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In chemical reactions existing bonds are broken and new bonds are made. This changes
the chemical energy of the atoms, and energy is exchanged between the chemical
system and the surroundings as heat. To measure this energy the term enthalpy, H, is

Enthalpy- the heat content of a…

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Hess's Law
It is not always possible to measure the enthalpy change of a reaction directly. Hess's law,
which is based on the law of the conservation of energy, gives a method for finding an
enthalpy change indirectly.

The enthalpy cycle shows two routes for
converting reactants to products. The…

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Measuring Enthalpy Changes
You cannot directly measure the enthalpy of a system but you can measure the heat
transferred to its surroundings. If a calorimeter is used the system is insulated thermally
form its surroundings. The change in temperature in the calorimeter is caused by the
enthalpy change, and can…

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The mass of the solid is not added to the mass of the solution. However, if the reactants
are solutions then the mass is equal to the total volume of the solutions.

Since the number of moles is needed to calculate molar enthalpy change, the reactant is
not in excess…

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The experimental determination of Hc for a fuel can easily be carried out. A known mass
of fuel is burnt in air to heat a known mass of water and the temperature change in the
water is recorded.

Two main sources of error in this experiment

Heat loss to the…

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3) Calculate the energy released when the bonds are made (Exothermic)

Bonds formed= 2(C=O) + 4(O-H) = (2 x 805)+(4 x 463) = 3462 KJ mol-1.

4) Add together the energy changes

H = (bonds broken) ­ (bonds formed)= 2646-3462 = -816 KJ mol-1.


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