Tuesday, June 5, 2012

The aim of the experiment is to find out which factors affect the decomposition of marble buildings by acid rain. This is the rate of reaction.

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BACKGROUND KNOWLEDGE

ACID RAIN

Acidity is measured using a pH scale, with the number 7 being neutral. A substance with a pH value of less than 7 is acidic, while one of a value greater than 7 is basic. The pH scale is logarithmic; that is, a substance of pH of 6 is 10 times more acidic than another with a pH of 7. Generally, the pH of 5.6 has been used as the baseline in identifying acid rain. Acid ran is defined as any rainfall that has an acidity level beyond what is expected in non-polluted rainfall. In essence, any precipitation that has a pH value of less than 5.6 is considered to be acid precipitation. This is a visual illustration of the pH scale



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One of the main causes of acid rain is sulphur dioxide. Natural sources that emit this gas are volcanoes, sea spray, rotting vegetation and plankton. However, the burning of fossil fuels, such as coal and oil, are largely to be blamed for approximately half of the emissions of this gas in the world. When sulphur dioxide reaches the atmosphere, it oxidizes to first form a sulphate ion. It then becomes sulphuric acid as it joins with hydrogen atoms in the air and falls back down to earth. Oxidation occurs the most in clouds and especially in heavily polluted air where other compounds such as ammonia and ozone help to catalyse the reaction, converting more sulphur dioxide to sulphuric acid. However, not all of the sulphur dioxide is converted to sulphuric acid. In fact, a substantial amount can float up into the atmosphere, move over to another area and return to earth unconverted.

Acid rain has been a major ecological concern for a few decades now. Until recently, little was known about acid rain. Many studies have been made to determine the chemistry of this ecological problem. Its full-blown effects have only been realized recently. Acid rain affects a lot of things, such as lakes and aquatic ecosystems, trees and soil, the atmosphere, buildings and architecture, and even humans.

RATES OF REACTION

For a reaction to occur

i. The particles must collide with each other

ii. The collision must have enough energy

COLLISION THEORY

All substances are made of particles. Particles can be atoms, molecules or ions. In order for a chemical reaction to occur, these particles must collide. The more collisions there are between particles in a given time, the faster the reaction.

FACTORS THAT AFFECT RATES OF REACTION

· TEMPERATURE

Higher temperature means faster rate of reaction because the particles have more energy, therefore they move faster so there are a greater number of collisions. As the temperature is increased, the ions in the reactants gain more kinetic energy, and so move faster, thus, there is a greater frequency of collisions and with a greater force i.e. they move more vigorously. So, there is a greater chance the reactants will react successfully.

· CONCENTRATION OF THE REACTANTS

If the concentration of the reactants is increased, there would be a greater number of particles in the same volume and so the molecules would collide more often. So, the energy of the collisions will remain the same (as long as the temperature is kept constant), but the molecules will collide more often (as there are more of them) and so there are greater successful reactions. Therefore increasing the concentration of the reactants would increase the rate of reaction.

· PRESENCE OF A CATALYST

A catalyst is a substance that increases the speed of a chemical reaction, without being used up in the reaction. So, if the mass of a catalyst is increased up to a point, the speed of the chemical reaction will increase, and so the rate of the reaction will increase.

· THE SURFACE AREA OF REACTANTS

Greater surface area means faster rate of reaction because the particles are ‘exposed’ more to take part in collisions. If something is powdered rather than in large chunks there would be a greater frequency of collisions, and consequently the rate of reaction would be increased.



· THE PRESENCE OF LIGHT

Some chemical reactions absorb light as they take place and this leads to an increased rate of reaction.

SECONDARY SOURCES

· Notes from teacher in note book

· Chemistry for GCSE � E.N Ramsden

· WWW.Google.Com search engine for information on acid rain

PRELIMINARY WORK

The amount of carbon dioxide given off when calcium carbonate reacted in different molarities of hydrochloric acid was measured. 50cm³ of HCl was poured into a conical flask, then grams of CaCO was sprinkled in and it was shut with a rubber bung, with rubber tubing connected to a gas syringe. Then the volume of gas inside the syringe was measured every 5 seconds. The results were noted down. This experiment was repeated with small and large chips of CaCO.



