ib chemistry on hess's law, enthalpy formation and combustion
TRANSCRIPT
http://lawrencekok.blogspot.com
Prepared by Lawrence Kok
Video Tutorial on Hess’s Law, Enthalpy Formation and Combustion.
CO(g)
∆H2
∆H2
∆H1 = -394
CO2(g)
C(s) + ½ O2(g)
CO(g) + ½ O2
C(s) + O2(g)
CO2(g)
A E
B C D
∆H6∆H5
∆H4∆H2
Hess’s Law
C(s) + O2(g) CO2(g)
Overall ∆H rxn is independent of its pathway
∆H rxn in series steps = sum of enthalpy changes for individual steps
∆H1
∆H2 ∆H4
∆H1
∆H3
Energy Level or Cycle Diagram to find ΔH
State function - property of system whose magnitude depend on initial and
final state
∆H3
A → D A → B → C → D
∆H1 = H2 + H3 + H4
∆H A → E is same regardless of its path
Final state
A → E A → C→ D → E
∆H1 = H2 + H3 + H4
A → E A → B → E
∆H1 = H5 + H6
Initial stateInitial state
Final state
∆H3 = -283
∆H1 = ∆H2 + ∆H3
Energy Level Diagram
22
1O
∆H2 = H1 - H3 = -394 +283 = -111 kJ mol-1
∆H3 = -28322
1O
Energy Cycle Diagram∆H1 = -394
∆H2 = H1 - H3 = -394 +283 = -111 kJ mol-1
Path not impt !!!!
∆H1 = ∆H2 + ∆H3
C(s) + O2 → CO2 (g) ∆H1 = -394
C (s) + ½ O2 → CO(g) ∆H2 = ??? CO(g) + ½ O2 → CO2 (g) ∆H3 = -283
+
Hess’s Law
Find ∆H cannot be measured
directly/experimentally
C(s) + 1/2O2 → CO(g) ∆H2
????
SO2(g)
∆H2
∆H2
∆H1 = -395
SO3(g)
S(s) + O2(g)
SO2(g) + ½O2
S(s) + 3/2O2(g)
SO3(g)
A E
B C D
∆H6∆H5
∆H4∆H2
Hess’s Law
S(s) + 3/2O2(g) SO3(g)
Overall ∆H rxn is independent of its pathway
∆H rxn in series steps = sum of enthalpy changes for individual steps
∆H1
∆H2 ∆H4
∆H1
∆H3
Energy Level or Cycle Diagram to find ΔH
State function - property of system whose magnitude depend on initial and
final state
∆H3
A → D A → B → C → D
∆H1 = H2 + H3 + H4
∆H A → E is same regardless of its path
Final state
A → E A → C→ D → E
∆H1 = H2 + H3 + H4
A → E A → B → E
∆H1 = H5 + H6
Initial stateInitial state
Final state
∆H3 = -98
∆H1 = ∆H2 + ∆H3
Energy Level Diagram
Find ∆H cannot be measured
directly/experimentally
∆H2 = H1 - H3 = -395 + 98 = - 297 kJ mol-1
∆H3 = - 9822
1O
Energy Cycle Diagram∆H1 = -395
∆H2 = H1 - H3 = - 395 + 98 = - 297 kJ mol-1
Path not impt !!!!
Hess’s Law
∆H1 = ∆H2 + ∆H3S(s) + 3/2O2 → SO3 (g) ∆H1 = -395
S(s) + O2 → SO2(g) ∆H2 = ???SO2(g) + ½O2 → SO3 (g) ∆H3 = -98
+
O2
S(s) + O2 → SO2(g) ∆H2
?????
N2(g) + 2O2(g)N2(g) + 2O2(g)
N2O4(g)
2NO2(g)
∆H1
N2O4(g)
∆H1
∆H2 = + 33
2NO2(g)
A E
B C D
∆H6∆H5
∆H4∆H2
Hess’s Law
N2(g) + 2O2(g) 2NO2(g)
Overall ∆H rxn is independent of its pathway
∆H rxn in series steps = sum of enthalpy changes for individual steps
∆H1
∆H2 ∆H4
∆H1
∆H3
Energy Level or Cycle Diagram to find ΔH
State function - property of system whose magnitude depend on initial and
final state
∆H3
A → D A → B → C → D
∆H1 = H2 + H3 + H4
∆H A → E is same regardless of its path
Final state
A → E A → C→ D → E
∆H1 = H2 + H3 + H4
A → E A → B → E
∆H1 = H5 + H6
Initial stateInitial state
Final state
∆H3 = + 9
Energy Level Diagram
Find ∆H cannot be measured
directly/experimentally
∆H3 = + 9
Energy Cycle Diagram∆H2 = + 33
∆H1 = H2 + H3 = -33 + 9 = - 24 kJ mol-1
Path not impt !!!!
Hess’s Law
∆H1 = ∆H2 + ∆H3
N2(g)+ 2O2 → 2NO2(g) ∆H2
= +33
2NO2 → N2+ 2O2 ∆H2 = - 33 N2(g) + 2O2 → N2O4(g) ∆H3 = + 9
+
2NO2(g) → N2O4(g) ∆H1 = ?
