They break down the rocks by reacting with, or dissolving, some of the materials in the rock. This is called chemical weathering. Rainwater is weakly acidic. This is because carbon dioxide gas dissolves in rain as it falls. The weak acid formed (carbonic acid) attacks rocks, mainly those containing calcium carbonate.
Most fossil fuels contain sulphur as an impurity. When we burn the fuel, the sulphur is oxidised. It turns into sulphur dioxide (SO 2) gas. Power stations burning coal or oil give off sulphur dioxide. This is the main cause of acid rain. The gas dissolves in rainwater, and reacts with oxygen in the air, to form sulphuric acid. Acid rain attacks buildings and metal structures. Limestone buildings are most badly affected. In my investigation, I am going to look at how Limestone might be weathered more quickly.
Limestone is a sedimentary rock, formed mainly from seashells. Chemically it is calcium carbonate (CaCO 3). Calcium carbonate can also be found in the form of marble and chalk Limestone is an important raw material and occurs naturally in many places in the UK. It thermally decomposes when heated strongly.
Calcium carbonate, in limestone and marble, reacts with acids. For the purpose of our experiment we will be using marble chips and hydrochloric acid.
Calcium carbonate + hydrochloric acid ? carbon dioxide + water + calcium chloride
Heat is given out in an exothermic reaction. We know this because the surroundings get warm. In an endothermic reaction, energy is taken in from the surroundings. The surroundings then have less energy than they started with, so the temperature falls. Making and breaking of bonds involves energy. You need to supply energy to break bonds, so bond breaking is endothermic. When new bonds are made energy is released to the surroundings, so bond making is exothermic
A chemical reaction can only occur between particles when they collide (hit each other). Particles may be atoms, ions or molecules. There is a minimum amount of energy which colliding particles need in order to react with each other. If the colliding particles have less than this minimum energy, then they just bounce off each other and no reaction occurs. This minimum energy is called the activation energy. The faster the particles are going, the more energy they have. Fast moving particles are more likely to react when they collide, as the collision may be successful in causing bonds to break. You can make particles move more quickly by heating them.
Factors affecting the rate of reaction
I have chosen to base my experiment on the effect of the concentration of acid on the rate of reaction of limestone.
If the concentration of any reactant in a solution is increased, the rate of reaction is increased. Increasing the concentration increases the probability of a collision between reactant particles because there are more of them in the same volume. Increasing the concentration of acid molecules increases the frequency at which they hit the surface of marble chips to dissolve them.
I predict that the rate of reaction will increase rapidly as the concentration of the acid is increased. At the higher concentrations the rate will level out and will not be able to react any faster. I think this will happen because there is more acid for the marble to collide with, increasing the rate. It will level out gradually because with higher concentrations there will only be the same amount of marble and the amount of acid can’t make any more difference.
To ensure safety in the lab I will be sure to wear a lab coat to protect my clothes. Acid is being used in this experiment, so I will wear protective eye goggles. Items will be removed from hazardous places where they would be likely to cause accidents. The utmost of care will be taken during this experiment for accurate results as well as for safety.
Prior to the investigation we did some preliminary experiments to decide on apparatus, timing of gas production, and chipping size.
Gas (cmï¿½) at 1M acid
We decided to use the large chippings because when we increase the concentration of the acid they will give an appropriate amount of gas within a given time.
We will use a 250cmï¿½ measuring cylinder to allow for the amounts of gas that will be produced in the higher concentrations.
We used this calculation to decide how much limestone to use. It shows that we arrived at an answer of 2g.
CaCO3 + 2HCl ? CaCl2 + H2O + CO2
1mole CaCO3 ? 1mole CO2
100g CaCO3 ? 24dm3 CO2
1g CaCO3 ? 240cm3 CO2
2g CaCO3 ? 480cm3CO2
When I do the experiment I am going to measure the rate of the reaction at 5 different concentrations so that I can demonstrate a proportional relationship between these two factors. I will also measure the factors that I hope to keep constant so that I can prove they did not change and affect the experiment
For the purpose of the experiment, we will be working in small groups of 2 or 3. Once all the necessary safety precautions have been taken, we will begin by setting up the apparatus. The water trough and measuring cylinder must be filled with water. The measuring cylinder will be placed upside down on top of the beehive stand inside the water trough. If there is any air at the top of the cylinder it will be measured and subtracted from the final result. 50cmï¿½ of hydrochloric acid will be measured using a small measuring cylinder. The acid and limestone chippings should be put in the conical flask and the delivery tube inserted immediately.
