Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: Emma on 12/01/2011 09:30:02
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Emma asked the Naked Scientists:
If you increase one reactant's concentration in an experiment, will that increase the rate of reaction?
What do you think?
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Potentially yes. If you are deficient in molecular collisions because one concentration is too low, this would increase the rate of molecular collisions and increase your reaction rate. Often though the rate of reaction stays the same unless you add a catalyst, you can also change the temperature.
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We discussed this question on our show
Dave - A reaction is normally to do with reacting two different substances together, so a type of molecule, we'll call molecule A and molecule B. If we just think about one molecule of molecule A, if you have a great big vat with only a few molecules of B in it, molecule A is going to wonder around gently throughout this huge vat, and because there's hardly any molecules of B in there, it’s almost never going to meet a molecule B. So, it’s very rarely going to react, so it’s going to have a very, very slow rate of reaction.
If you've got loads and load of molecule B, so billions and billions in there, it’s hardly going to have to travel any distance at all. So the time it takes for a single molecule of A to react is going to be much, much less. So the greater concentration of B there is, the higher the rate of reaction. Similarly of course, the more of A there are, the more times this is happening at once, so the rate of reaction is going to increase. So in general, total rate of reaction is related to the concentration of molecule A times the concentration of molecule B.
Click to visit the show page for the podcast in which this question is answered. (http://www.thenakedscientists.com/HTML/podcasts/show/2011.02.13/) Alternatively, [chapter podcast=3003 track=11.02.13/Naked_Scientists_Show_11.02.13_7913.mp3](https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.thenakedscientists.com%2FHTML%2Ftypo3conf%2Fext%2Fnaksci_podcast%2Fgnome-settings-sound.gif&hash=f2b0d108dc173aeaa367f8db2e2171bd) listen to the answer now[/chapter] or [download as MP3] (http://nakeddiscovery.com/downloads/split_individual/11.02.13/Naked_Scientists_Show_11.02.13_7913.mp3)
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Yes. You could investigate it for yourself using this method.
Rate of Reaction Investigation
We wanted to find out if the higher the concentration of a reactant, the higher the rate of reaction.
The reaction we investigated was acid reacting with magnesium. The equation for this is below:
hydrochloric acid + magnesium magnesium chloride + hydrogen
Equipment needed:
• 100 cm3 conical flask
• Stopwatch
• 25 cm3 measuring cylinder
• Hydrochloric acid at concentrations of: 0.6M, 0.8M, 1.0M, 1.2M and 1.4M
• Magnesium ribbon cut into 1cm pieces
Method:
1. Using the measuring cylinder, measure 25 cm3 of 0.6M hydrochloric acid into the conical flask.
2. Add a 1cm length of magnesium ribbon to the flask, swirl the mixture, and start the stopwatch.
3. Stop the stopwatch when the magnesium has completely reacted (i.e. completely disappeared) and record the time taken in a results table.
4. Rinse out the flask with water.
5. Repeat steps 1 – 4 with 0.8M, 1.0M, 1.2M and 1.4M of hydrochloric acid.
6. Repeat the whole investigation twice more.
You will need to keep the volume of acid the same and the starting temperature of the acid the same each time to make it a fair test.
Risk assessment:
Acid is an irritant and eye protection should be worn.
The hydrogen gas given off is flammable and should be kept away from open flames.