The temperature of a compound is a measure of the energy with which it moves/vibrates. Some compounds move more than others do at the same temperature. If a compound has a high temperature it vibrates with more energy than if it was colder. If a reaction is carried out at a high temperature the particles will move with more energy and collide with more energy, as such it is more likely that the activation energy will be met during a collision.
Surface area is the area over which the reaction takes place, when dealing with larger solids it can be increased by breaking or powdering it into smaller pieces. It increases the reaction rate by increasing the number of particles exposed, which makes the likelihood of a collision higher.
Catalysts are substances that increase the rate of reaction but remain chemically unchanged; they have been discovered for many reactions and are normally transition metals or compounds of transition metals. They provide a new faster reaction. Some catalysts like powdered platinum in the reaction between oxygen and hydrogen, work by weakening the bonds between one of the reactants and lowering the activation energy.
Sulphuric acid reacts with magnesium to form magnesium sulphate and hydrogen gas, as shown in the following equation:
Mg + H2SO4 → MgSO4 + H2
Since I also know magnesium sulphate dissolves in water this means there are two ways to measure the rate of reaction, time how long it takes for the magnesium sulphate to dissolve (i.e. the ribbon dissapears) or the hydrogen given off per second. I believe the former of these would best suit my purposes as it is easier to carry out.
Variables
From my background knowledge I know that there are four main things that could affect the problem these are:
The concentration of reactants
The temperature the reaction is carried out at
The surface area of a solid reactant
Whether or not a catalyst is used
Variable to be Investigated
From this list of variables I have chosen to investigate the concentration of the acid.
Prediction
I predict that:
“As the concentration of the acid increases so too shall the rate of reaction”
The Plan
In this experiment I shall replace the statue with magnesium ribbon and the acid rain with sulphuric acid. I shall test 0.33-2.00mol/dm3 in increments of 0.33mol/dm3. I shall do this by adding 30ml of acid to 5cm of magnesium ribbon and recording the time taken for the ribbon to be corroded away.
Independent Variable – Concentration of acid (mol/dm³)
Dependent Variable – Time taken to Corrode entirely
Controlled Variable – Surface area
– Acid used
– Temperature
– Length of magnesium
– Volume of acid
Safety
The following safety precautions should be taken:
- Long hair should be tied back
- Safety goggles should be worn at all times
- Report any spillage’s or breakage’s to the teacher immediately
- Use a test tube rack for holding the test tubes during the experiment
- Wash skin thoroughly if any contact is made with acid then report it to the teacher
Apparatus
The following are needed to carry out the experiment:
- A ruler
- A pair of scissors
- Two beakers
- A stop clock
- Two measuring cylinders
- A conical flask
Method
- Cut and measure 10 strips of magnesium ribbon each 3cm long
- Mix 0cm3 of water with 30cm3 of sulphuric acid of strength 2.0mol/dm3 and place 3cm magnesium ribbon
- Start a timer as soon as the magnesium touches the acid
- Record the time takenfor the magnesium to be entirely dissolved
- Work out the average time taken for the magnesium to dissolve and record it
- Repeat these steps mixtures:25cm3 acid + 5cm3 water
20cm3 acid + 10cm3 water
15cm3 acid + 15cm3 water
10cm3 acid + 20cm3 water
5cm3 acid + 25cm3 water
-
Record your results on graphs of time for reaction against concentration of acid and rate of reaction against concentration of acid
Blank Table of Results
Predicted Graph Results
Table of Results
Interpretation
My first graph, time for reaction to occur against concentration of acid, shows a negative correlation, with the curve of best fit similar to 1/x, meaning that as the acid became more concentrated the faster the reactants reacted. For example when the acid was concentration 1.66 mol/dm3 it took 22.5 seconds for the 3cm length of magnesium ribbon to be corroded, whereas when the concentration of acid was lowered to 0.66 mol/dm3 the time for the 3cm length of magnesium ribbon to be corroded increased to 188 seconds. The point for concentration 1.00 mol/dm3 does seem to be out of place, nearly taking as long to corrode the magnesium ribbon as the less concentrated 0.66 mol/dm3 point. As such I have concluded that this is an anomalous point, possibly due to error in measuring the acid and water.
