Standard Enthalpy Changes of Reaction Section 15.1.
-
Upload
laureen-franklin -
Category
Documents
-
view
230 -
download
0
Transcript of Standard Enthalpy Changes of Reaction Section 15.1.
![Page 1: Standard Enthalpy Changes of Reaction Section 15.1.](https://reader036.fdocuments.us/reader036/viewer/2022082400/5697c00b1a28abf838cc8594/html5/thumbnails/1.jpg)
Standard Enthalpy Changes of Reaction
Section 15.1
![Page 2: Standard Enthalpy Changes of Reaction Section 15.1.](https://reader036.fdocuments.us/reader036/viewer/2022082400/5697c00b1a28abf838cc8594/html5/thumbnails/2.jpg)
Introduction
Enthalpy change of formation: specified by ΔHθ
f is the heat change (at constant
pressure) on production of one mole of the pure substance from its elements in their standard state under standard thermodynamic conditions (298 K and 1 atm pressure)
![Page 3: Standard Enthalpy Changes of Reaction Section 15.1.](https://reader036.fdocuments.us/reader036/viewer/2022082400/5697c00b1a28abf838cc8594/html5/thumbnails/3.jpg)
More
Standard state: generally the most thermodynamically stable form of the pure element that exists under standard thermodynamic conditions
For example, for carbon it is graphite
![Page 4: Standard Enthalpy Changes of Reaction Section 15.1.](https://reader036.fdocuments.us/reader036/viewer/2022082400/5697c00b1a28abf838cc8594/html5/thumbnails/4.jpg)
Example
ΔHθf of silver bromide is the enthalpy
change for the reaction: Ag(s) + ½Br2(l) → AgBr(s) ΔHθ
f = -99.5kJ mol-1
Notice the formation of one mole and the standard states
2Ag(s) + Br2(l) → 2AgBr(s) makes 2 mol
Ag(s) + ½Br2(g) → AgBr(s) Br not in standard state
![Page 5: Standard Enthalpy Changes of Reaction Section 15.1.](https://reader036.fdocuments.us/reader036/viewer/2022082400/5697c00b1a28abf838cc8594/html5/thumbnails/5.jpg)
Calculations
Enthalpy change of a reaction = the sum of enthalpies of formation of products – sum of enthalpies of formation of reactants
ΔH = ΣΔHθf [products] – ΣΔHθ
f [reactants]
reactants products
elements
![Page 6: Standard Enthalpy Changes of Reaction Section 15.1.](https://reader036.fdocuments.us/reader036/viewer/2022082400/5697c00b1a28abf838cc8594/html5/thumbnails/6.jpg)
Example Problem #1
Calculate the enthalpy change of the following reaction:
3CuO(s)
+ 2Al(s)
→ 3Cu(s)
+ Al2O
3(s)
ΔHθf [CuO] = -155 kJmol-1
ΔHθf [Al
2O
3] = -1669 kJmol-1
See the board for the working out
![Page 7: Standard Enthalpy Changes of Reaction Section 15.1.](https://reader036.fdocuments.us/reader036/viewer/2022082400/5697c00b1a28abf838cc8594/html5/thumbnails/7.jpg)
Example Problem #2
Calculate the enthalpy change of the following reaction:
NH4NO
3(s) → N
2O
(g) + 2H
2O
(l)
ΔHθf [NH
4NO
3] = -366 kJ mol-1
ΔHθf [N
2O] = +82 kJ mol-1
ΔHθf [H
2O] = -285 kJ mol-1
![Page 8: Standard Enthalpy Changes of Reaction Section 15.1.](https://reader036.fdocuments.us/reader036/viewer/2022082400/5697c00b1a28abf838cc8594/html5/thumbnails/8.jpg)
Continued
ΔHθreaction
= ΔHθf,products
– ΔHθf, reactants
Have to consider 2 mol of water ΔHθ = (82 + (2) (-285)) - (-366) ΔHθ = (82 + (-570)) - (-366) ΔHθ = -488 + 366 ΔHθ = -122 kJ mol-1
![Page 9: Standard Enthalpy Changes of Reaction Section 15.1.](https://reader036.fdocuments.us/reader036/viewer/2022082400/5697c00b1a28abf838cc8594/html5/thumbnails/9.jpg)
Enthalpy Change of Combustion ΔHθ
c is the enthalpy change (at constant
pressure) when one mole of a pure substance undergoes complete combustion under standard thermodynamic conditions
Products of complete combustion are CO2
and H2O
Can be used to calculate the total enthalpy change only if both sides can be burnt in oxygen
![Page 10: Standard Enthalpy Changes of Reaction Section 15.1.](https://reader036.fdocuments.us/reader036/viewer/2022082400/5697c00b1a28abf838cc8594/html5/thumbnails/10.jpg)
Using Hess's Law elements compounds
+ O2
+ O2
combustion products
ΔHθ =ΣΔHθc [reactants] – ΣΔHθ
c [products]
Combustion is from the compound while formation is to the compound
![Page 11: Standard Enthalpy Changes of Reaction Section 15.1.](https://reader036.fdocuments.us/reader036/viewer/2022082400/5697c00b1a28abf838cc8594/html5/thumbnails/11.jpg)
Example Problem #3
Calculate the enthalpy change for the hydration of ethene according to the following equation:
C2H
4(l) + H
2O
(l) → C
2H
5OH
(l)
ΔHθc [C
2H
4] = -1409 kJ mol-1
ΔHθc [C
2H
5OH] = -1371kJ mol-1
ΔHθ = -1409 - (-1371) = - 38 kJ mol-1