practical 7- paper 2+3--> measuring rates of reaction

Question 1

explain how you could use a series of experiments to determine the order of this reaction with respect to A

Answer 1

step 1: preparation
- measure known volumes of A, B and C
- use colorimeter
- into separate containers

stage 2: procedure
- start timer at the point of mixing
- take series of colorimeter readings at regular time intervals
- use the same concentration of B and C
- same temperature
- repeat with different concentrations of A

step 3: results
- plot absorbance vs time and measure the gradient and time=0
- plot plot of gradient against volumes of A
description of interpreting order from shape of 1/time vs volume or concentration graph

Question 2

how to monitor the production of gas in a rates question

Answer 2

• measure 50cm3 of 1.o mol dm-3 HCl and add to the conical flask
• set up the gas syringe in the stand
• weigh 0.20g of magnesium
• add the magnesium ribbon to the conical flask, place the bung firmly into the top of the flask and start the stopwatch
• record the volume of hydrogen gas collected every 15 seconds for 3 minutes
• plot a volume of H2 gas produced against time

Question 3

continuous monitoring method
describe how you can determine the order with respect to NaOH in the hydrolysis of methylethanoate ester
CH3COOCH3 + NaOH–> CH3COONa + CH3OH

Answer 3

step 1
- add 50cm3 of 1 moldm-3 CH3COOCH3 to a 250cm3 beaker + 50cm3 of 1 moldm-3 NaOH and start the timer
- every 30seconds, trasnfer 5cm3 sample from mixture to c lean 250ml conical flask using a 10ml measuring cylinder and teat pipette
= quench the reaction by adding 20cm3 of cold water
- titrate the sample with 1molcm-3 HCl in burette to determine the concentration of NaOH

step 2:
- plot the concentration of NaOH vs time
- draw tangents at T=30 and T= 60 seconds and calculate the gradients
- rate= gradient which allows you to determine the rate of two different concentrations

stage 3:
- comapre the change in concentration to the change in rate
- if conc is x1/2 and the rate is x1/2- then the order is 1st as the concentration is directly proportional to the rate
- if conc is x1/2 and the rate has decreased by x1/4 then it is second order as concentration squared is directly proportionate to rate
- if conc is x1/2 and rate does not change then it is 0 order