Rate-Concentration Graphs and Initial Rates

Question 1

What do the different orders look like on rate-concentration graphs?

Answer 1

• Zero order: Horizontal line across the graph with zero gradient.
• First order: Straight diagonal line through the origin.
• Second order: Curve upwards starting at the origin with an increasing gradient.

Question 2

What are the rate equations for the orders regarding rate-concentration graphs?

Answer 2

• Zero order: rate = k[A]^0.
• First order: rate = k[A]^1.
• Second order: rate = k[A]^2.

Question 3

What happens to the reaction rate in a rate-concentration graph when their is a zero order?

Answer 3

It does not change with increasing concentration.

Question 4

How is the first order significant on a rate-concentration graph? How can rate constant be determined?

Answer 4

The rate is directly proportional to concentration for a first order relationship, so the rate constant can be determined by measuring the gradient of the straight line of this graph.

Question 5

How can the rate constant be found using a second order on a rate-concentration graph?

Answer 5

Because it is a curve, the rate constant cannot be obtained directly from this graph. So, you must plot a second graph of the rate against the concentration squared, the result is a straight line through the origin. The gradient of this straight line graph is equal to the rate constant k.

Question 6

What is the overall process of calculating the gradient of a rate-concentration graph?

Answer 6

1. Determine the order and the rate equation from the shape of the graph.
2. Determine the rate constant from the gradient: gradient = change in y (rate) / change in x (concentration). The gradient gives the rate constant of k.

Question 7

How do you use Log-Log graphs to find the order and the rate constant?

Answer 7

log(rate) = n x log[A] + log k
y = m x x + c

Question 8

What is the initial rate? How can it be found on a concentration-time graph?

Answer 8

The instantaneous rate at the start of a reaction when the time t=0. It can be found by measuring the gradient of a tangent drawn at t=0 on a concentration-time graph.

Question 9

What is a more suitable way of obtaining the initial rate of a reaction?

Answer 9

A clock reaction as it takes a single measurement when the reaction starts and can assume the average rate of reaction over the time for a precipitate to form or a colour change will be the same as the initial rate.

Question 10

What is the initial rate proportional to?

Answer 10

1/t.

Question 11

How can you use (iodine) clocks to obtain an initial rate of reaction?

Answer 11

1. The solution is colourless at the start and the time t is measured for the blue-black colour of the starch - iodine to disappear.
2. The initial rate is proportional to 1/t.
3. A graph of 1/t (proportional to the rate) against concentration is then plotted.
4. Further series of experiments are then carried out in which the concentration of one of the other reactants is changed.
5. From the results, the order with respect to each reactant is determined and a rate equation is written. The rate constant k can then be calculated.

Question 12

How is a rate-concentration graph set up from clock reactions?

Answer 12

• The concentration of a reactant is on the x-axis.
• 1/ts-1 is on the y-axis.