Questions and Answers
What do thermochemical equations show?
What is a thermochemical equation?
A chemical equation that also shows the enthalpy of a chemical reaction.
Which of the following rules apply to thermochemical equations?
What does △H represent?
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Endothermic reactions have a _____ value for △H.
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Exothermic reactions have a _____ value for △H.
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How do you calculate the net enthalpy?
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What is the equation for the initial combustion of liquid methane?
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How is net enthalpy related to initial enthalpy and enthalpy of vaporization?
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If oxygen is in excess, how much heat will the hand warmer produce when all the iron is oxidized?
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Study Notes
Thermochemical Equations
- Thermochemical equations depict both reactants and products, showcasing energy changes during a chemical reaction.
- Reactions can be classified as endothermic (positive ΔH) or exothermic (negative ΔH).
Definition of Thermochemical Equation
- A thermochemical equation includes the enthalpy change associated with a specific chemical reaction.
Rules for Thermochemical Equations
- All thermochemical equations must be balanced to reflect the law of conservation of mass.
- The physical states of reactants and products (solid, liquid, gas) are indicated in parentheses.
- The change in enthalpy (ΔH) is placed on the right side of the equation.
- ΔH can be expressed in kilojoules (kJ) or kilojoules per mole (kJ/mol).
- Endothermic reactions absorb heat, indicated by +ΔH; exothermic reactions release heat, indicated by -ΔH.
Change in Enthalpy (ΔH)
- ΔH represents the difference in thermal energy between reactants and products during a reaction.
Example Calculation of Heat Production
- For the reaction: 4Fe + 3O2 → 2Fe2O3 with ΔH = -3.926 kJ.
- Given parameters: Mass of iron = 100g, Molar Mass of Fe = 55.8 g/mol.
- Convert ΔH from kJ to kJ/mol: ΔH (kJ/mol) = 3926 kJ/4 mol Fe = 981.5 kJ/mol.
- Calculate moles of iron oxidized: Moles of Fe = Mass / Molar Mass = 100g / 55.8g/mol ≈ 1.79 mol.
Net Enthalpy
- Net enthalpy is determined by summing the enthalpies from initial combustion and any change of state.
Calculating Net Enthalpy
- Net enthalpy can be calculated if both the initial enthalpy and the enthalpy change are known.
- The initial enthalpy can also be derived from the net enthalpy when required.
Example of Enthalpy of Reaction
- Given the enthalpy of vaporization for methane (ΔH = 88 kJ) and net ΔH = -890 kJ.
- Unbalanced equation: CH4 + O2 → CO2 + H2O.
- Balanced equation: CH4 + 2O2 → 2CO2 + 2H2O.
- Initial enthalpy calculation: ΔH_initial = ΔH_net - ΔH_vaporization.
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Description
This quiz focuses on understanding thermochemical equations, highlighting their characteristics and the significance of enthalpy changes. Test your knowledge on the rules governing these equations, including balancing and notation of states. Perfect for chemistry students looking to reinforce their learning.
