unit 1b: reaction feasibility Flashcards
thermochemistry
the study of energy changes that take place during chemical reactions
energy changes can be used to predict
the feasibility of a reaction i.e. whether it will spontaneously take place
standard enthalpy of formation
the enthalpy change when one mole of a substance is formed from its elements in their standard states
standard state of a substance
its most stable state at standard conditions
standard conditions
298K (room temp=25degreesC)
standard enthalpy of formation for an element
zero (as there is no enthalpy change involved)
a spontaneous/feasible reaction
one which happens of its own accord (doesn’t need energy input)
spontaneous reactions tend towards…
the products rather than reactants
example of a spontaneous/feasible reaction
rusting
entropy
a measure of the degree of disorder in a system, the greater the entropy, the greater the disorder
state and entropy values
solids have low disorder and gases high disorder, changes of state involve large changes in entropy
an example of perfect entropy and zero disorder
a crystal lattice at absolute zero (0K)
temperature and entropy values
entropy increases and temperature increases
the second law of thermodynamics
the total entropy of a reaction system and its surroundings always increases for a spontaneous/feasible reaction
exothermic reactions (negative deltaH)
increases the entropy of the surroundings
endothermic reactions (positive deltaH)
decreases the entropy of the surroundings
the standard entropy of a substance
the entropy value for the substance in its standard state
units for entropy
JK^-1mol^-1
deltaG represents
the energy available ‘to do work’- the excess energy available beyond that needed just to make the reaction spontaneous
for a reaction to be feasible
deltaG must be negative
deltaG=0
reaction is just feasible/ at equilibrium
when deltaS is negative
the reaction is exothermic
deltaHf°
the standard enthalpy of formation
deltaS
entropy
deltaS must be positivite for
a spontaneous reaction
deltaS°
standard entropy change
deltaG
free energy
