Colorimeter: Monitoring colour change/ concentration. Amount of light absorbed by solution.
Experiment Method: Prepare standard solutions of known concentrations of solution. Choose
filter with complementary colour to coloured chemical. Zero colorimeter with water. Measure
absorbance readings of standard solutions. Plot calibration curve of absorbance against
concentration of solution.
Carry out reaction. Take absorbance readings of solution at time intervals. Use calibration curve
to determine concentration of solution at each time interval. Plot second graph of concentration
of solution against time to determine order of reaction
Concentration- Time Graphs: Gradient of concentration- time graphs is rate of reaction. Can use
graph to find order if all other reactant concentrations remain constant.
Zero vvOrder: A zero order reaction produces straight line with negative gradient. Reaction rate
does not change during reaction. Value of gradient = rate constant k.
First Order: First order reaction produces downwards curve. Reaction rate
slows down with time. If successive half- lives are constant, reaction is first
order, whereas second would vary. This pattern called exponential decay.
Half-life does not vary with initial concentration.
Second Order: Also downward curve but steeper at start and tailing off
more slowly. Will not be asked to find second order in exam.
All are negative gradient here, whereas ones below are through origin and
positive gradient.
Half- Life: t1/2 is the time taken for concentration of reactant to decrease to half its original value.
Calculating Half- Life: Draw across graph where concentration is half way. Once hit curve, draw
down to find time and work out time difference. Draw successive half- life at 0.5, 0.25 etc. Units
of half- life are seconds.
Or if know just the value of rate or concentration, divide it by two find its value after a half-life.
To find half-life, t1/2, from just knowing rate and concentration use t1/2 = ln 2/ k.
Rate Constant from Graph: Draw tangent on concentration- time graph. Calculate gradient to
get rate of reaction. Sub in rate and concentration where tangent drawn at in the rate equation
to find rate constant- if tangent drawn at 0s, sub in rate and conc at 0s, NOT the conc from
gradient drawn. Gradient = change in concentration/ change in time.
Can also this equation. k = ln2/ t1/2. This is more accurate.
If asks to do find rate of reaction after 40s, draw tangent at 40s.
Learn these graphs and lines.