16 - Energetics
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16.1 Standard enthalpy changes of reaction

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

Standard enthalpy change of formation -- The enthalpy change when 1 mol of a substance is made from its elements in their standard states eg C_(graphite) + 2H_2(g) -> 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 M⁺_(g) + X^-_(g) -> 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 - Temperature(in kelvin) x delta-S...spontaneity depends on deltaH, deltaS and the temp 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 x 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 x delta-S)

4) delta-G is never negative if delta-H is positive and delta-S negative.

Other Notes in this Category

1. 01 - Stoichiometry
2. 02 - Atomic Theory
3. 03 - Periodicity
4. 04 - Bonding
5. 05 - States of Matter
6. 06 - Energetics
7. 07 - Kinetics
8. 08 - Equilibrium
9. 09 - Acids and Bases
10. 10 - Oxidation and Reduction
11. 11 - Organic Chemistry
12. 12 - Atomic Theory
13. 13 - Periodicity
14. 14 - Bonding
15. 15 - States of Matter
16. 16 - Energetics
17. 17 - Kinetics
18. 18 - Equilibrium
19. 19 - Acids and Bases
20. 20 - Oxidation and Reduction
21. 21 - Organic Chemistry

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