Carsen Dietz
2021-26-10
Hess’s Law
Hypthesis: If able to determine the molar enthalpy of combustion of magnesium then enthalpy change can
ssssssssssssbe calculated using Hess’s Law.
Observations:
Reactant
Mass(g)
Magnesium Metal
Magnesium Oxide
Hydrochloric Acid
0.18
1.00
100g
Initial Temperature of HCl
(°C)
19
19
19
Final Temperature of Mixture
(°C)
28
27
NA
Analysis:
a) Reaction 1:
m= 100g
c= 4.18 J/°C
ΔT = (28-19)
= 9.0°C
Reaction 2:
m= 100g
c= 4.18 J/°C
ΔT = (27-19)
= 8.0°C
b) Reaction1:
Mg
0.18g Mg x
q=mcΔT
q=(100)(4.18)(9.0)
q=3762 J
q=3.762 KJ
q=mcΔT
q=(100)(4.18)(8.0)
q=3344J
q=3.344 KJ
1 𝑚𝑜𝑙
24.305𝑔 𝑀𝑔
3.762 𝐾𝐽
7.41𝑥10−3 𝑚𝑜𝑙 𝑀𝑔
= 7.41x10-3 mol Mg
=507.7 KJ/mol
qgained=-qlost
q=- 507.7 KJ/mol Mg
Reaction 2:
MgO
1.0g MgO x
1 𝑚𝑜𝑙
40.304𝑔 𝑀𝑔𝑂
= 2.48x10-2 mol MgO
3.344 𝐾𝐽
2.48𝑥10−2𝑚𝑜𝑙 𝑀𝑔𝑂
= 134.8 KJ/mol
qgained=-qlost
q= -134.8 KJ/mol MgO
c) Reaction 1: Mg(s)+2HCl(aq)→H2(g)+MgCl2(aq)
Reaction 2: MgO(s)+2HCl(aq)→H2O(l)+MgCl2(aq)
ΔH= -507.7 KJ/mol
ΔH= -134.8 KJ/mol
1
Reaction 3: H2(g)+ 2 O2(g)→ H2O(l
1
Mg(s +2
O2(g)→ MgO(s
1. Mg(s)+2HCl(aq)→H2(g)+MgCl2(aq)
2. H2O(l)+MgCl2(aq)→MgO(s)+2HCl(aq)
1
ΔH= -286 KJ/mol
3. H2(g)+2 O2(g)→ MgO(s
ΔH=?
ΔH= -507.7 KJ/mol
ΔH= 134.8 KJ/mol
ΔH= -286 KJ/mol
1
Mg(s + 2 O2(g)→ MgO(s
658.9 KJ/mol
d) Perecentage Error=
ΔHTarget= -
𝑎𝑐𝑡𝑢𝑎𝑙 𝑦𝑒𝑖𝑙𝑑−𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑦𝑒𝑖𝑙𝑑
𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑐𝑎𝑙 𝑦𝑒𝑖𝑙𝑑
x100%
−658.9 𝐾𝐽/𝑚𝑜𝑙−(−602 𝐾𝐽/𝑚𝑜𝑙)
=
−602 𝐾𝐽/𝑚𝑜𝑙
=9.45%
x100%
Error Analysis:
1. Human Error: The magnesium was stirred improperly. The mixture is heterogenous so it must be
sssssssssssssssssmixed vigorously for efficient mixing and to prevent hot spots, so that when the
ssssssssssssssssgthermometer is inserted in a hot spot that tempereature is not representative of the
ssssssssssssssss whole calorimeter.
2. Equipment Error: The calorimeter’s lid had a hole or crack in it. The hole or crack allows heat to
ssssssssssssssss escape out of the calorimeter causing the temperature not to increase fully,
ssssssssssssssss making the temperature inaccurate since it is not a fully closed system.
3. Human Error: Measuring the magnesium oxide inncorrectly. Not measuring enough magnesium
ssssssssssssssss oxide causes the mass to decrease for the reaction, therefore increasing the enthalpy
ssssssssssssssss change.