Speedy Cooking: Info Sheet

The chemical experiment that we did today used hydrochloric acid and marble chips. Marble chips are mostly made up of the chemical substance calcium carbonate, CaCO₃. Here is the stoichiometric (‘numerically’ balanced) equation for the reaction:

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

2 Hydrochloric + Calcium Carbonate Calcium Chloride + Carbon + Water

Acid (in solution) (solid marble chips) (in solution) Dioxide (gas) (liquid)

The rate of reaction is just the speed at which the substances on the left side of equation turn into the substances on the right side. The way we measured the rate of reaction today was measuring the how quickly carbon dioxide gas was produced in each experiment. For example, by comparing how much gas was produced in 1 minute with the strong acid, with how much gas was produced in 1 minute with the watered down acid, you can see that the watered down acid produced half as much gas in the same amount of time. This shows that the RATE of reaction was half as fast for the diluted (watered down) acid.

Number of Chips: The number of marble chips changes the rate of reaction because when you have more marble chips, there is more ‘stuff’ to react at one time, so there will be more gas produced in the same amount of time. If you want to make more potato soup, you must use more potatoes!

Size of Chips: If the same mass of marble chips is weighed out for bother experiments but one experiment uses larger pieces, this means that less surface area of the marble chip will be exposed to react with the acid. If we break the marble chips into smaller pieces, more marble chip surface will be exposed to react with the acid, and the rate of reaction will be faster. That is why if you want to cook potatoes more quickly, you cut the potato into smaller pieces.

Temperature: All substances are made up of millions of tiny little lumps called molecules, and these little lumps are just randomly zooming around. When we put substances together, two molecules can only react with each other if they happen to hit into each other while randomly zooming. So, an HCl molecule can only react with a CaCO3 molecule if they hit each other. If we raise the temperature, the molecules will feel more energetic, and move around more quickly. Imagine you are in a room with lots of blindfolded people. If everyone is walking, then the chance of you running into someone is low. But if everyone were to start running because they felt energetic, then the chance of bumping into someone is much higher. So, a reaction under high temperature will have a higher rate of reaction. Using the potato soup example again, the soup will cook faster on a stove at higher temperature.

Agitation: Agitation is disturbing the experiment physically, for example by stirring the reaction mixture, or shaking. This has a similar effect to raising the temperature. When you shake/stir the reaction mixture, the molecules in the mixture will be more likely to hit each other and cause the reaction to happen. So the rate of reaction will be faster if the reaction mixture is agitated. Think about cooking potatoes again: if you stir the potato soup with a spoon while you cook it, it cooks faster!

Concentration: The concentration means how many molecules of a chemical substance are ‘squished’ into one space. A high concentration means lots of molecules in a certain volume, and a low concentration means only a few molecules in the same volume. Now, we know that the rate of reaction will be fast if there is a high chance of molecules hitting each other in the reaction mixture. A high concentration is like putting lots of blindfolded people into a very small room – it’s very easy for them to bump into each other. So, the rate of reaction is much higher if the reactant concentration is high.

Conclusion: For a high rate of reaction, we should use small pieces, and if the reactant uses a liquid or a solution, then it should have a high concentration, and we should agitate the reaction mixture frequently and do the experiment at a high temperature. So, if we want to melt the chocolate very quickly, we should use a high temperature stove, and break the chocolate into very small pieces and stir it frequently.

Useful Resources:

Here are some websites and books that you can look at for more information!

http://www.gcsechemistry.com/rc.htm

http://www.wpbschoolhouse.btinternet.co.uk/page03/3_31rates.htm

http://www.chem4kids.com/reactions/time.html - This is a really fun and detailed page for all kinds of science and maths questions. Look around and browse if you want, or just click on the link for “Reactions in Chemistry” for further info on rates of reactions