Transcript Formation Reactions

Formation Reactions
Examples:
C(s) + O2(g)  CO2(g)
Na(s) + ½ Br2(l)  NaBr(s)
6C(s) + 6H2(g) + 3O2(g)  C6H12O6(s)
Formation Reactions
• The formation of 1 mole of a substance from
its elements in their standard state – i.e. their
most stable form at SATP – (what they are on
the periodic table)
Write the formation reaction for:
Sodium hydroxide
Na(s) + ½ O2(g) + ½ H2(g)  NaOH(s)
Aluminum phosphate
Al(s) + P(s) + 2O2(g)  AlPO4(s)
Potassium bromate
K(s) + Br2(l) + 3/2 O2(g)  KBrO3(s)
Standard Enthalpy of Formation ΔHfo
• The energy associated with a formation
reaction
• Units: kJ/mol
• Enthalpy of formation of an element already
in its standard state i.e. O2(g)
= zero
Look up the ΔHfo for
NaOH
KClO3
O2(g)
O(g)
I2(s)
I2(g)
I-(aq)
Hess’s Law Using ΔHfo
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
1) Write the formation reactions for each
substance in the above reaction.
2) How would you rearrange each equation to
make the target equation?
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
C(s) + 2H2(g)  CH4(g) x (-1)
ΔHfo x (-1)
O2(g)  O2(g)
ΔHfo
x(-1)
C(s) + O2(g)  CO2(g) x1
ΔHfo
x1
H2(g) + ½ O2  H2O(g) x2
ΔHfo x2
x(-1)
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
ΔHrxn = (1)ΔHfo CO2 + (2)ΔHfo H2O + (-1)ΔHfo CH4 + (-1)ΔHfo O2
ΔHrxn = [(1)ΔHfo CO2 + (2)ΔHfo H2O] –[(1)ΔHfo CH4 + [(1)ΔHfo O2]
ΔHrxn = [(n)ΔHfo CO2 + (n)ΔHfo H2O] –[(n)ΔHfo CH4 + [(n)ΔHfo O2]
ΔHrxn = Σn ΔHfo Products - Σn ΔHfo Reactants
ΔHrxn = [-393.5 + 2(-242)] – [-75 + 2(0)]
= -802.5kJ
ΔHrxn = Σn ΔHfo Products - Σn ΔHfo Reactants
Try:
1) CH3OH(l) + 3O2(g)  2CO2(g) + 4H2O(l)
ΔHrxn = -1692 kJ
2) 4NH3(g) + O2(g)  4NO2(g) + 6H2O(l)
ΔHrxn = -1396 kJ
3) What is the molar enthalpy of combustion of
propane?
ΔHrxn = -2220.5 kJ
4) The standard enthalpy of combustion of liquid
cyclohexane, C6H12, to liquid water and carbon
dioxide is -3824 kJ. What is the standard
enthalpy of formation of cyclohexane?
ΔHfo = -252 kJ/mol
Hess’s Law and Calorimetry
5) What mass of methane undergoing complete
combustion is required to heat 100.0 kg of water
from 5.0oC to 70.0oC in a gas water heater?
m = 480 g
Hess’s Law and Calorimetry
A 0.092 g sample of acetone is burned in 50.0 g
aluminum calorimeter containing 100.0 g of
water. The temperature rises from 20.0 oC to
25.0oC.
a) What is the molar enthalpy of combustion of
acetone, C3H8O?
b) Calculate the percent error.