How does concentration affect the rate of a reaction?
Increasing the concentration of the reactants will increase the frequency of collisions between the two reactants. So this is collision theory again. Although you keep the temperature constant, kinetic theory is 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.
If reaction is between a substance in solution and a solid, you just vary the concentration of the solution. The experiment is straightforward. If the reaction is between two solutions, you have a slight problem. You might find that the rate of reaction is limited by the concentration of the weaker solution, and increasing the concentration of the other makes no difference. What you need to do is fix the concentration of one of the reactants to excess. Now you can increase the concentration of the other solution to produce an increase in the rate of the reaction.
How does surface area affect a chemical reaction?
If one of the reactants is a solid, the surface area of the solid will affect how fast the reaction goes. This is because the two types of molecule can only bump into each other at the liquid solid interface, i.e. on the surface of the solid. So the larger the surface area of the solid, the faster the reaction will be.
Smaller particles have a bigger surface area than larger particle for the same mass of solid. There is a simple way to visualize this. Take a loaf of bread and cut it into slices. Each time you cut a new slice, you get an extra surface onto which you can spread butter and jam. The thinner you cut the slices, the more slices you get and so the more butter and jam you can put on them. This is "Bread and Butter Theory". By chewing your food you increase the surface area so that digestion can go faster.
A Fair Test:
To keep the investigation a fair test I will only change one factor, the factors I will keep the same are:
- The concentration of hydrochloric acid.
- The temperature of the reactants.
- Use the same cross.
- The quantity of the reactants.
The one factor I will vary will be the concentration of the sodium thiosulphate.
Apparatus
The equipment which I will use are:
- Conical flask
- Stop watch
- Hydrochloric acid
- Water
- Goggles
- Measuring cylinder
- Sodium thiosulphate
- Black crosses
Prediction
I predicted that the higher the concentration of sodium thiosulphate the faster the rate of reaction. I believe this because the concentration of a reactant varies the rate of reaction. In my opinion if water is mixed with less concentrated sodium thiosulphate, the reactants becomes very dilute causing the reaction to be slow. I therefore will be changing the amount of sodium thiosulphate and adding water so everything will add up to. For example, of sodium thiosulphate plus 20cm of water. This will equal 50cm altogether.
Method:
I will carry my experiment as follow:
- For safety I will wear goggles.
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I will mix the following quantities of water and sodium thiosulphate for instance of sodium thiosulphate with no water added.
- Place the conical flask on the top of a black cross.
- Mix the reactants in a conical flask.
- Then, add 5 cubic centimetres of hydrochloric acid to the solution in the flask.
- Immediately start the stop watch.
- When the cross is no longer visible, stop the stop watch and record the results in the table.
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Once I have done it for 50 cubic centimetres of sodium thiosulphate and no water, mix the next two quantities, of sodium thiosulphate and 10 cm’s of water as shown in the table (step 2).
- Once all the results for the quantities shown in step 2 table had been recorded, repeat the experiment three times.
Safety:
I will be using concentrated acid, so at all times I must be very careful and I must wear goggles. If any spills onto your hands it should be washed off immediately and reported to the teacher.
My Investigation:
I chose to use sodium thiosulphate as my concentration which I will vary because my preliminary work shows that this concentration was the fastest for the rate of reaction.
Preliminary:
I have determined to use 5 different concentrations of sodium thiosulphate and water; from 50ml the highest concentration to 10ml the lowest, because during the lessons prior to the start of the investigation we did some trials to choose of which concentration the rate of reaction was fastest. I recognised that the reaction involving sodium thiosulphate and hydrochloric acid took about 6 seconds when I used the ration 50:20. So I have decided to employ this ratio for all my trials.
To carry out this investigation I used a conical flask and I poured 50ml’s of sodium thiosulphate and 5ml of hydrochloric acid then cover the cross with the conical flask and record the time it looks for the cross to disappear. I then poured 40ml’s of sodium thiosulphate and 10ml of water and 5ml of acid until I had five different concentrations. During my trial I also measure the rate of reactions using marble chips and different hydrochloric acid concentrations and then measure the time taken to collect certain amount of gas in a given time.
Hypothesis:
Rates (chemical) take place out a variety of speeds. The rates of chemical reaction is an of how much of the reactants one by being converted into a given time. The water is mixed with less concentrated sodium.
Reaction Rate= Change in amount of substance
Time Taken
During any chemical reaction the concentrations of reactants decrease and the concentrations of products increase. The rate of reaction is the change in the concentration of reactants or products with time; and this change can be determined by measuring any convenient chemical or physical property of the reaction mixture (e.g., colour intensity, electrical conductivity, mass, pH, precipitate formation, temperature, or volume of gas).
Independent variables known to affect the rates of reactions include concentration, pressure, and surface area, the presence of an inhibitor, temperature, light, and the presence of a catalyst. The effect of these particular variables on reaction rates can be explained qualitatively by the use of collision theory; i.e., particles must collide before they react, and that a reaction will only occur if the colliding particles contain more than a certain minimum amount of energy – known as the activation energy.
