The enthalpy change of a reaction Essay Example for Free

The enthalpy change of a reaction Essay 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 800i 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. This is how I am going to carry out my experiment. For my experiment I will: 1. ) React calcium carbonate with hydrochloric acid. 2. ) React calcium oxide with hydrochloric acid. In both of these reactions I will measure the enthalpy changes by recording the starting temperature and following the reaction, the end temperature. By following this procedure, I can calculate the change in temperature and consequently the enthalpy change. The apparatus I will need for these reactions includes: 2 x Polystyrene Cups 250ml Beaker Digital Thermometer 50ml Measuring Cylinder I decided to use a polystyrene cup so I could reduce heat loss polystyrene is an insulator, whereas if I had used glass heat loss would have been a huge influence on results. I am using a beaker so that when I need to move the reactants, I do not influence any temperature change just through my body heat. Also, the thought of the solution spilling was not a pleasant one, so in case of spillage, the beaker would catch it. The digital thermometer is much more accurate than a conventional thermometer because it reads down to 1 decimal point. Safety Apparatus: Safety glasses and lab-coat. The quantities of materials I have available: HCl (2. 0 mol dm-3) : As much as required CaCO3 : Between 2. 4g and 2. 6g CaO : As much as required. In order to determine the quantities I will need to use in the experiment, I must carry out a few simple calculations. Here is the equation for the first reaction I will carry out, involving CaCO3 and HCl. 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). In the second reaction, involving Calcium Oxide and Hydrochloric acid, the equation is: CaO + 2HCl CaCl2 + H2O Again, like the first reaction, 2 moles of HCl are required to react with 1 mole of CaO. To keep things simple, I am going to use the same number of moles of CaO as CaCO3. Mass = Moles x Relative Formula Mass Mass of CaO needed = 0. 025 x 56 1. 4g Volume of HCl needed 0. 05 (twice as many as Ca0, as required) 2 0. 025 dm3 In order to carry out these experiments, the following directions should be carried out carefully. Method for reaction 1: CaCO3 and HCl. Add 2. 5g of CaCO3 to a polystyrene cup.   Place the polystyrene cup in a beaker.   Add 50cm3 of HCl to a 50ml measuring cylinder. Measure the temperature of the HCl.   Add the HCl to the CaCO3 and measure the end temperature. Once I have measured the end temperature I will compare it to the start temperature and calculate the temperature change. Method for reaction 2: CaO and HCl Add 1. 4g of CaO to a polystyrene cup. Place the polystyrene cup in a beaker.   Add 50cm3 of HCl to a 50ml measuring cylinder.   Measure the temperature of the HCl. Add the HCl to the CaO and measure the end temperature. Again, once I have measured the end temperature I will compare it to the start temperature and calculate the temperature change. Once the temperature changes have been calculated ? H needs to be found. 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 Show preview only The above preview is unformatted text This student written piece of work is one of many that can be found in our GCSE Patterns of Behaviour section.