18.2 concentration-time graphs

Question 1

how can concentration-time graphs be plotted?

Answer 1

continuous measurements taken during a reaction

Question 2

what is the word for continuous measurements being taken?

Answer 2

continuous monitoring

Question 3

what are two ways of continuous monitoring of reactions that produce a gas?

Answer 3

monitoring by gas collection
monitoring by mass loss

Question 4

how do you monitor rate with a colorimeter?

Answer 4

the wavelength of the light passing through a coloured solution is controlled using a filter
the amount of light absorbed by a solution is measured

Question 5

procedure for colorimeter

Answer 5

1) prepare standard solutions of known concentration of the coloured chemical
2) select a filter with the complementary colour of the chemical
3) zero the colorimeter with water
4) measure the absorbance readings of the standard solutions
5) plot a calibration curve of absorbance against concentration
6) carry out reaction
7) use the calibration curve to measure the concentration of chemical at each absorbance reading
8) plot a second graph of concentration of chemical against time which you can use to determine the order of reaction

Question 6

what does the gradient of a concentration-time graph show?

Answer 6

the rate of reaction

Question 7

how else can the order be deduced from a concentration-time graph?

Answer 7

the shape of the line for zero and first order reactions

Question 8

what shape is the line on a concentration-time graph for a zero order reaction?

Answer 8

produces a straight line with a negative gradient

Question 9

what shape is the line on a concentration-time graph for a first order reaction?

Answer 9

produces a downward curve with a decreasing gradient over time

Question 10

what is the half-life?

Answer 10

the time for the concentration of the reactant to halve

Question 11

what is the name for when a first order reaction has a constant half-life with the concentration halving ever half life?

Answer 11

exponential decay

Question 12

how can you work out the rate constant from the rate?

Answer 12

a tangent to the curve is drawn at a particular concentration
the gradient of the tangent gives the rate of reaction
rate constant can be calculated by rearranging rate equation

Question 13

how do you calculate the rate constant from the half-life?

Answer 13

k=ln2/t1/2