Enthalpy Changes Flashcards
Define ‘ Average bond enthalpy ‘
(Enthalpy change) when 1 mole
of gaseous covalent bonds is broken
State the equation for Percentage yield
% yield = (actual yield) x 100
(Theoretical yield)
Why might values obtained from an experiment differ from values stated in a data book ?
Heat loss
Not carried out under standard states in standard conditions
Incomplete combustion (Combustion reactions)
Evaporation of substance
State the definition for ‘ Standard enthalpy change of formation ‘
(Change in enthalpy) when 1 mol substance
in standard states and conditions
is formed from its pure constituent elements
State definition for ‘ Enthalpy change of combustion ‘
(Enthalpy Change) when 1 mole of substance
is burned in excess oxygen
under standard states
State definition / features of dynamic equilibrium
Closed system
Concentrations are constant
Rate of forward reaction is equal to the rate of the reverse reaction
State Hess’ Law
Enthalpy Change
Is independent of the path taken
What improvements can be made to an enthalpy of combustion reaction to get more accurate results ?
Insulate beaker with heating materials
Decrease distance between flame and beaker
Heat for longer
What is the main reason for the increase in reaction rate when an increase in temperature is introduced ?
Greater proportion of molecules with energy greater than or equal to the activation energy
What is the effect , on the rates of forward and reverse reactions , when a catalyst is introduced ?
Both rates increasing by the same factor
What is the enthalpy change for the condensation of a gas to a liquid ?
Negative
