GCSE Chemistry: Rates of Reaction (paper 2)

How does the slope give us an idea of the rate of reaction?

The steeper the slope, the faster the rate of reaction.

Equations for mean rate of reaction

Mean rate of reaction= Quantity of product formed/time taken

Mean rate of reaction= Quantity of reactant used/time taken

units can be in either mol/s g/s or cm³/s (for a gas)

Effect of concentration of reactants or the pressure of reacting gases on the rate of reaction

If a solution is made more concentrated, it means there are more reacting particles knocking about in the same volume of solvent.

Similarly, when the pressure of a gas is increased, it means that same number of particles occupy a smaller space.

This makes collisions between the reactant particles more frequent, thus increasing the rate of reaction

(higher concentration of reactant also leads to more product being produced as we started with more reactant molecules)

What’s collision theory?

Theory that states that chemical reactions can only take place when the reacting particles collide with each other. These collisions must have sufficient energy.

Effect of surface area of solid reactants on the rate of reaction

Particles in solution can only react with particles on the surface of the solid.

Breaking up the solid into smaller pieces increases the surface area of the solid reactant.

That means for the same volume of the solid, the particles around it will have more surface area to react with, increasing the frequency of collisions, thus increasing the rate of reaction.

Effect of temperature on rate of reaction

When temperature is increased the particles have more energy so they move faster.

Because they’re moving faster, the frequency of collisions increases, thus increasing the rate of reaction.

Also because the particles have more energy, more particles can overcome the activation energy barrier and collide successfully.

Effect of catalyst on the rate of reaction

A catalyst is a substance that increases the rate of reactions, without being used up.

Catalysts increase the rate of reaction by providing a different pathway for the reaction to happen that has a lower activation energy.

When a catalyst is present, particles require less energy to cross the activation energy barrier, which increases the frequency of successful collisions thus increasing the rate of reaction.

What is a reversible reaction

When in a chemical reaction, the products of the reaction can react to produce the original reactant.

Example of reversible reaction (thermal decomposition)

hydrated copper sulfate ⇌ anhydrous copper sulfate + water

CuSO₄.5H₂O(s) ⇌ CuSO₄(s) + 5H2O(l)

Key points about reversible reaction

• If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction.

• The same amount of energy is transferred in each case.

What is equilibrium?

The point at which the rates of the forward and backwards reactions in a reversible reaction are the same, and so the amounts of reactants and products in the reaction container don’t change.

How can equllibrium be achieved?

If the reaction takes place in a closed system.

What is L e Chatelier’s Principle

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

Effect of changing the concentration on a reversible reaction at equilibrium

• If you change the concentration of the reactants or products then the system is no longer at equilibrium.

• This means that the concentrations of all the substances will change until equilibrium is reached.

• If you increase the concentration of reactants, more products will be formed to reduce the amount of reactant-until equilibrium is reached again. (shifts to right)

• If you decrease the concentration of products, more reactants will react to increase the amount of product-until equilibrium is reached again.(shifts to right)

Effect of changing the temperature on a reversible reaction at equilibrium

• When the forward reaction is exothermic and the reverse reaction is endothermic

• If you increase the temperature of the system, then the equilibrium shifts to the left to reduce the temperature.

• That is because the reverse reaction is endothermic, so energy is taken in, causing the temperature to fall.

• This would decrease the amount of product and increase the amount of reactant.

• If you decrease the temperature of the system, then the equilibrium shifts to the right to increase the temperature.

• That is because the forward reaction is exothermic, so energy is released, causing the temperature to increase.

• This would increase the amount of product and decrease the amount of reactant.

Effect of changing the pressure on a reversible reaction at equilibrium (only for gases)

• If you increase the pressure on a reversible reaction at equilibrium, then the position of equilibrium shifts to the side with the smaller number of molecules.

• If you reduce the pressure, then the position of equilibrium shifts to the side with the larger number of molecules.

What does the pressure of gas depend on?

The number of molecules (not only thing though)

What is a hypothesis?

• A proposal that could explain a fact or an observation

• Must be testable

The disappearing cross reaction

sodium thiosulfate solution + hydrochloric acid → sulfur (solid)

What makes the solution go cloudy (turbid) in the disappearing cross reaction

• sulfur

• this turbidity can be used to see how long the reaction takes to finish

Describe the Rates of Reaction practical/disappearing cross

1. Use a measuring cylinder to put 10cm³ of sodium thiosulfate solution into the conical flask.

2. Place the conical flask onto a printed black cross.

3. Next add 10cm³ of hydrochloric acid into the conical flask. Swirl the solution and start a stopwatch.

4. Carefully observe through the top of the flask. After a certain time the solution will turn cloudy and stop the clock when you can no longer see the cross.

5. Carry out the experiment again using lower concentrations of sodium thiosulfate solution.

6. Then repeat the whole experiment and calculate the mean values for each concentration of sodium thiosulfate solution.

   (REMEMBER TO NOT COUNT ANOMOLIES)

How can a measurement be reproducible?

If it can be repeated by another person or using a different technique or equipment and still get the same result.

Problem with the disappearing cross experiment.

Different people have different eyesights, meaning that some people can see the cross for longer than others so they may not get the same result.

However, because all students use the same size printed cross, hopefully this problem won’t be too great.

Describe the second rate of reaction practical/ marble chips

1. Use a measuring cylinder to place 50cm³ of hydrochloric acid into a conical flask.

2. Then attach the conical flask to a bung and delivery tube.

3. Now place the delivery tube into a container filled with water.

4. Then place an upturned measuring cylinder also filled with water over the delivery tube.

5. Add a 3 cm strip of magnesium to the hydrochloric acid and start a stopwatch.

6. The reaction produces hydrogen gas which is trapped in the measuring cylinder. Every 10 seconds measure the volume of hydrogen gas in the measuring cylinder and do this until no more hydrogen is given off.

7. Repeat the experiment using different concentrations of hydrochloric acid.