Topic:
Rate of reaction Objective :
To show the effect of concentration on the rate of reaction. Introduction :
The rate of a reaction is the speed at which a reaction r eaction happens. If a reaction has a low rate, that means the molecules combine at a slower speed than a reaction with a high rate. Some reactions take hundreds, maybe even thousands, thousands, of years while others can happen in less than one second. The rate of reaction r eaction depends on the type of molecules that are combining. If you want to think of a very slow reaction, think about how long it took dinosaur bones to become fossils through breakdown. You can thank chemical processes in bacteria for most of those dinosaur bones in the museum. There is another big idea for rates of reaction called collision theory. The collision theory says that as more collisions in a system occur, there will be more combinations of molecules bouncing into each other. If there are a higher number of collisions in a system, more combinations of molecules can occur. occur. The reaction will go faster and the rate r ate of that reaction will be higher. Even though they are both liquids, think about how slowly molecules move in honey when compared to your soda. There are a lower number of collisions in the honey. Reactions happen - no matter what. Chemicals are always combining or breaking down. The reactions happen over over and over, but not always at the t he same speed. A few things affect the overall speed of the reaction and the number of collisions that can occur. Concentration: If there is more of a substance in a system, there is a greater chance that molecules will collide and speed up the rate of the reaction. If t here is less of something, there will be fewer collisions and the reaction will probably happen at a slower speed. Sometimes when you are in a chemistry lab, you will add one solution to another. When you want the rate of reaction to be slower, you will add only a few drops at a time instead of the entire beaker. Temperature: When you raise the temperature of a system, the molecules bounce around a lot more because they have more energy. When they bounce around more, they are more likely
to collide. That fact means they are also more likely to combine. When you lower the temperature, the molecules are slower and collide less. That temperature drop lowers the rate of the reaction. Back to the chemistry lab! Sometimes you will mix solutions in ice so that the temperature of the system stays cold and the rate of reaction is slower. Pressure: Pressure affects the rate of reaction, especially when you look at gases. When you increase the pressure, the molecules have less space in which t hey can move. That greater density of molecules increases the number of collisions. When you decrease the pressure, molecules don't hit each other as often. The lower pressure decreases the rate of reaction.
Materials And Apparatus :
1.
Stop watch
2.
10 ml measuring cylinder
3.
50ml measuring cylinder
4.
White paper
5.
2M hydrochloric acid
6.
0.25M sodium thiosulphate ( Na2S2O3)
7.
250ml conical flask
8.
Distilled water
Diagram :
Procedure :
1. 50ml of sodium thiosulphate solution is poured into a 250ml conical flask. 2. 5ml of the hydrochloric acid is added to the solution in the flask using 10ml measuring cylinder and the stopwatch is started at the same time. 3. The flask is swirled for one or twice. 4. The flask is placed over a piece of white paper with a dark cross-marked on it. 5. Looked vertically onto the cross and the time taken for the cross to disappear is recorded. 6. The experiment is repeated using 40, 30, 20, and 10ml of the sodium thiosulphate solution made up each time to a total volume of 50ml with distilled water, still using 5ml of hydrochloric acid 7. The time taken for the cross to disappear is taken consistently. 8. Two graphs were plotted with the concentration of the sodium thiosulphate solution against time and 1/time.
Results :
Experiment
1
2
3
4
5
Volume of sodium thiosulphate (ml)
50
40
30
20
10
Volume of water (ml)
0
10
20
30
40
Volume of hydrochloric acid(ml)
5
5
5
5
5
Concentration of sodium thiosulphate(M)
0.25
0.20
0.15
0.10
0.05
Time taken(s)
13
17
24
30
80
1/time (s-1)
0.077
0.059
0.042
0.033
0.016
Discussion :
In this experiment, sodium thiosulphate (Na2S2O3) is the source of the thiosulphate ions, and hydrochloric acid will be used as the source of hydrogen ions. The chemical reaction between sodium thiosulphate and hydrochloric acid results in sulphur being precipitated slowly.
The equation is as below, Na2S2O3(aq) + 2HCL(aq)
2NaCl(aq) + SO2 (aq) + H2O(l) + S(s)
Sulfur dioxide is a gas at room temperature, but is very soluble in water. Sulfur is a water insoluble solid. As a result, the solution first becomes cloudy and then opaque. In this reaction, sulphur was formed a very find solid particle and causes the solution to change from colourless to yellowish cloudy. The time taken for a certain quantity of sulphur to form and cause the „X‟ mark to disappear is used to determine the rate of reaction. 1 Rate of reaction ∝
time taken for the mark ‘X’ to disappear
The exact amount of sulphur is not measured but I assumed that quantity of sulphur will be produced in order to obscure the „X‟ mark on the filter paper below the conical flask is the same.
Question 1. What do the graph of concentration against time and concentration against 1/time indicate about the effect of concentration on a rate of reaction? Ans: From the plotted graph of rate of reaction versus concentration of sodium thiosulphate, the concentration of the solution will be directly proportional to (1/time), and the concentration of the solution will be decreasing linearly with (time). 2. Write an equation for the reaction used in this experiment. Ans: Na2S2O3(aq) + 2HCL(aq)
2NaCl(aq) + SO2 (aq) + H2O(l) + S(s)
3. What other solution can you use to replace the sodium thiosulphate to conduct the same experiment? Ans: Aqueous silver nitrate solution can be use to replace sodium thiosulphate solution.
Precautions :
1. Do not swirl the conical flask more than two times. 2. Wear rubber gloves and lab coat throughout the experiment. 3. Look perpendicular at the conical flask to observe when the mark ”X” disa ppear 4. Make sure all the apparatus are clean and dry before adding sodium thiosulphate.
Conclusion:
The aim for this investigation is to investigate what is the effect of the concentration of sodium thiosulphate (Na2S2O3) on the rate of reaction with Hydrochloric Acid (HCl). The experiment concludes than“when the concentration of sodium thiosulphate incr easing, the rate of reaction will increase, the less time taken for the “X mark to disappear.” A graph is plotted with of rate of reaction versus concentration of sodium thiosulphate, the time will decrease when the concentration was increased. The results obtained from the plotted graph of rate of reaction versus concentration of sodium thiosulphate, shows that “it directly proportional” is proven true because the relationship was linear because the graph shows a straight line.
References :
1. Lab manual 2. http://en.wikipedia.org/wiki/Reaction_rate 3. http://www.chm.davidson.edu/vce/kinetics/RateOfReaction.html 4. http://www.chem4kids.com/files/react_rates.html 5. http://amrita.vlab.co.in/?sub=2&brch=191&sim=345&cnt=1
