3.4 Chemical Energy Flashcards
Enthalpy; Hess's Law; Bond Enthalpy (14 cards)
3.4a- Enthalpy
Define: “Ethalpy of Combustion”
- The enthalpy change when one mole of the substance burns completely in oxygen
- CH4 + 2O2 → CO2 + 2H2O
3.4a- Enthalpy
What is the enthalpy value for Combustion reactions always?
- Negative
- ∴ Exothermic
3.4a- Enthalpy
Define: “Enthalpy of Solution”
- The energy change that occurs when one mole of substance completely dissolves in water
- Can be exo- or endothermic
- NaCl + H20 → Na+ Cl-
3.4a- Enthalpy
How are the reactions:
* Exothermic
* Endothermic
- If energy is lost to surroundings
- If energy is taken in from surroundings
3.4- Enthalpy
Define: “Enthalpy of Neutralization”
- The amount of energy given out when one mole of water is formed in a neutralization reaction
- HCl + NaOH → NaCl + H2O
3.4a- Enthalpy
What will the enthalpy of neutralization reactions always be?
- Negative
- ∴ Exothermic
3.4a- Enthalpy
Disadvantages of the Spirit Burner Method?
- When the lid is removed, some alcohol may evaporate ∴ mass burned may appear higher
- Enthalpy requires complete combustion, orange flame inidcates otherwise ∴ mass burned may appear higher
- Some heat can be lost to surroundings ∴ not all heat to water
3.4b- Hess’s law
What is Hess’s law?
The enthalpy change in converting reactants into products is the same regardless of the rout by whcih the reaction takes place.
3.4b- Hess’s law
Define: “Enthalpy of Formation”
- The energy change that occurs when** one mole** of a substance is formed from its elements in their natural states
- C + 2H2 → CH4
3.4c- Bond enthalpy
What is bond breaking always?
Endothermic
3.4c- Bond enthalpy
What is bond making always?
Exothermic
3.4c- Bond enthalpy
What is Mean Molar Bond Enthalpy
Average values quoted for bonds which occur in different molecular environments
3.4c- Bond Enthalpy
What is molar bond enthalpy for molecule XY
The energy required to break one mole of XY bonds
3.4c- Bond Enthalpy
How to calculate ΔH
ΔH= Σ ΔH bonds broken + ΔH bonds made
