[18.3] rate-concentration graphs and initial rates

Question 1

what is needed for rate-concentration graphs to be plotted?

Answer 1

measurements of rate of reaction at different concentrations

Question 2

describe the graph produced from a zero order reactant

Answer 2

• horizontal straight-line with zero gradient
• k = y intercept
• reaction rate does not change with increasing conc

Question 3

describe the graph produced from a first order reactant

Answer 3

• positive straight-line graph through origin
• k = gradient
• rate is directly proportional to conc

Question 4

describe the graph produced from a second order reactant

Answer 4

upward curve with increasing gradient

Question 5

how do you find the rate constant in a rate-concentration graph for a second order reactant?

Answer 5

• graph is a curve, so k cannot be directly obtained from this graph
• plot a second graph of rate against conc²
• this results in a straight line through the origin
• gradient of this straight line = rate constant

Question 6

what is a clock reaction used for and why is this useful?

Answer 6

• to find the initial rate of a reaction by taking a single measurement
• more convenient

Question 7

how do you measure initial rate from a clock reaction?

Answer 7

• time t from the start of an experiment to measure for a visual change to be observed eg. colour or precipitate
• average rate is assumed to be same as initial rate
• initial rate is proportional to 1/ t
• clock reaction is repeated several times with different concs and values of 1 / t are calculated for each experiment

Question 8

describe how to use an iodine clock reaction

Answer 8

• aqueous iodine is orange-brown
• starch is added
• iodine turns blue-black
• measure time from start of reaction to when colour appears