Rate-Concentration Graphs

Question 1

Describe how a rate-concentration graph looks for a zero order reaction.

Answer 1

• horizontal straight line with gradient zero
• y-intercept = k

Question 2

Describe how a rate-concentration graph looks for a first order reaction.

Answer 2

• straight line graph through the origin
• gradient = k

Question 3

Describe how a rate-concentration graph looks for a second order reaction.

Answer 3

• upward curve with an increasing gradient (exponential)

Question 4

How can initial rate be calculated?

Answer 4

• gradient of tangent at t=0 on a concentration-time graph
• proportional to 1 / time

Question 5

Describe how the iodine clock procedure can be used to determine the order of reaction and hence, the rate equation.

Answer 5

1. Using pipettes, add 5cm3 of Potassium Iodide, 2cm3 of sodium thiosulfate, and 2cm3 of starch into a conical flask and mix well
2. Add 2cm3 of potassium peroxodisulfate to the conical flask and start the stopwatch.
3. Stop stopwatch when solution turns blue/black and record time.
4. Repeat experiment with different concentrations of potassium iodide
5. Calculate initial rate using (2x10^-3) / time and plot graph of initial rate against iodide concentration
6. Calculate gradient of the graph and deduce the order of reaction with respect to iodide ions
7. Use this to determine the rate equation
   —> rate = k[I-][S2O8^2-]