Concentration and Reaction Rates
Concentration (of solutions)
Fact:
Surface area, concentration and pressure all have the same effect on reaction rate (an increase leads to a faster reaction rate). This is because in each case an increase in the property leads to an increase in the number of collisions in that phase of matter.
As the concentration of the reactants increases, so does the reaction rate.
Fact:
Optional Experiment: Concentration and reaction rate
Aim
To determine the effect of reactant concentration on reaction rate.
Apparatus
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Concentrated hydrochloric acid \((\text{HCl})\), magnesium ribbon
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Two beakers, two test tubes and a measuring cylinder.
Method
Warning:
Do not get hydrochloric acid (\(\text{HCl}\)) on your hands. We suggest you use gloves and safety glasses whenever handling acids and handle with care.
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When diluting a solution remember that if you want a 1:10 solution (1 part original solution in 10 parts water) measure \(\text{10}\) \(\text{cm$^{3}$}\) of water in a measuring cylinder and pour it into a beaker, then add \(\text{1}\) \(\text{cm$^{3}$}\) of the original solution to the beaker as well. \(\text{2}\) parts concentrated acid to \(\text{20}\) parts water will also be a 1:10 solution. Remember to always add the acid to the water, and not the other way around.
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Prepare a solution of 1 part acid to 10 parts water (1:10). Label a test tube A and pour \(\text{10}\) \(\text{cm$^{3}$}\) of this solution into the test tube.
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Prepare a solution of 1 part acid to 20 parts water (1:20). Label a test tube B and pour \(\text{10}\) \(\text{cm$^{3}$}\) of this solution into the test tube.
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Take two pieces of magnesium ribbon of the same length. At the same time, put one piece of magnesium ribbon into test tube A and the other into test tube B, and pay close attention to what happens.
Make sure that the magnesium ribbon is long enough so that your hand is not close to the \(\text{HCl}\).
The equation for the reaction is:
\(2\text{HCl}(\text{l}) + \text{Mg}(\text{s})\) \(\to\) \(\text{MgCl}_{2}(\text{s}) + \text{H}_{2}(\text{g})\)
Results
Write down what happened (what did you observe?) in each test tube.
Questions and discussion
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Which of the two solutions is more concentrated, the \(\text{1}\):\(\text{10}\) or \(\text{1}\):\(\text{20}\) hydrochloric acid solution?
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In which of the test tubes is the reaction faster? Suggest a reason for this.
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How can you measure the average rate of this reaction?
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Name the gas that is produced?
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Why is it important that the same length of magnesium ribbon is used for each reaction?
Conclusions
The \(\text{1}\):\(\text{10}\) solution is more concentrated and therefore this reaction proceeds faster. The greater the concentration of the reactants, the faster the average rate of the reaction. The average rate of the reaction can be measured by the rate at which the magnesium ribbon disappears.
Optional Video: Reaction of Hydrochloric Acid and Magnesium
Explanation:
The greater concentration of the reactant means that there are more particles of reactant (\(\text{HCl}\)) per unit volume of solution. Therefore the chance that \(\text{HCl}\) particles will collide with the \(\text{Mg}\) particles will be higher for the solution with the greater concentration. The number of successful collisions per unit time will be higher and so the rate of the reaction will be faster.
Optional Video: Concentration and Reaction Rates
In this project the learners should design their own experiment in the following format:
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Aim
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Apparatus
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Method
They can also perform the experiment and write up results and conclusions as well.
This experiment should focus on the effect of concentration on the rate. The easiest way to do this is to vary the concentration of the vinegar and keep the mass of baking soda constant.
Optional Project: Concentration and rate
Design an experiment to determine the effect of concentration on rate using vinegar and baking soda.
Hint: mix water and vinegar to change concentration but keep the total volume constant.
This lesson is part of:
Energy and Chemical Reactions