As the graph showed a curve of 1/x I also graphed the concentration of acid against the rate reaction which give me a graph with a positive correlation and a line of best fit going through the y-axis at. For example the rate of reaction for acid of concentration 0.66 mol/dm3 was only 0.013sec whereas the rate of reaction for acid of concentration 1.33 mol/dm3 was 0.033sec. Of course the point for concentration 1.00 mol/dm3 is still off the line of best fit and is still an anomalous point with a reaction rate of 0.015sec when it should be closer to 0.023sec. I will now use the formula M=(Y2-Y1)-(X2-X1), were M is the gradient and X and Y are points on the line, to calculate the gradient of the line
M=(Y2-Y1)-(X2-X1)
M=(0.053-0.013)/(2-0.66)
M=0.040/1.33
M=0.03
Since I know the gradient is 0.03 I can calculate the y-intercept and thus find the equation of the line:
y=mx+c
0.013=0.03*0.66+c
0.013=0.02+c
c=-0.007
This means the equation of the line of best fit is y=0.03x–0.007. Since it does not go through the origin it means that the concentration of acid is not directly proportional to the rate of reaction and that some other factor affects it. In this case I believe that it is due to there being more Mg molecules than H2SO4 molecules in the less concentrated solutions. This would mean that there would not be enough molecules of sulphuric acid to entirely react the magnesium into magnesium sulphate and as such the time to for the magnesium to ‘disappear’ would be theoretically infinite resulting in a rate of reaction of zero.
Conclusion
What these graphs show us is that as the concentration of the acid rain goes up so too does the speed with which magnesium and magnesium alloys corrode. This affects the rate of reaction because the more concentrated a reactant is, the more particles there are that can take part in the reaction. This means there shall be a greater number of collisions per second between the reactants, and the more collisions there are the more likely it is that they will collide with enough energy and in the right way to react. Another factor, which could contribute to more acidic acid rain at this time of year, is the increase of traffic on the roads, again due to the colder weather, since it is too cold to walk places and more people use cars. When the cars fuel is combusted it also gives off some sulphur dioxide which adds to the pollution. The acid rain may also take the form of snow or fog.
This relates to the problem because it means that one of the possible reasons it corrodes more at different times of the year is due to more acidic acid rain being produced, which would be caused by increased air pollution from the burning of fossil fuels. The time of year this is most likely to happen is winter when, due to the decrease in temperature, more fuel is burned to produce heat energy to warm houses which means more SO2 emissions and as such more concentrated acid in acid rain.
Acid rain is currently a subject of great controversy because of widespread environmental damage for which it has also been blamed, injuring crops and forests, and threatening or depleting life in freshwater lakes. However the 1990 amendments to the Clean Air Act of 1967 put in place regulations to reduce the release of sulphur dioxide from power plants to 10 million tons per year by January 1, 2000. This amount is about one-half the emissions of 1990.
However I also know that if a compound has a high temperature it vibrates with more energy than if it was colder. If a reaction is carried out at a high temperature the particles will move with more energy and collide with more energy, as such it is more likely that the activation energy will be met during a collision. Increasing the rate of reaction. This means that there are other factors that should be investigated however if the statue is galvanised with zinc, which is more reactive than magnesium, it would still reduce the corrosion of the statue because the sulphuric acid will react with that instead of the magnesium, this is called sacrificial protection.
Evaluation
These results support my hypothesis which was:
“As the concentration of the acid increases so too shall the rate of reaction”
My second graph shows this to be the case with a line of best fit going straight through the origin. The reason the rate of reaction increases with concentration is because the more concentrated a reactant is, the more particles there are that can take part in the reaction. This means there shall be a greater number of collisions per second between the reactants, and the more collisions there are the more often there shall be a successful collision, so the reactants will react faster.
My results were fairly accurate with all but one point near the line of best fit on each graph. Since both graphs are really showing the same information it will only be necessary to work out the percentage of accuracy there is in my results. I will use the last point on the second graph as it is furthest from the line of best fit. The actual point was 0.044 for rate of reaction when it should have been nearer 0.039. So:
(0.039/0.044)*100=Percentage of accuracy
0.886*100=88.6%
As such I am 88.6% certain of my results.
If I was doing the experiment again I would record three reactions for each concentration of acid to try and get a better average, and I would also use a burette to measure volume of water and sulphuric acid in each solution to improve the accuracy of my results. I would also increase the volume of acid used to 60ml and see if this causes the line of best fit on the second graph to be closer to the origin.