C2bL3  Heating Up

Key Words

Accurate - measured properly and carefully with no mistakes.

Factors  - conditions that affect an experiment

Fair - keeping all factors the same except for the variable you are changing and the variable that changes as a result.

Reliable - data that are averages of at least three different tests using the same conditions.  They can be repeated.

Trial experiment - you often try out the experiment to see what happens, which gives you an idea of what measurements you can take.

 

Test Yourself

Homework
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Grade E

You will have done this experiment:

Sodium thiosulphate + hydrochloric acid  ®  Sulphur dioxide + sulphur + sodium chloride + water

 

Na2S2O3 (aq) + 2HCl (aq) ®  SO2 (g) + S (s) + 2NaCl (aq) + H2O (l)

 

The sulphur is the precipitate.

 

You find the end point by putting the flask on a piece of paper onto which a black cross is drawn.  The end point is when the cross disappears.  We can record the time taken at different temperatures:

  • The temperature is the independent variable (the factor that you change).

  • The time is the dependent variable (the factor that changes as a result).

We can then draw a graph that will look like this:

 

We find that the hotter the solution, the faster the reaction.

Grade C

This experiment is often used for an investigation.  You need to do a trial experiment (sometimes called a preliminary experiment) so that you can see:

  • How long it takes for the reaction to happen at a particular temperature;
  • What concentration the solutions should be;
  • What volumes there should be;
  • How many points you will take.

You need to consider not just how the test well be fair, but also how you will make sure that the data are accurate and reliable.

The graph above tells us:

  • As the temperature rises, the time decreases.
  • An increase of 10 oC reduces the time by 50 %.
  • Changing from 20 oC to 30 oC has more of an effect on the time than from 30 oC to 40 oC.

The results can be explained by collisions.  As it gets warmer, the frequency of collisions and the kinetic energy of each collision increase.  Therefore there are more successful collisions.

If the rate doubles with a 10 oC rise in temperature, we can say that the frequency of successful collisions is doubling too.

The graph is an idealised curve.  It is unlikely that your results will fit exactly.  Instead they will be scattered about the curve, and you will have to draw a line of best fit (not always easy).  You will need to decide if any results are anomalous (don't fit into the pattern).

Grade A

The are no notes at this level.