An increase in concentration, or in pressure (for gases), increases the number of collisions between the particles. An increase in the surface area of reactants, which are either solids or immiscible liquids, increases the number of particles exposed to collisions. An inhibitor decreases the number of collisions between the particles, effectively by reducing either the concentration or the surface area of reactants or catalyst. An increase in temperature results in an increase in the kinetic energy of the particles, so more will have the required activation energy for successful collisions. Light energy absorbed by reactants is transduced into chemical energy, so more of these particles will have the required activation energy for successful collisions. A catalyst decreases the activation energy of a reaction, so more particles will have the required energy for successful collisions.
Analysis
First I worked the average and then I plotted the marks on the graph. As the concentration of sodium thiosulphate decreases the rate of reaction increases. The increase of sodium thiosulphate decreases the speed of particles so decrease of the concentration of sodium thiosulphate increases the speed of particles meaning the rate of reaction is longer. The faster the particles move, the greater the number of collisions, and therefore the rate of the reaction decrease. The higher the concentration of sodium thiosulphate the faster the rate of reaction.
Looking at my results I can see that the higher the concentration of sodium the faster the rate of reaction. You could see that between 50ml and 20ml of sodium thiosulphate the rate of reaction increased. So it went higher and higher meaning when there was 10ml of sodium thiosulphate it took longer to react. From my results you could see that if the sodium thiosulphate decreased it took longer to react. This shows that if we continued and increased the amount of sodium thiosulphate it will react very fast and therefore the rate of reaction will decrease. The faster the particles move, the greater the number of collisions, and therefore the rate of the reaction decreases.
In my investigation I found out that the higher the concentration of sodium thiosulphate the faster the rate of reaction.
I think this because the concentration of a reactant varies the rate of reaction. In my opinion if water is mixed with less concentrated sodium thiosulphate, the reactants becomes very dilute causing the reaction to be slow.
My predictions match my results because I predicted that the higher the concentration of sodium thiosulphate the faster the rate of reaction. I worked it out by using my head. I started to think about this experiment then I thought if there is more concentrated sodium thiosulphate and less water it will react faster. The particles will collide but because there is less water the rate of reaction will be faster and quicker. If there is just a bit more concentrated sodium thiosulphate than water it will take a bit more time to react because there is nearly an equal amount of each reactant so the particles will collide together but because it’s equally the same it will take a bit longer. The concentration of sodium thiosulphate will react with water and will react not fast but not slow. This would happen if there is only an equal amount of each reactant.
Evaluating My Evidence
I believe that the experiment was successful but some of the results were unexpected or unreliable. In my graph I can see that there was one result that was anomalous. It was out of line. When I drew the line of best fit I could see that result was wrong. But that does not matter because it still increased like it was supposed but it just wasn’t enough. This was at 30ml of sodium thiosulphate. The rest of my results were accurate. I think that this anomalous occurred because I stopped the stop watch too early which I wasn’t meant too. If I had to do it again I will have stopped the stop watch at an accurate time so my results wouldn’t show any anomalous. My graph shows as the concentration of sodium thiosulphate increased the rate of reaction was faster.
I believe I carried out my investigation very well. I used all the equipment I was supposed to. I also set the experiment correctly. I managed to get most of the timings accurate. I used a stop watch to do this. I also carried out the amount of reactants correctly and accurately. The concentration of sodium thiosulphate, water and hydrochloric acid were very accurate when I measured them.
I can see from my graph that if there is less concentration of sodium thiosulphate the rate of reaction would be slow but if the concentration was higher the rate of reaction will be fast. At 50ml I could see that the time to react was extremely fast. As the concentration of sodium thiosulphate decreased it took longer to react. At 40ml it took 2.7 seconds longer to react than 50ml. Then I had to decrease the amount of sodium thiosulphate by 30ml and it took 1.6 second longer than the previous result so now it was 12.6 seconds. When I reduced the amount of sodium thiosulphate to 20ml the time it took nearly doubled. It went up by 8.4 seconds. So the result was 21 seconds. The time for the black cross to disappear started to increase making the rate of reaction slower. The next result was extremely high. The time it took previously doubled not twice but more than four times. When I measured the concentration of sodium thiosulphate it was 10ml and when I did the experiment the time it took to react was very long and slow. It took 84.6 seconds to react. The time for the cross to disappear was very exceptionally high. My graph shows a curve going down from 10ml to 50ml. Meaning the higher the concentration of sodium thiosulphate the faster it took to react so the rate of reaction was extremely fast.
If I had to do the experiment again I will probably measured the amount of the concentrated sodium thiosulphate by every 5ml and increasing to 10ml, 15ml until 50ml. So it will go up in 5’s instead of 10’s. I would also test each result five times and then work out its average rather than doing it three times. This would make my results correct and accurate. Next time I did the experiment I could investigate how the surface area or the temperature affects the rate of reaction. The reason is the surface area of the solid will affect how fast the reaction goes. This is because the two types of molecule can only bump into each other at the liquid solid interface, i.e. on the surface of the solid. So the larger the surface area of the solid, the faster the reaction will be. Smaller particles have a bigger surface area than larger particle for the same mass of solid.