Any chemical reaction results in the breaking of some bonds (needing energy) and the making of new ones (releasing energy). Obviously some bonds have to be broken before new ones can be made. Activation energy is involved in breaking some of the original bonds.
Where collisions are relatively gentle, there isn't enough energy available to start the bond-breaking process, and so the particles don't react.
The Maxwell-Boltzmann Distribution
Because of the key role of activation energy in deciding whether a collision will result in a reaction, it would obviously be useful to know what sort of proportion of the particles present have high enough energies to react when they collide.
In any system, the particles present will have a very wide range of energies. For gases, this can be shown on a graph called the Maxwell-Boltzmann Distribution which is a plot of the number of particles having each particular energy.
The area under the curve is a measure of the total number of particles present.
The Maxwell-Boltzmann Distribution and activation energy
Remember that for a reaction to happen, particles must collide with energies equal to or greater than the activation energy for the reaction. We can mark the activation energy on the Maxwell-Boltzmann distribution:
The large majority of the particles don't have enough energy to react when they collide. To enable them to react we either have to change the shape of the curve, or move the activation energy further to the left. This is described on other pages.
Kinetic Theory: -
Only at absolute zero are the particles in a substance completely stationary. Above zero the particles start to vibrate. Eventually they will reach a point where their energy is so great that the bonds holding them together break apart and the substance melts. As more heat is applied the particles in a liquid move around faster and faster and will eventually be given enough energy to turn into a gas.
There are a number of factors, which effect the rate of reaction
Concentration: -
For many reactions involving liquids or gases, increasing the concentration of the reactants increases the rate of reaction. In a few cases, increasing the concentration of one of the reactants may have little noticeable effect of the rate.
Cases where changing the concentration affects the rate of the reaction
The same argument applies whether the reaction involves collision between two different particles or two of the same particle.
In order for any reaction to happen, those particles must first collide. This is true whether both particles are in solution, or whether one is in solution and the other a solid. If the concentration is higher, the chances of collision are greater. When a solution is more concentrated there are more particles of solute present in the same volume of solution. The particles will therefore be closer together and more likely to collide and react. When the solution is diluted with more water the solute particles are more spaced out and so are less likely to collide and react.
Cases where changing the concentration doesn't affect the rate of the reaction
Where a catalyst is already working as fast as it can: -
Suppose you are using a small amount of a solid catalyst in a reaction, and a high enough concentration of reactant in solution so that the catalyst surface was totally cluttered up with reacting particles.
Increasing the concentration of the solution even more can't have any effect because the catalyst is already working at its maximum capacity.
In certain multi-step reactions: -
Suppose you have a reaction, which happens in a series of small steps. These steps are likely to have widely different rates - some fast, some slow.
For example, suppose two reactants A and B react together in these two stages:
The overall rate of the reaction is going to be governed by how fast A splits up to make X and Y. This is described as the rate determining step of the reaction.
If you increase the concentration of A, you will increase the chances of this step happening for reasons we've looked at above.
If you increase the concentration of B, that will undoubtedly speed up the second step, but that makes hardly any difference to the overall rate. You can picture the second step as happening so fast already that as soon as any X is formed, it is immediately pounced on by B. That second reaction is already "waiting around" for the first one to happen.
Surface Area: -
In reaction between magnesium metal and dilute hydrochloric acid. The reaction involves collision between magnesium atoms and hydrogen ions.
The equation for this reaction is:
When a solid reacts with a liquid or gas, the size of the particles of solid will affect the rate of the reaction.
Larger lumps have a smaller surface area than the same mass of smaller pieces. Powders contain small particles of solid, which have a very large surface area and can therefore react very quickly.
Temperature: -
As you increase the temperature the rate of reaction increases. As a rough approximation, for many reactions happening at around room temperature, the rate of reaction doubles for every 10°C rise in temperature.
Increasing the collision frequency: -
Particles can only react when they collide. If you heat a substance, the particles move faster and so collide more frequently. That will speed up the rate of reaction.
