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A Investigation into the Rate of Chemical Weathering of Marble

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Introduction

Georgia Smith 11V January 2002 Chemistry Coursework A Investigation into the Rate of Chemical Weathering of Marble Planning Chemical weathering is where acids in the atmosphere weather away certain materials, such as marble. The acids often come from pollution like car fumes and smoke. This acid is absorbed into the atmosphere and then, as part of the weather cycle, it is 'rained' back down and erodes the surfaces of materials. The acid involved in chemical weathering can slowly weather away at statues and buildings. Rate is a measure of how fast or slow something is. Rate of reaction describes how fast or slow a chemical reaction is. For chemists who are making a product, it is important to know how long the reaction takes to complete before the product is produced. Rate is a measure of a change that happens over a single unit time. This unit of time is most often a second, a minute or an hour. This investigation is going to be carried out in the chemistry lab so it is essential to imitate the process of chemical weathering in the most accurate way possible. The particles which actually react with the marble are the H+ ions, these reactant ions originally come from hydrochloric acid. The best way to imitate the chemical weathering of marble is by using acid to replicate acid rain. Metal Carbonate + Acid ----- Metal Salt + Water + Carbon Dioxide CaCO + 2HCl ----- CaCl + H O + CO This experiment is based on the Collision Theory. Reactions occur when particles in the reacting solid, liquid or gases collide with each other. Here, the reaction occurs when the acid particles collide with the calcium carbonate particles in the marble. Not all the collisions result in a reaction; sometimes the particles just bounce off each other. For the H+ ions to react they must have a minimum amount of energy called the activation energy. The molecules' energy comes from the kinetic (movement) energy they have. ...read more.

Middle

11 x 2 = 22 (20) 20 x 2 = 40 (38) 33 x 2 = 66 (60) This illustrates why I predict that by doubling the acid (H+) concentration the frequency of collision will double and therefore the rate of reaction between the H+ ions and the marble chips will double. When the acid is more concentrated there are more particles of the acid present in the same volume. This means that particles will be closer together and more likely to collide with the marble chips and react. When more water is added, the acid becomes more dilute. Acid particles become more spaced out, and are less likely to react. The effect of concentration on reaction rate: Particles are far apart and are less likely to meet and react. More particles are present in the same volume, so they are closer together and more likely to meet and react. Method 1. Firstly I will measure out five lots of 3g of medium sized marble chips using the electronic balance for accuracy. I will ensure that there are roughly 13 to 14 chips in each lot so that they all have about the same surface area. 2. Next I will measure out the different concentrations of hydrochloric acid and distilled water solution using the measuring cylinder, each to exactly 60ml of solution in total. It is necessary to use distilled water because tap water may have other chemicals or an acidic trace in it which would effect the results of the experiment. 3. I will then put a set of the marble chips into the bottom of the conical flask which is attached to the gas syringe by a rubber stopper and a rubber tube. Before I actually begin the experiment it is important to push all of the air out of the gas syringe otherwise it will not begin from the actual starting point. ...read more.

Conclusion

It was impossible to stop some of the CO escaping from the conical flask between the acid hitting the marble chips and putting the rubber bung into the top of the flask - so readings at the beginning may have been slightly inaccurate. Despite these potential inaccuracies in my method, my results a very useful because they still indicate a clear pattern from which I was able to confidently draw a conclusion, which concurred with the main part of my prediction. The method I used was simple to execute, could be carried out within the time available and produced results of reasonable accuracy. The quality of my results could be improved by doing more extensive experiments and investigating the effect of proportionality between the surface area and the volume of solution. A larger mass of chips could be used with a greater volume of solution so more variations could be made with the experiment. Some of the variables were very difficult to control; like the surface area of the chips and the temperature. I feel that I did all that was possible to make these variables consistent though. The chips were weighed accurately and also counted to insure there was roughly the same number in each experiment - this could be made more reliable by using chips specially made to have the same surface area e.g. balls of CaCO . I could also have taken the temperature of the solution at the beginning of each experiment so I could know whether or not it was having any effect on the reaction, or even put everything into a water bath to sustain the same temperature. The experiments should ideally be carried out on the same day if they are going to be averaged because conditions and equipment should always be the same to make the investigation as fair as possible. To gain more reliable results my results could be pooled with groups who did the same experiment so there is a greater number to gain an average from, but all the experiments included must be completed to the same standard of fairness and accuracy. ...read more.

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