RESULTS



The amount of CO given off by g of small limestone in a 50 cm solution of Hydrochloric acid

Time (s) Vol of H given off (.0 M) Vol of H given off (1.5 Mol) Vol of H given off (1.0 M)

5 4 0

10 0 5

15 15 0 7

0 0

5 6 5 1

0 4 10 14

5 40 1 16

40 48 1 18

45 54

50 60 6 5

55 67 0 7

60 74 5 0

65 81

70 87 4 6

75 5 46 40

80 100+ 50 44

85 55 46

0 57 50

5 6 5

100 66 56

105 70 58

110 74 6

115 77 65

10 80 67

15 84 70

10 86 74

15 76

140 78

145 8

150 86

155 88

160 0

165 7



The amount of CO given off by g of large limestone in a .0 Mol solution of Hydrochloric acid

Time (S) Volume of H

5 5

10 8

15 10

0 1

5 18

0 1

5 4

40 8

45 1

50 5

55 8

60 40

65 4

70 47

75 50

80 55

85 60

0 6

5 68

100 70

105 7

110 7

115 8

10 85

15 0

10 5

EXPERIMENTAL PREDICTION

I think that the rate of reaction will be proportional to the concentration. For example, as the concentration is doubled, the rate of reaction will also double. So the marble will react much quicker when the molarity of the solution is higher. Therefore the amount of carbon dioxide given off is proportional to the concentration of the solution.

FACTORS TO VARY AND CONTROL

There are five factors that I could have controlled. These are

· Surface area � Medium sized chips

· Mass � grams of CaCO

· Temperature � Room temperature

· Catalyst � None used

The factor that I have decided to change is the concentration of the hydrochloric acid. There are a number of reasons for me making this choice. The main reason is that some of the other choices would not have been possible; I cannot vary temperature, as it is dangerous to heat acid and I cannot change the surface area as it would be too inaccurate. I didn’t choose to use a catalyst, as there is no catalyst that affects this particular reaction. I didn’t change the mass of the marble chips as I thought it would be more interesting to change the concentration.

RANGE AND NUMBER OF OBSERVATIONS

I will do 5 ranges of molarities; 0.5M; 1M; 1.5M; 1.75M; M. For each of these ranges, I will do repeats concurrently and then find an average.

ACCURACY OF OBSERVATIONS

I will repeat the reaction for each molarity times and then work out the average. This means that my results will be much more accurate as any anomalies would be counted out. The readings should be quite accurate as the gas cylinders are very accurate apparatus. However they are only accurate to every degree.

EXPERIMENTAL PROCEDURE

APPARATUS LIST

· Calcium carbonate chips

· 0.5, 1.0, 1.5, 1.75, .0 Molar solutions of hydrochloric acid

· A gas syringe with a capacity of a 100 cm

· A flat-bottomed conical flask

· Pipette

· Distilled water

· Measuring cylinder

· Weighing scale to weigh the calcium carbonate chips

· Clamp stand

· Beakers

· Rubber bung tubing

· Stopwatch



METHOD

The reactant that I will vary is the concentration of the hydrochloric acid. The method of varying the concentration would be to dilute the hydrochloric acid. With hydrochloric acid, it would be possible to dilute it as it is liquid itself. In this experiment 0.5, 1.0, 1.5 and .0 molar solutions of hydrochloric acid are used. This can be done by adding parts of distilled water to the molar solution of hydrochloric acid. This must be done accurately for a fair test.

Approximately grams of calcium carbonate then should be weighed out with a weighing scale. Once both of these are measured the syringe is clamped tightly and the calcium carbonate is added to the hydrochloric acid in a conical flask. The rubber bung is then placed on quickly for the carbon dioxide not to escape.

A stopwatch is used to measure volume of carbon dioxide every 5 seconds until there is a complete reaction. This procedure is repeated 5 times for each of the molar solutions of hydrochloric acid; 0.5; 1.0; 1.5; 1.75 and .0. After the results are taken down the rates of reaction are worked out.

SAFETY

Safety glasses must be worn during the whole experiment as the hydrochloric acid is a volatile substance and spits when reacting with the calcium carbonate.



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