7 Equilibrium Flashcards
How do one-way and reversible reactions differ in terms of reactant-to-product conversion?
Some reactions go to completion where the reactants are used up to form the products and the reaction stops when all of the reactants are used up
In reversible reactions the products can react to reform the original reactants
What symbol is used to show a reversible reaction?
To show a reversible reaction, two half arrows are used: ⇌
What is dynamic equilibrium?
In a dynamic equilibrium the reactants and products are dynamic (they are constantly moving)
What properties does dynamic equilibrium have?
In a dynamic equilibrium:
1. the rate of the forward reaction is the same as the rate of the backward reaction in a closed system
2. the concentrations of the reactants and products is constant
3. There is no change in macroscopic properties such as colour and density as they depend on concentration
What is a closed system?
A closed system is one in which none of the reactants or products escape from the reaction mixture
What is an open system?
In an open system some matter is lost to the surroundings
When can equilibrium be reached, in solution?
When a reaction takes place entirely in solution, equilibrium can be reached in open flasks
When can equilibrium be reached, in gas?
If the reaction involves gas, equilibrium can only be reached in a closed system
Are the concentrations of the reactants and products equal at equilibrium?
NO
they’re not equal, but CONSTANT i.e. stay the same!
What is the equilibrium expression?
The equilibrium expression is an expression that links the equilibrium constant, Kc, to the concentrations of reactants and products at equilibrium taking the stoichiometry of the equation into account
What is the equilibrium expression for the reaction?
aA + bB ⇌ cC + dD
Kc = [C]^c * [D]^d / [A]^a * [B]^b
What state of matter is ignored in the equilibrium expression?
Solids are ignored in equilibrium expressions
Is Kc of a reaction general?
The Kc of a reaction is specific to a given equation
What does the size of Kc tell us?
The size of Kc tells us how the equilibrium mixture is made up with respect to reactants and products
What does it mean if Kc > 1?
If Kc > 1, the concentration of products is greater than the concentration of reactants and we say that the equilibrium lies to the right hand side
What does it mean if Kc <1?
If Kc < 1, then the concentration of reactants is greater than the concentration of products and we say that the equilibrium lies to the left hand side
Does Kc vary with temperature?
yes
Kc is a constant at a specified temperature
Since temperature can affect the position of equilibrium it follows that Kc is dependent on temperature
What is Kc dependent on apart from temperature?
In the previous section we saw that the concentrations of the substances are raised to the power of the coefficients from the balanced equation
This means the Kc expression is dependent on a specific equation
Give an example of how Kc is dependent on the stoichiometry of the equation, using ammonia formation?
½N2(g) + 1½H2(g) ⇌ NH3(g)
Kc = [NH3] / [N2]^1/2 * [H2]^3/2
If you double the stoichiometry the equation becomes
N2(g) + 3H2(g) ⇌ 2NH3(g)
The new Kc expression for this reaction is then:
[NH3]^2 / [N2] * [H2]^3
Therefore how does doubling the equation affect Kc?
You can probably see that when we double the coefficient the new Kc is the square of the original value
new Kc = (original Kc)^2
How does Kc change when the equation is reversed?
Kc becomes the reciprocal of the original Kc value:
i.e. 1/Kc
What is the reaction quotient?
The reaction quotient, Q, is the ratio of products and reactants for a reaction that has NOT yet reached equilibrium
What is the equation for Q?
[products]/[reactants]
What can Q tell us?
It is a useful concept because the size of Q can tell us how far a reaction is from equilibrium and in which direction the reaction proceeds
What does it mean if Q = Kc?
If Q = Kc then the reaction is at equilibrium, no net reaction occurs
What does it mean if Q < Kc?
If Q < Kc the reaction proceeds to the right in favour of the products
What does it mean if Q > Kc?
If Q > Kc the reaction proceeds to the left in favour of the reactants
Therefore what can we figure out with Q, if we have the concentrations of substances?
Using values of the concentrations of the substances present we can work out if a reaction is at equilibrium or not, as Kc = Q at equilibrium
What does the position of equilibrium refer to?
The position of the equilibrium refers to the relative amounts of products and reactants in an equilibrium mixture.
What does it mean if equilibrium shifts to the left?
When the position of equilibrium shifts to the left, it means the concentration of reactants increases
What does it mean if equilibrium shifts to the right?
When the position of equilibrium shifts to the right, it means the concentration of products increases
What is Le Chatelier’s principle?
Le Chatelier’s principle says that if a change is made to a system at dynamic equilibrium, the position of the equilibrium moves to minimise this change
What is Le Chatelier’s principle used to predict?
The principle is used to predict changes to the position of equilibrium when there are changes in temperature, pressure or concentration
How does an increase in concentration of reactant affect the position of equilibrium?
Equilibrium shifts to the right to reduce the effect of an increase in the concentration of a reactant
How does an decrease in concentration of reactant affect the position of equilibrium?
Equilibrium shifts to the left reduce the effect of decrease in the concentration of a reactant
When is a change in pressure relevant to equilibrium position?
Changes in pressure only affect reactions where the reactants or products are gases
How does an increase in pressure affect the equilibrium position?
equilibrium shifts in the direction that produces the smaller number of molecules of gas to decrease the pressure again
How does a decrease in pressure affect the equilibrium position?
equilibrium shifts in the direction that produces the larger number of molecules of gas to increase the pressure again
How does an increase in temperature affect the equilibrium position?
equilibrium moves in the endothermic direction to reverse the change
How does a decrease in temperature affect the equilibrium position?
equilibrium moves in the exothermic direction to reverse the change
What is the effect of a catalyst on the position of equilibrium?
A catalyst is a substance that increases the rate of a chemical reaction (they increase the rate of the forward and reverse reaction equally)
Catalysts only cause a reaction to reach its equilibrium faster
Catalysts therefore have no effect on the position of the equilibrium once this is reached
Is Kc affected by changes in concentrations?
NO
If all other conditions stay the same, the equilibrium constant Kc is not affected by any changes in concentration of the reactants or products
Does a change in pressure affect Kc?
NO
A change in pressure only changes the position of the equilibrium (see Le Chatelier’s principle)
If all other conditions stay the same, the equilibrium constant Kc is not affected by any changes in pressure of the reactants and products
How does Kc change with temperature for an endothermic reaction?
Changes in temperature change the equilibrium constant Kc
For an endothermic reaction such as:
2HI -> H2 + I2
An increase in temperature = [H2] and [I2] increases & [HI] decreases
Because [H2] and [I2] are increasing and [HI] is decreasing, the equilibrium constant Kc INCREASES
How does Kc change with temperature for an EXOTHERMIC reaction?
the equilibrium constant Kc decreases
Does a catalyst affect Kc?
If all other conditions stay the same, the equilibrium constant Kc is not affected by the presence of a catalyst
A catalyst speeds up both the forward and reverse reactions at the same rate so the ratio of [ products ] to [ reactants ] remains unchanged
Does a catalyst affect Kc?
If all other conditions stay the same, the equilibrium constant Kc is not affected by the presence of a catalyst
A catalyst speeds up both the forward and reverse reactions at the same rate so the ratio of [ products ] to [ reactants ] remains unchanged
