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To Determine the Enthalpy Change of a Reaction.

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TO DETERMINE THE ENTHALPY CHANGE OF A REACTION PLANNING. CaCO3 --> CaO + CO2 This is the equation for the reaction I intend to find the enthalpy change for. As this particular reaction is very difficult to measure the enthalpy change by carrying it out (the decomposition of CaCO3 needs a temperature of over 800�C), I must use another method and it uses Hess' Law. Hess' Law States: "The enthalpy change for any chemical reaction is independent of the intermediate stages, provided the initial and final conditions are the same for each route." CaCO3 CaO + CO2 CaCl2 + CO2 + H2O In simpler terms, the main point of Hess' Law is that the total enthalpy change for the indirect route of a reaction is the same as the direct route, i.e. ?H1 = ?H2- ?H3. The diagram above represents a Hess Cycle or a Thermochemical Cycle. By knowing the enthalpy changes in two parts of the cycle, it is possible to calculate the third part and complete the cycle. ...read more.


CaCO3 + 2HCl --> CaCl2 + H2O + CO2 This equation shows that 2 moles of HCl are needed for every 1 mole of CaCO3 to react. Using this information I can calculate the volume of HCl I will need in the reaction. I first need to find the number of moles of CaCO3 will be in the reaction: Number of Moles = Mass Relative Formula Mass No. of moles of CaCO3 --> 2.5g 100 --> 0.025 moles of CaCO3 Now I need to calculate the minimum amount of HCl I will need in the reaction. (Adding more would not affect the results because rate of reaction has no relevance to the experiment. All that is important in this reaction is that it is completed.) Volume = Moles Concentration Volume of HCl --> 0.05 (twice as many as CaCO3, as required) 2 --> 0.025 dm3 of HCl I now know that the quantities I require, in the first experiment, are as follows: CaCo3 - About 2.5g HCl - 50cm3 (25cm3 is minimum - any less will affect results) ...read more.


Once all of the enthalpy changes have been calculated, follow the equation ?H1= ?H2- ?H3 Risk Assessment The substances which I am using are both fairly dangerous and have the potential to cause serious damage. Hydrochloric acid can cause burns and is irritating to the respiratory system. The molarity I am using is only irritant but this is still not pleasant. The only form of protection that is really needed while using HCl is to wear safety glasses. Calcium oxide causes burns and is irritating to the eyes, skin and respiratory system. Again, only eye protection is really needed with the use of this substance. By using my plan, I expect to get accurate and consistent results as I have calculated the exact amounts of substances needed and factors which will affect results, such as heat loss. Nothing in my experiment will give inaccurate or varied results as long as the method is followed strictly, i.e. correct and precise measurements of substances. The only way the experiment can be inaccurate therefore is through human error. Sources used in devising my plan: * Haz-Cards. * Chemical Ideas, Second Edition - Burton, Holman, Lazonby, Pilling, Waddington. * Class work notes ...read more.

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