Firstly, the Kinetic Theory. This theory states that all elements and molecules have a certain amount of energy in the form of kinetic energy. This means that all particles are moving. Some particles have more energy than others and are therefore moving around faster. If you heat these particles up they will absorb the thermal energy supplied and transfer it into more kinetic energy. This means that every particle has more kinetic energy and is therefore moving even faster.
Collision theory explains why a chemical reaction will actually happen between two chemicals. When two different particles collide they can do one of two things. They can simply bounce off one another or react. They will bounce off one another if the energy of the collision is small. (The particles do not have much kinetic energy) This kind of collision is called an unsuccessful collision. If the two particles collide with a great amount of energy (the particles have lots of kinetic energy) then a reaction between the two particles will occur. This is called a successful collision. All chemical reactions have a minimum energy requirement for a successful collision to occur. This amount of energy is called the activation energy for the reaction. Different reactions have different activation energies.
If you heat up the hydrochloric acid you are giving the hydrochloric acid particles more kinetic energy. This means that the collisions between particles will be more frequent and that more of these collisions will result in a successful collision. This increases the reaction rate for two reasons:
- The number of collisions taking place will increase.
- The number of collisions where the energy of the colliding particles, exceeds the energy of activation for the reason will increase.
Cold Atoms Hot Atoms
When the temperature is increased the particles all move quicker. If they are moving faster, they are going to have more collisions.
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Concentration increases the number of collisions:
Anything that increases the number of collisions between particles will increase the rate of a reaction. Concentration is a measure of how much solute e.g. hydrochloric acid is dissolved in a given volume of water. We can change the concentration of a solution very easily by diluting with water. With increase in concentration, there will be more particles in a given volume and so more collisions will take place. The rate of reaction will therefore increase.
To understand this better, I will explain the reaction between hydrochloric acid (HCL) and marble chips (Calcium Carbonate – CaCO3) and how concentration will affect the rate of the reaction. An increase in the concentration of hydrochloric acid will cause the rate of reaction between hydrochloric acid and calcium carbonate to increase.
To understand a chemical reaction we must visualize what is occurring in the beaker. When you add to CaCO3 the HCl will collide with the CaCO3 chips. Only if the HCl collides with sufficient energy will a reaction occur. For a successful collision to occur the HCl must have a certain amount of energy called the activation energy of the reaction. Not all HCl molecules will have this amount of energy; therefore not all collisions will result in a successful collision. If you increase the concentration of the HCl then you are increasing the number of HCl molecules in the reaction mixture. This means that a greater proportion of the molecules have enough energy to result in a successful collision. Hence, concentration affects the rate of a chemical reaction. The larger the concentration, the more likely it is for the reactant particles to collide. This is the COLLISION THEORY OF CHEMICAL REACTIONS. The more particles of reactant there are in a fixed volume of solution, the more likely it is that reactant particles will collide and a reaction occurs.
The general rule for concentration is the rate doubles as the concentration doubles (they are directly proportionate).
Low concentration High concentration
(low pressure) (high pressure)
The diagram above explains that if the solution is made more concentrated it means that there are more particles of reactant knocking about between the water molecules, which makes collisions between the important particles more likely. In a gas solution, increasing the pressure (concentration) means the molecules are more squashed up together so there are going to be more collisions, resulting in increase in the reaction rate.
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Size of solid particles (or Surface area) increases collisions:
This factor only affects reactions where a solid is involved. It means that in an experiment, if one of the reactants is a solid then breaking it up into smaller pieces will increase its surface area and there will be more useful collisions and more molecules of carbon dioxide that are made.
Increasing the pressure on the reaction mixture can speed up reaction that involves gases. This then has the same effect as increasing their concentration. Doubling the pressure halves the volume and doubles the concentration.
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A Catalyst increases the number of collisions:
A catalyst is a substance, which alters the rate of reaction, without being used up and remains chemically unchanged at the end of the process. Catalysts in biological processes are called enzymes. Most of the times, catalysts are used to speed up the reaction in a solution.
