Enthalpy Flashcards
Define Enthalpy change
Enthalpy change is the amount of heat energy that is either given out or taken in from the surroundings, during a reaction, at a constant pressure.
State the formula to calculate enthalpy
Q = m c Δ T
State the formulas used to calculate enthalpy change of combustion
Q = m c Δ T / 1000 (to convert from J to KJ)
Moles = Mass/Mr
Q/Moles
State the 3 types of enthalpy change
- Standard enthalpy change of combustion ΔHc
- Standard enthalpy change of formation ΔHf
- Standard enthalpy change of reaction ΔHr
State Hess’ law
The enthalpy change for a chemical reaction is the same, regardless of route taken from reactants and products
State the equations used in Hess’ law
ΔH formation questions
ΔH formations of reactants + ΔH = ΔH formations of products
ΔH = ΔH formations of products - ΔH formations of reactants
ΔH = SUM ΔH formations of products - SUM ΔH formations of reactants
Describe how to apply Hess’ law on combustion questions
- Use the formula ΔH + formation of reactants = ΔH formations of products
- Rearrange the equation to find ΔH
Define average bond enthalpy
Average bond enthalpy is the energy needed to break one mole of a specific bond in a molecule in the gaseous state.
Whats significant about the energy required to break a bond and the energy released when that bond is formed
The value is the same
What are average bond enthalpies used for
To determine the enthalpy change of a reaction
In what state are average bond enthalpies determined for in molecules
A gaseous state
Why do we invert the sign on the bond enthalpies of the products, and not the reactants
Products - exothermic
Reactants - endothermic
Describe how to calculate the enthalpy change of the whole reaction
Add the bond enthalpies of the reactants and the products
If the enthalpy change of the reaction is negative, what does this tell us
It’s an exothermic reaction
Define standard enthalpy of formation
The standard enthalpy of formation is the enthalpy change when one mole of a substance is formed from its constituent elements, under standard conditions, where all substances are in their standard state.
