4.6 The rate and extent of chemical change - GCSE Chemistry AQA

What is the mean rate of reaction formula?

Quantity of reactant used / time taken

Quantity of product formed / time taken

Unit: g/s or cm³/s

What does a reaction graph look like? (Interpreting reaction graphs)

At the start, the gradient is steepest as the reaction is at its fastest because there is a high concentration of reactant particles

The curve gets less steep as the reaction is slowing down as reactants are used up

The reaction stops when the line plateaus because one of the reactants has been completely used up

How do you calculate rate of reaction at a specific time on a curved graph?

Draw a tangent at that specific point and calculate the gradient

What is collision theory?

Chemical reactions can only occur when reacting particles collide with each other an with sufficient energy (activation energy)

Activation energy - The minimum amount of energy that particles must have to react

What factors affect the rate of a chemical reaction?

Concentration
Pressure
Surface Area
Temperature
Catalysts

Why does concentration affect the rate of reaction?

Increasing concentration increases the rate as there are more particles in the same volume, so the particles are closer together leading to more frequent successful collisions to reach activation energy

Why does pressure affect the rate of reaction?

Increasing pressure increases the rate as it causes there to be more gas particles per unit of volume so there are more frequent collisions therefore more successful collisions

Why does surface area affect the rate of reaction?

Increasing surface area increases the surface are to volume ratio increasing the rate as more particles are exposed at the surface and are available to react resulting in more frequent successful collisions

Why does temperature affect the rate of reaction?

Increasing temperature increases the rate as particles have more kinetic energy therefore move faster, leading to more frequent collisions

Due to the increased energy, a higher proportion of collisions have the required activation energy to react

Why does a catalyst affect the rate of reaction?

Adding a catalyst increases the rate as they provide an alternative reaction pathway that has a lower activation energy. This means more particles have enough energy to react when they collide

What is the relationship between the rate and the concentration/pressure?

The rate is directly proportional to concentration. If you double the number of particles in a fixed volume, you double the frequency of collisions

What is a catalyst?

A substance that increases the rate of a chemical reaction but is not used up during the process (mass remains the same)

Biological catalysts are called enzymes

How do Catalysts work?

Catalysts provide an alternative pathway for the reaction

This alternative pathway has a lower activation energy

Therefore, a higher proportion of collisions have sufficient energy to result in a reaction

What is a reversible reaction?

The products can react together to reform the original reactants

If the forward reaction of a reversible reaction is exothermic the reverse reaction is…

endothermic
The amount of energy transferred in each direction is exactly the same

Describe the reversible reaction involving copper sulfate, including the color change

Hydrated copper sulfate (blue -(heat)→ anhydrous copper sulfate (white) + water

Anhydrous copper sulfate (white) + Water → hydrated copper sulfate (blue)
Forward reaction is endothermic, the reverse is exothermic

When is dynamic equilibrium reached?

In a closed system, when the forward and reverse reactions occur at the same rate the concentrations of reactants and products remain constant

What is Le Chatelier’s principle?

If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract change and restore the equilibrium

Describe the effect of changing the concentration of reactant and product on the position of the equilibrium

If the concentration of a reactant is increased, more products will form until equilibrium is reached again (Equilibrium will move to right)

The equilibrium will shift to the side that uses up the added substance

Describe the effect of changing the temperature on the position of equilibrium

Increasing temperature will cause the equilibrium to shift in the endothermic direction to absorb more heat

Describe the effect of changing pressure on the position of equilibrium

An increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules.

Describe the effect of a catalyst on the position of the equilibrium

No effect
It just speeds up both forward and backward reactions equally
Equilibrium is achieved faster

What is the method of measuring gas production to investigate how concentration affects the rate of reaction? 6 marks

Measure a set volume of dilute hydrochloric acid using a measuring cylinder and add it to a conical flask

Add a set length of magnesium ribbon to the flask and immediately attach a gas syringe or a delivery tube leading to a measuring cylinder inverted in water

Start a stopwatch the moment the magnesium is added

Record the volume of hydrogen gas produced at regular intervals until the reaction stops

Repeat the experiments using different concentrations of hydrochloric acid while keeping the volume of acid and length magnesium ribbon the same

Disappearing cross method 6 marks

Measure a set volume of sodium thiosulfate solution into a conical flask

Place the flask on a piece of paper with a black cross drawn o it

Add a set volume of dilute HCl and start a stopwatch immediately

Swirl the flask once and look down through the top of the flask

Stop the clock the exact moment the black cross is not longer visible (masked by sulfur precipitate)

Repeat using different concentrations of sodium thiosulfate keeping the depth of liquid and volume of acid constant

State a hypothesis for RP5 and identify the independent variable

as the concentration of reactant increases, the rate of reaction will increase

The concentration of the reactant - Independent variable

Volume of gas produced per unit of time / the time taken for cross to disappear - dependent variable

What is the main source of error in the disappearing cross experiment?

Human subjectivity - different people may disagree on the exact moment the cross disappears - use the same observer for all tests