6. Chemical Equilibria Flashcards
What is a reversible reaction?
A reaction that can take place in both directions and is therefore incomplete
Why is the reaction rate fast at the beginning of a reversible reaction?
There are only reactants
How is a dynamic equilibrium reached in a reversible reaction?
- beginning - only reactants so fast rate
- reactants produce products so forward reaction slows
- products then react to form reactants
- continues until equilibrium reached
What happens at dynamic equilibrium?
- rates of forward and reverse reactions are the same
* net concentrations of the components of the reaction mixture remain constant
What condition must be true for dynamic equilibrium?
Reaction takes place in a closed system
If the concentration of products at equilibrium is high compared to that of the reactants, where does equilibrium lie?
To the right
If the concentration of reactants at equilibrium is high compared to that of the products, where does equilibrium lie?
To the left
What can change the position of equilibrium?
- temperature
- concentration
- pressure
What principle can be used to explain which way equilibrium will shift?
Le Chatelier’s
What does Le Chatelier’s principle state?
That if a factor affecting the position of equilibrium is altered, the position of equilibrium shifts to oppose the effect of the change
Which way does equilibrium shift when the temperature of a reaction is increased?
In the endothermic direction (in order to ↓ the temperature)
Which way does equilibrium shift when the temperature of a reaction is decreased?
In the exothermic direction (in order to ↑ the temperature)
The reaction N₂ + 3H₂ ⇌ 2NH₃ has a -ve enthalpy change. What happens when the temperature is increased?
- equilibrium shifts left
- in endothermic direction
- to take in heat energy to oppose the change and ↓ temperature
Would a high or low temperature increase the yield of ammonia? [N₂ + 3H₂ ⇌ 2NH₃]
Lower - will shift equilibrium to produce more products - but rate will be slower
When will changing pressure have an effect on equilibrium?
When reactions involve gases
Which way does equilibrium shift when the pressure of a reaction is increased?
In the direction with fewer moles of gas to oppose the change (i.e. ↓ pressure)
Which way does equilibrium shift when the pressure of a reaction is decreased?
In the direction with the most moles of gas to oppose the change (i.e. ↑ pressure)
What happens if pressure is increased but the number of moles is the same on both sides of the reaction?
The position of equilibrium remains unaffected, but equilibrium will be reached faster
What is the position of equilibrium for reactions in the liquid/aqueous phase affected by?
Concentration, not pressure
What happens to equilibrium when the concentration of a reactant is increased?
Shift position to the right
What happens to equilibrium when the concentration of a product is increased?
Shift position to the left
In the equation BiOCl + 2HCl ⇌ BiCl₃ + H₂O, what happens when more acid is added?
Equilibrium will shift right to counteract the increase in HCl
In the equation BiOCl + 2HCl ⇌ BiCl₃ + H₂O, what happens when more water is added?
Equilibrium will shift left to counteract the increase in H₂O
How does a catalyst increase the rate of reaction?
- lowering activation energy
* provide alternate route for the reaction
What happens to equilibrium when a catalyst is added?
Position not affected, but rate increases so time to reach equilibrium decreases
What happens if pressure is changed in a reaction with gases and solids as reactants, and only gases as products?
Only count the moles of gas when deciding which way equilibrium shifts
How can ethanol be produced industrially?
Ethene (produced from cracking of crude oil) can be hydrated
In the equation to produce ethanol using hydration of ethene [C₂H₄ + H₂O ⇌ C₂H₅OH], what is the theoretical yield?
100%
Will the reaction C₂H₄ + H₂O ⇌ C₂H₅OH have an 100% yield in reality?
No
Why may some reversible reactions with a theoretical yield of 100% not actually have 100% yield?
May produce some side reactions
What is the initial yield of ethanol?
5%
What could the yield of the production of ethanol by hydration of ethene be increased by?
- ↑ pressure
- ↑ concentration of steam as its cheaper than ethene
- ↓ temperature
What temperatures are used in the hydration of ethene to ethanol? Why?
- 573K and 6500kPa
* compromised between best yield and fastest rate
What is the equilibrium constant?
The ratio of concentrations of the reactants and products in a reaction mixture
What can the equilibrium constant be denoted by?
Kc
What does the c in Kc represent?
Molar concentration
When does the value of Kc remain constant?
Only if the temperature remains constant
For any reaction that reaches equilibria, how can the equation be written (with regard to Kc)?
ᵃA + Bᵇ ⇌ ᶜC + ᵈD
How can the equilibrium constant be written using the equation ᵃA + Bᵇ ⇌ ᶜC + ᵈD (provided temperature is constant)?
Kc = [C]ᶜ [D]ᵈ / [B]ᵇ [A]ᵃ
How can Kc be shown in an expression?
Kc = products/ reactants
What is the units for Kc?
Vary but worked out by cancelling units for each term in the expression
What does homogenous mean?
All reactants and products are in the same phase
What happens to the equilibrium position if Kc is relatively large?
It lies well over to the product (right) side
What happens to the equilibrium position if Kc is relatively small?
It lies well over to the reactant (left) side
How will change in temperature affect Kc?
Will ↑ or ↓ depending on whether forward reaction is exothermic or endothermic
Where will equilibrium lie if Kc > 1?
Concentration of products > reactants so equilibrium lies to the right
Where will equilibrium lie if Kc < 1?
Concentration of reactants > products so equilibrium lies to left
If temperature increases for an exothermic reaction, what happens to Kc?
Decreases
If temperature increases for an endothermic reaction, what happens to Kc?
Increases
If temperature decreases for an exothermic reaction, what happens to Kc?
Increases
If temperature decreases for an endothermic reaction, what happens to Kc?
Decreases
Will increasing or decreasing concentration have an effect on the value of Kc?
No
Why will a change in concentration not affect Kc?
Kc is dependent on concentrations of reactants and products changing
e.g. if concentration of reactants is increased, then equilibrium will shift until equilibrium is restored - so no overall change in concentration of reactants or products
Will increasing or decreasing pressure have an effect on the value of Kc?
No
Will adding a catalyst have an effect on the value of Kc?
No - increases rate of forward and reverse reactions by the same amount
Which reactions are regarded as going to completion?
Those with Kc having a value greater than 10¹⁰
What reactions are regarded as not taking place at all?
Those with Kc having a value less than 10⁻¹⁰
