surface area-to-volume ratio increase reaction rate?

A greater proportion of the particles are exposed at the surface → more particles can collide with the other reactant → collision frequency increases.

How does increasing temperature affect the rate of reaction?

Increasing temperature increases the rate of reaction.

Why does increasing temperature increase reaction rate?

Particles gain kinetic energy → they move faster and collide more frequently → a greater proportion of collisions have energy greater than or equal to the activation energy → there are more successful collisions per second → the rate increases.

Why does temperature affect reaction rate more than simply increasing collision frequency?

Increasing temperature increases both collision frequency and the energy of collisions → a greater proportion of particles have enough energy to overcome activation energy → the number of successful collisions increases significantly.

Why is there not a directly proportional relationship between temperature and reaction rate?

Increasing temperature does not simply increase collision frequency by a fixed proportion → it also changes the proportion of particles with energy greater than or equal to the activation energy.

What is collision theory?

Chemical reactions can occur only when reacting particles collide with each other with sufficient energy to react.

What is a successful collision?

A collision between reacting particles with enough energy to overcome the activation energy and result in a reaction.

What is an unsuccessful collision?

A collision in which the particles do not have enough energy to overcome the activation energy → no reaction occurs.

What is activation energy?

The minimum amount of energy that particles must have to react.

Why don't all collisions result in a reaction?

Some collisions do not have enough energy to overcome the activation energy → the particles do not react.

How does increasing concentration affect collisions according to collision theory?

More particles are present in the same volume → collision frequency increases → there are more successful collisions per second → the rate increases.

How does increasing pressure affect collisions according to collision theory?

Gas particles are closer together → collision frequency increases → there are more successful collisions per second → the rate increases.

How does increasing surface area affect collisions according to collision theory?

More particles on the surface are exposed → collision frequency increases → more successful collisions occur per second → the rate increases.

How does increasing temperature affect collisions according to collision theory?

Particles have more kinetic energy → they move faster and collide more frequently → more collisions have energy greater than or equal to the activation energy → more successful collisions occur → the rate increases.

What happens to the kinetic energy of particles when temperature increases?

The average kinetic energy of the particles increases.

Why does increasing temperature result in more particles having sufficient energy to react?

The particles have a greater average kinetic energy → a greater proportion of particles have energy equal to or greater than the activation energy → more collisions are successful.

What is a catalyst?

A substance that changes the rate of a reaction without being used up in the reaction.

How does a catalyst increase the rate of a reaction?

It provides a different reaction pathway with a lower activation energy → more particles have enough energy to react → more successful collisions occur per second → the rate increases.

Does a catalyst get used up during a reaction?

No. A catalyst is not used up during the reaction.

What happens to activation energy when a catalyst is used?

The activation energy is lower because the catalyst provides a different reaction pathway.

Why does lowering activation energy increase reaction rate?

More particles have enough energy to overcome the lower activation energy → a greater proportion of collisions are successful → the rate increases.

How is a catalyst shown on a reaction profile?

The catalysed pathway has a lower peak → the activation energy is lower.

Does a catalyst change the energy of the reactants or products?

No. It provides an alternative pathway with a lower activation energy.

Does a catalyst change the overall energy change of a reaction?

No. The energy difference between reactants and products remains unchanged.

Why is a catalyst not included in the chemical equation?

The catalyst is not used up during the reaction → it does not form part of the overall reactants or products.

How can a catalyst be identified from its effect on a reaction?

It increases or changes the rate of reaction but is not used up → it provides an alternative reaction pathway.

What is an enzyme?

A molecule that acts as a catalyst in a biological system.

Why are enzymes catalysts?

They increase the rate of biological reactions by providing an alternative pathway with a lower activation energy.

Why do different reactions require different catalysts?

Different reactions involve different reactants and reaction pathways → a catalyst must provide a suitable alternative pathway for the specific reaction.

What is a reversible reaction?

A reaction in which the products can react to produce the original reactants.

How are reversible reactions represented?

Using a reversible reaction symbol ⇌.

What does the forward reaction do?

It converts the reactants into products.

What does the reverse reaction do?

It converts the products back into the original reactants.

What can happen if the conditions of a reversible reaction are changed?

The position of equilibrium can change so that the system responds to the change.

What is dynamic equilibrium?

A state in a closed system where the forward and reverse reactions occur at the same rate and the concentrations of reactants and products remain constant.

When is dynamic equilibrium reached?

In a closed system, when the forward and reverse reactions occur at exactly the same rate.

Why must equilibrium be established in a closed system?

Reactants and products must be prevented from escaping → otherwise their concentrations could continually change → a stable equilibrium cannot be maintained.

Does a reaction stop at dynamic equilibrium?

No. The forward and reverse reactions continue to occur, but at equal rates.

Why do the concentrations remain constant at equilibrium?

The forward and reverse reactions occur at the same rate → reactants are converted into products at the same rate that products are converted back into reactants.

What does dynamic mean in dynamic equilibrium?

The forward and reverse reactions are still occurring even though the overall concentrations remain constant.

What does equilibrium mean in chemistry?

A state where the forward and reverse reactions occur at equal rates in a closed system.

What happens to the concentrations of reactants and products at equilibrium?

They remain constant, but they are not necessarily equal.

Are the concentrations of reactants and products necessarily equal at equilibrium?

No. They are constant, but they can be different from each other.

What happens to the rate of the forward reaction as equilibrium is approached?

It generally decreases as reactants are used up.

What happens to the rate of the reverse reaction as equilibrium is approached?

It increases as more products are formed.

What happens when the forward and reverse reaction rates become equal?

Dynamic equilibrium is established.

What is Le Chatelier's Principle?

If a system at equilibrium is subjected to a change in conditions, the system responds in a way that counteracts the change and restores equilibrium.

What does Le Chatelier's Principle predict?

The direction in which an equilibrium shifts when conditions such as concentration, temperature or pressure are changed.

HIGHER TIER: What happens when the concentration of a reactant is increased at equilibrium?

The equilibrium shifts in the direction that uses up the added reactant → more products are formed until equilibrium is restored.

HIGHER TIER: Why does increasing the concentration of a reactant produce more products?

The increased reactant concentration disturbs equilibrium → the system responds by favouring the forward reaction → more reactant is converted into product → equilibrium is restored.

HIGHER TIER: What happens when the concentration of a product is increased at equilibrium?

The equilibrium shifts in the direction that uses up the added product → more reactants are formed until equilibrium is restored.

HIGHER TIER: Why does increasing the concentration of a product favour the reverse reaction?

The increased product concentration disturbs equilibrium → the system responds by favouring the reaction that removes the added product → more reactants are formed.

HIGHER TIER: What happens when the concentration of a reactant is decreased?

The equilibrium shifts in the direction that produces more of the removed reactant.

HIGHER TIER: What happens when the concentration of a product is decreased?

The equilibrium shifts in the direction that produces more of the removed product → more reactants react → more products are formed.

HIGHER TIER: What happens to the equilibrium position when the temperature is increased for an endothermic forward reaction?

The equilibrium shifts towards the products → more products are formed.