Thermochemistry Worksheet 2: Solutions and Explanations
This guide provides the step-by-step solutions for Thermochemistry Worksheet 2. These problems cover enthalpy, heat capacity, calorimetry, and Hesss Law. Understanding these calculations is essential for mastering energy flow in chemical reactions.
Key Concepts Covered
- Specific Heat Capacity (q = mcT): Determining energy required to change temperature.
- Calorimetry: Calculating heat exchange between systems in a lab setting.
- Enthalpy of Reaction: Calculating H using standard heats of formation.
- Hesss Law: Calculating overall enthalpy by manipulating multiple reaction steps.
Problem 1: Heat Transfer Calculations
Question: How much heat is required to raise the temperature of 50.0 g of water from 20.0C to 80.0C? (Specific heat of water = 4.18 J/gC)
Solution:
Use the formula q = mcT
m = 50.0 g
c = 4.18 J/gC
T = 80.0C - 20.0C = 60.0C
q = (50.0 g) * (4.18 J/gC) * (60.0C) = 12,540 J or 12.54 kJ
Problem 2: Calorimetry
Question: A 25.0 g piece of metal at 100.0C is dropped into 100.0 g of water at 25.0C. The final temperature of the system is 28.5C. What is the specific heat of the metal?
Solution:
Heat gained by water = Heat lost by metal
q_water = (100.0 g)(4.18 J/gC)(28.5 - 25.0) = 1,463 J
q_metal = -1,463 J
-1,463 J = (25.0 g)(c_metal)(28.5 - 100.0)
-1,463 = (25.0)(-71.5)(c_metal)
c_metal = -1,463 / -1,787.5 = 0.818 J/gC
Problem 3: Hess's Law Application
Question: Calculate the H for the reaction: C(s) + 2H2(g) CH4(g), given the following combustion data:
- C(s) + O2(g) CO2(g) (H = -393.5 kJ)
- H2(g) + 1/2O2(g) H2O(l) (H = -285.8 kJ)
- CH4(g) + 2O2(g) CO2(g) + 2H2O(l) (H = -890.3 kJ)
Solution:
Target: C(s) + 2H2(g) CH4(g)
- Keep eq 1: C + O2 CO2 (H = -393.5)
- Multiply eq 2 by 2: 2H2 + O2 2H2O (H = 2 * -285.8 = -571.6)
- Reverse eq 3: CO2 + 2H2O CH4 + 2O2 (H = +890.3)
Summing these: (-393.5) + (-571.6) + 890.3 = -74.8 kJ
Study Tips for Thermochemistry
When working through these problems, always pay attention to the units. Ensure that mass is in grams and temperature is consistently handled in either Celsius or Kelvin. Most importantly, keep a close watch on the sign of H; a negative value indicates an exothermic process, while a positive value indicates an endothermic process. If you find your answer has the wrong sign, re-check your subtraction (Final - Initial) or ensure you have correctly reversed the reaction direction in Hess's Law problems.
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