A2 chemistry.

How do I solve (aiii) using half lives?

Also, can I simply determine the initial rate for both the reactions of ethyl ethanoate, make a table, use 0.2 moldm-^3 as the initial? conc for both rows and determine the order of reaction of HCl?

The mark scheme says one could use a half life ratio method too, which I hope someone can explain as well?

You use the graph to work out the order of reaction with respect to ethyl ethanoate.

Each half life is the amount of time taken for the concentration to halve. If the half lives are constant then it is first order. If they increase, its 2nd order generally but it could be 3rd order etc.

For example if the concentration at the start is 1.00, look at how long it takes to get from 1.00 to 0.50, then look at how long it takes to get from 0.50 to 0.25 etc. If these are the same then it will be first order like i said. If each next half life is twice as long, then its 2nd order.

The numbers don't have to be exact e.g. 20 mins, 21 mins, 22mins. You're reading off experimental data and off a rough graph. If they are still roughly the same, it is likely to be first order.

For aiii. I'm not sure if you even use half lives or can. You will have two curves on the graph, one where [HCl] is 0.1 and one where it is 0.2

From that look at the rates and see if doubling the concentration doubled the rate. If it did, then it would be first order, if it quadruped the rate its 2nd order etc. If it had no effect, then 0 order.

I do that by drawing a tangent at 0.20 mol dm-^3 and determining the initial rate for both the curves, then checking by how much did the increase in [HCl] increase the rate to obtain the order of HCl with respect to the reaction?