F6 Energy and thermochemistry

Question 1

Heat:

Work:

Answer 1

Heat: energy transfer as a result of temperature difference

Work: energy change that results in motion against a force

Question 2

Specific heat capacity Cs:

Molar heat capacity Cm:

Answer 2

Specific heat capacity Cs: raising the temperature of 1 g by 1 K (J K-1 g-1)

Molar heat capacity Cm: raising the temperature of 1 mol by 1 K (J K-1 mol-1)

Question 3

enthalpy (ΔH)

Answer 3

The heat released during a reaction at constant pressure (DP = 0)

Question 4

Hess’s law

Answer 4

The total enthalpy change for a chemical reaction is independent of the path by which the reaction occurs, provided that the starting and finishing states are the same for each reaction path.

Question 5

bond dissociation enthalpy D,(A-B)

Answer 5

The bond dissociation enthalpy D, (A-B) is the enthalpy change for breaking an A-B bond under standard conditions in the gas phase.

Question 6

2KNO3(s) + S(s) + 3C(s) -> K2S(s)+ N2(g) + 3CO2 (g).

Vad är entalpiändringen för reaktionen ovan?

Answer 6

∆rH=Hprod – Hreakt = (HCO2 + HN2 + HK2S) – (HC + HS + HKNO3) = (-393.53 + 0-428.41) kJ –(0+0- 494.6*2) kJ = -619.7 kJ

Question 7

2KNO3(s) + S(s) + 3C(s) -> K2S(s)+ N2(g) + 3CO2 (g).

Är reaktionen ovan spontan vid 298 K?

Answer 7

Spontan om ∆G<0.
∆rG = Gprod - Greakt = (3-394.4 + 0 -340.5)kJmol-1 –(30 +0 -2*393.3)kJmol-1 =
-1523.7+786.6 kJmol-1 = -737.1 kJ.
-> ja, spontan

Question 8

Anta reaktionen då svaveldioxid oxiderar i gasform: SO2(g) + 1⁄2 O2(g) -> SO3(g). Beräkna reaktionens entalpiändring vid 25 °C. Är reaktionen exo- eller endotermisk?

Answer 8

[ΔHf(SO3 (g))] - [ΔHf(SO2 (g)) + 0.5ΔHf(O2 (g))]= [1(-395.7)] - [1(-296.8) + 0.5(0)] = -98.9 kJ (per mole of SO2)
-> Exotermisk