WJEC AS Chemistry- Energetics

Complete revision notes from a WJEC new text book on energetics. 

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Energetics
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
used.
Enthalpy- the heat content of a system.
Enthalpy Change- the heat added to a system at a constant pressure, and thus the
overall energy exchange with the surroundings.
Standard enthalpy of formation - the enthalpy change when one mole of the
compound forms from its elements under standard conditions with the elements and the
compound in their standard state.
Standard enthalpy of a reaction - is the energy transferred when the molar
quantities or reactants as stated react under standard conditions (1 atmospheric
pressure, 298k, substances in their most stable conditions, solutions with concentration of
1 mol dm3)
Standard enthalpy of combustion - the enthalpy change when one mole of a
substance burns completely in oxygen under standard conditions. All combustion
reactions are exothermic therefore the enthalpy change is always negative.
The principle of conservation of energy states that energy cannot be created or
destroyed only changed from one form to another. Therefore if heat is released in a
reaction, the amount of energy that leaves the system is exactly the same as the
amount of energy that goes into the surroundings.
Enthalpy on its own cannot be measured, although enthalpy changes can.
H = Hproducts ­ Hreactants
For a reaction that releases heat, the enthalpy change is negative. This is an
exothermic reaction.
For a reaction that absorbs heat, the enthalpy change is positive. This is an
endothermic reaction.
Enthalpy change for reaction depends on the conditions, so in reactions standard
enthalpy changes are measured under fixed conditions. These are 298 K (25'C) and 1 atm
(101 kPa.)
All elements in their standard state have a standard enthalpy change of formation of 0
Kj mol-1.

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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 first is a
direct route and the second an indirect route via
the formation of an intermediate.…read more

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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 be measured with a thermometer if the mass and the specific
heat capacity are known.…read more

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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 and must be measured accurately. So the mass of a solid or the
concentration of a solution must be known.
The coffee cup calorimeter.…read more

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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 surroundings5
Incomplete combustion of fuel
Bond Enthalpy
Bond Enthalpy gives information about the strength of a covalent bond. Bond enthalpies
show how much energy is needed to break different covalent bonds.…read more

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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.…read more

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