dynamic Equilibria Flashcards
What is a dynamic equilibria
A chemical system is in equilibrium when:
- The concentrations of reactants and products remains constant
- The rate of the forward reaction is the same as the rate of the reverse reaction
what are the conditions of dynamic equilibria
- reversible
- in a closed sysstem
Describe how concentrations of products and reactants affects the equilibria and where it lies.
- reactions producing high con of products = equilibrium lies to the right
- reactions producing high conc of reactants and lower products = equilibrium lies to the left
what is lee chateliers principle
“When a system in dynamic equilibrium is subjected to a change, the equilibrium position will shift to minimise the change”.
what is the effect of pressure on equilibrium
- If pressure is increased, position of eqm. moves to side with fewest moles of gas
- This reduces the total moles of gas, and minimises the change in pressure.
- decreased pressure= moves to the side with more moles
What is the effect of concentration on equilibrium
- If concentration of reactant is increased, position of eqm. moves to decrease the concentration of reactant, so moves to the RHS
- If concentration of product is increased, position of eqm. moves to decrease the concentration of product, so moves to the LHS
Effect of temperature on equillibrium
- If temperature is increased, position of eqm. moves in the endothermic direction, to take in heat and minimise the temperature change
- If temperature is decreased, position of eqm. moves in the exothermic direction, to release heat and minimise the temperature change
effect of catalyst on equillibrium
- Catalysts do not affect the position of equilibrium
- Catalysts lower the activation energy.
- They speed up both the forward and reverse reaction.
- Catalysts increase the rate at which equilibrium is established, but not the position of equilibrium
define homogenous equillibrium
An equilibrium in which everything present is in the same phase
define heterogenous equillibrium
An equilibrium in which substances are in different phases
how do you find the equilibrium constant (Kc)
products/reactants
What does Kc show, and what does the size of the number show (large and small)
- Kc is showing ratio of products : reactants
- if Kc is large it means there are more products than reactants
- More products than reactants
- Forward reaction is favoured
- Equilibrium position is towards right hand side
- if Kc is small it means there are more reactants than products
- More reactants than product
- Reverse reaction is favoured
- Equilibrium position is towards left hand side
What is Kc affected by
Only affected by temperature
How does temperature affect Kc
- In exothermic:
- increasing temp moves equilibrium to endothermic direction (LHS) so more reactants formed
- Kc decreases
- In endothermic:
- increasing temp moves equillibrium to endothermic direction (RHS) so more products formed
- Kc increases
Calculate Kc from initial concentration
PCl5 + H2O = 2HCl + POCl3
At 100 degrees, a 2dm3 flask contains 0.075mol of PCl5; 0.05 mol of H2O; 0.75 mol of HCl; 0.5 mol of POCl3.
- mol/vol = conc
- divide all numbers by 2 dm3
- Kc- [0.375]*2 x [0.25]/[0.0375]x[0.025]= 37.5 moldm.-3
Write the equilibrium expression for each of the following equilibrium reactions, as shown. State the units of the equilibrium constant in each case.
a. H2(g) + I2(g) ⇌ 2HI(g)
b. N2O4(g) ⇌ 2NO2(g)
c. CH3OH(l) + CH3COOH(l) ⇌ CH3COOCH3(l) + H2O(l)
d. N2(g) + 3H2(g) ⇌ 2NH3(g)
Kc = [NH₃]² / ([N₂][H₂]³) Units: mol⁻²·dm⁶
e. 2SO2(g) + O2(g) ⇌ 2SO3(g)
f. 4HCl(g) + O2(g) ⇌ 2H2O(g) + 2Cl2(g)
g. 4NH3(g) + 5O2(g) ⇌ 4NO(g) + 6H2O(g)
a. H2(g) + I2(g) ⇌ 2HI(g)
Kc = Kc = [HI]² / ([H₂][I₂]) Units: none
b. N2O4(g) ⇌ 2NO2(g)
Kc = [NO₂]² / [N₂O₄] Units: mol·dm⁻³
c. CH3OH(l) + CH3COOH(l) ⇌ CH3COOCH3(l) + H2O(l)
Kc = [CH₃COOCH₃][H₂O] / ([CH₃OH][CH₃COOH]) Units: none
d. N2(g) + 3H2(g) ⇌ 2NH3(g)
Kc = [NH₃]² / ([N₂][H₂]³) Units: mol⁻²·dm⁶
e. 2SO2(g) + O2(g) ⇌ 2SO3(g)
Kc = [SO₃]² / ([SO₂]²[O₂]) Units: mol-1 dm3
f. 4HCl(g) + O2(g) ⇌ 2H2O(g) + 2Cl2(g)
Kc = [H₂O]²[Cl₂]² / ([HCl]⁴[O₂]) Units: ,mol-1 dm+3
g. 4NH3(g) + 5O2(g) ⇌ 4NO(g) + 6H2O(g)
Kc = [NO]⁴[H₂O]⁶ / ([NH₃]⁴[O₂]⁵) Units: mol dm-3
2NO2(g) ⇌ N2O4(g)
2.00 moles of NO2 are placed in a 1.00 dm3 flask and allowed to react. At equilibrium 1.80 moles of NO2 are present.
a. write the equilibrium expression
b. determine the value for Kc and state the units
ICE Table:
Initial: (NO2: 2.00)(N2O4: 0.00 )
Change: (NO2: 0.2)(N2O4: 0.1)
Equil (mol): (NO2: 1.80)(N2O4: 0.1)
Equi conc (moldm-3): (NO2: 1.80)(N2O4: 0.1)
a. Kc = [0.1] / [1.80]2
b. Kc = [0.1] / [1.80]2 = 0.03 mol-1 dm+3
what is Kp and what units is it measured in
equilibrium constant for the gas phase
Kpa
What does the table look like for working out Kp
Initial moles
Change in moles
Equillibrium moles
Mole fraction
Partial pressure
How do you do mole fraction
Mole/total moles
How do you do Partial pressure
mole fration x total pressure
1 mol of HI was left to decompose at 100kPa. At equilibrium 0.778 moles of HI remained. What is the value of Kp?
2HI = I2 + H2
Kp = 11.1 x 11.1 / 77.82 = 0.0204 (no units)
