COLORFUL CHEMISTRY

Some chemical reactions are very fast and others are very slow. We want to speed up some reactions, for example the conversion of polluting gases in car exhausts to less harmful chemicals. To do this cars are fitted with a catalytic converter. These contain catalysts. Catalysts are substances that speed up chemical reactions without getting used up themselves. The catalyst is still there when the reaction has finished. This makes catalysts very useful as they can be used again and again. They are used in lots of industrial processes so that the product can be made faster. Commonly, transition metal salts are used as catalysts. They provide new routes for chemical reactions with lower energy barriers than the original reaction routes. In this experiment you will be using a cobalt (II) salt as a catalyst for a chemical reaction and observing the changes as it speeds up the reaction.

Materials

• 0.5M potassium sodium tartrate solution
• 2M hydrogen peroxide
• 0.1M solution of any cobalt (II) salt
• 250 ml beaker
• Graduated cylinders
• Bunsen burner
• Tripod and gauze

Instructions

1. Pour 10 ml potassium sodium tartrate solution into the 250 ml beaker. Click on the graduated cylinder and drag to beaker to add contents.
2. Add 10 ml hydrogen peroxide into the same beaker.
3. Slowly heat the solution to boiling.
4. Mover temperature slider to turn on and adjust the heat. Look for any signs of reaction.
5. When the solution is boiling turn off the burner.
6. Add 1 ml cobalt salt.
7. Note your observations.

1. What effect does heating usually have on the rate of a chemical reaction? Did raising the temperature make this reaction occur?
2. What happened when you added the catalyst – the cobalt (II) salt?
3. Why was the color at the start and at the end the same?

1. Reactions occur faster at higher temperatures because the reacting particles have a greater average kinetic energy. Therefore there will be more frequent collisions between particles and a greater proportion of these collisions will have energy greater than the minimum required to cause the reaction. This minimum requirement is called the activation energy. In this reaction, even boiling the solutions does not provide enough energy to cause a reaction. The activation energy is very high.
2. The reaction occurred when the catalyst (the cobalt (II) salt solution) was added. The mixture fizzed up as a gas was expelled and the solution changed color. The precise color changes will depend on the cobalt salt used. However, at least three different colors should be observed in quick succession. When the reaction has occurred, the end color is the same as that of the cobalt (II) salt solution used.
3. The color was the same at the end as at the start because the cobalt (II) salt was acting as a catalyst. A catalyst is not used up during a reaction. Although it may change during a reaction, it is reformed at the end – causing the original color to return.
4. The cobalt catalyst caused the color changes. As a catalyst, cobalt (II) provided an alternative route for the reaction with a lower activation energy. This enabled the reaction to occur. Cobalt (II) provided this alternative by taking part in the reaction and, as it did so, changing its oxidation state. Cobalt has a different color for each of its oxidation states. Each color seen was a stage in the new reaction route. The color returned to the original color at the end when cobalt returned to oxidation state II.