Sketch of a typical Graph for Rate of Reaction:
Time
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Gradient 1 represents the original fairly slow reaction.
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Gradient 2 & 3 represent the reaction, taking place quicker but with the same initial amounts.
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In Gradient 4, the increase could be due to any of these:
- increase in temperature.
- increase in concentration or pressure.
- solid reactant crushed up into smaller bits.
- catalyst added.
Hypothesis or Prediction:
To make a prediction for my investigation I will have to ask myself the question: What will happen to the rate of the reaction if I increase the concentration of Hydrochloric Acid? The answer to the question is my prediction. The next thing to do is to explain the reasons as to why would the reaction go faster, if I increase the concentration of Hydrochloric Acid.
After studying the various factors that affect reaction rate between Hydrochloric Acid (HCL) and Calcium Carbonate, I have been able to predict my investigation as to what would happen if the concentration of HCl is different at various stages while performing my experiment. I believe that more concentrated the acid, the more gas (carbon dioxide) will be produced, and will be collected faster as the reaction will take place faster i.e. higher the concentration of acid particles, the chance for activation energy increases. The activation energy is the amount of energy required for the reactants to successfully react. The collision theory is evidence to help with my Hypothesis, knowing the solution is more concentrated, it makes the particles more reactive, I predict that with the increase in the concentration of hydrochloric acid, more carbon dioxide will be produced, and will vary with the change in the concentration.
As this happens the rate of reaction slowly decreases and levels at the same point. The key point however, is that, the higher concentration reaction will achieve this in a shorter amount of time, hence my graph will have a steeper initial gradient and will level off to 0 once the reaction stops. Hence, more diluted is the solution, the lesser is the amount of carbon dioxide gas produced. I would expect to obtain the diagram of my experiment something as shown below, (if I dilute the HCL acid with different measurements of water at each stage).
Volume of acid (cm3)
Apparatus & Safety:
Test tube
Stopper
Rubber-tubing
Syringe
Timer
Top-pan balance or mass balance
Marble chips
Dilute Hydrochloric Acid
Distilled water
2 Clamps
Safety:
Before starting with performing the experiment, we have to ensure, to follow the outlined safety procedures:
Laboratory Safety – Protective clothing
- Students must wear protective glasses or safety goggles when diluting concentrated acids, bases, or heat chemicals. There is always the danger of hot or caustic materials being splashed into the eyes.
- Students should wear strong shoes and take care of unbuttoned long sleeves and long hair.
- Students must wear protective clothing, e.g. gloves, aprons, safety goggles when chemicals are being handled. Teachers should be aware of students who wear contact lenses.
Laboratory Safety – Experimental procedures
- While performing the experiment, students should ensure to set the things in the middle of the table and not at the side because it may fall off.
- Read the label on reagent bottles twice to avoid errors.
- Always follow exactly the instructions for chemical experiments.
- Do not substitute another chemical for chemicals specified in the experiment.
- Clean up the area after use.
Method:
Since I have written my hypothesis (prediction with explanation) and outlined the apparatus and safety measures, my next step is to decide how to do the experiments.
This experiment that I will be performing is to measure the rate of reaction (amount of carbon dioxide gas produced) by using solutions of different concentrations. The effect of changing the concentration of a solution on the reaction rate can be followed by measuring the volume of gas given off over a period of time. I will be performing my experiment five times by using different volumes of solutions of hydrochloric acid of different concentrations added to the same mass of small marble chips. A typical set of apparatus for performing the experiment is shown below, with the proposed method:
Diagram:
- Take the necessary precautions to start with the experiment.
- Firstly, accurately measure on the top-pan balance marble chips separately for 5 experiments and keep aside.
- Then, attach the test-tube to one clamp and the syringe to another clamp. Next, attach the rubber tubing of the test-tube to the syringe.
- Next, in the first step, add the required amount of Hydrochloric acid into the test-tube, without adding any water. Then add a fixed amount of marble chips into it and fix the stopper immediately.
