Equations Topic 5/15

Question 1

Heat transfer equation

Answer 1

Q = mcΔT => ΔH = mcΔT / n

where
Q = heat transfer
m = mass
c = heat capacity
ΔT = change in temperature
ΔH = change in enthalpy
n = number of moles

Question 2

Change in enthalpy at STP (enthalpies of formation)

Answer 2

ΔHθ = ΣHθf (product) - ΣHθf (reactant)

where
ΔHθ = change in enthalpy at STP
ΣHθf = sum of the enthalpies of formation

Question 3

Change in enthalpy at STP (bond enthalpies)

Answer 3

ΔHθ = ΣE (broken) - ΣE (formed)

where
ΔHθ = change in enthalpy at STP
ΣE = sum of enthalpies

Question 4

Entropy

Answer 4

ΔSθreaction = ΣSθ (product) - ΣSθ (reactant)

where
ΔSθr = change in entropy at STP
ΣSθ = sum of entropies at STP

Question 5

Spontaneity

Answer 5

ΔG = ΔH - TΔS

where
ΔG = change in Gibbs free energy
ΔH = change in enthalpy
T = temperature
ΔS = change in entropy

**If G is negative, the reaction is spontaneaous

Question 6

Change in enthalpy (with number of moles)

Answer 6

ΔH = Q / n

where
ΔH = enthalpy change (J mol^-1)
Q = energy (kilojoules / kJ)
n = number of moles (mol)

Question 7

Lattic enthalpy ionic model

Answer 7

ΔHølat = (Knm) / (Rx n+ + Ry m-)

where
K = a constant that depends on the geometry of the lattice
n & m = are charges of the ions
R = radius of each respective ion
X = metal
Y = non-metal

Question 8

Equation that links solution enthalpy, lattice enthalpy and enthalpies of hydration, for any ionic compound.

Answer 8

ΔHøsol (XY) = ΔHølat (XY) + ΔHøhyd (X+) + ΔHøhyd (Y−)

Question 9

Entropy change for surroundings

Answer 9

ΔS (surroundings) = - ΔH (system) / T; unit: J K^-1 mol^-1

Question 10

Calculating total entropy change

Answer 10

ΔS (total) = ΔS (system) + ΔS (surroundings) > 0
(substituting entropy change for surroundings)

ΔS (total) = ΔS (system) + - ΔH (system) / T > 0

Question 11

Exothermic reactions

Answer 11

Increase the entropy of the surroundings as heat is dispersed into the surroundings. This clearly allows for ΔS (total) to be greater than zero, as entropy increases.

Question 12

Endothermic reactions

Answer 12

There is a decrease in the entropy of the surroundings, but this is compensated for by a greater increase in the entropy of the system, once again allowing for ΔS (total) to be greater than zero.

Question 13

The Second Law of Thermodynamics

Answer 13

States that:

• total entropy of an isolated system can never decrease.
• total entropy of an isolated system is constant only if all processes are reversible.
• this system will spontaneously move to a state of maximum entropy.