unit 2 Flashcards
What is equilibrium
the state attained in a reversible reaction, when the rate of the forward reaction equals the rate of the reverse reaction.
the concentration if the reactants and products are constant.
a catalyst has no effect.
the equilibrium constant
aA + bB ⇌ cC + dD
K = [C]^c [D]^d / [A]^a [B]^b
calculations involving equilibrium constants
the equilibrium constant has no units
if K >1, equilibrium lies to the right, favouring products
if K<1, equilibrium les to the left, favouring reactants
K gives no indication of rate
calculations involving equilibrium constants further points
-the concentrations of pure solids and pure liquids at equilibrium are taken as a contestant and given a value of 1 in equilibrium expression
-the value of K can be affected by changing temps.
-the value of K cannot be affected by changes in pressure or concentration.
increasing temp does what to the value of K
increases it
if value of K is lower at a higher temp
reaction in exothermic, by lowering temp eq goes right
Standard enthalpy of formation (∆Hf)
The enthalpy change involved when one mole of a substance is formed from its elements in their standard states
eqn used to calculate standard enthalpy change for a chemical reaction
ΔHf = Σ ΔH products - Σ ΔH reactants
what is the value of the enthalpy of formation of an element
0
standard state
the standard state of a substance is its most stable form at a pressure of 1atm at a specific temperature usually taken as 298 K or 25c
entropy
a measure of the amount of disorder
symbol for entropy
S
standard entropy
entropy at 298K, 1atm
units for standard entropy
JK-1 mol-1
the standard entropy for solids
lower compared to liquids and gas
link between temp and entropy
as temp goes up entropy goes up
at what temp do molecular vibrations stop, giving a substance 0 entropy
0K
third law of thermodynamics
at 0k, the particles in a solid are no longer vibrating are perfectly ordered. such a solid is described as a perfect crystal. so entropy of a perfect crystal at 0K is zero
predicting an increase/decrease in entropy
negative entropy when the reactants have a gas (high entropy), the products are solid (low entropy)
calculating entropy change
ΔS = ΣS prod - ΣSreact
feasible reaction
one which tends towards the products.
eqn that links Gibbs free energy, enthalpy, entropy and temp
ΔG=ΔH-TΔS
if ΔG < 0
reaction is feasible/spontaneous
second law of thermodynamics
defines the condition for a feasible reaction.
- for a reaction to be feasible, the total entropy for a reaction is positive
Free energy
At eq:
ΔG=0
when ΔG<0:
the forward reaction is feasible, with a greater concentration of products. K>1
when ΔG>0
the reverse reaction is feasible, there is a greater concentration of reactants. K<1
reaction mechanism and the rate determining step RDS
The rate of a multi step reaction is determined by the slowest step, the RDS
a reaction mechanism/ rate eqn cannot be determined from an equation alone. the order and rate eqn can only be determined from experimental results.
rate depends on
the concentration of reactants in the slow step.
