Energetics Flashcards
(13 cards)
define enthalpy change
the heat change at constant pressure
what are the standard conditions needed for standard enthalpy change
- 298K
- 100kPa
define Standard enthalpy of combustion
- the enthalpy change when 1 mol of a substance is completely burned in oxygen under standard conditions with all substances in standard states
what does a hess cycle look like when using enthalpy of formation data and what is the equation used?
- elements in their standard state at bottom
- the arrows point upwards
- Δ H Formation = ( Δ H f ) Products − ( Δ H f ) Reactants
what does a hess cycle look like when using enthalpy of combustion data and what is the equation used?
- products at the bottom
- the arrows point downwards
- Δ H Combustion = ( Δ H f ) Reactant − ( Δ H f ) Products
define Bond Dissociation Energy
- enthalpy change to break the bond in 1 mol of gaseous molecules to form gaseous atoms
what equation links bond enthalpy with enthalpy change
ΔH = Σ bond energies broken - Σ bond energies made
what are equation links heat energy with enthalpy change
- ΔH = q/n
q = heat energy (kilojoules)
n = number of moles
what are equation links heat energy with specific heat capacity
- q = mc∆t
q = heat energy (joules)
m = mass (grams)
c = specific heat capacity (4.18 JK⁻¹g⁻¹
∆t = change in temp
what is the enthalpy of formation of an element
- zero by definition
define Standard enthalpy of formation
- the enthalpy change when 1 mol of a substance is formed from its elements under standard conditions with all substances in standard states
why would the enthalpy of combustion determined experimentally be different than that calculated using enthalpies of formation.
- Incomplete combustion
- Evaporation of fuel
- Experiment not completed under standard
conditions
Suggest two disadvantages of using a glass beaker on a tripod and gauze instead of a copper calorimeter
- Glass is a poorer conductor than copper
- Tripod and gauze would reduce heat transfer