∆H1 = ∆H2 + ∆H3
∆H1 = H2 + H3 = -33 + 9 = - 24 kJ mol-1
inverse
2NO2(g) → N2O4(g) ∆H = -24
∆Hf θ
(reactant)∆Hf
θ
(product)
Using Std ∆Hf θ formation to find ∆H rxn
∆H when 1 mol form from its element under std condition
Na(s) + ½ CI2(g) → NaCI (s) ∆Hf θ = - 411 kJ
mol -1
Std Enthalpy Changes ∆Hθ
Std condition
Pressure 100kPa
Temp 298K
Conc 1M All substance at std states
Hess’s Law
Std ∆Hf θ formation
Mg(s) + ½ O2(g) → MgO(s) ∆Hf θ =- 602 kJ
mol -1
Reactants Products
O2(g) → O2 (g) ∆Hf θ = 0 kJ
mol -1
∆Hrxnθ = ∑∆Hf
θ(products) - ∑∆Hf
θ(reactants)
∆Hf θ
(products)∆Hf
θ
(reactants)
∆Hrxnθ
Elements
Std state solid gas
2C(s) + 3H2(g)+ ½O2(g) → C2H5OH(I) ∆Hf θ =- 275
kJ mol -1
1 mole formed
H2(g) + ½O2(g) → H2O(I) ∆Hf θ =- 286
kJ mol -1
Std state solid gas 1 mol liquid
For element Std ∆Hf θ formation = 0
Mg(s)→ Mg(s) ∆Hf θ = 0 kJ
mol -1
No product form
Using Std ∆Hf θ formation to find ∆H of a rxn
PDF version
Click here chem database (std formation enthalpy)
Online version
Click here chem database (std formation enthalpy)
C2H4 + H2 C2H6
Find ΔHθ rxn using std ∆H formation
Reactants Products
2C + 3H2
ElementsC2H4 + H2 → C2H6
∆Hrxnθ
∆Hrxnθ = ∑∆Hf
θ(pro) - ∑∆Hf
θ(react)
∆Hrxnθ = Hf
θ C2H6 - ∆Hf
θ C2H4+ H2
= - 84.6 – ( + 52.3 + 0 ) = - 136.9 kJ mol -1
2CH4(g) + 4O2(g) → 4H2O + 2CO2(s) ∆Hcθ = - 890
x 2 = - 1780 kJ mol -1
Std ∆Hf θ formation to find ∆H rxn ∆H when 1 mol form from its element under std
conditionNa(s) + ½ CI2(g) → NaCI (s) ∆Hf
θ = - 411 kJ mol -1
Hess’s Law Std ∆Hf θ
formation
Mg(s) + ½ O2(g) → MgO(s) ∆Hf θ =- 602 kJ
mol -1
Reactants Products
∆Hrxnθ = ∑∆Hf
θ(products) - ∑∆Hf
θ(reactants)
∆Hf θ
(products)∆Hf
θ
(reactants)
∆Hrxnθ
ElementsStd state solid gas 1 mole formed
Total amt energy released/absorbed α mol reactants
CH4(g) + 2O2(g) → 2H2O + CO2(s) ∆Hcθ = - 890
kJ mol -1
ΔH reverse EQUAL in magnitude but opposite sign to ΔH forward
Na+(g) + CI_
(g) → NaCI(s) ∆Hlattθ = - 770 kJ
mol -1
NaCI(s) → Na+(g) + CI_
(g) ∆Hlattθ = + 770
kJ mol -1
H2(g) + ½O2(g) → H2O(I) ∆Hf θ =- 286
kJ mol -1
H2(g) + ½O2(g) → H2O(I) ∆Hcθ =- 286
kJ mol -1
Compound NaF NaCI NaBr NaI
Hf θ(kJ mol-
1)-573 -414 -361 -288
More ↑ – ve formation↓
More ↑heat released to surrounding↓
More ↑ energetically stable (lower in energy)↓
Do not decompose easily
Subs Na2
OMgO
AI2O3
Hf θ -
416-
602-
1670
Subs P4O10 SO3 CI2O7
Hf θ -
3030-390 +250
1 mole formed
2 mole formed x 2
О
О
∆Hfθ formation vs ∆Hc
θ combustion
∆H Form - std state liquid
∆H Comb - std state liquid
More –ve – more stable
Across Period 3↓
∆H – more ↑ –ve ↓
Lower in energy↓
Oxides more stable
Across Period 3↓
∆H – more ↑ +ve ↓
Higher in energy↓
Oxides less stable – decompose easily
∆Hf = ∆Hc
∆Hrxn
C2H4 + H2 C2H6
C2H6 + 3.5 O2 2CO2 + 3 H2O
∆Hf θ (reactant) ∆Hf
θ (product)
Hess’s Law
Std ∆Hf θ formation to find ∆H rxn
Reactants Products∆Hrxnθ
∆Hf θ
(product)∆Hf
θ
(reactant)
Elements
∆Hf θ - 85 0 - 393 - 286
Reactants Products
∆Hrxnθ = ∑∆Hf
θ(pro) - ∑∆Hf
θ(react)
∆Hrxnθ = - 1644 - ( - 85 )
= - 1559 kJ mol -1
- 85 0 - 858 - 786
