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# enthalpy change

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Introduction

Determining the Enthalpy change for the thermal decomposition of Calcium Carbonate The aim of this investigation is to determine the enthalpy change for the following reaction: CaCO3 � CaO + CO2 (s) (s) (g) The above reaction takes place when calcium carbonate undergoes thermal decomposition. I will calculate the enthalpy change of the reaction by an indirect method based on Hess's law. Enthalpy change ?H is the measure of heat change at a fixed pressure and temperature. It is known as the standard molar enthalpy change of a reaction and is measured in kilojoules per mole. Hess's law states that the total enthalpy change for a chemical reaction is the same, whatever route is taken, provided that the initial and final conditions remain constant. The diagram below outlines how I expect to calculate the enthalpy change of the reaction using an indirect method. ?H3 CaCO3(s) CaO(s) + CO2(g) HCl HCl ?H1 ?H2 CaCl2 (aq) Calcium oxide and calcium carbonate both react readily with 2mol dm? � hydrochloric acid solutions. The temperature changes during these two reactions can be measured and the enthalpy changes ?H1 and ?H2 can be calculated. ?H3 can be calculated using the Hess's cycle above as ?H1- ?H2 = ?H3 Below are the results tables that I recorded after carrying out the procedure for both calcium carbonate and calcium oxide. ...read more.

Middle

Number of Moles = Mass of Element (g) / Relative Atomic Mass of Element = 2.6 / 100.1 = 0.025974025 Here the mass of the element is 2.6 and relative atomic mass is the sum of the relative atomic masses of each of the elements of which calcium carbonate is composed of. Relative atomic mass of Calcium carbonate: CaCO3 = 40.1+12 + (16 x 3) = 100.1 ?H1= 0.22092 / 0.025974025 = 8.504650319 KJ mol? � ?H1= -8.505 KJ mol? � (to 3.dp) The value for the enthalpy change is negative because energy is lost to the surroundings hence it is an exothermic reaction. I will now calculate the enthalpy change that occurs between the reaction of calcium oxide and hydrochloric acid. ?H= mc?T ?H2= 51.4 x 4.2 x 11 = 2374.68 J = 2.37468 KJ Number of Moles in CaO= 1.4 / 56.1 = 0.024955436 ?H2= 2.37468 / 0.024955436 = 95.15682275 KJ mol? � ?H2= -95.157 KJ mol? � (to 3.dp) Now that I have calculated ?H1 and ?H2 I am able to calculate ?H3. ?H3= ?H1- ?H2 = -8.505-(-95.157) ?H3= 86.652 KJ mol? � From the result above we can see that the reaction when calcium carbonate is heated is an endothermic reaction. An endothermic reaction is when energy has been taken in from the surroundings. ...read more.

Conclusion

or limitations of the equipment used. Probably the biggest measurement error that occurred in the experiment was with regards to the temperature change that take place during a reaction. This was because it was virtually impossible to get a more reliable answer than that to the nearest whole number. It would have been very unlikely that the true values of the temperatures to be a whole value therefore this would have caused error in my result. To solve this problem I could have used a different type of thermometer, such as a digital thermometer that gives a reading which is more accurate than that to the nearest whole number. Another measurement error that could have taken place was when measuring the masses of my substances. I only measured the masses of the substance to one decimal place, which wasn't very accurate to give me a more reliable result I should have measured masses up to more significant figures. To make the experiment more accurate I could have repeated the investigation more times, from the results of each separate experiment I could then have been able to calculate an average of my enthalpy changes. Overall I feel I that the experiment went relatively well as I have done some research and found that other people have come to the same conclusion as me, which is that the thermal decomposition of calcium carbonate is an endothermic reaction. ...read more.

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