Practical investigations
Read our general notes on Risk Assessment
I am doing A2 coursework on the reaction between an acid and a metal. I have found out some rates from concentration/time graphs for the reaction between HCl and Magnesium. I have plotted the points for a rate graph, but am unsure about whether or not the line of best fit needs to go through zero. If I plot the line from zero, only one point goes through the line. But if I do not join the line to nought, and start from a bit after zero on the x axis, most points are on the line. I don’t know if the line has to go through nought!
Ulex replies
I am assuming that you have found the rate of reaction at a number of concentrations and have plotted rate (y axis) against concentration. If this is so then
Rate = k [A]n where [A] is the reactant concentration and n is the order with respect to A
It therefore follows that, if the concentration is zero, the rate must also be zero. Sadly, the graph has to pass through the origin.
You might consider the possibility that your concentrations are not what you thought they were – e.g because you didn’t check the concentration of the original solution which you diluted to obtain all the other solutions. This might explain why your best-fit line doesn’t pass through the origin. In any case, it is surely more important that the line is straight so that first order is indicated. If the line were an obvious curve, the order is not 1.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
back to Practical investigations
Rate this page or react
Share your views on this page, 1 ratings so far
, rated at 
updated: 19 January 2006