x 2 x 3
C2H4 + H2 C2H6
∆Hrxnθ Reactants Products
2C + 3H2
∆Hf θ + 52 o - 85
Reactants Products
∆Hrxnθ = ∑∆Hf
θ(pro) - ∑∆Hf
θ(react)
∆Hrxnθ = - 85 - ( + 52 )
= - 137 kJ mol -1
C2H6 + 3.5 O2 2CO2 + 3 H2O
2C + 3H2 + 3.5O2
Energy Level Diagram
2CO2 + 3H2O
2C + 3H2 + 3.5O2
C2H6 + 3.5 O2
∆Hf θ = -85
∆Hf θ = -
393
∆Hf θ = 0
2C + 3H2 + 3.5O2 Elements
Reactants
Products
∆Hf θ = -
286
∆Hrxn= (- 393 x 2 + -286 x 3) – (- 85 + 0) = - 1559 kJ mol-1
x 2
x 3
C2H4 + H2 Reactants
C2H6
∆Hrxn = - 85 – ( + 52 + 0 ) = - 137 kJ mol-1
2C + 2H2 + H2
∆Hf θ = +
52
∆Hf θ = 0
∆Hrxn
Products
2C + 3H2 Elements
∆Hf θ = -
85
Elements
∆Hf θ (reactant) ∆Hf
θ (product)
∆Hf θ
(reactant)∆Hf
θ
(product)
Using Std ∆Hf θ formation to find ∆H rxn
Hess’s Law
Reactants Products
∆Hrxnθ = ∑∆Hf
θ(products) - ∑∆Hf
θ(reactants)
∆Hf θ
(products)∆Hf
θ
(reactants)
∆Hrxnθ
Elements
2H2S + SO2 3S + 2H2O
Find ΔHθ rxn using std ∆H formation
Reactants Products
3S + O2 + 2H2
Elements
∆Hrxnθ
∆Hrxnθ = ∑∆Hf
θ(pro) - ∑∆Hf
θ(react)
∆Hrxnθ = - 572 - ( - 338 )
= - 234 kJ mol -1
2H2S + SO2 → 3S + 2H2O∆Hf
θ - 20.6 - 297 0 - 286
- 41.2 - 297 0 - 572
x 2 x 2
Reactants Products
Using Std ∆Hf θ formation to find ∆H rxn
Reactants Products∆Hrxnθ
∆Hf θ
(product)∆Hf
θ
(reactant)4C + 12H2 + 9N2 + 10O2
Elements
∆Hf θ + 53 - 20 - 393 - 286 0
Reactants Products
∆Hrxnθ = ∑∆Hf
θ(pro) - ∑∆Hf
θ(react)
∆Hrxnθ = - 5004 - ( +112 )
= - 5116 kJ mol -1
4CH3NHNH2 + 5N2O4 4CO2 + 12H2O + 9N2
4CH3NHNH2 + 5N2O4 4CO2 + 12H2O + 9N2
+ 212 - 100 - 1572 - 3432 0
x 4 x 5 x 4 x 12
NH4NO3 N2O + 2H2O∆Hrxn
θ Reactants Products
N2 + 2H2 + 3/2O2
NH4NO3 N2O + 2H2O
∆Hf θ - 366 + 82 - 286 x 2
Reactants Products
∆Hrxnθ = ∑∆Hf
θ(pro) - ∑∆Hf
θ(react)
∆Hrxnθ = - 488 - ( - 366 )
= - 122 kJ mol -1
Elements
∆Hc θ
(reactant)∆Hc
θ
(product)
∆Hcθ combustion to find ∆H formation of
hydrocarbon
∆H when 1 mol completely burnt in oxygen under std condition
Std Enthalpy Changes ∆Hθ
Std condition
Pressure 100kPa
Temp 298K
Conc 1M All substance at std states
Hess’s Law
Std ∆Hc θ combustion
C(s) + O2(g) → CO2(g) ∆Hcθ = - 395 kJ
mol -1
Reactants Products
∆Hrxnθ = ∑∆Hc
θ(reactant) - ∑∆Hc
θ(product)
∆Hcθ
(product)∆Hc
θ
(reactant)
∆Hrxnθ
Combusted products
1 mole combusted
H2(g) + ½O2(g) → H2O(I) ∆Hc θ = - 286 kJ
mol -1
Std ∆Hc θ combustion to find ∆H of a rxn
PDF version
Click here chem database (std combustion enthalpy)
Online version
Click here chem database (std combustion enthalpy)
Find ΔHθ formation using std ∆H comb
Reactants Products
2CO2 + 3H2O
∆Hrxnθ
∆Hrxnθ = ∑∆Hc
θ(react) - ∑∆Hc
θ(pro)
∆Hrxnθ = Hc
θ ( C + H2 ) - ∆Hc
θ C2H6
= 2 x (-395) + 3 x (-286) – (-1560) = - 88 kJ mol -1
H2(g) + ½O2(g) → H2O(I) ∆Hc θ = - 286 kJ
mol -1
C(s) + O2(g) → CO2(g) ∆Hcθ = - 395 kJ
mol -1
Combustion H2 = formation of H2O∆Hc = ∆Hf
Combustion C = formation of CO2
∆Hc = ∆Hf
Using combustion data
2C(s) + 3H2(g) → C2H6(g) ∆Hf θ = -
84 kJ mol -1
2C(s) + 3H2(g) + ½O2(g) → C2H5OH(I) ∆Hf θ = -
275 kJ mol -1
2C + 3H2 C2H6
+ 3.5O2 + 3.5O2
x 2 x 3
How enthalpy formation hydrocarbon obtained ?
