TOPIC 6 - Rate + Extent of Chemical Change

Why does the mixture become cloudy (with a solid product in a solution)?

the product is a precipitate/ insoluble substance

What is meant by repeatable?

getting similar results under the same conditions by the same students with different repeats

2 SO2(g) + O2(g) ֖ 2 SO3(g)
Explain why the percentage yield of sulfur trioxide in this reaction is greater if the pressure is higher

there are more moles of gas on the left; so the position of equilibrium shifts to the right

Suggest two reasons why a higher pressure is not used

• the yield is already high 
  -  more energy required 
  -  risk of explosion is increased

•   higher income from increased yield is outweighed by the extra expenditure 

•   increased cost of safety precautions

What is the effect of adding a catalyst on the equilibrium position of a reaction?

no effect

The concentrations of the substances in this reaction do not change at dynamic equilibrium - why?

the forward and reverse reactions are taking place at exactly the same rate

How does an equation show that the reaction is reversible?

It contains the symbol ⇌

What happens when the reaction reaches equilibrium?

the mass of each substances doesn't change; the rates of the forward + reverse reactions are equal

ways of changing the rate of reaction between gases

change pressure; change temp; add a catalyst

The balance always read 0.2 g before being used - What type of error was caused by the balance?

a systematic error

The rate of the reaction with methanoic acid is greater than the rate of the reaction with ethanoic acid. Explain why

• methanoic acid has a lower pH

• so has a higher concentration of H+ ions

• so more frequent collisions

What two improvements could student A make to the method to give valid results?

Put the conical flask in a water bath at a constant temperature; use same mass of MGO2 each time

Why do fine particles have a greater rate of reaction than coarse particles?

the surface area of the fine particles is greater; so more frequent collision with the other particles

The student draws a new cross for each experiment. Suggest why this might give inaccurate results

the cross might might darker/ paler; a different size

If more reactant is added, how and why does the position of Eq shift?

the position of Eq shifts towards the products; so that the increase in concentration of reactants is reduced (counteract the change)

If all the reactants are liquids, will a change in pressure affect equilibrium?

No change; because no gases are present

What is meant by equilibrium?

In a close system; the rate of the forward and backward reactions are equal