Bumpkin, I wrote a long post to this a couple of days ago (when I got back from skiing) in response to this, and the database had a "slight problem". So, here I try again:
The prac I would suggest involves the reaction of a solution of sodium thiosulfate (Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub>) with hydrochloric acid. The net ionic equation is:
S<sub>2</sub>O<sub>3</sub><sup>2-</sup> (aq) + 2H<sup>+</sup> (aq) ---> S (s) + SO<sub>2</sub> (g) + H<sub>2</sub>O (l)
You need a solution of sodium thiosulfate that is around the 0.2 - 0.25 molL<sup>-1</sup>, and a solution of hydrochloric acid that is around the same concentration. Get 5 identical beakers (100 mL), and prepare solutions as follows:
Beaker 1: 25.0 mL of the sodium thiosulfate solution
Beaker 2: Thoroughly mix 20.0 mL of the sodium thiosulfate solution with 5.0 mL of water
Beaker 3: Thoroughly mix 15.0 mL of the sodium thiosulfate solution with 10.0 mL of water
Beaker 2: Thoroughly mix 10.0 mL of the sodium thiosulfate solution with 15.0 mL of water
Beaker 2: Thoroughly mix 5.0 mL of the sodium thiosulfate solution with 20.0 mL of water
Each solution should be the same volume, and hence have the same depth (necessary as a control - hence the need for identical beakers).
Now, get a piece of whit paper, and draw on it a black cross. Place beaker 1 over the cross, and look down through the solution at the cross. Add into the solution 5.0 mL of the hydrochloric acid, and time (with a stopwatch) how long it takes from adding the acid until you can no longer see the cross due to the formation of the yellow sulfur solid. Repeat for beakers 2 to 5, timing in each case until you can't see the cross.
Now, since Rate is inversely proportional to time, you should find that the times get longer as the concentration of thiosulfate ions in the initial mixture decreases. A plot of concentration of thiosulfate (y-axis) against time for cross to be obscured (x-axis) whould be roughly a rectangular hyperbola. You can calculate rate as 1 / time, and plot rate (y-axis) against initial concentration of thiosulfate (x-axis) - this gaph should be linear, and the line of best fit should extrapolate through the origin. The shows that:
Rate of Reaction = k[S<sub>2</sub>O<sub>3</sub><sup>2-</sup>]
although you don't know whether k depends in any way on the concentration of acid, H<sup>+</sup>.
The effect of temperature can also be demonstrated with this reaction. Prepare four beakers with 20.0 mL of sodium thiosulfate solution and 30.0 mL of water. Place one in an ice bath, another in warm water, a third n hot water, and leave the fourth at room temperature. Allow them to satnd for a while. Then, for each one in turn, dry the outside of the beaker, measure its temperature (thermometer), and add 5.0 mL of hydrochloric acid and record time for the cross to be obscured. You will find that as temperature increases, the time taken for the cross to be obscured decreases, and you could draw a graph to illustrate the effect of temperature on rate.
