energetics Flashcards
define enthalpy change
the change in heat energy at a constant pressure
what are the standard conditions for enthalpy change
-temperature at 298 K
-pressure at 100 kPa
define standard enthalpy of combustion
the enthalpy chnage when one mole of a substance is completely burned in oxygen under standard conditions, and all substances in standard states
define enthalpy of formation
the enthalpy change when one mole of a compound is formed from its elements under standard conditions, all substances in standard states.
what are the seven diatomic atoms
H2 O2 N2 F2 CL2 BR2 I2
define Hess’s law
energy cannot be created or destroyed but can be converted from one form into another
the enthalpy change in a reaction is independent of the route taken and depends only on the initial and final states
how can you calculate enthalpy of combustion
AH of reactants - AH of products
assuming all values are combustion values
how can you calculate enthalpy of formation
AH of products - AH of reactants
assuming all values are formation values
bond dissociation energy
enthalpy change to break the bond in one mole of gaseous molecules, to form gaseous atoms
mean bond enthalpy
the enthalpy required to break one mole of a covalent bond averaged over a range of compounds
how can you calculate the enthalpy change in mean bond enthalpy
AH = [AH of bonds broken] -[AH of bonds formed]
equation for entropy (AS)
and whats the units for entropy
AS= S products- S reactants
J K-1mol-1
how to calculate gibbs free energy (AG)
AG= AH-T(AS)
how can you tell if a reaction is feasible or not in gibbs free energy
-if the AG is less than or equal to 0 (-ve value) then its feasible
-if its greater than 0 (+ve value) then it is not feasible
-the more negative the more feasible the reaction
entropy definition
the measure of disorder in a system.if a reaction has an increase in disorder it is considered more feasible
e.g gases have more disorder than solids as they are more random and scattered.
the more +ve the entropy the more disordered so more feasible
what two factors affect entropy
-change in state (if more gaseous increases disorder)
-change in moles( if moles on products increases more disorder)
