energetics Flashcards
What is enthalpy change?
The heat energy change measured at constant pressure
What does the sign of delta H tell you ?
Positive value for endothermic process (heat absorbed)
Negative value for exothermic process (heat evolved)
What are the standard conditions for standard enthalpy changes ?
pressure =100kpa
temperature= 298k
What is hess’s law?
The enthalpy change of a reaction is independent of the route taken
What is standard enthalpy change of formation?
The enthalpy change when one mole of a substance is formed from its elements under standard conditions and with all substances in their standard states
What is standard enthalpy change of combustion ?
The enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions and with all substances in their standard states.
What is bond dissociation enthalpy?
The enthalpy change for
A-B(g) -> A(g) + B (g)
Gaseous molecules Gaseous atoms
What is mean bond enthalpy?
The enthalpy change when 1 mole of covalent bonds is broken averaged over a range of different compounds.
What is activtation energy ?
The energy needed to break bond in reactants
what is the equation for heat energy ?
q= m (g) c( 4.18 j/g/k) change in temp ( k or degrees)
q=mct
What is the equation for delta h?
-q / (moles x1000)
How do you calculate enthalpy changes for delta h using bond enthalpies ?
( energy needed to break all the bonds in the reactants) -
(energy released when new bonds are formed in the products )
Why are delta h values calculated using mean bond enthalpies only approximations?
This is because the values of the actual bond enthalpies for the bonds in the compounds may be a little different from the mean bond enthalpies used in the calculation?
