Standard Heats of Formation Worksheet
Explore the concept of standard heats of formation, calculate enthalpy changes, and understand Hess's Law with this comprehensive worksheet for Grade 9 Science.
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Standard Heats of Formation
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Read each question carefully and provide your best answer. Show all your work for calculation problems.
What is Enthalpy?
Enthalpy (H) is a thermodynamic property of a system. It is the sum of the internal energy (U) of the system and the product of its pressure (P) and volume (V). In simpler terms, it's a measure of the total heat content of a system. When a chemical reaction occurs, the change in enthalpy (ΔH) represents the heat absorbed or released during the reaction at constant pressure.
Standard Enthalpy of Formation (ΔH°f)
The standard enthalpy of formation (ΔH°f) is the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states at 25°C (298 K) and 1 atm pressure. The standard enthalpy of formation for an element in its standard state is defined as zero.
1. Which of the following statements best describes the standard enthalpy of formation (ΔH°f)?
The heat released when a compound decomposes.
The enthalpy change when one mole of a compound is formed from its elements in their standard states.
The total heat content of all reactants in a reaction.
The energy required to break all bonds in a compound.
2. What is the standard enthalpy of formation (ΔH°f) for a pure element in its standard state?
Positive
Negative
Zero
Variable
3. The standard state for most elements is defined at a temperature of and a pressure of .
4. A reaction with a negative ΔH is called an reaction, meaning it heat.
5. According to Hess's Law, the total enthalpy change for a reaction is of the pathway taken.
6. Using the following standard enthalpies of formation, calculate the enthalpy change (ΔH°) for the combustion of methane (CH4):
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
ΔH°f [CH4(g)] = -74.8 kJ/mol
ΔH°f [CO2(g)] = -393.5 kJ/mol
ΔH°f [H2O(l)] = -285.8 kJ/mol
7. Explain how Hess's Law is applied in calculating the enthalpy change of a reaction.
8. The standard enthalpy of formation for a compound is always a negative value.
True
False
9. A reaction that absorbs heat from its surroundings is an endothermic reaction.
True
False