Effect of Concentration + Pressure

Question 1

why does increasing the concentration/pressure for solutions/gases increase the rate?

Answer 1

• there are more particles per unit volume
• so the collision frequency is higher
• so there are more successful collisions per unit time

Question 2

how can increasing concentration be accomplished?

Answer 2

by dissolving more solute particles

Question 3

how can increasing pressure be accomplished?

Answer 3

by making the reaction vessel smaller

Question 4

what is the disappearing cross experiment?

Answer 4

• the effect of concentration on rate can be demonstrated by reacting sodium thiosulphate solution with hydrochloric acid

Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + S(s) + SO2(g) + H2O(l)

Question 5

what happens during the disappearing cross experiment?

Answer 5

• in this reaction, a solid (sulphur) is slowly produced
• after a while you can’t see through the mixture any longer
• this is measured by observing a black cross under the mixture

Question 6

what are the variables of the disappearing cross experiment?

Answer 6

input = the concentration of the Na2S2O3 (aq)
output = the time taken for the cross to disappear
control:
- total volume of solution
concentration of HCl (aq)
- temperature

Question 7

what equation is used to calculate rate from the times taken from the disappearing cross experiment?

Answer 7

rate (s^-1) = 1/time taken (s)

Question 8

what are the results of the disappearing cross experiment on a graph?

Answer 8

• the graph shows a straight line through the origin
• this is because rate is proportional to concentration
  e.g. double the concentration = double the rate