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
What does changing the amount/concentration of substance affect the value of?
the value of Q
How is the value of Kc maintained?
the composition of the equilibrium mixture changes by shifting its position
Explain what happens in equilibrium when reactant is added.
equilibrium shifts right
increases collisions involving reactants
rate of forward reaction increases
Explain what happens in equilibrium when product is added.
equilibrium shifts left
decreases collisions involving reactants
rate of forward reaction decreases
Give an example of where equilibrium has biological relevance.
Osmosis in erythrocytes - equilibrium position must be equal on both sides of the membrane
In a hypotonic solution, they will become lysed as water moves in
In an isotonic solution, they will maintain their shape as water moves in and out at an equal rate
In a hypertonic solution, they will become shrivelled as water moves out
How does changing pressure affect reactions?
only affects reactions with different numbers of gas molecules on each side
the more particles you have in a given volume, the greater the pressure they exert
if pressure is increased, the system moves to decrease it, towards the side with fewer gas molecules
(pressure favours the direction of fewer gas molecules)
Give an example of where the pressure in equilibrium has biological relevance.
i.e. Haemoglobin binding to and releasing oxygen molecules
Hb + O₂ ⇌ Hb-O₂
How does adding a catalyst affect equilibrium?
does not affect the position of equilibrium
affects the rate of both forward and backward reaction equally
however, it does increase the rate of attainment of equilibrium
Give an example of when a catalyst is used in attaining a biological equilibrium (using the formula equation shown on the document).
Fructose 1,6-phosphate ⇌ Dihydroxyacetone phosphate + Glyceraldehyde 3-phosphate
Explain the effect of temperature on Kc.
temperature is the only thing that can change the value of Kc
altering temperature affects the rate of both backward and forward reactions but it alters the two rates to different extents
hereby, the equilibrium moves producing a new Kc value
the direction of movement depends on the sign of the enthalpy change
How does the ΔH symbol affect the direction equilibrium will shift?
a negative ΔH means the forward reaction is exothermic and the backward reaction will decrease temperature so it will shift the equilibrium to the left-hand side
a positive ΔH means the forward reaction is endothermic and the backward reaction will increase temperature so it will shift the equilibrium to the right-hand side
Complete the following table on changes and equilibrium:
Condition: Change: Reaction: Concentration Pressure Temperature // Catalyst
Complete the following table on changes and equilibrium:
Condition: Change: Reaction:
Concentration Add reactant Forward
Remove reactant Back
Add product Back
Remove product Forward
Pressure Increase Towards fewer
moles of gas
Decrease Towards more
moles of gas
Temperature Endothermic reaction → K changes
Increase ← K changes
Decrease
Exothermic reaction ← K changes
Increase → K changes
Decrease
//
Catalyst Increases rates equally No effect
For a reaction at equilibrium, draw out the product and reactant lines on a:
a) concentration-time graph
b) rate-time graph
a) see document
b) see document