As soon as the delivery tube has been positioned in the test tube, we will slide the end under the beehive stand and begin the stopwatch. We have chosen to use a beehive stand for accuracy in reading the measuring cylinder. The other option was to tilt the measuring cylinder so that the end of the delivery tube would fit under. This might lead to misreadings and inaccurate results. A hand must be kept on the measuring cylinder at all times, otherwise it would fall off due to the pressure from the gas. We will time the reaction for a maximum of 10 minutes, or as long as it take to reach 250cmï¿½. This was found in the preliminary work to be an adequate time to collect enough gas to demonstrate the rate of reaction for a particular concentration. All measurements will be taken from the bottom of the meniscus.
My preliminary work helped me to decide the quantity of limestone to use. I needed to discover how much gas different amounts would produce, so that a measurable amount is produced. If too much gas were produced, the measuring cylinder could be full in too short a time. This would completely devastate the results. If not enough gas were produced, this would produce inaccuracy in measuring and the results would not be very reliable.
During this experiment several factors will need to be kept constant so that only the change in concentration affects the acid and limestone. So that the test is fair, gas will be collected for the same maximum time each concentration. The experiment will be carried out at room temperature throughout, because a change would affect the number of successful collisions. Concentration is the controlled variable for this experiment. It will be easy to keep the acid at constant concentration, as the acid is not topped up throughout the 10 minutes. The lab technicians in school prepare the acid. We trust them to achieve the correct concentration.
Time in minutes
Volume of gas in cmï¿½
The graph shows that the rate of reaction increased as the concentration of the acid was increased. It was lowest in 1m acid as expected, but highest in 1.75m instead of 2m.
1m acid – reacted at a speed of approximately 20cmï¿½ per minute. It went in a straight line but did not fill the entire measuring cylinder. At the end of the 10 minutes the final volume was 189cmï¿½. The results show that this concentration was the slowest of the 5 concentrations I tested. None of the results were particularly far from the line of best fit.
1.25m acid – reacted at a speed of 30cmï¿½ per minute, though it was slower in the beginning. The final volume was 221cmï¿½, which did not fill the measuring cylinder. There is an anomaly at 8 minutes where there is a slight dip in the rate of reaction.
1.5m acid – this reaction was only slightly faster than the 1.25m acid. At the end of the experiment it had only produced 7cmï¿½ more than the 1.25m acid. Their lines of best fit have almost identical slopes.
1.75m acid – reacted faster than the previous concentrations, as expected. The 5th minute result is significantly slower than the rest of the results, which fit the curve of best fit.
2m acid – started off slow but picked up to an average rate of 35cmï¿½ per minute. It was unanticipated that the 2m acid would be slower than the 1.75m acid, which happened. All of the points but the 4th minute fit a smooth curve.
Though the rate of reaction increases as the concentration of acid increases, the results are not as I expected. The 1.25m and 1.5m are suspiciously close together, which leads me to believe that they may have been the same concentration. Also 1.75m reacted faster than 2m, which should not have happened. It is doubtful that any of the acids but 1m were as they were labelled.
The rate of reaction increases as the concentration of acid increases because the particles collide more often with a greater concentration. There are more particles of acid in the same volume when the concentration is increased. Increasing the concentration of acid molecules increases the frequency at which they hit the surface of marble chips to dissolve them.
I think the method I used was fairly accurate though the results were not as expected. The few anomalous results within single graphs are not drastically far from the line/curve of best fit; the majority of points are fit the pattern.
One problem with the method was the beehive stand. Gas had a tendency to collect under it, causing inaccurate readings. This affected the results by the potential volume of gas not being in the measuring cylinder at the time it was produced.
A measuring cylinder is not very accurate. The air at the top when beginning the experiment can jeopardise the results by making reading inaccurate. Although the amount of air at the top at the start can be measured and subtracted from final results, a gas syringe would be more effective.
If I did the experiment again I would repeat the experiment a further amount of times in order to gain an accurate mean. I would also use acids mixed on different days to eliminate the possibility of acid that is not the right concentration as happened in this experiment. I think my results are sufficiently reliable to draw my conclusion from, when compared to textbook examples. To find out more about the rate of reaction of limestone, I could compare the effect of concentration with the effects of particle size and temperature.