Measuring Temperature and Enthalpy Change Practical

 Measuring Temperature in Enthalpy Experiments  

- Use a polystyrene cup with a lid to minimise heat loss or gain from the surroundings.  

- Stir the reaction mixture with a stirring rod to ensure even distribution of thermal energy.  

- Ensure the thermometer bulb is fully immersed in the reaction mixture.  

- For greater accuracy, use an electronic temperature sensor with data logging software to plot temperature changes.  

Flame Calorimeter (for improved accuracy):  

1. Spiral chimney made of copper (a good conductor of heat).  

2. Flame is enclosed to reduce heat loss.  

3. Fuel burns in pure oxygen rather than air, ensuring complete combustion.  

Enthalpy Change Assumptions  

When calculating enthalpy change, the following assumptions are made:  

- All solutions have the same heat capacity as water.  

- Heat absorbed by the calorimeter/apparatus is ignored.  

- Reaction or dissolving is complete and not slow.  

- The density of the solution is taken to be the same as that of water.  

- No evaporation of water occurs.  

- Room temperature remains unchanged.  

- Combustion of fuel is complete.  

Calculations  

- Measure the mass of the solid used.  

- Calculate moles using:  

  n = Mass/M_r

- Energy change:  

  q = m x c x Delta T 

  where:  

  - m = mass of solution  

  - c = specific heat capacity  

  - Delta T = temperature change  

- Express enthalpy change in kJ/mol.  

- Convention:  

  - Exothermic → negative value  

  - Endothermic → positive value  

Accuracy Points (with Explanation)  

- Wash containers with the solution to be used, then dry the cup.  

- Allow water to stand for some time before use.  

- Stir liquid continuously for uniform temperature distribution.  

- Place polystyrene cup inside a beaker for extra insulation and support.  

- Clamp thermometer securely, ensuring bulb is immersed.  

- If both reactants are solutions, measure their temperatures before mixing and use the average temperature.  

- Polystyrene beakers are effective calorimeters because they are good insulators and have high specific heat capacity.  

- Add powdered salt rapidly to minimize heat loss.  

- Use a lid on the polystyrene cup to maximize temperature change.  

- Always use a large scale for precise measurement.