18.2 Concentration-time Graphs
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What methods of continuous monitoring can be used to measure the rate of a reaction?
• gas collection • mass loss • colorimetry
Describe a method using colorimetry, which can be used to determine the initial rate of the reaction below: I2 + CH3COCH3 ⟶ CH3COCH2I + I- + H+
• prepare standard solutions of I2, (each of different concentration) • calibrate the colorimeter using water • place a filter which has a complementary colour to the solution. e.g blue/green, since iodine solutions are orange-brown • measure the absorbance readings of each I2 solution • plot a calibration curve, (indicates absorbance readings of each concentration of I2) • carry out the reaction, taking absorbance readings at regular time intervals. • compare the absorbance readings to the calibration curve to determine the concentration of I2 at each time. • plot a graph of I2 concentration against time. • find the gradient of the tangent at t = 0, which is the rate.
What is half-life?
time taken for concentration of a reactant to half
How can you differentiate between a concentration-time graph of a 1st & 2nd order reaction?
1st order has constant half life 2nd order has increasing half life
How can you find the value of k from a 1st order concentration-time graph?
USING RATE: draw tangent @ specific concentration & find gradient. gradient = rate. sub in rate & concentration into rate equation to find k. USING HALF-LIFE: find half life. k = ln(2)/half life
What is the equation for finding k using the half life?
k = ln(2)/half life