Hess's Law Worksheet - Rancho High School
Hess's Law Worksheet
1. Calculate ΔH for the reaction: C2H4 (g) + H2 (g) --> C2H6 (g), from the following Data.
|C2H4 (g) + 3 O2 (g) --> 2 CO2 (g) + 2 H2O (l) |ΔH = -1411. kJ |
|C2H6 (g) + 3½ O2 (g) --> 2 CO2 (g) + 3 H2O (l) |ΔH = -1560. kJ |
|H2 (g) + ½ O2 (g) --> H2O (l) |ΔH = -285.8 kJ |
2. Calculate ΔH for the reaction 4 NH3 (g) + 5 O2 (g) --> 4 NO (g) + 6 H2O (g), from the following Data.
|N2 (g) + O2 (g) --> 2 NO (g) |ΔH = -180.5 kJ |
|N2 (g) + 3 H2 (g) --> 2 NH3 (g) |ΔH = -91.8 kJ |
|2 H2 (g) + O2 (g) --> 2 H2O (g) |ΔH = -483.6 kJ |
3. Find
ΔH° for the reaction 2H2(g) + 2C(s) + O2(g) --> C2H5OH(l), using the following thermochemical data.
|C2H5OH (l) + 2 O2 (g) --> 2 CO2 (g) + 2 H2O (l) |ΔH = -875. kJ |
|C (s) + O2 (g) --> CO2 (g) |ΔH = -394.51 kJ |
|H2 (g) + ½ O2 (g) --> H2O (l) |ΔH = -285.8 kJ |
4. Calculate
ΔH for the reaction CH4 (g) + NH3 (g) --> HCN (g) + 3 H2 (g), given:
|N2 (g) + 3 H2 (g) --> 2 NH3 (g) |ΔH = -91.8 kJ |
|C (s) + 2 H2 (g) --> CH4 (g) |ΔH = -74.9 kJ |
|H2 (g) + 2 C (s) + N2 (g) --> 2 HCN (g) |ΔH = +270.3 kJ |
5. Calculate ΔH for the reaction 2 Al (s) + 3 Cl2 (g) --> 2 AlCl3 (s) from the Δata.
| 2 Al (s) + 6 HCl (aq) --> 2 AlCl3 (aq) + 3 H2 (g) | ΔH = -1049. kJ |
| HCl (g) --> HCl (aq) | ΔH = -74.8 kJ |
| H2 (g) + Cl2 (g) --> 2 HCl (g) | ΔH = -1845. kJ |
| AlCl3 (s) --> AlCl3 (aq) | ΔH = -323. kJ |
| | |
Hess's Law Worksheet Solutions
|1. | | |
| |C2H4 (g) + 3 O2 (g) --> 2 CO2 (g) + 2 H2O (l) |ΔH = -1411. kJ |
|flip |2 CO2 (g) + 3 H2O (l) --> C2H6 (g) + 3½ O2 (g) |ΔH = +1560. kJ |
| |H2 (g) + ½O2 (g) --> H2O (l) |ΔH = -285.8 kJ |
| |C2H4 (g) + H2 (g) --> C2H6 (g) |ΔH = -137. kJ |
2.
|x2 |2 N2 (g) + 2 O2 (g) --> 4 NO (g) |ΔH = 2 (-180.5 kJ) |
|flip x2 |4 NH3 (g) --> 2 N2 (g) + 6 H2 (g) |ΔH = -2 (-91.8 kJ) |
|x3 |6 H2 (g) + 3 O2 (g) --> 6 H2O (g) |ΔH = 3 (-483.6 kJ) |
| |4 NH3 (g) + 5 O2 (g) --> 4 NO (g) + 6 H2O (g) |ΔH = -1628. kJ |
3.
|flip |2 CO2 (g) + 2 H2O (l) --> C2H5OH (l) + 2 O2 (g) |ΔH = -1(-875. kJ) |
|x2 |2 C (s) + 2 O2 (g) --> 2 CO2 (g) |ΔH = 2 (-394.51 kJ) |
|x2 |2 H2 (g) + O2 (g) --> 2 H2O (l) |ΔH = 2 (-285.8 kJ) |
| |2H2(g) + 2C(s) + O2(g) --> C2H5OH(l) |ΔH° = -486. kJ |
4.
|flip x½ |NH3 (g) --> 1/2 N2 (g) + 3/2 1H2 (g) |ΔH = -½(-91.8 kJ) |
|flip |CH4 (g) --> C (s) + 2 H2 (g) |ΔH = -1(-74.9 kJ) |
|x½ |1/2 H2 (g) + C (s) + 1/2 N2 (g) --> HCN (g) |ΔH = ½(270.3 kJ) |
| |CH4 (g) + NH3 (g) --> HCN (g) + 3 H2 (g) |ΔH = +256.0 kJ |
5.
|Ø |2 Al (s) + 6 HCl (aq) --> 2 AlCl3 (aq) + 3 H2 (g) |ΔH = -1049. kJ |
|x6 |6 HCl (g) --> 6 HCl (aq) |ΔH = 6 (-74.8 kJ) |
|x3 |3 H2 (g) + 3 Cl2 (g) --> 6 HCl (g) |ΔH = 3 (-1845. kJ) |
|flip x2 |2 AlCl3 (aq) --> 2 AlCl3 (s) |ΔH = -2 (-323. kJ) |
| |2 Al (s) + 3 Cl2 (g) --> 2 AlCl3 (s) |ΔH = -6387. kJ |
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Note on Significant Figures: When adding or subtracting numbers, determine which of the numbers has the fewest decimal places. The answer to the calcu慬楴湯猠潨汵敢眠楲瑴湥琠桴慳敭渠浵敢景搠捥浩污瀠慬散lation should be written to the same number of decimal places.
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