Indirect determination of enthalpy change of decomposition of sodium hydrogen carbonate by thermochemical measurement and Hess's Law.

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Chemistry – CH1 Practical Assessment

Written Account of Practical Assessment


Indirect determination of enthalpy change of decomposition of sodium hydrogen carbonate by thermochemical measurement and Hess’s Law.

Background Information:

Many standard enthalpy changes of reaction cannot be measured directly, and we therefore have to employ an indirect approach using energy cycles. This method relies on Hess’s Law, which states that if a change can be brought about by more than one route, then the overall enthalpy change for each route must be the same, provided that the starting and finishing conditions are the same for each route. We could say that this is a consequence of the first law of thermodynamics, which states that energy can neither be created or destroyed in chemical reactions, and so energy changes for a reaction must be the same, whether it takes place in one step or in a whole series of steps.

I will be measuring the enthalpy changes of reaction of sodium hydrogencarbonate and sodium carbonate with dilute hydrochloric acid. By applying Hess’s Law to the results, I will be able to calculate the enthalpy change of decomposition of sodium hydrogencarbonate, (ΔH3), which is impossible to measure directly.

The cycle below shows how this is done:

The method involves carrying out reactions in separate experiments in insulated calorimeters, calculating the heat absorbed or evolved – allowing for heat losses to the surroundings – and scaling up to molar amounts.

Risk Assessment

  • Hydrochloric Acid (2M) is a low hazard, but may still cause harm if it comes into contact with eyes or broken skin. If the hydrochloric acid comes into contact with skin, notify supervisor and wash affected area immediately with water.

  • Should spillage occur, again notify supervisor and dilute with water before mopping up.

  • Sodium carbonate is an alkaline metal and poses a hazard. Should it come into contact with skin, wash immediately with copious amounts of water.

  • Throughout the experiment safety goggles and a lab coat should be worn. Eyes are a highly sensitive region of the body and care should be taken to protect them.

  • Care should be taken when using the thermometer. It is important that it is not broken or damaged, as it contains mercury, and the vapour from mercury is a cumulative poison.

  • Care should be taken not to inadvertently pierce the bottom of the polystyrene cup (calorimeter) with the thermometer.

  • Steady the cup with the thermometer in it with a laboratory clamp if necessary, so that it will not tip over.

  • At the end of the experiment, small quantities of the chemicals can be diluted with running water and run to waste. This should be done with care.

Apparatus & chemicals needed:

  • Simple calorimeter (polystyrene cup with lid to act as an insulating vessel, lid has a perforation to fit thermometer)
  • Thermometer (graduated in 0.5°C divisions)
  • Accurate weighing scales (correct to three decimal places)
  • Weighing bottle (plastic, with lid)
  • 25 cm3 burette
  • Lab clamp
  • Graph paper

  • Dry sodium hydrogencarbonate (3.0g)
  • Dry anhydrous sodium carbonate (2.0g)
  • 100cm3 of 2M hydrochloric acid

Implementation & Analysis

Procedure for Experiment 1: with sodium hydrogen carbonate

Initially, I gathered all the necessary apparatus together and set up my calorimeter. I ensured that the polystyrene cup was clean and intact, that the lid was secure and was pierced correctly so that I could fit a thermometer through the hole. A graphical representation of the apparatus can be shown as follows:

Using a 25 cm3 burette, I accurately measured 50cm3 of 2M hydrochloric acid, and carefully poured this into my calorimeter. I then put the thermometer into place and recorded the temperature of the acid at 30 s intervals for the next three minutes. I recorded the results of this in a table (please see separate sheet).

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I then accurately weighed the plastic weighing bottle on scales accurate to three decimal places. The weight came to 6.136g. I then added 3.0g of dry sodium hydrogen carbonate, ensuring that the weight of NaHCO3 was slightly over the stated 3.0g. This will help to minimise error, as some of the NaHCO3 might be left in the weighing bottle when we add it to the acid, or could stick to the sides of the calorimeter, whereas we want to ensure that all of the stated 3.0g of NaHCO3 react with the acid. The weight of the bottle plus the NaHCO3 was 9.339g.


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