C6 Two Rates Experiments (page 145) Flashcards
Magnesium and HCl react to produce what gas?
H2
Magnesium and HCl react to produce H2 gas, explain the process
1) Start by adding a set volume of dilute hydrochloric acid to a conical flask and carefully place on a mass balance.
2) Now add some magnesium ribbon to the acid and quickly plug the flask with cotton wool.
3) Start the stopwatch and record the mass on the balance. Take readings of the mass at regular intervals.
4) Plot the results in a table and work out the mass lost for each reading. Now you can plot a graph with some time on the x-axis and loss of mass on the y-axis.
5) Repeat with more concentrated acid solutions. Variables such as the amount of magnesium ribbon and the volume of acid used should be kept the same each time - only change the acid’s concentration. This is to make your experiment a fair test.
6) The three graphs show that a higher concentraction of acid gives a faster rate of reaction.
(see graph on page 145)
regarding the Magnesium and Hcl experiment to produce H2 gas, why do you put cotton wool in the top of the flask?
It lets the gas escape but stops the acid spitting out.
Sodium Thiosulfate and HCl produce a cloudy Precipitate, explain the process of this experiment?
1) These two chemicals are both clear solutions. They react together to form a yellow precipitate of sulfur.
2) Start by adding a set volume of dilute sodium thiosulfate to a conical flask.
3) Place the flask on a piece of paper with a black cross drown on it. Add some dilute HCl to the flask and start the stopwatch.
4) Now watch the black cross disappear through the cloudy sulfur and time how long it takes to go.
5) The reaction can be repeated with solutions of either reactant at different concentrations. (Only change the concentration of one reactant at a time though). The depth of the liquid must be kept the same each time.
6) These results show the effect of increasing the concentration of HCl on the rate of reaction, when added to an excess of sodium thiosulfate. (see diagram on page 145).
7) The higher the concentration, the quicker the reaction and therefore the less time it takes for the mark to disappear.
8) One sad thing about this reaction is that it doesn’t give a set of graphs. Well i think it’s sad all you get is a set of readings of how long it took till the mark dissappeared for each concentration.
A student carried out an experiment investigating the effect of changing the HCl concentration on the rate of reaction between HCl and Mg. State two factors that she shoould have kept constant? (2 marks)
E.g. volume of HCl added (1 mark), mass of magnesium used (1 mark)
You need to learn the methods involved i these experiments - but remember, other reactions can also be used to investigate the four factors that affect rate. You might see different experiments in your exams, or the same ones but measuring a different factor - so watch out.
