Practical investigations
Read our general notes on Risk Assessment
My teacher told me that the initial rate was the very first, linear section of the graph of volume of hydrogen produced against time. I collected plenty of hydrogen, up to about 600 seconds each experiment, (some were less because there was a significant temperature change). Is the initial rate the very first section of the graph? If so, does it sound right that my initial rates are found under 30 seconds?
Could I take the initial rate by finding the gradient of the graph (volume of hydrogen/time)? I think because I am finding such small initial rates, the volume that I collected was so small (for instance 2.5 cm3) that my initial rate won't be very accurate?
210208
Corrie writes........
There are two ways of using the gas volumes to get at a 'initial rates'. One way is to measure the gas volume as a function of time, as you have done, and then plot the volume-time graph and use the slope of the first (more or less linear, hopefully) part of the graph as a measure of the rate.
The other way is to select a volume of gas to collect which is less than the final volume and during the collection of which the acid concentration does not change significantly (or the temperature rise significantly) and time how long it takes to collect. Then use 1/time as proportional to rate for a series of experiments in which the acid concentration is changed. This way it is not necessary to plot a volume-time graph each time.
I can't quite follow why some of your volumes are so small. Let's say you used 0.1 g of Mg each time. Then you would expect a final volume of about 100 cm3 of hydrogen, using an excess of acid. This final volume should not change, although it may take a lot longer to collect as you dilute the acid, PROVIDED you are using an excess of acid each time despite diluting it.
Have you had a look at some of the data in the references contained in the two Re:act Tutorials on Metal + Acid Investigations - you can find them via the Practical Investigations link on the Re:act Homepage.
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, 0 ratings so far
updated: 21 February 2008
