5 Chemical Equilibrium Flashcards
Define ‘equilibrium.’
a state in which there are no observable changes over time
What are the 4 main requirements of equilibrium?
a closed system
constant pressure and temperature
reversible process/reaction
rates of opposing changes are equal (i.e. Le Chatalier’s principle)
Which two things describe an equilibrium system?
dynamic (in constant motion)
reversible (can approach from either direction but both reactants and products are present at all times)
Looking at the following reaction, at what point has it reached equilibrium?
aA + bB → cC + dD
when the rate of the forward reaction equals the rate of the backward/reverse reaction (formation of reactants and products is the same)
quantities of reactants and products do not change any further
State the ‘Law of Mass action.’
the rate of a reaction is proportional to the concentration of reactants
What does the reaction quotient, Q, relate to?
the concentrations of products and reactants to each other at “any point” in a reaction
For the general reaction: aA + bB → cC + dD state the formula for Q.
Q = [C]^c [D]^d/ [A]^a[B]^b
= [Products]/[Reactants]
What is Kc?
the equilibrium constant
the value of the reaction quotient, Q, at equilibrium
does not depend on initial concentrations of reactants and products
For the general reaction: aA + bB ⇌ cC + dD state the formula for Kc.
Kc = [C]^c [D]^d/ [A]^a[B]^b
= [Products]/[Reactants]
State the three stages of writing out equilibrium expressions.
- Write the balanced equation
- Write the products as the numerator and the reactants as the denominator (do not include solids/solvents and square brackets [X] mean concentration, use them for Kc but not Kp)
- Write the coefficient of each substance in the balanced equation as exponents
What is ‘homogenous’ equilibrium? Give an example.
when reactants and products are in the same physical state
i.e. N₂(g) + H₂(g) → NH₃(g)
so all these components go into the Kc/Kp expression
What is a ‘heterogeneous’ equilibrium? Give an example.
when reactants and products are in different physical states i.e. CaCO₃(s) ⇌ CaO(s) + CO₂(g) here only gases would be used: Kc = [CO₂] Kp = P(CO₂)
For each of the following reactions write an expression for Kc:
a) CO₂(g) + H₂(g) → CO(g) + H₂O(l)
b) 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)
c) SnO₂(s) + 2CO(g) → Sn(s) + 2CO₂(g)
a) Kc = [CO]/[CO₂][H₂]
b) Kc = [H₂]^4/[H₂O]^4
c) Kc =[CO₂]^2/[CO]^2
What 2 things does Kc allow us to do?
predict how a reaction will progress to achieve equilibrium
calculate the concentration of reactants and products at equilibrium
Calculate Kc and its units using the following information:
H₂(g) + I₂(g) → 2HI(g)
A closed system initially containing 1.00x10^-3M H₂ and 2.00x10-3M I₂ at 448℃ is allowed to reach equilibrium.
The equilibrium concentration of HI is 1.87x10^-3M.
Use the ICE method:
H₂ I₂ HI
I(mol) 1.00x10^-3 2.00x10^-3 0
C(mol) -1/2 x (1.87x10^-3) -1/2 x (1.87x10^-3) +1.87x10^-3
E(conc) 0.065x10^-3 1.065x10^-3 1.87x10^-3
Kc = [1.87x10^-3]/ [0.065x10^-3][0.065x10^-3]
= 51
Units = M^2/M^2 → NO UNITS
How do we find the units for Kc or Kp?
By putting units into Kc/Kp brackets and cancelling them out like fractions.
For each of the following relationships between Q and k values, state the initial state of the reaction and the change in equilibrium:
a) Q = 0
b) Q < k
c) Q = k
d) Q > k
e) Q ∞ k
a) initial state = all reactants
change = shift right
b) initial state = mostly reactants
change = shift right
c) initial state = equilibrium
change = none
d) initial state = mostly products
change = shift left
e) initial state = all products
change = shift leftWhat can a Kc tell us?
the equilibrium position of a reaction and how far a reaction progresses
What does a small Kc value tell us?
the reaction favours reactants
equilibrium lies to the left-hand side
little reaction occurs
What does a large Kc value tell us?
the reaction favours products
equilibrium lies to the right-hand side
reaction nearly complete
State the formula for finding ΔG from Kc.
ΔG = -RTln(Kc)
R - gas constant (8.314J/mol/K)
T - temperature (K)
ln(Kc) - Log to the base 10
State Le Chatalier’s principle.
when a change is applied to a system in dynamic equilibrium, the system reacts in such a way as to oppose the effect of the change
What does a change in equilibrium conditions do?
upsets the equilibrium of the system
a system under stress will shift to release the stress
there will be a change in the rate of the forward or reverse reaction to return the system to equilibrium
Does reactant/product concentration affect a Kc value?
the Kc value is not affected by this at a constant temperature