- As the reaction takes place, bubbles will start to come out of the test-tube into the measuring syringe. The inner part of the syringe will start moving, thus showing the gas being produced. Record the results in ml for the gas (carbon dioxide) collected after one minute. Now, wash the test-tube and the syringe and following the same procedure as above, repeat the experiment several times, but by using different volume of Hydrochloric acid and diluting it with a certain measure of water each time, to get a more accurate average for each concentration. Note down the results of gas given out in one minute.
- Readings of the volume of gas collected after each minute will be made until the reaction is complete for each experiment.
- When I have my results, I will put them in a table, then plot a graph to support my experiment.
Fair Test:
When we plan, we must make it a FAIR TEST. For this I will have to control all the variables I am not going to change. In my experiment, the temperature and volumes of calcium carbonate will be kept the same. The only thing I will change is the volume of the HCL acid. Each time, I will decrease the volume of HCL acid by 2cm3 and dilute it with 2cm3 of water in each reaction and this process will be repeated several times, in order to get a more accurate average for each concentration. By doing so, the line on my graph will have an initial steep gradient but eventually will level off to a flat level, once the reaction is complete. The reason why it will level off is due to the fair testing.
The next step is to get an accurate measure of the speed of a reaction. It means how would I collect and measure the gas. This is very important in order to get a fair test for my investigation.
To measure the speed of a reaction experiment, it should involve the use of a gas syringe to measure the volume of gas given off.
Experiment:
- Firstly, accurately measure on the top-pan balance 3g of marble chips separately for 5 experiments and keep aside.
- Then, attach the test-tube to one clamp and the syringe to another clamp. Next, attach the rubber tubing of the test-tube to the syringe.
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1. Add the 10cm3 of Hydrochloric acid into the test-tube. Do not add any water. Then add 3g of marble chips into it and fix the stopper immediately. Reaction will start taking place and bubbles will come out of the test-tube into the measuring syringe. The inner part of the syringe will start moving, thus showing the gas being produced. Noted that after one minute the volume of gas (carbon dioxide) produced was 85 ml. Washed the test-tube and the syringe and following the same procedure as above, for starting the second experiment.
2. Take 8 cm3 of HCL acid, dilute with 2cm3 of water and same amount of marble chips i.e. 3g and repeated the experiment. Noted the volume of carbon dioxide produced this time after one minute was 40 ml.
3. Take 6 cm3 of HCL acid, dilute with 4 cm3 water. 3g marble chips. Noted the reaction and volume of CO2 produced this time in after one minute was 32 ml.
4. Take 4cm3 of HCL acid, dilute with 6cm3 water, add 3g of marble chips and note the CO2 gas produced. It was 23 ml this time. Noted the results.
5. Take 2cm3 of HCL acid, dilute with 8cm3 water; add 3g of marble chips. Noted the reaction and CO2 produced. It was 10 ml.
- Readings of the volume of gas collected after each minute will be made until the reaction is complete for each experiment.
- I have recorded the results as shown in the table below, and plotted a graph to support my experiment.
Table of Results & Graph:
(Please see last page for the graph)
Conclusion:
What did I find in my experiment?
The experiment above purely explains the reaction between hydrochloric acid (HCL) and marble chips (Calcium Carbonate – CaCO3) and how concentration has an effect on the reaction rate. An increase in the concentration of hydrochloric acid will cause the rate of reaction between hydrochloric acid and calcium carbonate to increase.
To understand a chemical reaction we must visualize what is occurring in the beaker. When you add to CaCO3 the HCl will collide with the CaCO3 chips. Only if the HCl collides with sufficient energy will a reaction occur. For a successful collision to occur the HCl must have a certain amount of energy called the activation energy of the reaction. Not all HCl molecules will have this amount of energy; therefore not all collisions will result in a successful collision. If you increase the concentration of the HCl then you are increasing the number of HCl molecules in the reaction mixture. This means that a greater proportion of the molecules have enough energy to result in a successful collision. Hence, concentration affects the rate of a chemical reaction. The larger the concentration, the more likely it is for the reactant particles to collide, and higher will be the amount of carbon dioxide gas produced.
