A Investigation into the Rate of Chemical Weathering of Marble

Authors Avatar
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. When you warm a substance its particles gain more kinetic energy and their average velocity increases. Therefore reactions are more likely to take place at higher temperatures.

Temperature is not the only variable to consider in the investigation. There are many others:

* Type of Acid - An acid is an acid because it produces H (hydrogen ions) in solution. The acids that are available are; hydrochloric acid, nitric acid, sulphuric acid, citric acid and ethanoic acid. Some of these are strong acids which means that they completely split up into ions to give the highest possible concentration of H+ ions. Others are weak acids and do not completely split up into ions. The effect of varying the acid would show which acids are most destructive on the marble.

* Concentration of Acid - The concentration of an acid is usually quoted in 'moles per litre'. The hydrochloric acid in the in the lab is 2mols/litre i.e. 2mols HCl (73g) per litre of acid solution. The other acids vary in their concentration, for example, sulphuric acid is quoted at 1mole/litre.

The effect of varying the concentration of one of these acids, by diluting it further with distilled water to create a range of concentrations, would display whether or not the concentration of the acid effects the rate of chemical weathering of marble. Altering the concentration of the acid can affect the rate of reaction because, in theory, increasing the H+ ion concentration will increase the frequency of collision. The table below illustrates how an experiment could be performed using this variable:

Acid (ml)

Water (ml)

Total Vol (ml)

Acid %

30

0

30

00

20

0

30

67

0

20

30

33

* Temperature of the Acid - The temperature of the acid could be lowered by adding ice to the solution or increased by heating it over a flame. It is important to remember that 20 C is not twice as hot as 10 C so the particles do not necessarily move twice as fast. This is a scientific experiment so the best scale to use would be the Kelvin Scale. If the temperature were varied it would show whether or not temperature has an effect on the rate of chemical weathering of marble.
Join now!


* Surface Area of Marble - This is the most difficult variable to control because to compare the effect of varying surface area it would be necessary to estimate the surface area involved. The only scale that can be used is small/medium/large and this is highly inaccurate because one person's opinion may be different from another and the marble chips available are generally very similar sizes, there is only a few millimetres difference between 'small' and 'large'.

Using smaller pieces of marble would increase the reaction rate because this increases the total surface area which is where ...

This is a preview of the whole essay