It turns out that the frequency of two-particle collisions in gases is proportional to the square root of the Kelvin temperature. If you increase the temperature from 293 K to 303 K (20°C to 30°C), you will increase the collision frequency by a factor of:
That's an increase of 1.7% for a 10°c rise. The rate of reaction will probably have doubled for that increase in temperature - i.e. an increase of about 100%. The effect of increasing collision frequency on the rate of the reaction is very minor.
The important effect is that as the particles have more energy when heated up, any collisions are more likely to result in the particles reacting - this has a considerable effect on the reaction rate. This is supported by the kinetic theory.
First Method: -
Apparatus 1:
The apparatus we will be using are listed as follows:
- Gas syringe
- Flask
- 6 concentrations of dilute hydrochloric acid (ranging from 0.5 to 5 molar)
-
18 pieces of 3.5cm2 magnesium ribbon
- plastic tube attached to rubber bung
- small glass measuring cylinder
- stop watch
- Measure 50ml of 1 molar hydrochloric acid with a measuring cylinder.
- Arrange the equipment as shown in the above diagram
- Add one piece of magnesium to the flask, replace the bung as quickly as possible and start the stopwatch.
- Record the volume of hydrogen at appropriate times.
- Repeat this procedure with different concentrations of acid; repeat each one three times to get accurate results.
Revised Method: -
We tried out the previous method for one experiment and we found out that it didn't work very well, it's very time consuming and we couldn't obtain accurate results. Therefore we decided to slightly alter the method.
Apparatus 2: -
- Beehive
- Trough
- Flask
- Plastic tube attached to rubber bung
- Measuring cylinder
- 5 concentrations of dilute hydrochloric acid (ranging from 1 to 4 molar)
- 12 pieces of 3.5cm magnesium ribbon.
- Stopwatch
- Small glass measuring cylinder
- Measure 50ml of 1 molar hydrochloric acid with the small glass measuring cylinder.
- Arrange the equipment as shown in the above diagram
- Add one piece of magnesium to the flask, replace the bung as quickly as possible and start the stopwatch.
- Record the volume of hydrogen at appropriate times.
- Repeat this procedure with different concentrations of acid; repeat each one three times to get accurate results.
- To make 1.5 molar acid, mix 75% (15ml) 2 molar with 25% (5ml) water. To make 4molar acid, mix 80% (16ml) 5 molar acid with 20% (4ml) of water
Safety and fairness:
For this experiment, I will make sure everything is done with safety and fairness. Throughout the whole experiment all Lab rules must be followed, to make sure the experiment is fair I will repeat each concentration three times and take the average for my graph and conclusion. The apparatus should be kept the same for each experiment to ensure all results are taken without any advantages or disadvantages. Everything in the experiment should be kept the same apart from the concentration of hydrochloric acid, which I will change for each experiment.
Results:
Evaluation: -
The results I obtained from the experiment were reliable enough to prove that my prediction was correct. The first graph clearly shows that as you increase the concentration of the acid the rate of reaction increases because as the concentration is increased the curves become steeper. This supports my prediction that if you give the particles more energy by increasing the concentration the rate of reaction increases as this allows more collisions due to a higher concentration of acid particles, as there are more collisions it's more likely for there to be more reactions.
There were however some anomalous results in my experiment, which shows that the experiment was not done as accurately as possible and there were many sources of error in my method.
We were told to drop the magnesium into the flask, replace the bung as quickly as possible and start the stopwatch at the same time. This could've been very inaccurate because the magnesium and hydrochloric acid will start reacting before the bung is replaced. This could've been improved if there was a glass wall to separate the magnesium and the acid and so all we had to do was shake the flask and they would start reacting. This would've been much more precise as the stopwatch would be started as soon as the reaction started.
Another inaccuracy was that we done the experiment on three different days and in two different labs, this might have been inaccurate because the temperature may have not been the same on different days and in different rooms. This could've been improved if we measured the temperature of the acid every time.
It may have also been more accurate if we had weighed the pieces of magnesium instead of just measuring the length of it. It would've also been better if we had increased the amount e.g. 1g.
The pressure of the hydrogen gas given off could've also been measured to make sure it was constant so it wouldn't affect the results in any way.
Overall our results were not accurate enough to make any firm conclusions, they were also quite limited as we only done four concentrations of acid, this could've also been improved if we had more time to obtain more results.