-790 -858 - 1371
2 C + 3H2 + 3.5O2 C2H5OH
∆Hc θ (reactant) ∆Hc θ (product)
∆Hrxnθ = ∑∆Hc
θ(react) - ∑∆Hc
θ(pro)
= 2 x (-395) + 3 x (-286) – (-1560) = - 88 kJ mol -1
Reactants Products
2 C + 3H2 + 3.5O2 C2H5OH
2C + 3H2 C2H6
2C + 3H2 C2H6
∆Hcθ comb to find ∆Hf formation of
hydrocarbon
∆Hrxn
Hess’s Law
Reactants Products∆Hrxnθ
∆Hc θ
(product)∆Hc
θ (reactant)
∆Hc θ -395 -286 - 1560
Reactants Products
- 790 -858 - 1560
x 2
∆Hrxnθ
Reactants Products
Energy Level Diagram
∆Hc θ = -395
∆Hc θ = - 1560
∆Hc θ = -286
Combusted products
Reactants
Products
∆Hrxn = (- 395 x 2 + -286 x 3) – (-1560 ) = - 88 kJ mol-1
Reactants
∆Hrxn
Products
∆Hc θ = -
1371
2CO2 + 3H2O
x 3
2C + 3H2
C2H6
2CO2 + 3H2O 2CO2 + 3H2O
x 2 x 3
2 C + 3H2 + 3.5O2
2CO2 + 3H2O
∆Hc θ -395 -286 - 1371
∆Hrxnθ = ∑∆Hc
θ(react) - ∑∆Hc
θ(pro)
= 2 x (-395) + 3 x (-286) – (-1371) = - 275 kJ mol -1
x 2 x 3
C2H5OH
∆Hc θ = -395 x 2
∆Hc θ = -286 x 3
2CO2 + 3H2OCombusted products
2CO2 + 3H2O
∆Hrxn = (- 395 x 2 + -286 x 3) – (-1371 ) = - 275 kJ mol-1
+ 3.5O2 + 3.5O2
+ 3.5O2 + 3.5O2
3C2H2 C6H6
∆Hc θ (reactant) ∆Hc θ (product)
∆Hrxnθ = ∑∆Hc
θ(react) - ∑∆Hc
θ(pro)
= 6 x (-395) + 3 x (-286) – (-3271) = + 49 kJ mol -1
Reactants Products
3 C2H2 C6H6
6C + 3H2 C6H6
6C + 3H2 C6H6
∆Hcθ comb to find ∆Hf formation of
hydrocarbon
∆Hrxn
Hess’s Law
Reactants Products∆Hrxnθ
∆Hc θ
(product)∆Hc
θ (reactant)
∆Hc θ -395 -286 - 3271
Reactants Products
-2370 -858 - 3271
x 6
∆Hrxnθ
Reactants Products
Energy Level Diagram
∆Hc θ = -395
∆Hc θ = - 3271
∆Hc θ = -286
Combusted products
Reactants
Products
∆Hrxn = (- 395 x 6 + -286 x 3) – (-3271 ) = + 49 kJ mol-1
Reactants
∆Hrxn
Products
∆Hc θ = -
3270
6CO2 + 3H2O
x 3
6C + 3H2
C6H6
6CO2 + 3H2O 6CO2 + 3H2O
x 6 x 3
3 C2H2
6CO2 + 3H2O
∆Hc θ -1300 -
3270
∆Hrxnθ = ∑∆Hc
θ(react) - ∑∆Hc
θ(pro)
= 3 x (- 1300) – (-3270) = - 630 kJ mol -1
x 3
C6H6
∆Hc θ = -1300 x 3
6CO2 + 3H2OCombusted products
6CO2 + 3H2O
∆Hrxn = (- 1300 x 3 ) – (-3270 ) = - 630 kJ mol-1
-3900 - 3270
Combustion C2H2
+ 7.5O2 + 7.5O2
+ 7.5O2 + 7.5O2
CH2 =CHCH3 + H2 → CH3CH2CH3
CH2 = CHCH3 + H2 CH3CH2CH3
Reactants Products
∆Hc θ (reactant) ∆Hc θ (product)
∆Hrxnθ = ∑∆Hc
θ(react) - ∑∆Hc
θ(pro)
= ( -1411) + (-286) – (-1560) = - 137 kJ mol -1
C2H4 + H2 C2H6
∆Hcθ comb to find ∆Hf formation of
hydrocarbon
∆Hrxn
Hess’s Law
Reactants Products∆Hrxnθ
∆Hc θ
(product)∆Hc
θ (reactant)
∆Hc θ -1411 -286 - 1560
Reactants Products
∆Hrxnθ
Reactants Products
Energy Level Diagram
∆Hc θ = -1411
∆Hc θ = - 1560
∆Hc θ = -286
Combusted products
Reactants
Products
∆Hrxn = (-1411) + (-286) – (-1560) = - 137 kJ mol-1
Reactants
∆Hrxn
Products
∆Hc θ = -
2222
2CO2 + 3H2OC6H6
2CO2 + 2H2O 2CO2 + 3H2O
3CO2 + 4H2O
∆Hc θ - 2060 -286
- 2222 ∆Hrxn
θ = ∑∆Hc θ(react) - ∑∆Hc
θ(pro)
= (-2060) + (-286) – (-2222) = - 124 kJ mol -1
∆Hc θ = -2060
3CO2 + 3H2OCombusted products
3CO2 + 4H2O
∆Hrxn = (-2060) + (-286) – (-2222 ) = - 124 kJ mol-1
Combustion CH2 =CHCH3
C2H4 + H2 C2H6
C2H4 + H2
+ H2O
Combustion C2H4
CH3CH2CH3
CH2=CHCH3 + H2
∆Hc θ = -286
+ H2O
+ 3.5O2 + 3.5O2
+ 5O2 + 5O2
∆Hc
Std ∆Hf θ formation to find ∆H rxn
2H2S + SO2 → 2H2O + 3S ∆Hrxn = ?
