How the Concentration of a Solution Affects Rates of Reaction.

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How the Concentration of a Solution Affects Rates of Reaction

In this investigation I am going to be investigating the rate of reaction between Hydrochloric Acid (HCl) and Calcium Carbonate (CaCO3).

There are several factors that affect rates of reaction, and I could investigate any of these. These factors are:

* Surface Area

* Concentration

* Temperature

* Affects from Catalysts

Surface Area:

The rate of reaction is affected by the surface area of what is being reacted; in this case the surface area of the calcium carbonate. If the particles of calcium carbonate have a greater surface area (i.e. using many small particles rather a large chunk) the reaction time will decrease; and larger particles would take longer to react.

This can be explained using the collision theory. For the reaction between hydrochloric acid and calcium carbonate to occur, the particles in the chemicals must collide with each other. The more the collisions occur, the faster the reaction.

An object with greater surface area gives a greater number of exposed particles to collide with others.

This diagram (taken from the 'Chemistry For You' textbook) shows this:

You can also further demonstrate this by putting numbers to it. Imagine having a 1000cm3 block. Each side of the cube will have an area of 100cm2. The total surface area of the cube would be 6*100 = 600cm2.

Now let's imagine that I cut this 1000 cm3 into lots of smaller 1 cm3 blocks. I would need exactly 1000/1 = 1000 of these smaller blocks to give me the same volume as the larger block. All of the 1000 smaller blocks have the same volume as the single larger block; however their collective surface area is far greater.

Each side of the smaller blocks would have a surface area of 1cm2, and therefore the total surface area of a single small cube would be 6*1 = 6cm2. There are 1000 of these smaller blocks in one single large one, so 6*1000 = 6000cm2 is the total surface area of all 100 small blocks.

The surface area of the small blocks is 100 times greater than the surface area of the single large block.

Thus, we can see that there is a much larger amount of particles exposed for collisions to happen, and therefore more chance for a reaction to occur.

[Source: Chemistry For You - Lawrie Ryan]

Concentration:

The concentration of the hydrochloric acid will also affect the rate of reaction in this investigation. An acid of a higher concentration will have a faster rate of reaction than a lower concentration.

This is once again can be explained using the collision theory.

A higher concentration of acid would have more particles of acid in a specific volume of water than a lower concentration would.

Here is a small diagram to show this:

(A) (B)

Beaker (A) (imagine as being 1 molar) has half as many acid particles in the same volume of water as beaker (B) (imagine as being 2 molar), meaning it is a lower concentration. There is also half as much chance of the acid particles hitting the marble chip in beaker (A) as in beaker (B), therefore the rate of reaction should be half as much in a 1 molar (lower) concentration as in a 2 molar (higher) concentration.

[Source: Chemistry For You - Lawrie Ryan]

Temperature:

Temperature can have a large affect on rates of reaction. As the temperature raises, reaction rates increase, and as it drops, the rates decrease.

This can partly be explained again by collision theory. As the temperature increases, the particles gain more energy than they had before. As this energy increases, the particles move faster than at lower temperatures. When the particles are travelling faster, there are a greater number of collisions, and the collisions start the reactions.
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There is also another factor, however. Some particles do not have enough energy to start a reaction, even if they are colliding with other particles, they will just bounce off. The greater energy from the temperature increase will increase the chances of a reaction happening when they collide.

[Source: Chemistry For You - Lawrie Ryan]

Catalysts:

Catalysts are substances which can speed up reactions.

So if you add a catalyst, then the rate of the reaction rises, the time taken to react drops.

All reactions have an 'activation energy', which is the energy ...

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