• Join over 1.2 million students every month
  • Accelerate your learning by 29%
  • Unlimited access from just £6.99 per month

Determination of the Enthalpy Change of a Reaction

Extracts from this document...


Determination of the Enthalpy Change of a Reaction Determine the enthalpy change of the thermal decomposition of calcium carbonate by an indirect method based on Hess' law. Using the proposed method of obtaining results, these values were gathered: Reaction 1: CaCO3(s) + 2HCl(aq) ?Cl2(aq) + CO2(g) + H2O(l) Experiment Number Mass of CaCO3 (g) Temperature Change (?) 1 2.50 2 2 2.55 2 1/6 3 2.50 2 1/4 4 2.53 2 1/6 5 2.47 2 � 2.51 2.12 Reaction 2: CaO(s) + 2HCl(aq) ?Cl2(aq) +H2O(l) Experiment Number Mass of CaO (g) Temperature Change (oK) 1 1.30 9 1/2 2 1.36 10 1/3 3 1.46 11 4 1.35 10 1/6 5 1.40 10 1/2 � 1.37 10.3 � in both cases represents the mean of the data. Using the equation for enthalpy change: ?H = mc?T Where: m = Mass of liquid to which heat is transferred to (g) c = Specific heat capacity of aqueous solution (taken as water = 4.18 J.g-1.K-1) ?T = Temperature change (oK) We can thus determine the enthalpy changes of reaction 1 and reaction 2 using the mean (�) of the data obtained. Reaction 1: ?H = 50 x 4.18 x -2.12 ?H = -443.08 This value is for 2.51g of calcium carbonate, not 100.1g which is its molecular weight. Therefore: ?H = -443.08 x (100.1 / 2.51) = -17670.2 J.mol-1. ?H = -17.67 kJ.mol-1. Reaction 2: ?H = 50 x 4.18 x -10.3 ?H = -2152.7 This value is for 1.37g of calcium oxide, not 56.1g which is its relative molecular mass. Therefore: ?H = -2152.7 x (56.1 / 1.37) = -88150.7 J.mol-1. ?H = -88.15 kJ.mol-1. Hess' law states that: 1"The total enthalpy change for a chemical reaction is independent of the route by which the reaction takes place, provided initial and final conditions are the same." This means that therefore the enthalpy change of a reaction can be measured by the calculation of 2 other reactions which relate directly to the reactants used in the first reaction and provided the same reaction conditions are used, the results will not be affected. ...read more.


These are that the specific heat capacity of all solutions to which a temperature change has occurred is taken as the value for water. This is relatively high due to its high stability from its hydrogen bonding. Therefore with solutions with a lower specific heat capacity, less thermal energy is required to raise the temperature of the solution and the enthalpy change is not as great as suspected. Also the density of the solution is judged as water which has a density of 1. That is, for every 1 cm3 of liquid, it has a mass of 1 gram. This is because Density = Mass / Volume. However, no solutions have a density of 1 unless of course they are pure water. This means that there could, in fact be a larger amount of molecules to which the thermal energy is transferred to. Therefore with solutions with greater density, more thermal energy is required to raise the temperature of the solution and the enthalpy change is greater than suspected. Taking into account all of these potential areas of limitation of the method, if the experiment were to be repeated, it would have to include as many methods in which to prevent these sources of error to occur. This would include: conducting the experiment in a vacuum flask, using a digital thermometer to display an exact change in temperature of the solution, the flask to contain a method of introducing the solid reagent without loss of heat, provide standard conditions under which to conduct the experiment and use the true specific heat capacity and density of hydrochloric acid when calculating enthalpy changes. Results: For CaCO3: T1 = 17 T2 = 19 DT= 02 using 2.57g of CaCO3 For CaO: T1 = 18 T2 = 27 DT= 09 using 1.39g of CaO Analysis: In order to determine the enthalpy change for the thermal decomposition of calcium carbonate, we must work out the enthalpy changes for both the reactions of calcium carbonate and calcium oxide with hydrochloric acid. ...read more.


