Investigating rates (Practical)

Question 1

Describe the method of a practical to test the effect of surface area on rate using:

• conical flask
• measuring cylinder
• marble chips (small, medium, large)
• cotton wool
• dilute hydrochloric acid
• scale accurate to 0.01g
• stop watch

Answer 1

• prepare 3 sets of different sized marble chips
• each set needs the same mass
• add a measured volume of acid to the flask
• seal with cotton wool
• place flask and marble chips on scale
• record the mass
• remove cotton wool and add chips
• start the stop watch and replace the cotton wool
• record the mass every 30s
• repeat steps with the other sets of chips

Question 2

mean rate of reaction

Answer 2

change in mass/chosen time

Question 3

A student investigates the effect of acid concentration on the rate of this reaction:
Na2S2O3(aq) + 2HCl(aq) -> 2NaCl(aq) + H2O(l) + SO2(g) + S(s)

the reaction mixture becomes cloudy as sulfur forms
the student measures the time taken for it to become too cloudy to see a cross drawn on a piece of paper underneath the flask

in each experiment he uses the same volume and concentration of sodium thiosulfate solution
he uses the same volume of diluted acid each time

explain three steps that the student takes to obtain results that allow a fair comparison

Answer 3

• same volume of sodium thiosulfate solution
• same concentration of sodium thiosulfate solution
• same amount of cloudiness (measured using the disappearing cross)
• same volume of diluted acid

also affect the rate of reaction/measured rate of reaction