CHEMISTRY 3.4
WORKSHEET ELEVEN
1.
(a)
ANSWERS
HEAT OF COMBUSTION & FORMATION
2.
(a)
(b)
The energy change when one mole of the substance is formed from its
elements with all reactants and products in their standard states.
The energy change when one mole of the substance is completely burnt
with all reactants and products in their standard states.
The states at room temperature (25oC) and pressure (1 Atm)
Zero
3.
(a)
∆rHo = 52.3 - 226.7 = -174.4 kJ mol-1
(b)
∆rHo = (-1260) - 2(-277.7) - 2(-393.5)
= -1260 + 555.4 + 787
= 82.4 kJ mol-1
(c)
∆rHo = (-1096) - (-110.5)
(b)
= -985.5 kJ mol-1
4.
∆rH = -2712 = 4(-393.5) + 5(-285.8) - ∆fHo(C4H9OH)
-2712 = -1574 - 1429 - ∆fHo(C4H9OH)
∆fHo(C4H9OH) = -291 kJ mol-1
5.
(a)
C4H10 (g) + 6.5 O2(g) → 4CO2 (g) + 5H2O(l)
(b)
-2882 = 4(-393.5) + 5(-285.8) - ∆fHo(C4H10)
∆fHo(C4H10) = -1574 - 1429 + 2882
= -121 kJ mol-1
(c)
Energy is required for bond breaking and energy is released when
bonds are made. In combustion reactions, mainly C-O and H-O bonds
are made while mainly C-C and C-H bonds are broken. The greater
stability of C-O and H-O bonds compared to C-C and C-H means that
there is always an overall release of energy in combustion.
(a)
C8H18 (g) + 12.5 O2(g) → 8CO2 (g) + 9H2O(l)
(b)
-5509 = 8(-393.5) + 9(-285.8) - ∆fHo(C8H18)
∆fHo(C4H10) = -3148 - 2572.2 + 5509
= -211.2 kJ mol-1
(a)
(b)
C2H5OH (l) + 3.5 O2(g) → 2CO2 (g) + 3H2O(l)
∆cHo = 2(-393.5) + 3(-285.8) - (-277.7)
= -787 - 857.4 + 277.7
= -1366.7 kJ mol-1
6.
7.