18.3 Rate-concentration Graphs & Initial Rates

Question 1

How can you determine the rate constant from a zero order rate-concentration graph?

Answer 1

rate constant = y intercept

y = k[A]⁰ = k

Question 2

How can you determine the rate constant from a first order rate-concentration graph?

Answer 2

gradient = rate

rate = k[A], y = mx

where m = k, since k is constant

& x = [A], since [A] is variable

Question 3

How can you determine the rate constant from a second order rate-concentration graph?

Answer 3

plot a graph of rate against concentration²

transposes curve to straight line

gradient of straight line = k

Question 4

How can you find out the order of a reaction using a clock reaction?

Answer 4

• Measure time taken for visual change to occur
  
  • E.g formation of precipitate/colour change
• Assuming rate stays fairly constant, instantaneous rate = average rate over whole reaction
  
  • Initial rate ∝ 1/t (greater value of t gives slower rate)
• Clock reaction repeated several times with different concentrations of reactants
  
  • Plot graph of concentration against 1/t, plotting a different graph for each reactant
• Order with respect to each reactant is determined based on shape of graph