Hello

-425.93 -92.30 -407.27 -285.83
Hess's Law: ΔH(rxn) = Sum[ΔH(f) products] - Sum[ΔH(f) reactants]
ΔH(rxn) = [(1 mol)(-407.27 kJ/mol) + (1 mol)(-285.83 kJ/mol)] - [(1 mol)(-425.93 kJ/mol) + (1 mol)(-92.30)]
= - 359.47 kj/molWhy are there energy changes when a chemical reaction takes place? Do physical state changes involve energy changes? Exothermic energy changes and endothermic energy changes in chemical reactions are described, and exothermic reactions and endothermic reactions are discussed in terms of bond energies - including calculations of energy transfers for GCSE/IGCSE/O Level and basic stuff for GCE Advanced Level AS students. Methods of obtained vales for energy changes in chemical reactions are described and how to do the calculations based on calorimeter experiment results. Also discussed are activation energies, reaction profiles, catalysts.

If you are an A Level student, these pages will help as an introduction, but then you should follow the A Level links to the more advanced revision notes and calculation methods.

Advanced Level Energetics–Thermochemistry – Enthalpies of Reaction, Formation & Combustion

Sub-index for ENERGY CHANGES (GCSE/IGCSE/O level)

1. Heat changes in chemical/physical changes - exothermic and endothermic

2. Reversible reactions and energy changes

3. Activation energy and reaction profiles

4. Catalysts and activation energy

5. Introduction to bond energy/enthalpy calculations

6. Calorimeter methods of determining energy changes

7. Energy transfer calculations from calorimeter results

Advanced Level Energetics-Thermochemistry - Enthalpies of Reaction, Formation & Combustion
This method 6.1 is can be used for any non-combustion reaction that will happen spontaneously at room temperature involving liquids or solid reacting with a liquid. The reactants are weighed in if solid and a known volume of any liquid (usually water or aqueous solution). The mixture could be a salt and water (heat change on dissolving) or an acid and an alkali solution (heat change of neutralisation). It doesn't matter whether the change is exothermic (heat released or given out, temperature increases) or endothermic (heat absorbed or taken in, temperature decreases). See calculations below.
The main problem is heat loss from the calorimeter, though a poorly conducting cup and a sealing lid to stop convection in air, both help reduce the heat loss and quite accurate results can be obtained. You can also stand the plastic cup in a beaker of cotton wool to provide even greater thermal insulation.

From the temperature rise or temperature fall and the heat capacity of the water you can calculate how much heat was released or how much heat energy was absorbed by a specific quantities of chemicals reacting (or even the energy transfer when a salt dissolves in water, which can be exothermic or endothermic).

You can use this method for adding a solid to water or a solution or mixing two solutions of reactants. In either case you must carefully measure the initial temperature of all the reactants.

(c) doc b This method 6.2 is specifically for determining the heat energy released (given out) for burning fuels. The burner is weighed before and after combustion to get the mass of liquid fuel burned. The thermometer records the temperature rise of the known mass of water (1g = 1cm3).
The heat from the fuel combustion heats up the water. From the heat capacity of the water and the temperature rise you can calculate how much heat was released by a specific mass of fuel.

You can use this system to compare the heat output from burning various fuels. The bigger the temperature rise, the more heat energy is released. See calculations below for expressing calorific values.

This is a very inaccurate method because of huge losses of heat e.g. radiation from the flame and calorimeter, conduction through the copper calorimeter, convection from the flame gases passing by the calorimeter etc. BUT, at least using the same burner and set-up, you can do a reasonable comparison of the heat output of different fuels. You can simple hydrocarbons like hexane, alcohols like ethanol and even vegetable oils and it is possible to do a crude calibration using a fuel of known energy output.