∆Hf θ - 21 x 2 - 297 - 286 x 2 0
∆Hrxnθ = ∑∆Hf
θ(pro) - ∑∆Hf
θ(react)
∆Hrxnθ = - 572 - ( - 339 )
= - 233 kJ mol -1
Reactants Products
IB Question ∆Hf and ∆Hc
1 2
2 Pb(NO3)2 → 2PbO + 4 NO2 + O2 ∆Hrxn = ? ∆Hf
θ - 444 x 2 - 218 x 2 + 34 x 4 0
Std ∆Hf θ formation to find ∆H rxn
Reactants Products
∆Hrxnθ = ∑∆Hf
θ(pro) - ∑∆Hf
θ(react)
∆Hrxnθ = - 300 - ( - 888 )
= + 588 kJ mol -1
∆Hcθ comb to find ∆Hf formation of C6H6
(Benzene)
6C + 3H2 → C6H6 ∆H form= ?∆Hc
θ -395 x 6 -286 x 3 - 3271
∆Hrxnθ = ∑∆Hc
θ(react) - ∑∆Hc
θ(pro)
= 6 x (-395) + 3 x (-286) – (-3271) = + 49 kJ mol -1
Reactants Products
2C + 3H2 + 3.5O2 → C2H5OH ∆Hform= ? ∆Hc
θ -395 x 2 -286 x 3 - 1371
∆Hrxnθ = ∑∆Hc
θ(react) - ∑∆Hc
θ(pro)
= 2 x (-395) + 3 x (-286) – (-1371) = - 275 kJ mol -1
∆Hcθ comb to find ∆Hf formation of C2H5OH
(Ethanol)
Reactants Products
Combustion C / H2
to CO2 and H2O
C2H4 + H2 → C2H6 ∆H rxn = ?
∆Hcθ comb to find ∆H rxn of
C2H6
∆Hrxnθ = ∑∆Hc
θ(react) - ∑∆Hc
θ(pro)
= ( -1411) + (-286) – (-1560) = - 137 kJ mol -1
∆Hc θ -1411 -286 - 1560
Reactants Products
Find ∆Hc using ∆H provided
CH2 = CHCH3 + H2 CH3CH2CH3
3CO2 + 4H2O
CH2 =CHCH3 + H2 → CH3CH2CH3
∆Hc θ x -286
- 2222
∆Hrxnθ = ∑∆Hc
θ(react) - ∑∆Hc
θ(pro)
- 124 = x + (-286) – (-2222) x = - 2060 kJ mol -1
Reactants Products
∆H = -2222
∆H = - 124
CH2 = CHCH3 + H2 → CH3CH2CH3 ∆H = -124CH3CH2CH3 + 5O2 → 3CO2 + 4H2O ∆H = -2222H2 + ½ O2 → H2O ∆H = -28.6Combustion of
hydrocarbon
Formation Enthalpy
use
NOT Formation Enthalpy
use
Diamond unstable respect to graphite↓
Kinetically stable (High Ea)↓
Wont decompose spontaneous
∆H = - 98
∆H = - 187
H2O(I) + 1/2O2(g)
H2O2(I)
H2(g) + O2(g)
∆H2= -111
∆H1 = -394
CO2(g)
C(s) + ½ O2(g)
CO(g) + ½ O2
C(s) + O2(g)
CO2(g)
∆H3 = -283
Energy Level Diagram
Energy Cycle Diagram
Energetic stability vs Kinetic stability
C(s) + O2(g) → CO2 ∆H = - 394CO(g) + 1.5O2(g) → CO2 ∆H = - 283C(s) + 1.5O2(g) → CO ∆H = - 111
Lower in energy ( -ve ∆H)↓
Thermodynamically more stable
∆H = - ve
All are thermodynamically stable (-ve ∆H)↓
More heat released to surroundingLower in energy
↓Both oxides (CO2/CO) are thermodynamically
↓Stable with respect to their element (C and O2)
H2(g) + O2(g) → H2O2 ∆H = - 187H2O2(I) → H2O + O2 ∆H = - 98
C(diamond) → C (graphite)
C(diamond) → C (graphite) ∆H = - 2
Diamond forever
H2O2 unstable respect to H2O/O2
↓Kinetically stable (High Ea)
↓Wont decompose spontaneous
All are thermodynamically stable (-ve ∆H)↓
More heat released (lower energy)↓
H2O2 thermodynamically more stable with respect to its elements H2/O2
↓H2O2 unstable with respect to H2O and O2 Will decompose to lower energy (stable)
High Activation energyKinetically stableWont decompose
∆H = - ve
H2O (I) + O2
H2O2(I)∆H = - ve
High Activation energyKinetically stableWont decompose
C + O2 energetically unstable respect to CO2
↓Kinetically stable (High Ea)
↓Wont react spontaneous unless ignited!
