The more collisions between particles in a given time, the faster the reaction.
The acid particles can only react with the marble chips when they collide. E.g.
If this is a 1m solution of acid And this is a 2m solution. There are twice as many acid particles
in the same volume of water.
The acid particles move randomly through the water. As you increase the concentration of the acid, there are more acid particles in the same volume. Therefore there is a greater chance of acid particles colliding, and reacting, with particles on the surface of the marble. You therefore increase the rate of reaction.
Effect Of Surface Area.
As the marble chips are solid, the particle size (or surface area) is an important variable. Surface area is a measure of how much surface is exposed. If the solid reactant i.e. the marble chips, is broken down into smaller pieces - it has a greater surface area. The hydrochloric acid particles can only collide and react with the solids surface area, so the greater the surface area the more collisions occur, therefore the more successful collisions occur.
As we increase the surface area, we increase the rate of reaction.
This diagram indicates the outer surfaces where collisions with a liquid can take place. In the large lump there are fewer collisions per second leading to a slow rate of reaction. 4 smaller lumps of a solid have a larger surface area therefore more collisions can take place per second producing a faster reaction rate.
Effect Of Temperature.
This is a variable that we are unable to control as we cannot control the temperature in a school laboratory. I will measure the temperature every time I do an experiment, which should be at room temperature. I am doing this because:
As we increase the temperature, we increase the rate of reaction.
This happens because as we heat the particles they have more energy, and move around more quickly. As they travel faster, there are more collisions in a certain time. Therefore, reactions get faster as we raise the temperature.
Another reason why the rate increases could be because some colliding particles just bounce off of each other, they don’t “bang” hard enough together to start a reaction simply because they don’t have enough energy. However at higher temperatures, the particles are moving faster and they crash together harder. Therefore there are more successful collisions, which produces a reaction.
Therefore raising the temperature not only makes the particles collide more often in a certain time, it also makes it more likely that the collisions result in a successful reaction. Because there are more, effective collisions temperature has a large effect on rates of reaction. If you raise the temperature by just 10°C you could roughly double the rate of reactions.
Reaction at 30°C Reaction at 40°C
Effect Of Stirring.
My experiment also involves a liquid so stirring is also an important variable. I have decided against stirring, as it is impossible to control the rate at which stirring is occurring unless a machine is doing it. Stirring could also bring about the breaking down on the solid (i.e. the marble chips) with the glass rod, increasing the surface area and causing this to be an unfair test.
Effect Of Catalysts.
Definition: A catalyst is a substance, which speeds up a chemical reaction. At the end of the reaction the catalyst is chemically unchanged.
Catalysts increase the rate of a reaction by helping break chemical bonds in reactant molecules and provide a 'different pathway' for the reaction. This effectively means the Activation Energy is reduced.
Look at the energy level diagram for the exothermic reaction (above left), once the reaction starts it provides the energy itself to keep the reaction going. Catalysts make it easier for particles to react, so therefore the rate of reaction increases.
I have decided to leave out the involvement of a catalyst as it might interfere with the set-up of the reaction and leave me unable to tell what caused the increase or decrease in the rate of reaction (the catalyst or the concentration of Hydrochloric acid).
Effect Of Light.
Light can effect particular reactions and act as a catalyst. As well as acting as an electromagnetic wave, light can be considered as energy “bullets” (called photons), which have sufficient “impact” to break chemical bonds – in the same way a catalyst does. Light can also cause particles to gain kinetic energy. This means that the particles have more energy that required for “activation energy” so more successful collisions will occur.
Effect Of Volume.
If you have an increased amount of volume there will be an increased amount of acid particles. Therefore there will be more collisions; therefore there is a heightened chance of more successful collisions occurring.
Effect Of Mass.
The larger the mass, the more particles there are to react with. This could cause the rate of reaction to slow down dependant on the volume of acid included. If there is a small mass of marble chip and a large volume of acid then the rate of reaction will increase.
To make sure that my experiments are fair I shall try and keep all of the variables constant:
- Same mass of marble chips
- Same volume of Hydrochloric acid.
- The experiment shall be performed in the same room, with the lights on.
- No catalyst is to be used.
