Unit 2) 1) Equilibria , Factors Affecting Equilibrium And The Equilibrium Constant Flashcards
What does it mean by “ a reaction is in equilibrium”
Some reactions never go to completion but are in a state of dynamic equilibrium
A chemical reaction is said to be in equilibrium when the rate of the forward reaction is equal to the rate of the reverse reaction : so the forward reaction and E verse reaction do not stop
At equilibrium the concentration of reactants and products will be ________ but not ________
At equilibrium the concentration of reactants and products will be constant but not equal
What is a closed system
In a closed system heat is able to be ________ to or from surroundings
For an equilibrium to establish the reaction must take place in a closed system : this is where the chemicals are trapped and nothing is added or removed , but heat is allowed to be transferred to or from the surroundings
What did Henri le chatelier say about equilibrium
“If a system is at equilibrium and a change is made in any of the conditions ,then the system responds to counteract the change as much as possible”
- in other words if you make a change to a reaction at equilibrium , the system will alter the concentrations to minimise the effect until equilibrium is re established.
How can the position of equilibrium be altered
- altering concentration of reactant or product
- altering pressure (increasing or decreasing )
- altering temperature (increasing or decreasing )
Effect of concentration on equilibrium position
Increasing concentration of reactant - equilibrium shifts right
Increasing equilibrium of product - equilibrium shifts left
Effect of temperature on equilibrium position
Increasing temperature - equilibrium shifts in direction of the endothermic reaction
Decreasing temperature- equilibrium shifts in direction of the exothermic reaction
Effect of pressure on equilibrium position
Increase in pressure - equilibrium shifts to side with least moles of gas
Decrease in pressure - equilibrium shifts to side with most moles of gas
Composition of equilibrium mixtures
Look at notes for examples and explanation
What is equilibrium constant
The equilibrium constant is a mathematical relationship that shows how the concentration of the products vary with the concentration of the reactants
What symbol does the equilibrium constant have , what symbol does it have when in terms of concentration
Equilibrium constant is - K
In terms of constant Kc
Equilibrium constant of general equation aA + bB —— > < cC + dD
Kc = [C]^c x [D]^d / [A]^a x [B]^b
- where the brackets indicate the concentration of species present at equilibrium , this is then raised to the power of the number of moles of that species from the balanced equation
(Look in notes it makes more sense)
What is a homogenous equilibrium
In a homogenous equilibrium , all the species present at equilibrium are all in the same state
Worked out examples of equilibrium constant expressions
Look in notes ! Important
Does K have units
No , K does not have units
What happens when a pure solid / pure liquid is present at equilibrium
Example
When a pure solid / liquid is present at equilibrium it’s concentration takes a value of 1
This is NOT true for aqueous solutions
Example in notes
What is a heterogenous equilibria
- example
- in a heterogenous equilibria , not all the species present are in the same state
- an example of a heterogenous equilibrium is heating calcium carbonate in a closed system so that the carbon dioxide gas produced cannot escape and equilibrium is established : look in notes for details
What can affect the position of equilibrium , but not the equilibrium constant
Changing the concentration or pressure may affect the position of equilibrium, but it does not affect th equilibrium constant ,K.
What happens to equilibrium when concentration or pressure is changed (in terms of constant )
When the concentration (or pressure in a chemical equilibrium involving gases) is changed , the position of equilibrium also changes - until the ratio of products and reactants return to original value (position of equilibrium changes until numerical value of K is de established )
What effect does a catalyst have on position of equilibrium and value of K
- presence of a catalyst has no effect on the position of equilibrium
- therefore a catalyst has no effect on the numerical value of the equilibrium constant K
Effect of temperature on equilibrium constant K
The value of equilibrium constant , K , is dependent on temperature
Increase in temperature effect on K
- in endothermic reactions
- in exothermic reactions
- in an endothermic reaction , increase in temp favours products , so ratio of products / reactants increases so K increases
- in endothermic reactions increase in temp causes increase in yield of products so value of K increases
- in an exothermic reaction , an increase in temp favours the reactants so ratio of products /reactants decreases and therefore K decreases.
- in exothermic reactions an increase in temp causes a decrease in the yield of the product and the value of K decreases
What does the equilibrium constant value give indication of
Since the equilibrium constant is the ratio of concentration of products divided by the concentration of reactants, it’s actual value gives guidance to the extent of a reaction once it has reached equilibrium
The greater the value of Kc the ________ the concentration of products compared to reactant
Greater
- in other words the further the reaction has gone to completion
Value of Kc an extent of reaction
Much less than 1 (or 10-3 ) - position of equilibrium is to the left (effectively no reaction )
Approximately 1- neither side is favoured , significant quantities of reactants and products at equilibrium
Much greater than 1 - to the right , reaction is effectively complete
