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Topic 2.1

ENERGETICS

Measuring and Calculating Enthalpy Changes
Mean Bond Dissociation Enthalpies
Hess' Law
























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ENTHALPY CHANGES

1. Exothermic and endothermic reactions

When a chemical reaction takes place, the products and reactants have different stabilities and
thus there is a change in potential energy. However since total energy is always conserved, any
change in potential energy must be balanced by an equal and opposite change…

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2. Standard enthalpy changes

The change in chemical potential energy during a chemical reaction is known as the enthalpy
change for that reaction. It is given the symbol H.

By convention, if the reaction is exothermic (ie heat is given out) the enthalpy change is said to
be negative: H…

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3. Standard enthalpies of formation and combustion

The term "standard enthalpy of reaction" can be used to describe any chemical reaction, but
there are some important reactions which have special names:

The standard enthalpy of formation of a substance is the enthalpy change when one mole of
that substance is…

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4. Calculating enthalpy changes from temperature changes

Enthalpy changes are generally measured by carrying out a reaction under controlled conditions
in a laboratory and measuring the temperature change.

The amount of heat required to change the temperature of a system by 1K is known as the heat
capacity of a…

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MEAN BOND ENTHALPIES

During a chemical reaction, the bonds in the reactants are broken. This is an endothermic
process energy is required to do this. After the bonds have been broken, however, the bonds
in the products are formed. This is an exothermic process energy is released when this
happens.…

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These bond enthalpies are mean values the exact strength of a bond depends on its
environment. Even the strengths of the same type of bond in the same molecule may vary:
Eg in water: HOH(g) HO(g) + H(g) H = +502 kJmol1
HO(g) H(g) + O(g) H = +427 kJmol1…

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Thus bond enthalpies calculated from different reactions may vary slightly. They also do not take
intermolecular forces into account. Mean bond enthalpies thus only give you approximate
values for enthalpy changes.

b) Solids and liquids

In giant covalent substances, all the covalent bonds have to be broken before free gaseous…

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Determining mean bond enthalpies from given data:

If the enthalpy change for a reaction is known, and most of the bond enthalpies are known, it is
possible to calculate the mean bond enthalpy of a particular bond:

Eg The enthalpy of formation of methane is known to be 76 kJmol1.…

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HESS' LAW

Hess' Law states that "the enthalpy change for a chemical reaction depends only on the
initial and final states and is independent of the path followed".

In other words whichever route, however direct or indirect, by which the reaction proceeds, the
overall enthalpy change for the reaction will…

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