C8

Question 1

Collision theory

Answer 1

Explanation of chemical reactions in terms of reacting particles colliding with sufficient energy for a reaction to take place

• Particles collide more often
• Particles collide with more energy

Question 2

Activation energy

Answer 2

Ea: minimum energy needed for a reaction to take place

Question 3

Reversible reaction

Answer 3

Reaction which the products can reform the reactants

Question 4

Le Chárelier’s Principle

Answer 4

If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change

Question 5

Anhydrous

Answer 5

Without water

Question 6

Closed System

Answer 6

Stystem which no matter enter or leaves

Question 7

1… Equilibrium

Answer 7

The point in a reversible reaction at which the forward and backward rates of reaction are the same. Therefore, the amounts of substances presents in reacting mixture remain constant… rate of forward reaction equals rate of reverse reaction

• Concentration increased, more products will be formed until Equilibrium reached again
• Concentration decreased, more reactants will react until Equilibrium reached agaim

e.g.

• car moving at constant speed
• 2 people balancing on see saw
• 2 objects at equal temperature

Question 8

2… Equilibrium

Answer 8

Question 9

Calculate rate of reaction

Answer 9

• Reactants used up as they make products
• Products of reaction are made

Question 10

Reversible Reaction examples

Answer 10

• Ammonia + Hydrogen Chloride <=> Ammonium chloride
• Hydrogen copper sulfate <=> Anhydrous copper sulfate + Water

Question 11

Concentration and Rate of Reaction Required Practical (see video and a4 document on Student Shared)

Answer 11

Investigation into how the concentration of a solution affects the rate of a chemical reaction

There are two parts to this practical which investigate how the rate of reaction can be measured.

Activity 1: Observing colour change

You will react sodium thiosulfate with hydrochloric acid. You will then find out how the rate of reaction changes as the thiosulfate solution becomes more dilute.

Activity 2: Measuring the volume of gas produced

You will react magnesium ribbon and hydrochloric acid. You will then find out how the rate of reaction is affected by the concentration of the acid.

Risk assessment

• Safety goggles should be worn throughout.

Method

Activity 1: Observing colour change

You are provided with the following:

• 40 g/dm³ sodium thiosulfate solution
• 2.0 M dilute hydrochloric acid
• 10 cm³ measuring cylinder
• 100 cm³ measuring cylinder
• 100 cm³ conical flask
• printed black paper cross
• stopclock.

Read these instructions carefully before you start work.

1. Use a measuring cylinder to put 10 cm³ sodium thiosulfate solution into the conical flask.
2. Use the measuring cylinder to then add 40 cm³ water.This dilutes the sodium thiosulfate solution to a concentration of 8 g/dm³.
3. Put the conical flask on the black cross.
4. Put 10 cm³ of dilute hydrochloric acid into the 10 cm³ measuring cylinder.
5. Put this acid into the flask. At the same time swirl the flask gently and start the stopclock.
6. Look down through the top of the flask.
7. Stop the clock when you can no longer see the cross.

Take care to avoid breathing in any sulfur dioxide fumes. (see image below)

Write the time it takes for the cross to disappear in the first blank column of the table such as the one below. Record the time in seconds.

You will need to multiply any minutes by 60 and then add the extra seconds.

Concentration of sodium thiosulfate in g/dm³

Time taken for cross to disappear in seconds

First trial

Second trial

Third trial

Mean

8

16

24

32

40

Repeat steps 1‒7 four times, but in step 1 use:

• 20 cm³ sodium thiosulfate + 30 cm³ water (concentration 16 g/dm³)
• 30 cm³ sodium thiosulfate + 20 cm³ water (concentration 24 g/dm³)
• 40 cm³ sodium thiosulfate + 10 cm³ water (concentration 32 g/dm³)
• 50 cm³ sodium thiosulfate + no water (concentration 40 g/dm³

​​8. Then repeat the whole investigation (steps 1–7) twice more.

Record the results in the second and third blank columns of the table.

9. Calculate the mean time for each of the sodium thiosulfate concentrations. Leave out anomalous values from your calculations.

Record the means in the fourth blank column.

10. Plot a graph with:

• ‘mean time taken for cross to disappear in seconds’ on the y-axis
• ‘Sodium thiosulfate concentration in g/dm³’ on the x-axis

Draw a smooth curved line of best fit.

What can you say about the effect of the independent variable (concentration) on the dependent variable (time taken for the cross to disappear)? What were your control variables?

Compare your results with those of others in the class. Is there evidence that this investigation is reproducible?

Question 12

What are the 5 key factors that affect the rate of reaction?

Answer 12

1. Surface Area
2. Temperature
3. Concentration
4. Pressure
5. Catalysts

Question 13

1. Surface area

Answer 13

• When a solid reacts in a solution, size of pieces of solid affects rate of reaction
• rate of a chemical reaction increases if the surface area to volume ratio of any solid reactant is increased
• increases frequency of collisions between reacting particles
• Larger the suface area (smaller the item) the quicker the reaction

This is because if it was a big object, the particles inside it are not in contact with the reactant particles in the solution, making them not react/reaction takes longer.

Smaller lumps have more of solution covering the solution, making it quicker

• Smaller the size of pieces of a solid material, the larger the surface area to volume ratio.

Question 14

2. Temperature

Answer 14

• Reactions happen quicker as temperature increases
• Increase the temperature increases the rate of reaction
  
  • Particles collide more frequently
  • Collide more energetically
  • More collisions are occuring in a given time results in a reaction
    
    • higher proportion of particles have energy greater than Ea

Question 15

3. Concentration

Answer 15

• Increase the concentration of reactants in solutions increases frequency of collisions between particles, so increases rate of reaction.
  
  • because more particles of reactants moving around the same volume of solution
• More ‘crowded together’ reactant products are, more likely it is that they will collide -increased frequency of collision means its a faster reaction

Question 16

4. Pressure

Answer 16

• Increasing the pressure of reacting gases also increases frequency of collisions, so increases rate of reaction
  
  • Same effect as concentration
• Increased pressure squashes the gas particles closer together. More gas in given space
  
  • Increases chance of more collisons and reactions, increasing rate of reaction

Question 17

5. Catalysts

Answer 17

• A substance that speeds up a chemical reaction by providing a different pathway for the reaction that has a lower Ea. The Catylst is chemically unchanged at the end of the reaction.
• Don’t increase the frequency of collisions between reactant particles
• Don’t make collisions more energetic
• Increase rates of reaction by providing an alternative reaction pathway to products, lowering Ea from with it not have a catalyst (see reaction profile below)

Positives

Negatives

Can speed up reactions - reducing energy costs by using this

Cataysts used in chemical eventually become poisoned - don’t work anymore.

But these are most effective, so can end up being cheaper by them not needing to pay for extra energy needed without one.

Very expensive metals: Gold, Platinum, Palladium.

Can be used over and over again

Help environment (don’t need to burn fossil fuels for higher temps/pressures)(stops more CO₂ entering atmosphere when burnt, helping to stop climate change)