C + O2
CO2(g)
High Activation energyKinetically stableWont decompose
C graphite thermodynamically more
stable with respect diamond↓
Will diamond decompose to graphite
∆H = - veDiamond
Graphite
∆H = - 50 – (+12) = - 62 kJ mol -1
∆H = - 50 kJ mol -1
Cold water = 50g
CuSO4 .100H2O
CuSO4 (s) + 95H2O → CuSO4 .100H2O
CuSO4 (s) + 100H2O → CuSO4 .100H2O
Mass cold water add = 50 gMass warm water add = 50 gInitial Temp flask/cold water = 23.1 CInitial Temp warm water = 41.3 CFinal Temp mixture = 31 C
CuSO4 (s) + 5H2O → CuSO4 .5H2O ∆H = ?
Slow rxn – heat lost huge – flask used.
Heat capacity flask must be determined. CuSO4 (s) + 5H2O → CuSO4 .5H2O
1. Find heat capacity flask
Ti = 23.1C
Hot water = 50g T i = 41.3C
No heat loss from system (isolated system)
↓ ∆H system = O
Heat lost hot H2O=Heat absorb cold H2O + Heat absorb flask (mc∆θ) (mc∆θ) (c ∆θ)
50 x 4.18 x (41.3 – 31) = 50 x 4.18 x (31-23.1) + c x (31-23.1)
Water Flask CuSO4
Heat capacity flask, c = 63.5JK-1
Mass CuSO4 = 3.99 g (0.025mol)Mass water = 45 gT initial flask/water = 22.5 CT final = 27.5 C
Mass CuSO4 5H2O = 6.24 g (0.025mol)Mass water = 42.75 gT initial flask/water = 23 CT final = 21.8 C
2. Find ∆H CuSO4 +100H2O → CuSO4 .100H2O3. Find ∆H CuSO4 .5H2O + 95H2O → CuSO4 .100H2O
Hess’s Law
CuSO4 + H2O → CuSO4 .100H2O CuSO4 .5H2O + H2O → CuSO4 .100H2O
Water Flask CuSO4 5H2O
∆Hrxn = Heat absorb water + vacuum (mc∆θ) + (c∆θ)
∆Hrxn = Heat absorb water + vacuum (mc∆θ) + (c∆θ)
∆Hrxn= 45 x 4.18 x 5 + 63.5 x 5∆Hrxn= - 1.25 kJ
∆Hrxn= 42.75 x 4.18 x 1.2 + 63.5 x 1.2∆Hrxn= + 0.299 kJ
0.025 mol = - 1.25 kJ 1 mol = - 50 kJ mol-1
0.025 mol = + 0.299 kJ 1 mol = + 12 kJ mol-1
CuSO4 (s) + 5H2O → CuSO4 .5H2O
CuSO4 .100H2O
∆H = + 12 kJ mol -1
AssumptionNo heat lost from system ∆H = 0Water has density = 1.0 gml-1
Sol diluted Vol CuSO4 = Vol H2OAll heat transfer to water + flask
Rxn slow – lose heat to surrounding Plot Temp/time – extrapolation done, Temp correction
limiting
Enthalpy change ∆H - using calorimeter
without temp correction
Lit value = - 78 kJ mol -1
CONTINUE
Time/m
0 0.5
1 1.5
2 2.5
3 3.5
4
Temp/C 22
22 22 22 22
27 28 27 26
∆H = - 60 kJ mol -1
CuSO4 .100H2O
CuSO4 (s) + 95H2O → CuSO4 .100H2O
CuSO4 (s) + 100H2O → CuSO4 .100H2O
CuSO4 (s) + 5H2O → CuSO4 .5H2O ∆H = ?
CuSO4 (s) + 5H2O → CuSO4 .5H2O
Water Flask CuSO4
Mass CuSO4 = 3.99 g (0.025 mol)Mass water = 45 gT initial mix = 22 CT final = 28 C
Mass CuSO4 5H2O = 6.24 g (0.025 mol)Mass water = 42.75 gT initial mix = 23 CT final = 21 C
Find ∆H CuSO4 + 100H2O → CuSO4 .100H2OFind ∆H CuSO4 .5H2O + 95H2O → CuSO4 .100H2O
Hess’s Law
CuSO4 + H2O → CuSO4 .100H2O CuSO4 .5H2O + H2O → CuSO4 .100H2O
Water Flask CuSO4 5H2O
∆Hrxn = Heat absorb water + flask (mc∆θ) + (c∆θ)
∆Hrxn = Heat absorb water + flask (mc∆θ) + (c∆θ)
∆Hrxn= 45 x 4.18 x 6 + 63.5 x 6∆Hrxn= - 1.5 kJ
∆Hrxn= 42.75 x 4.18 x 2 + 63.5 x 2∆Hrxn= + 0.48kJ
0.025 mol = - 1.5 kJ 1 mol = - 60 kJ mol-1
0.025 mol = + 0.48 kJ 1 mol = + 19 kJ mol-1
CuSO4 (s) + 5H2O → CuSO4 .5H2O
CuSO4 .100H2O
∆H = + 19 kJ mol -1
∆H = - 60 – (+19) = - 69 kJ mol -1
Rxn slow – lose heat to surrounding Plot Temp/time – extrapolation done, Temp correction
limiting
Enthalpy change ∆H using calorimeter
Data collection
Temp correction – using cooling curve for last 5 m
time, x = 2initial Temp = 22 C
final Temp = 28 C
Extrapolation best curve fit y = -2.68x + 33 y = -2.68 x 2 + 33 y = 28 (Max Temp)
Excel plot
CuSO4 + H2O → CuSO4 .100H2O(Exothermic) Heat released
CuSO4 .5H2O + H2O → CuSO4 .100H2O(Endothermic) – Heat absorbed
Temp correction – using warming curve for last 5 m
Time/m
0 0.5
1 1.5
2 2.5
3 3.5 4
Temp/C 23
23 23 23 23
22 22 22.4 22.7
initial Temp = 23 C
time, x = 2
final Temp = 21 C
Excel plot
Extrapolation curve fit y = + 0.8 x + 19.4 y = + 0.8 x 2 + 19.4 y = 21 (Min Temp)
Lit value = - 78 kJ mol -1
with temp correction
∆H = - 113 kJ mol -1
Enthalpy change ∆H using calorimeter
Cold water = 50 g
MgSO4 .100H2O
MgSO4.7H2O + 93H2O → MgSO4 .100H2O
MgSO4 + 100H2O → MgSO4 .100H2O
Mass cold water add = 50 gMass warm water add = 50 gInitial Temp flask/cold water = 23.1 CInitial Temp warm water = 41.3 CFinal Temp flask/mixture = 31 C
MgSO4 (s) + 7H2O → MgSO4 .7H2O ∆H = ?
