Evaluation Page 12
Graphs, Charts and Diagrams
Scatter Graph showing Volume of Gas against time Page 5
Line Graph showing Gas given off against Time Page 6
Graph with rate calculations Page 7
Graph showing Rate against time after 20 seconds Page 8
Diagram to show Collision Theory Page 9
Raw Data Page 10
The Task
Marble buildings are corroded by acid rain. The rate of corrosion is affected by various factors, the reaction can be affected by temperature, concentration, pressure, surface area and in some circumstances light can also be a factor. We have investigated the rate of corrosion on marble chips with Hydrochloric acid to find out more about acid rain erosion.
Background Knowledge
The principle chemicals that produce acid rain are sulphur dioxide (SO2), nitrogen oxides (NO2 ) including nitric oxide and nitrogen dioxide. Natural as well as human activities are responsible for the production of these atmospheric pollutants. Natural processes include bacterial action in soils, volcanic eruptions and degassing from oceanic plankton. SO2 from volcanic eruptions can significantly affect the Earth’s atmospheric chemistry and climate. Major phases of volcanic activity in the geological past have ejected vast quantities of SO2 into the atmosphere.
The noxious effects of pollution caused by human activity, particularly the burning of coal, have been recognised for centuries, although the association with acid rain was not realised until the nineteenth century. In England, during the reign of Edward I, the use of coal washed on shore from exposed beds on the sea floor was prohibited and even punishable by death! Most atmospheric pollutants are linked to industrial processes, for example smoke from factories, SO2 and NOx from power stations, pollen from agricultural activities, dust from building construction and asbestos from insulating material. Of these the emissions from factories and power stations have been linked to acid rain. Approximately 120 million tonnes of SO2 is emitted annually around the globe from these principal sources and other industries using oil and coal as fossil fuels. The effects on the environment are amplified as a result of the traditional concentration of industry and power production in certain areas. For example, the concentration of power stations along the Trent valley in Britain has resulted in an annual emission of some 600,000 tonnes of SO2 from this area alone.
The chemical reactions that lead to acid rain begin as energy from sunlight in the form of photons which hit ozone molecules (O2) and single reactive oxygen atoms. These oxygen atoms react with water molecules (H2O) to produce electrically charged, negative, hydroxyl radicals (HO). It is these hydroxyl radicals that are responsible for oxidising sulphur dioxide and nitrogen oxide to produce sulphuric acid and nitric acid. Additionally, the formation of nitric acid can trigger further reactions, which release new hydroxyl radicals to generate more sulphuric acid. The usual way in which these acids reach the ground is in water droplets from clouds as rain. However, sulphuric acid can condense to form microscopic droplets that contribute to the many recorded acid hazes, mists and fogs experienced over urban districts. Some of the particles will settle to the ground or vegetation can absorb some of the SO2 gas directly from the atmosphere. The latter process has been described as dry deposition as distinct from true acid rain or wet deposition.
Temperature inversions, common in urban areas, can concentrate pollutants near the ground and bring about persistent smog. The London smog of December 1952 was the most infamous of recent times. It covered a radius of 30km from the centre and current estimates of the pH of the smog range from 1.4 to 1.9 (more acidic than lemon juice!). 4,000 deaths were attributed to bronchial infections resulting from this smog. One other notable acid deposition incident in Britain included an acid mist of pH2 which covered much of the Lincolnshire and Norfolk coastline on 9th September 1989 and which was caused by heavy industry and vehicle emissions in Germany and Poland. The leaves on thousands of trees were scorched overnight and aluminium implements corroded. Such incidents, however, are not unusual. Unusually high concentrations of sulphur and nitric acid have been recorded in Athens, Mexico City, Tokyo and Los Angeles on a regular basis.
Experimental Prediction
The factor we will be investigating is concentration; my prediction is that if the concentration is doubled then the rate will double this means that the two are proportional. Our secondary factor is temperature; my prediction for this is that if the temperature is doubled then the rate will double.
The idea behind my predictions is the collision theory, particles of the different substances, in this case marble and hydrochloric acid, move around and when they collide they react. There are ways to make reactions faster by using factors such as heat, pressure, concentration and catalysts. We can explain how these work with the collision theory. When you heat a reaction you give the particles more energy to move faster and so these faster moving particles are more likely to collide. With concentration you increase the number of particles as you increase the concentration and so make collisions more likely. I believe that both of these factors are proportional to the speed of reaction, this means that whatever you do to the temperature or concentration, you will do to the rate of reaction.
Safety
To be safe in the lab we removed all objects from the surface we were working on and then removed our blazers and tucked our ties inside our shirts to make sure nothing went in the experiment. We wore safety goggles at all times when anybody in the class was working and we used safety mats to protect the desks.
Factors to Vary and Control
In this investigation I will investigate the affect of concentration on the rate of reaction between hydrochloric acid and marble chips. We will vary the concentration and record the results but keep all other factors the same (temperature, pressure, light and surface area).
Range and Number of Observations
To get a fair number of observations we will take each result three times and take the average. We will use 0.25M, 0.5M, 0.75M, 1.0M, 1.5M and 2.0 Moralities to get a good range and we will take readings every 10 seconds up to 180 seconds to make sure we have all the readings we need.
Accuracy of Observations
To reach a good level of accuracy we will take readings every three times and then take an average so that if one result is out than the other two will compensate for this and make the result more accurate. The results will be accurate to two decimal places on the gas syringe and to one second on the stop clock.
Preliminary Work
In the preliminary work we investigated the size of marble chips, timings of results and concentrations of acid. The marble chips had three sizes, small, medium and large. The small chips went too fast for us to take our readings with the higher concentrations, the medium chips were good for most reactions but were a bit fast with the highest concentrations and too slow for the lowest concentrations. The large chips were the best for the higher concentrations but were useless with the lower ones so we chose to use the medium chips.
At the start of the preliminary work we agreed to take readings every 10 seconds and choose the concentrations and size of chips to fit the readings. When we chose the medium chips we saw that the most likely place for an error in our results was in the higher concentrations so we chose only a few high ones and more lower ones.
Experimental Procedure:
Equipment List:
Gas Syringe 500ml Conical Flask Measuring Cylinder
Hydrochloric acid Distilled Water Marble Chips
Electric Scales Stop Clock
To start with we cleared everything away and followed the precautions I outlined in the safety section. After this we collected together our equipment and set it up as shown in the diagram.
Next we weighed out 10g of medium marble chips and 10ml of the acid. We added the acid into the conical flask and then my partner would add the marble chips and put the bung in the flask whilst I would start the clock. He would then read out the results every 10 seconds and I would record them in the table.
We measured the results in gas given off, an alternative way of recording results would have been to have the conical flask on the scales and to record the mass loss.
