energetics Flashcards
define enthalpy change
overall energy exchanged with the surroundings when a change happens at constant pressure (100kpa) and the final temperature is the same as the starting temperature.
define specific heat capacity
the enthalpy in joules which heats 1Kg of substance by 1K
what are standard conditions
1mol
100Kpa
298K
define standard enthalpy of combustion
enthalpy change when 1mol of substance completely burns in excess 02 under standard conditions 298K 100Kpa
define standard enthalpy change of formation
enthalpy change when 1mol of substance is formed from its elements in their standard states under standard conditions 298K 100Kpa
define standard enthalpy change of neutralisation
the enthalpy change to form 1mol of water by the reaction of an acid and an alkali under standard conditions 298K 100Kpa
define standard enthalpy change for a reaction
the enthalpy change when molar quantities as stated in the equation react under standard conditions 298K 100Kpa
enthalpy (deltaH) =
energy transferred (q) KJ / mol
energy transferred (q) =
mass of substance heated x specific heat capacity x temperature change
overall enthalpy change =
energy to break bonds (+ve) + energy to make bonds (-ve)
endothermic reaction
more energy needed to break bonds than make new ones
enthalpy is positive
heat taken in from surroundings
exothermic reaction
more energy needed to make new bonds than break existing ones
enthalpy is negative
heat is given out
what is Hess’s law
energy in a reaction system must be conserved as energy cannot be created or destroyed.
therefore overall enthalpy change for a reaction is the same regardless of the route taken.
define bond enthalpy
energy required to break one mole of bonds in a gaseous state, under standard conditions
298K 100Kpa
what is Gibbs free energy
indicator of reaction feasibility
if deltaG is 0 or negative then reaction is feasible
how to calculate Gibbs free energy
enthalpy - (Temperature x entropy)
(deltaH - T deltaS)
how to calculate minimum temperature a reaction is spontaneously feasible at
make deltaG equal to zero
0 = enthalpy - (temperature x entropy of surroundings)
why would a reaction not happen even if it was thermodynamically feasible
kinetic factors
for example activation energy