Slow rxn – heat lost huge – flask used.
Heat capacity flask must be determined. MgSO4(s) + 7H2O → MgSO4 .7H2O
1. Find heat capacity flask
Ti = 23.1 C
Hot water = 50 g T i = 41.3 C
No heat loss from system (isolated system)
↓ ∆H system = O
Heat lost hot H2O=Heat absorb cold H2O + Heat absorb flask (mc∆θ) (mc∆θ) (c ∆θ)
50 x 4.18 x (41.3 – 31) = 50 x 4.18 x (31-23.1) + c x (31-23.1)
Water Flask MgSO4
Heat capacity flask, c = 63.5JK-1
Mass MgSO4 = 3.01 g (0.025mol)Mass water = 45 gT initial mix = 24.1 CT final = 35.4 C
Mass MgSO4 7H2O = 6.16 g (0.025mol)Mass water = 41.8 gT initial mix = 24.8 CT final = 23.4 C
2. Find ∆H MgSO4 +100H2O → MgSO4 .100H2O3. Find ∆H MgSO4 .7H2O + 93H2O → MgSO4 .100H2O
Hess’s Law
MgSO4 + H2O → MgSO4 .100H2O MgSO4 .7H2O + H2O → MgSO4 .100H2O
Water Flask MgSO4 .7H2O
∆Hrxn = Heat absorb water + flask (mc∆θ) + (c∆θ)
∆Hrxn = Heat absorb water + flask (mc∆θ) + (c∆θ)
∆Hrxn= 45 x 4.18 x 11.3 + 63.5 x 11.3∆Hrxn= - 2.83 kJ
∆Hrxn= 41.8 x 4.18 x 1.4 + 63.5 x 1.4∆Hrxn= + 0.3 kJ
0.025 mol = - 2.83 kJ 1 mol = - 113 kJ mol-1
0.025 mol = + 0.3 kJ 1 mol = + 12 kJ mol-1
MgSO4 (s) + 7H2O → MgSO4 .7H2O
MgSO4 .100H2O
∆H = + 12 kJ mol -1
∆H = - 113 - 12 = - 125 kJ mol -1
AssumptionNo heat lost from system ∆H = 0Water has density = 1.00g ml-1
Sol diluted Vol MgSO4 = Vol H2OAll heat transfer to water + flask
Rxn fast – heat lost to surrounding minimizedDont need to Plot Temp/time
No extrapolation
Error AnalysisHeat loss to surroundingHeat capacity sol is not
4.18Mass MgSO4 ignored
Impurity presentMgSo4 already hydrated
limiting
∆H = - 286 kJ mol -1
∆H = - 442 kJ mol -1
Enthalpy change ∆H using calorimeter
Cold water = 50g
Mass cold water add = 50gMass warm water add = 50gInitial Temp flask/cold water = 23.1CInitial Temp warm water = 41.3CFinal Temp flask/mixture = 31C
Mg(s) + ½ O2 → MgO ∆H = ?
Slow rxn – heat lost huge – flask is used.
