Practical investigations
Read our general notes on Risk Assessment
Could you help me with my coursework please...basically for the second experiment I'm finding the Ea between magnesium and HCl, H2SO4, ethanoic acid and phosphoric acid. But what I don't understand is why I'm doing that. Also..how am I supposed to find out the concentrations to use and volumes of each acid, and the mass of Mg ribbon to use..and the temperatures to use?
I really don't get what I'm supposed to do..and what to measure...I think I'm supposed to measure the temperature, total time taken for the reaction to complete...and then plot a graph of Ln(rate) against 1/Temp in Kelvins... Please is there any way you can help??
211208
Ulex replies
OK, let’s take one thing at a time!
Ea, as you know, is the activation energy for a reaction. This is related to the energy input into the slowest step in the reaction. In the case of a reaction involving an acid, you might expect that Ea would have something to do with how strong the acid is. The selection of acids you mention includes two strong acids, hydrochloric and sulphuric, and two weaker ones, phosphoric and ethanoic. You are basically finding out whether there is any such relationship between Ea and strength.
What you need to do is to perform a series of measurements with each acid in turn, keeping everything constant except the temperature. Begin with the acid you used in the first experiments. Choose the conditions used in the slowest experiment in the series – at higher temperatures the rate increases dramatically, so start with a slow one. Measure the temperature of the acid and do a fresh measurement of the time of the reaction at this temperature.
Now repeat the measurement with fresh materials but keep the volume of acid, the quantity of magnesium, the concentration of the acid constant but, before you put the magnesium into the acid, raise the temperature of the acid by about 10o C. Go on repeating the experiment at higher and higher temperatures. You may find that reaction times get very short above 60°C.
(TIPS: don’t try to get the temperature exactly on a particular value; get it about right and then record the actual value. Secondly, stir the mixture during the reaction – funny things happen to the rate at higher temperatures if you don’t, because of hydrogen bubbles getting in the way between the acid and the metal. Consider taking two readings of temperature for each experiment, one before adding the metal, the other immediately the reaction is over; the idea of this is to check that the temperature hasn’t changed during the reaction; take an average value if it has varied.)
Now do the whole thing again with one of the other acids, keeping the volume, etc conditions exactly the same as for the first acid. Repeat for the remaining acids.
You plot the graph for each acid as you said in your question. It should be a straight line – draw the best one you can, The gradient of this line is equal to Ea/R from which you can calculate Ea.
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: 23 December 2008
