5.1.3: Rate-concentration graphs

Question 1

How can you use initial rates to determine the order with respect to each reactant?

Answer 1

• Carry out a reaction several times.
• Each time vary the concentration of one of the reactants
• Obtain the initial rate for each of these different concentrations.
• Plot a rate-concentration graph for each reactant.

Question 2

Why is the initial rate only considered in a clock reaction?

Answer 2

-The visual changes that occur are assumed to occur as the reaction first happens, when the rate is generally fastest.

Question 3

In clock reactions, the time taken for a visible event to occur is inversely proportional to the initial rate. What does this mean and how can this be expressed mathematically?

Answer 3

• The shorter the time taken for the change, the faster the reaction must be.
• rate (direct. prop.) 1/t

Question 4

What is a good approximation of initial rate when a graph is plotted?

Answer 4

1/t

Question 5

The reaction between sodium thiosulfate and hydrochloric acid is an example of a clock reaction

How is can the rate-conc. be drawn from this?

Answer 5

1. A cross is drawn on a piece of paper and placed under a beaker,
2. The reactants are then added together in the beaker.
3. The products form a cloudy solution as the precipitate is formed.
4. A stopwatch is used to time how long it takes for the cross under the beaker to be obscured by the solution.
5. The experiment is carried out using various conc. of HCl and Na₂S₂O _3.
6. A graph of 1/t is plotted.

Question 6

If a rate-conc. graph is first order with respect to a given reactant, how can it be used to find k?

Answer 6

K=rate/conc.

This is the same as calculating the gradient (dy/dx)