The rate and extent of chemical change

What is the rate of a chemical reaction?

A measure of how quickly the reactants are converted into products

What is the formula for calculating the rate of a chemical reaction?

rate = mass of product produced or lost / time taken

rate = volume of product produced or lost / time taken

What are the different ways of measuring the rate of a reaction?

1. Precipitation and colour change

2. Change in mass

3. Volume of gas gained or lost

1. Precipitation - measure the time it takes for a mark (e.g black cross under flask) to disappear

2. Colour change - measure the time it takes for a colour change to occur

3. In both cases calculate the rate using 1/time taken

What are the issues with measuring the rate of a reaction by observing colour change and precipitation?

-Subjunctive

-can’t use the results to plot a rate of reaction graph

How can you calculate the rate of a reaction by measuring the change in mass?

1. Use a balance to monitor how the mass changes over time

2. Take measurements at regular intervals

3. Plot the results on a graph

4. Draw a tangent and calculate its gradient to find the rate at a particular time

What is the most accurate way to measure the rate of a reaction and why?

Change in mass because the balance is the most accurate measuring device

How can you use a gas syringe to measure the rate of a chemical reaction?

1. Use the gas syringe to monitor how the volume of gas changes over time

2. Take measurements at regular intervals

3. Plot the results on a graph

4. Draw a tangent and calculate its gradient to find the rate at particular time

What factors affect the rate of a chemical reaction?

1. Temperature

2. Concentration of solution or pressure of gas

3. Surface area

4. Presence of a catalyst

How does increasing the temperature increase the rate of a reaction?

1. As temperature increases, particles move around faster because they have more kinetic energy

2. This means they will collide more frequently

3. They will also collide more frequently

4. They will also collide with more energy, so the collision is more likely to have enough energy to react successfully (overcome the activation energy)

How does increasing the concentration or pressure increases the rate of a reaction?

-As the concentration of a solution (or the pressure of a gas) increases, the number of particles per unit of volume will increase

-This will increase the frequency of collisions and so increases the rate of reaction

How does surface area of a solid affects the rate of a reaction?

1. If a reactant is solid, breaking it into smaller pieces increases the surface area to volume ratio

2. This means, for the same volume of solid, the particles will have a greater area over which they can collide, so will collide more frequently

3. More frequent collisions result in a greater number of reactions

What is collision theory?

States that for particles to react, they have to collide with sufficient energy, and at the right orientation

What is activation energy?

The minimum amount of energy that particles require to react together

Why do particles need activation energy?

To break the bonds of the reactants, so the reaction can begin

How does a catalyst increase the rate of reaction?

-Catalysts speed up reactions by lowering the activation energy required for a reaction to occur

-They do this by giving an alternative route (reaction pathway) that requires less energy

On a concentration/time graph of a chemical reaction, what does a steep gradient of the line indicate?

a steep line signifies that the rate of reaction was fast

why do concentration/time graphs level out?

the graph levels out as the reactants are used up

what is the formula to find the gradient of a line?

gradient = change in y / change in x

how do you find the rate of reaction at a particular point on a concentration/time graph?

1.Draw a tangent to the line at that point

2.Calculate the gradient of the tangent by picking 2 points on the line and doing ‘change in y / change in x’

3.The gradient of this tangent is the rate

What is a reversible reaction?

reaction which can go both forwards and backwards

During a reversible reaction, what happens as the reaction progresses and the reactants react?

1.The concentrations of reactants fall

2.The forward reaction slows down

3.The concentrations of products rise

4.The backwards reaction speeds up

when is a system at equilibrium?

when the rate of the forward reaction is equal to the rate of the backward reaction

what is a dynamic equilibrium?

where both the forward and backward reactions are happening, but there is no overall affect, because they are at the same rate - product is breaking down just as quickly as it is being formed, so there is no overall change in the concentrations of the reactants or products

what condition is required for a reaction to reach equilibrium?

the reaction must take place in a closed system (where nothing can enter or leave)

what does it mean if the position of equilibrium lies to the right?

the concentration of the products is greater than the concentration of the reactants

what does it mean if the position of equilibrium lies to the left?

the concentration of the reactants is greater than the concentration of the products

does equilibrium mean the concentration of products and reactants are equal?

no: an equilibrium may lie to the left or the right, and that tells us which has the greater concentration

what factors alter the position of equilibrium?

-temperature

-pressure (affects gases)

-concentration of reactants and products

what is Le Chatelier’s Principle?

if you change the conditions of a reversible reaction at equilibrium , the system will try to counter that change in order to restore the equilibrium

why is Le Chatelier’s principle useful?

allows you to predict the affect of any changes made to a system

according to Le Chatelier’s principle, what happens if you decrease the temperature?

the position of equilibrium will shift towards the exothermic direction, to produce more heat (and counteract the decrease in temperature)

according to Le Chatelier’s principle, what happens if you increase the temperature?

the position of equilibrium will shift towards the endothermic direction in order to decrease the temperature (and counteract the increase in temperature)

according to Le Chatelier’s principle, what happens if you increase the pressure?

the equilibrium will shift to the side with fewer molecules of gas, to reduce the pressure (and counteract the increase in pressure)

according to Le Chatelier’s principle, what happens if you decrease the pressure?

the equilibrium will shift to the side with more molecules of gas, to increase the pressure (and counteract the decrease in pressure)

according to Le Chatelier’s principle, what happens if you increase the concentration of the reactants?

the equilibrium will shift to the right (products side) to reduce the reactant concentrations and restore equilibrium

according to Le Chatelier’s principle, what happens if you decrease the concentration of the reactants?

the equilibrium will shift to the left (reactants side) to increase the reactant concentrations and restore equilibrium

according to Le Chatelier’s principle, what happens if you add a catalyst?

it will have no effect on the position of equilibrium