Scientific Explanation:
The results and the graph show that the reaction rate is fastest at the start of the reaction. The faster the reaction, the steeper is the graph. During the reaction, the rate decreases, and the slope of the graph levels off. Eventually, the reaction stops (reaction rate is 0g/min and the graph become flat. (Gradient or slope is = 0).
How does concentration affect the rate of a reaction?
This is because of the collision theory and activation energy, taking place.
Increasing the concentration of the reactant (HCL) will increase the frequency of collisions between the two reactants. So this is collision theory. Even the kinetic theory applies to this experiment when you vary the concentration. Although I kept the temperature constant, kinetic theory was relevant. This is because the molecules in the reaction mixture have a range of energy levels. When collisions occur, they do not always result in a reaction. If the two colliding molecules have sufficient energy they will react.
All chemical reactions have a minimum energy requirement for a successful collision to occur. This amount of energy is called the activation energy for the reaction. Different reactions have different activation energies.
When I increased the concentration of the HCl then I am increasing the number of HCl molecules in the reaction mixture. This means that a greater proportion of the molecules have enough energy to result in a successful collision. Hence, concentration affects the rate of a chemical reaction. The larger the concentration, the more likely it is for the reactant particles to collide, and higher is the amount of gas produced. This is the COLLISION THEORY OF CHEMICAL REACTIONS.
Explanation:
When the concentration of reactant was increased, the particles were closer together. There were more collisions so the rate of reaction increased as shown in the table and graph. The speed at which the gas is given off is a measure of the rate of reaction.
This proves that, If I double the volume of the acid I am using, I will double the volume of carbon dioxide produced in the reaction with marble chips when it has reached completion. My prediction can be scientifically argued that by increasing the volume of acid, you increase the number of particles of one of the reactants, thus increasing the product output (CO2 gas).
Referring to my prediction:
My results and graph supports my hypothesis. The results were summarized in a table; hence, I was able to compare them at a glance. On the graph the steeper the curve, the faster the rate of reaction. Also, from the graph I was able to work out the average rate of reaction for the gas produced. The higher the concentration of acid the faster the rate of the reaction and higher the amount of gas (CO2) produced.
Evaluation:
Overall, I am pleased with my experiment and results. I think my results were fairly accurate and proved my hypothesis. However, I needed to give more evidence to my prediction. This would take the form of a comparison between two or three separate sets of results, to enable get a more accurate average and be able to explain that concentration does affect the rate of reaction.
If I were to do this experiment again, I would probably make it more accurate by using a Burette instead of a syringe, to measure the amount of gas given off, because a burette is accurate to 0.1 ml. Also, the readings of the volume of gas collected after each minute should have been made until the reaction is complete for each experiment. It means, the reaction should stop in each case when the marble chips, has completely dissolved.
So, I feel that given full time for the chips to dissolve in each experiment, would have given me much more accurate measurements and show at which points the rate of reaction was at best and the curve line of reaction on the graph.
From this graph it may well be clear that one or two individual results, or a whole set of readings, may not fit very closely to the trend - anomalous results! The measurements may not be that accurate because, this experiment was done on two different days, i.e. one set of results was done on a different day when the temperature in the room may have been different. To ensure that there is place for improvement and if I had to do the experiment all over again, I will have to do the following to improve the accuracy of my results:
- Wait for final measurement until the reaction is complete and the marble chip has completely dissolved.
- I will ensure that the experiment is done on one day itself, because a change in temperature does effect the rate of reaction.
- I will ensure that the test-tube and syringe is completely dry, before I start with the new experiment.
In many reactions it is not convenient to measure concentration - instead some other property is monitored which represents the concentration directly, e.g. loss in mass, production of gas, change in color, change in pH, change in conductivity or change in pressure.
If I increase both variables, (temperature and concentration) the rate of reaction will definitely increase.