- I will not stir my experiments.
- I am unable to control the temperature so I will therefore perform my experiments at room temperature, measuring the temperature before every experiment.
- I will attempt to keep the surface area of the marble chips constant.
- Repeat each experiment twice and take an average. If there are anomalies, repeat and discount the original data gained.
- Have a control i.e. adding 0% or a 0.0 concentration to the calcium carbonate.
I will also:
- Clean my apparatus before launching another experiment with a different concentration.
- Ensure not to contaminate the acids by using separate pipettes for each of the acids.
- If I feel it is necessary, I will repeat the experiment to gain an average, to make my results more accurate.
- Wear safety goggles as I know from previous knowledge that the acid can fizz and spit out.
- Wear a labcoat to protect my clothing and me.
Method.
There are various different ways in which I could measure the rate of reaction for Hydrochloric acid and Calcium Carbonate.
1. Measuring the gas using a gas syringe:
2. Measuring the mass of gas given off using scales:
- Collecting the gas over water:
There is also, downward delivery, upward delivery etc.
We know that carbon dioxide is more dense than air and less dense than water; therefore we have the choice between downward delivery, a gas syringe, measuring the gas given off on scales or collecting the gay over water. I have decided to measure the gas given off over water.
In order to perform this experiment I will need the following:
- A conical flask
- A bung with a delivery tube attached
- Hydrochloric acid
- Marble chips (calcium carbonate)
- Scales
- A stop clock
- Thermometer
- Pipette
- Distilled water
- Water container
- 100cm³ to place upside down in water
- 2 x 50cm³ to make up concentrations
- Tables to record results
- Labcoat
- Safety Goggles
Preliminary Data.
Before doing the actual experiment I did a quick “test run” to make sure my experiment was practical and there wouldn’t be any faults when I did the actual experiment. I also did a preliminary experiment to work out how much Hydrochloric acid and Calcium Carbonate I would need to use.
It was impossible to take readings before 30 seconds, as I forgot to let air into the top of the measuring cylinder. This is something I have noted not to do in my actual experiment.
I have decided to use 50cm³ of Hydrochloric acid and 3g of Calcium Carbonate in the form of marble chips.
What I’m Going To Do.
- Set up the apparatus as shown in the diagram above.
- Take temperature of the room.
-
Make up the 5 concentrations I am going to use ±: 0%, 10%, 20%, 30%, 40% concentration (or 0, 0.2, 0.4, 0.6, 0.8 Moles).
- Place 10g of marble chips (with roughly the same surface area) at the bottom of the conical flask.
- Fill the container with water
- Place 100cm³ measuring cylinder under water, till the measuring cylinder is filled with water up to the 10cm³ mark*.
- Place the delivery tube in the water, and place inside the upside down measuring cylinder.
- Add the first concentration (i.e. 0% or 0 Mole i.e. distilled water)
- Place bung in the conical flask.
- Start the clock.
-
Read off measurements** every 10 seconds for 2 minutes.
- Plot a rate of reaction graph.
*This is because it is impossible to take readings before the 10cm³ mark, as there are no markings.
**The measurements should be read accurately to the nearest mm³
± With concentration it is important to mention moles. I will be given HCL 3mdl-3 as well as distilled water. As I am doing to repeat each experiment, both with 50cm³ worth of solution I will save time by doing 1 batch of 100cm³ solution and then dividing it into two. With this in mind, I will create my solutions by following this table:
The figures highlighted in the blue refer to my experiment; the other figures demonstrate how the equations I used to figure out the other information referring to my experiment.
Prediction Graph.
This diagram represents what the graph would look like if I carried out the whole experiment. I have decided the graph looks like this, as we are collecting something (i.e. carbon dioxide) rather than losing something (i.e. mass) therefore the graph is positive. There will become a point where there is no more carbon dioxide given off, therefore it will continue in a straight line, as shown above.
As I am only interested in the initial rate of reaction, it is the steeper part of the curve that I need to look at. See below left.
I will find out the gradient of the initial parts of these curves and then plot these on another graph. See above right. This graph will prove my initial theory:
As we increase the concentration, the rate of reaction increases.
