Chemistry Paper 2 : Rates of Reactions

What does the slope look like on a graph with a fast rate of reaction?

It is steeper.

What does a less steep slope tell us about a reaction?

The reaction is slowing down, the rate of reaction is decreasing. Fewer reacting molecules available because most have already reacted and turned into product.

What does a slope with a flat line tell us about rate of reaction?

The reaction has stopped, all reactant molecules have reacted.

How to calculate the mean rate of reaction for products

Mean rate of reaction = quantity of product formed/time taken.

How to calculate mean rate of reaction for reactant used?

Mean rate of reaction = quantity of reactant used/time taken.

What is the collision theory?

Chemical reactions can only take place when the reacting particles collide with each other. The collisions must have sufficient energy.

How is the rate of a chemical reaction determined?

But the frequency of successful collisions.

What is cloudiness in a solution known as?

Turbidity

How to do disappearing cross required practical?

1. Measure 10cm3 of sodium thiosulfate in measuring cylinder and pour into conical flask.

2. Place conical flask on top of printed black cross.

3. Add 10 cm3 of hydrochloric acid into flask.

4. Swirl solution and start stop watch, look down at flask.

5. Stop clock when we can no longer see cross.

6. Carry out experiment again with lower concentrations or sodium thiosulfate.

7. Repeat experiment and calculate mean values for each concentration.

What values shouldn't be included when calculating mean?

Anomalous results.

How can something be reproducible?

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

What is an issue with disappearing cross experiment that affects results? How could you prevent it.

People have different eyesights so some see it longer than others → use the same sized printed cross.

How to measure volume of gas produced by a reaction: required practical?

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

2. Attach conical flask to a bung and delivery tube.

3. Place delivery into container filled with water.

4. Place upturned measuring cylinder also filled with water over the delivery tube.

5. Add 3cm strip of magnesium to hydrochloric acid and start stop clock.

6. Reaction produced gas which is trapped in measuring cylinder. Measure volume of hydrogen gas in cylinder, until no more hydrogen given off.

7. Repeat experiment with different amounts of hydrochloric acid.

What do both practicals show about the concentration?

The greater the concentration of chemical reaction, the faster the reaction takes place.

Are the rate of reaction practicals showing that the finding is reproducible?

Yes

What happens to rate of reaction as the surface area is increased of a solid reactant?

The larger the surface area (could be done by splitting solid up into pieces), the higher the rate of reaction (more collisions).

What is the surface area : volume of smaller blocks of solid reactant compared to larger blocks.

Smaller: greater sa:v

Larger: smaller sa:v

Why can measuring the volume of a gas be difficult with a measuring cylinder? What can you use instead?

The bubbles can be quite rapid. Use a gas syringe which is more accurate

Why is a cotton wool used when measuring mass of carbon dioxide gas lost? What does it prevent?

Allows carbon dioxide gas to escape, but prevents acid from splashing out of flask → if it did splash out, the mass would fall more than it should (anomalous)

What is an exothermic reaction?

Energy is released from reaction to surroundings (product is smaller/lower on the graph)

What is the activation energy?

The minimum amount of energy needed for the particles to react.

Where is activation energy on endo/exothermic graph?

on line with the reactant and then to the top of arch

Would particles with low energy colliding allow it to overcome the activation energy barrier?

No, they wouldn't collide successfully so no reaction would occur.

Would particles with large energy collide and overcome activation barrier?

Yes, they have enough energy to overcome activation barrier and can collide successfully. Reaction takes place.

What does increasing the temperature do to rate of reaction?

Higher kinetic energy gained so more frequent successful collisions. More particles can overcome activation barrier. RATE PROPORTIONAL TO TEMP.

What is a catalyst?

Increased rate of chemical reaction without being used up during reaction.

Why can catalysts save money?

They carry out reactions quickly without needing to increase temperature.

Why can catalysts be reused in the reaction?

They aren't used up.

How to catalysts increase rate?

Provide different pathway for the reaction that has a lower activation energy.

Why is activation lower with a catalyst?

Less energy is needed to cross activation energy barrier. So more frequent successful collisions (increasing rate of reaction).

Why are catalysts not included in chemical equation for a reaction?

They are not used up in the reaction. Catalysts are not a reactant.

What is the catalyst for living organisms?

Enzymes.

How can you change direction of reversible reactions?

Changing conditions

If a reversible reaction is exothermic in one direction, what will the other one be?

Endothermic. Same amount of energy transferred in each case.

What is equilibrium?

Forward and backwards reactions will take place at the exact same rate in a closed system.

What stays constant in dynamic equilibrium?

The concentration of reactions and products.

What happens to the system when it's at equilibrium and conditions are changed? What is this scientifically known as?

The system counteracts the change. This is Le chateliers Principle

What happens if we increase temperature in a system?

The system counteracts the change.

What happens when we increase pressure on a reversible reaction at equilibrium?

Position of the equilibrium shifts to the side with the smaller number of molecules.

What happens when we reduce the pressure on a reversible reaction at equilibrium?

Position of equilibrium shifts to the side with the larger number of molecules.

If the number of molecules is the same on both sides, what does changing pressure do to the position of equilibrium?

Has no effect.