Chemistry: Rate & extent of chemical change (Paper 2 Topic 6)

What is the graph for a chemical reaction - Product formed?

What is the graph for a chemical reaction - Reactants used?

what is the formula to calculate mean rate of reaction?

How do we calculate the rate of reaction?

Drawing a tangent

Straight line at a point

Difference in y divided by difference in x

What is Collision Theory?

Chemical reactions an only take place when reacting particles collide with each other

The collisions must have sufficient energy

Rate of a chemical reaction is determined by freq of successful collisions

Frequency - No. of successful collisions per second

What is the effect of concentration on the rate?

• Higher concentration:

• The number of collisions (per sec) will be higher

• Therefore higher frequency

• Higher freq means quicker rate of reaction

• (Rate is proportional to frequency)

What is a hypothesis?

• Proposal that could explain a fact or observation

• Must be testable

What is the Investigating rate of reaction practical (liquid) process?

1. Measure 10cm³ of sodium thiosulphate into conical flask

2. Put flask onto black cross

3. Add 10cm³ of hydrochloric acid

4. Swirl solution, start stopwatch

5. Observe through top of flask, stop timer when cross is invisible

6. Repeat with lower concentrations of sodium thiosulphate solution

7. Calculate mean values

What affects the accuracy of this experiment?

• A measurement is reproducible if it can be repeated

• Different people have different eyesights

• May not get the same result

• Same size cross should reduce mistakes

What is the investigating rate of reaction practical (Gas) process?

1. Measure 50cm³ dilute hydrochloric acid into conical flask

2. Attach to bung and delivery tube

3. Add 3cm strip of magnesium to acid

4. Start stopwatch

5. Record amount of gas in measuring cylinder every 10s

6. Repeat with different concentrations of hydrochloric acid

What are the Investigating rate of reaction practical results?

• The greater the concentration of a chemical in a reaction, the faster the reaction takes place

• This finding is reproducible (2 experiments)

What is the Ratio of surface area:volume effect on rate?

Smaller sized blocks have a larger surface area:volume ratio

More particles on the surface

More collisions per second

Higher rate

What effect does temperature have on rate?

• Particles need activation energy to react

• If 2 particles with little energy collide, a reaction will not occur

• Increasing the temp increases the energy of the particles

• Increases freq of collisions & rate

• Rate of reaction is proportional to the temperature

What is a catalyst? Why are they commonly used?

Increase the rate of chemical reactions without being used up

Saves money bc reactions occur quickly without temp increase

Not used up, saves money

Increase the rate by providing a different pathway for the reaction that has a lower activation energy

3 key facts about catalysts

• Not included in chemical equation (Not used up)

• Different reactions need different catalysts

• Enzymes are catalysts in living organisms

What is a common reversible reaction?

Heat is added to go ‘forwards’ (endothermic) & released to go ‘backwards’ (exothermic)

If it is exothermic one way, its endothermic the other

The same amount of energy is transferred

What is equilibrium?

If a reversible reaction is in a container, the reactants and products can’t escape

Eventually the reverse reactions will occur at the same rate

What is Le Chatelier’s Principle?

• If a reaction is at equilibrium and a change is made to the conditions, the reaction responds to counteract the change

(Ep 9 ‘concentration…’ freesciencelessons rates of reaction)

What affect does temperature change have on equilibrium?

(ep 10 ‘temperature…’ freesciencelessons rates of reaction)

What affect does pressure change have on equilibrium?

• Increase pressure, position of equilibrium shifts to side with less molecules

• Decrease pressure, position of equilibrium shifts to side with more molecules

• If reaction is equal on both sides, pressure change has no effect