Heat capacity flask must be determined. Mg + ½ O2 → MgO
1. Find heat capacity flask
Ti = 23.1C
Hot water = 50g T i = 41.3C
No heat loss from system (isolated system)
↓ ∆H system = O
Heat lost hot H2O=Heat absorb cold H2O + Heat absorb Flask (mc∆θ) (mc∆θ) (c ∆θ)
50 x 4.18 x (41.3 – 31) = 50 x 4.18 x (31-23.1) + c x (31-23.1)
HCI Flask Mg
Heat capacity flask, c = 63.5JK-1
Mass Mg = 0.5 g (0.02 mol)Vol/Conc HCI = 100 g, 0.1MT initial mix = 22 CT final = 41 C
Mass MgO = 1 g (o.o248 mol)Vol/Conc HCI = 100 g, 0.1MT initial mix = 22 CT final = 28.4 C
2. Find ∆H Mg + 2HCI → MgCI2 + H2
3. Find ∆H MgO + 2HCI → MgCI2 + H2O4. Find H2 + ½ O2 → H2O
Hess’s Law
∆H Mg + 2HCI → MgCI2 + H2
∆Hrxn = Heat absorb water + flask (mc∆θ) + (c∆θ)
∆Hrxn = Heat absorb water + flask (mc∆θ) + (c∆θ)
∆Hrxn= 100 x 4.18 x 19 + 63.5 x 19∆Hrxn= - 9.11kJ
∆Hrxn= 100 x 4.18 x 6.4 + 63.5 x 6.4∆Hrxn= -3.1 kJ
0.02 mol = - 9.11 kJ 1 mol = - 442 kJ mol-1
0.0248 mol = - 3.1 kJ 1 mol = - 125 kJ mol-1
∆H = - 125 kJ mol -1
∆H = - 442 – 286 + 125 = - 603 kJ mol -1
AssumptionNo heat lost from system ∆H = 0Water has density = 1.00g ml-1
Sol diluted Vol HCI = Vol H2OAll heat transfer to water + flask
Rxn fast – heat lost to surrounding minimizedDont need to Plot Temp/time
No extrapolation
Error AnalysisHeat loss to surrounding
Heat capacity sol is not 4.18
Mass MgO ignoredImpurity present
Effervescence cause loss Mg
+ 2HCI
MgCI2 + H2
HCI Flask MgO
+ ½ O2
MgCI2 + H2O
+ 2HCI
MgCI2 + H2O
∆H MgO + 2HCI → MgCI2 + H2O
Mg + ½ O2 → MgO
MgCI2 + H2
+ 2HCI
MgCI2 + H2O
+ ½ O2
limiting
MgCI2 + H2O
Data given
2KHCO3(s) → K2CO3 + CO2 + H2O
∆H = + 51.4 kJ mol -1
Enthalpy change ∆H for rxn using calorimeter
Cold water = 50g
Mass cold water add = 50 gMass warm water add = 50 gInitial Temp flask/cold water = 23.1 CInitial Temp warm water = 41.3 CFinal Temp flask/mixture = 31 C
2KHCO3(s) → K2CO3 + CO2 + H2O ∆H = ?
Slow rxn – heat lost huge – vacuum flask is used.
Heat capacity of flask must be determined.
1. Find heat capacity flask
Ti = 23.1C
Hot water = 50g T i = 41.3C
No heat loss from system (isolated system)
↓ ∆H system = O
Heat lost hot H2O=Heat absorb cold H2O + Heat absorb flask (mc∆θ) (mc∆θ) (c ∆θ)
50 x 4.18 x (41.3 – 31) = 50 x 4.18 x (31-23.1) + c x (31-23.1)
HCI Flask KHCO3
Heat capacity flask, c = 63.5JK-1
Mass KHCO3 = 3.5 g (0.035 mol)Vol/Conc HCI = 30 g, 2MT initial mix = 25 CT final = 20 C
Mass K2CO3 = 2.75 g (0.02 mol)Vol/Conc HCI = 30 g, 2MT initial mix = 25 CT final = 28 C
2. Find ∆H 2KHCO3 + 2HCI → 2KCI + 2CO2 + 2H2O3. Find ∆H K2CO3 + 2HCI → 2KCI + CO2 + H2O
Hess’s Law
∆Hrxn = Heat absorb water + flask (mc∆θ) + (c∆θ)
∆Hrxn = Heat absorb water + flask (mc∆θ) + (c∆θ)
∆Hrxn= 30 x 4.18 x 5 + 63.5 x 5∆Hrxn= + 0.9 kJ
∆Hrxn= 30 x 4.18 x 3 + 63.5 x 3∆Hrxn= -0.56 kJ
0.035 mol = + 0.9 kJ 1 mol = + 25.7 kJ mol-1
0.02 mol = - 0.56 kJ 1 mol = - 28 kJ mol-1
∆H = - 28 kJ mol -
1
∆H = +51.4 – (-28) = + 79 kJ mol -1
AssumptionNo heat lost from system ∆H = 0Water has density = 1.00g ml-1
Sol diluted Vol HCI = Vol H2OAll heat transfer to water + vacuum
Rxn fast – heat lost to surrounding minimizedDont need to Plot Temp/time
No extrapolation
Error AnalysisHeat loss to surrounding
Heat capacity sol is not 4.18
Mass of MgO ignoredImpurity present
Effervescence cause loss Mg
+ 2HCI
HCI Flask K2CO3
2KCI + 2CO2 + 2H2O
+ 2HCI
+ 2HCI
limiting
2KHCO3(s) → K2CO3 + CO2 + H2O
2KCI + CO2 + H2O
2KHCO3 + 2HCI → 2KCI + 2CO2 + 2H2O K2CO3 + 2HCI → 2KCI + CO2 + H2O
x 2
2KCI + 2CO2 + 2H2O 2KCI + CO2 + H2O
+ 2HCI
Acknowledgements
Thanks to source of pictures and video used in this presentation
Thanks to Creative Commons for excellent contribution on licenseshttp://creativecommons.org/licenses/
Prepared by Lawrence Kok
Check out more video tutorials from my site and hope you enjoy this tutorialhttp://lawrencekok.blogspot.com