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Revision:Energetics - 16

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TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Chemistry > Energetics - 16

16.1 Standard enthalpy changes of reaction

16.1.1

Standard state -- 101 kPa, 298 K (or 1 atm, 25 degrees Celsius).

Standard enthalpy change of formation -- The enthalpy change when 1 mol of a substance is made from its elements in their standard states eg

\mathsf{C_{(graphite)} + 2H_{2(g)} \longrightarrow CH_{4(g)}}.

(molecules, like H2 are considered to be 'standard state'. Fractions of mols -> ie fractions in coefficients, may also be used if necessary as 1 mol must be produced).


16.1.2

If a reaction can be expressed in terms of changes of formation (and bond enthalpies as in SL) then add up all the \Delta H values to get the \Delta H for the reaction.


16.2 Lattice enthalpy

16.2.1

Lattice enthalpy - the enthalpy change when 1 mol of crystals (ie an ionic lattice) is formed from its component particles at an infinite distance apart. ie:

\mathsf{M^+_{(g)} + X^-_{(g)} \longrightarrow MX_{(s)}}.

The value of lattice enthalpy is assumed to be negative for the separation of the lattice, and positive for the formation of the lattice.


16.2.2

As above, lattice enthalpies just add another type of reaction to those which can be shown on the Born-Harber cycle.


16.2.3

lattice enthalpy increases with higher ionic charge and with smaller ionic radius (due to increased attraction).


16.3 Entropy

16.3.1

Factors which increase disorder in a system : Mixing of particles, change of state to greater distance between particles (solid --> liquid or liquid --> gas), increased particle movement (temperature), increased number of particles (when more gas particles are produced, this generally outweighs all other factors).


16.3.2

Predict the sign of \Delta S (the change in entropy) for a reaction based on the above factors. \Delta S is positive when entropy increases (more disorder) and negative when entropy decreases (less disorder).


16.3.3

The standard entropy change can be calculated by subtracting the absolute entropy of the reactants from that of the products.


16.4 Spontaneity of a reaction

16.4.1

Reactions which release heat (and so increase stability) tend to occur. Reactions which increase entropy (\Delta S is positive) tend to occur, but neither can be used to accurately predict spontaneity alone.


16.4.2

when \Delta G is negative, the reaction is spontaneous, when it's positive, their reaction is not.


16.4.3

\Delta G = \Delta H - T \Delta S


spontaneity depends on \Delta H, \Delta S and the temp (T, in Kelvin) at which the reaction takes place (or doesn't as the case may be).


16.4.4

Yeah...well...stick numbers into that equation above...


16.4.5

4 possibilities...

  1. \Delta G is always negative when \Delta H is negative and \Delta S is positive.
  2. \Delta G is negative at high temperatures if \Delta H is positive and \Delta S is positive (ie an endothermic reaction is spontaneous when T \Delta S is greater than \Delta H)
  3. \Delta G is negative at lower temperatures if \Delta H is negative and \Delta S is negative (exothermic reactions are spontaneous if \Delta H is bigger than T \Delta S)
  4. \Delta G is never negative if \Delta H is positive and \Delta S negative.

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