There were also a number of other problems with the experiment, but these were of lower significance than those mentioned above. The most easily noticed one of these is the measuring cylinder - we were instructed to use a 250cm3 measuring cylinder to measure out 50cm3 of 2 mol dm-3 HCl. This could have produced an error, as it is using a large cylinder to measure out a relatively small amount of acid. However, whilst meaning that this variable was not entirely under control during the experiment, as the acid was in excess, it is not especially crucial that exactly 50cm3 was used - it does not affect the readings taken. Another variable was the concentration of the acid. This could have affected the results, by the concentration not being accurate. If this were the case, however, then the error would have been systematic, and so will have appeared in both sets of results, as the acid was drawn from the same bottle. Another one of these small errors is the accuracy of the balances - the readings given may not have been entirely accurate. However, this slight inaccuracy (?0.004g) pales in the face of the other experimental problems, so it can be safely disregarded. These problems were not the only ones with the experiment - the major factor being that it was only performed once, with no repeats - these values were taken to be correct, with no comparisons made. This could easily be rectified by performing a suitable number of repetitions - for example, 4 repetitions could be made, and an average taken. This would vastly improve the reliability of the end results, as the average would more accurately reflect the true temperature change. Overall, there were a large number of problems with this experiment, and correspondingly, there are a large number of things that I would like to change if I were to be able to repeat this experiment. The experiment was successful I that results were obtained, but I suspect that these results are vastly different to the actual values. ...read more.

The above preview is unformatted text

This student written piece of work is one of many that can be found in our AS and A Level Organic Chemistry section.

Found what you're looking for?

  • Start learning 29% faster today
  • 150,000+ documents available
  • Just £6.99 a month

Not the one? Search for your essay title...
  • Join over 1.2 million students every month
  • Accelerate your learning by 29%
  • Unlimited access from just £6.99 per month

See related essaysSee related essays

Related AS and A Level Organic Chemistry essays

  1. Marked by a teacher

    Experiment to Determine Acidities of Wine. The purpose of this experiment is to ...

    5 star(s)

    Since 2 moles of NaOH reacts with 1 mole of Tartaric Acid, 8.535x10-3 moles of NaOH will react with 4.268x10-3 moles of Tartaric Acid. Therefore there are 4.268x10-3 moles of Tartaric Acid in 100cm3 of wine, so the concentration of acid in the wine can be calculated.

  2. Thermal Decomposition of Copper Carbonate

    Protect against physical damage. Isolate from incompatible substances. Sweep up and shovel into suitable containers for disposal. Copper(II) oxide [CuO(s)] 1.62x103moles Release of toxic metal fumes during decomposition. Keep in a tightly closed container, stored in a cool, dry, ventilated area.

  1. Find the enthalpy change of combustion of a number of alcohol's' so that you ...

    error, which we are going to use later when we collect our results. It will help minimise the amount of error in the experiment. % Error: Electronic Balance It is set to 2 decimal places and therefore reads to the nearest 0.01g.

  2. Investigating the Enthalpy Changes of Combustion of Alcohols.

    Using the data logger definitely improved the accuracy.) The results I obtained from the experiments (for the enthalpy of combustion) are nowhere near the theoretical amount that I should have got (2) (even taking into account the above percentage errors).

  1. The aim of this experiment is to produce Aspirin. This is an estrification in ...

    The purity of a substance can be determined by its melting point. A pure substance has a specific melting point. If the melting point of the same substance is measured and is found to be lower than the standard value, the substance is not pure since impurities lower the melting point of a substance.

  2. investigating the amount of ascorbic acid present in fruit

    Pestle and Mortar To grind the sample of fruit. Electronic Balance To weigh out accurately 10g of ascorbic acid and 4g of KI to make out my solutions. Glass Stirring Rod To stir the mixture of KI, soluble starch and ascorbic acid together with distilled water.

  1. Comprehensive and Detailed Chemistry notes

    When this occurs, the pH of rainwater becomes quite low, as low as 3.5-4.0. The following are reactions that occur which cause rain to be acidic. Sulfur dioxide reacts with rain in the atmosphere forming sulfurous acid: SO2 (g) + H2O (l)

  2. The aim of this experiment is to investigate the enthalpy change of combustion for ...

    molecular structure the estimated enthalpy change of combustion is therefore the same as the same bonds are being broken as they are formed in both alcohols. I therefore predict that positioning of the OH has no effect on the enthalpy of combustion.

  • Over 160,000 pieces
    of student written work
  • Annotated by
    experienced teachers
  • Ideas and feedback to
    improve your own work