C2bL4  Grind up to Speed up

Key Words

Rate - how fast something goes.

Particles  - little pieces of the substance.

Surface area - the total area for the particles.

Grade E

You will have done this experiment:

Calcium carbonate + hydrochloric acid  ®  calcium chloride  + carbon dioxide + water

CaCO_{3 (s)} + 2HCl _(aq) ®  CaCl_{2 (aq)} + CO_{2 (g)} + H₂O _(l)

As carbon dioxide is a gas, we can measure the gas loss on a top pan balance.  It has to be a balance that can measure to 0.01 g (expensive).

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

We find that the smaller the chips, the faster the rate of 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 particle size;
• What volume the hydrochloric acid should be;
• What the mass of the marble chips is;
• 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:

• How much carbon dioxide is made by each size of particle in the first minute.
• The total mass of the gas is the same, as the mass of marble chips is the same.
• The gradient gives us the rate of reaction. 

Rate of reaction = mass loss ÷ time

The results can be explained by collisions.  The reaction only happens at the surface of the chips.  If we grind up the chips more finely, we have a bigger surface area for the reaction to happen.  With a large surface area there are more successful collisions

The total amount of gas produced in the end is the same.

The reaction is the fastest at the beginning because there are lots of unreacted particles to collide.  As more particles have reacted, there are fewer unreacted particles, so the chances of a successful collision are less.

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.