Rate of Reaction (AQA GCSE Chemistry)

An advantage of measuring the rate of a reaction via the formation of a precipitate is that no specialist equipment is needed.

The typical units for expressing rate of reaction in terms of moles are moles per second, mol/s.

To convert mass per unit time to moles per unit time, divide the mass per unit time by the molar mass of the substance.

The molar gas volume is 24,000 cm³/mol.

To convert volume per unit time to moles per unit time for gases, divide the volume per unit time by the molar gas volume (24,000 cm³/mol).

False.

There is no direct way to measure moles, so it is necessary to calculate the rate using mass or volume first.

The equation for converting mass to moles is:

moles = mass / M_r

The first step in calculating rate of reaction in mol/s when given time in minutes is to convert minutes to seconds.

True.

The molar gas volume is the same for all gases at room temperature and pressure.

Line A shows magnesium powder reacting with acid and line B shows magnesium ribbon reacting with acid.

The reactions takes 30 seconds to complete. The point at which the line plateaus is when the reaction is complete.

Line A is magnesium reacting with concentrated acid, and line B is magnesium reacting with dilute acid.

The reaction is the fastest at the beginning of a reaction. Between 0 and 10 seconds.

True.

The gradient of the line in a rates graph is equal to the initial rate of reaction.

False.

The steeper the gradient of the line in a rates graph then the faster the rate of reaction.

A horizontal line in a rate graph indicates that the reaction has finished.

The line on the graph that represents the reaction that takes place at a higher temperature is A.

The line on the graph that represents the reaction that takes place with a catalyst is A.

The lines on the graph that indicate a reaction that has taken place between acid and

• powdered calcium carbonate: A

• lumps of calcium carbonate: B

To calculate the gradient at a specific point on a curved graph:

• Draw a tangent at that point

• Determine the change in y

• Determine the change in x

• The gradient = change in y / change in x.

False.

The gradient varies at different points along a curved rate graph.

The gradient represents the rate of reaction at a specific point in time.

True.

A steeper tangent indicates a faster rate of reaction at that point.

You can compare rates of reaction at different points on a curved graph by calculating and comparing the gradients of tangents at those points.

The equation for calculating gradient is:

gradient = change in y / change in x

The five factors that can affect the rate of a reaction are:

• The concentration of the reactants in solution

• The pressure of reacting gases

• Surface area of solid reactants

• The temperature of the reaction 

• The presence of a catalyst

Magnesium powder has a greater surface area.

Increasing the concentration will increase the rate of reaction.

False.

Adding a catalyst to a reaction will increase the rate.

Decreasing temperature decreases the rate of reaction.

True.

It is of economic interest to have a higher rate of reaction as this implies a higher rate of production and hence a more efficient and sustainable process.

Increasing surface area increases the rate of reaction.

The time for a reaction to complete decreases when temperature is increased.

True.

Decreasing particle size has the same effect as increasing surface area.

The amount of product formed remains the same when a catalyst is added, but it forms more quickly.

Collision theory explains how rates of reaction depend on the frequency and energy of collisions between reactant particles.

The minimum energy required for reactant particles to undergo a chemical reaction.

True.

Increasing the concentration of a solution will increase the number of reactant particles in a given volume, allowing more frequent and successful collisions per second.

Increasing temperature:

• Increases the kinetic energy of the particles,

• Leading to more frequent and successful collisions with energy greater than the activation energy.

Increasing the surface area of a solid reactant increases the rate of reaction because more particles are exposed to the other reactant, producing a higher number of collisions per second.

The number of collisions is proportional to the number of particles present.

A successful collision is a collision between reactant particles that leads to the formation of product molecules.

The symbol for activation energy is E_a.

False.

Not all collisions result in a chemical reaction.

Unsuccessful collisions happen when the colliding species do not have enough energy to break the necessary bonds.

A catalyst is a substance that increases the rate of a reaction without being used up itself.

A catalyst speeds up the rate of a reaction by providing an alternative pathway with lower activation energy.

False.

Catalysts are not used up in the reaction.

False.

Although catalysts are not part of the overall reaction, they may be written over the arrow in reaction equations, indicating the conditions.

True.

Only small amounts of a catalyst are needed in a reaction as the catalyst is not used up.

You can show that a substance is a catalyst by:

• Weigh the catalyst before the reaction

• Weigh the catalyst after the reaction

• The mass should remain the same

The letter which represents the activation energy for a reaction using a catalyst is C.

True.

Enzymes are nature's catalysts, biological substances that catalyse reactions in living cells.