What conditions are essential to reach equilibrium?
- The forward and reverse reaction occur at the same rate
- It requires a closed system
- The concentrations of reactants and products at equilibrium are constant
- No changes in macroscopic properties (temperature, pressure, density, colour)
Why do the forward and reverse reactions need to occur at the same rate?
- This is a form of dynamic equilibrium
- Reactants are continuously changed to products and they are continuously changed to reactants
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Why do the concentrations of reactants and products need to be constant?
- This is because the rate of the forward and backward reactions are equal
- Equilibrium can be approached from two directions
- In the forward reaction the concentrations of products increase, in the forward reaction the concentrations of reactants decrease
- At equilibrium concentrations are constant, not necessarily equal
Draw graph
Why does equilibrium require a closed system?
- The reactants and products cannot escape from the reaction vessel
- In open systems, matter can escape
What is physical equilibrium? What conditions are required?
- Physical changes in a closed system
- E.g. rate of evaporation of the water is greater than the rate of condensation
- It is a dynamic equilibrium
- Rates of forward and backward reaction equal
- Volume of liquid at equilibrium is constant
- Closed system
- No macroscopic properties
What do you calculate at equilibrium for any reaction?
- The equilibrium constant Kc using the equilibrium concentrations of reactants and products
- Need to also write the equilibrium constant expression:
aA + bB ⇌ cC + dD (a,b,c,d represent stoichiometric coefficients) - A change in temperature changes the value of the Kc value
How do you write an equilibrium constant expression?
- Products on the top, reactants on the bottom
- Square brackets, show concentrations (mol/dm^3) of the reactants and products at equilibrium
- Powers are stoichiometric coefficients
Check book
What does the value of Kc determine?
- Value of Kc at a particular temperature is a useful indication as to how far a reaction had gone toward completion (on the right side)
- High value, reaction almost complete, most reactants converted to products. At equilibrium, higher concentration of products than reactants
- Low value, forward reaction barely proceeded, only small proportion of reactants formed to products (lies to the left)
- Magnitude is temperature dependent
How does the value of Kc reflect the position of the reaction?
Check book
How can the equilibrium constant be manipulated?
Structure of equilibrium expression depends on the direction it is written is (products on left or right side). Occur in a reversible reaction:
- Reversing the equation
- Changing the coefficients in the balanced equation
- Reactions in sequence
How does reversing the equation manipulate Kc?
- Reversing the reaction, the value Kc will be the inverse/reciprocal of the value for the forward reaction
How does changing the coefficients in balanced equations manipulate Kc?
- If coefficients are doubled, Kc is squared
- If coefficients are halved, Kc square root taken
How does reactions in sequence manipulate Kc?
- Value of Kc is equal to the product of the individual values for the different steps
Kc = Kc1 × Kc2 × Kc3
Only the temperature can change the value of Kc.
What is the reaction quotient?
- Qc is a measure of the (relative) amounts of reactants and products for a reaction that has not yet reached equilibrium
- Predicts in which direction a reaction will proceed to reach equilibrium
How do you calculate Qc?
- Using the same formula as for Kc (check book)
- However, in this case non-equilibrium concentrations of reactants and products are used
What do you calculate with the Kc and Qc values?
- Comparing the value of Qc and Kc, indicates the direction in which the reaction must proceed to reach equilibrium
- When Qc < Kc reaction is not yet at equilibrium, proceeds to the right
- When Qc = Kc reaction at equilibrium
- When Qc > Kc reaction is not yet at equilibrium, proceeds to the left
What does Le Chatelier’s principle determine?
- Allows to predict in which direction the position of equilibrium will shift
- System will respond to minimize the effect of the change, applied at a state of equilibrium
What changes can be made to a reaction at equilibrium?
- Concentration
- Pressure of gases
- Temperature (also change the value of Kc)
- (Catalyst)
- Shift the equilibrium position to the left or right
What does changing the concentration do to a reaction at equilibrium?
- Decreasing concen. of products, increase concen. of reactants, move to the right
- Decrease concen. of reactants, increase concen. of products, move to the left
Draw graphs in book
What does changing the pressure do to a reaction at equilibrium?
- Only effects equilibrium if reactants and products are in gaseous states
- Pressure can be changed by adding or removing gaseous reactant or products
- Changing the volume of the system
- V ∝ 1/P
- Adding an inert gas will have no effect on the equilibrium position
In what direction must the reaction shifts when the pressure is changed?
- Increase pressure, shift to side with less gas moles
- Decrease pressure, shift to side with more gas moles
- Reactions with same number of gaseous molecules on each side, changes in pressure will have no effect
- Use ‘gaseous molecules’ term, use coefficients of these molecules
What does changing the temperature do to a reaction at equilibrium?
- Consider the enthalpy change (∆H)
- When the temperature is increased, the reaction will shift to the endothermic side
- When the temperature is decreased, the reaction will shift to the exothermic side
- In the exam, they will tell you which direction of the reversible reaction is exothermic/endothermic
What happens during a reaction, when the temperature is changed?
- If the temp. is increased, the position of equilibrium will shift to the endothermic side (direction of either forward or backward reaction)
- When forward reaction is endothermic, the added heat will be absorbed or when exothermic temperature is increased
