Chapter 7: Equilibrium Flashcards
What are reversible reactions?
Reactions that can go either way
What happens at equilibrium?
- Forward and backwards reactions happening at the same rate
- Takes place in a closed system - no matter, energy or pressure changes
- Both reactant and product are present
- Does not mean the same amount of reactants and products
- Concentration of the reactants and products is not be the same at equilibrium
Is it always possible to observe a change in a chemical reactions?
No. At equilibrium there are no observable macroscopic changes
What happens in dynamic equillibrium?
Macroscopic properties are constant (concentrations of all reactants and products remains constant) and the** rate of the forward reaction is equal to the rate of the reverse reaction**
What are the 5 characteristics of the equilibrium state?
- Macroscopic properties are constant at equilibrium - at equilibrium the concentrations of all reactants and products remain constant
- At equilibrium the rate of the forward reaction is equal to the rate of the reverse reaction
- Equilibrium can be attained only in a closed system - equilibrium is never reached if the product/reactant escapes and it has no opportunity to recombine.
- All species in the chemical equation are present in the equilibrium reaction mixture
- Equilibrium can be attained from either direction - equilibrium can be reached if a reaction is flipped
How do we know if a concentration has reached equilibrium?
when it does not change anymore
graph: the conc of H2 decreases at first, until it levels off when equilibrium is reached
What is physical equilibria?
Equilibria involving a change in state
(e.g. liquid-vapour)
What process is evaporation?
an endothermic processes
Consider a volatile liquid, such a bromine, in a closed container. How is equilibrium reached?
- beginning: no molecules of vapour above the liquid
- molecules escape from the fluid (evaporation)
- As molecules of vapour appear, these strike the surface of the liquid and some re-enter it (this process is condensation)
- At first the rate of condensation is low, but as the number of molecules in the vapour phase increases the rate of condensation increases
- Eventually the rate of condensation becomes equal to the rate of evaporation and nothing more appears to change
How is this a dynamic equilibrium?
because the rate of evaporation and the rate fo condensation are equal
How is this an example of phase equilibrium?
when the rate of evaporation (vaporisation) is equal to th rate of condensation, the colour of vapour remains constant and a state of equilibrium has been reached. This is known as a phase equilibrium because it involves a change in phase (state)
What does the position of equilibrium refer to?
to the relative amounts of reactants and products present at equilibrium
What is the Le Chatelier’s principle
If a system at equilibrium is subjected to a change, the position of equilibrium will shift in order to minimise the effect of the change
What does Le chatelier’s principle allow us to predict?
in which direction the position of equilibrium will shift
Where is the position of equilibrium shifted in a heat vs cool reaction mixture?
Heat reaction mixture: position of equilibrium is shifted in the endothermic direction
Cool reaction mixture: position of equilibrium is shifted in the exothermic direction
Consider this following reaction: which way will the position of equilibrium shift to when the pressure increases?
This reaction involves the conversion of two molecules of gas (on the left-hand side) to three molecules of gas (on the right-hand side). As the pressure is increased, the position of equilibrium shifts to the left-hand side - i.e. the side with fewer gas molecules - to minimise the effect of the change
Consider this following equilibrium. What effect does pressure have on this?
Because there is the same number of molecules of gas on both sides, changing the pressure has no effect on the position of equilibrium.
Consider the decomposition of calcium carbonate. What effect does increasing the pressure have on the position of equilibrium?
There is only one molecule of gas on the right-hand side but none on the left-hand side - therefore increasing the pressure causes the position of equilibrium to shift to the left
Consider this system at equilibrium. What happens to the equilibrium when you add more more acid and more OH- (alkali)?
Adding more acid, the position of equilibrium shifts to the right to use up the excess acid and so minimise the effect of the change.
If we add alkali (OH-) to the solution, they will react with the H+ ions in the solution to form water. This then shifts the position of equilibrium to the left in order to minimise the effect of the change by replacing the H+ ions
In general, what happens if you increase the concentration of one of the species in an equilibrium mixture?
The position of equilibrium shifts to the opposite side to reduce the concentration of this species.
What is the Kc constant
It is the equilibrium constant for a particular reaction at a particular temperature
the ‘c’ indicates that this equilibrium constant is expressed in terms of concentrations
How do you calcuate the Kc?
What information does an equilibrium constant provide?
Provides information about how far a reaction proceeds at a particular temperature
If the Kc are all much greater than 1 = reaction proceed almost toally towards the product
If Kc is very much less than 1 = the reaction hardly proceeds at all towards the products (the position of equilibrium lies a long way to the left)
What is the reaction quotient, Q?
the ratio of the concentrations of the reactants and products (raised to the appropriate powers) at any point in time.
An expression for Q is exactly the same as that for the equilibrium constant - except that the concentrations are not equilibrium concentrations
What do these mean?
How does equilibrium constant get affected by these changes to the reaction equation?
How can the equilibrium constant be affected?
The value of the equilibrium constant for a particular reaction is only affected by a change in temperature
How does the equilibirum constant expression for this reaction change to the effect of increasing temperature?
The reaction is exothermic in the forward direction, so an increase in temperature causes the position of equilibrium to shift in the endothermic direction - that is, the position of equilibrium shifts to the left.
The concentration on the top of the expression for Kc decreases and the concentration on the bottom increases. Therefore, the over value of Kc decreases
How does the equilibirum constant expression for this reaction change to the effect of increasing temperature?
This is an endothermic reaction
as the termperature increases the position of the equilibrium shifts in the endothermic direction in orde to take in heat and reduce the effect of the change
the position of the equilibrium is shifted to the right so that More NO and less N₂ and O₂ are present at equilibrium
this results in the value of the equilibrium constant decreasing
For exothermic and endothermic reactions, how does the value of the equilibrium constant change as temperature increase?
For an exothermic reaction, the value of the equilibrium constant decreases as the temperature is increased
For an endothermic reaction, the value of the equilibrium constant increases as the temperature is raised
How do catalyst affect the rate of forward and reverse reactions?
A catalyst is a substance that increases the rate of forward and reverse reactions equally without being permanently changed in the process
They provide an alternative pathway of lower activation energy (Ea) for the reaction. Reduces the activation energy for the forward and backward reaction. affect both reactions equally.
How does the presence of a catalust affect the position of equilibrium or the value of the equilibrium constant?
The presence of a catalyst does not affect the position of equilibrium or the value of the equilibrium constant; it only reduces the time taken to reach equilibrium
Why dont catalyst appear in a chemical equation?
Because they are the same at the beginning as at the end of the reaction
What information does the equilibrium constant give?
give us info about how far a reaction goes towards completion (that is, about the extent of the reaction)
no info on the rate
What 4 conditions can have an effect on the position of equilibrium and the Kc?
Pressure
Concentration
Catalyst
Temperature
What are the effect of changes in 4 conditions on the position of equilibrium and the value?
What is pressure’s effect on the position of equilibrium and Kc?
If a reaction involves a change in number of gas molecule:
increase pressure -> shift to the direction that gives a decrease in the number of gas molecules
no change on Kc
How does the concentration affect position of equilibrium?
Shift in order to use up any substance that has been added or replace any substance that has been removed from the equilibrium mixture
Add H to the left - shift to the right so more H is used
No effeect on Kc
What effect does a catalyst have on the position of equilibrium and Kc?
none
What effect does temperature have on th eposition of equilibrium and Kc?
Increase temp -> shift to endo direction
Decrease temp -> shift to exo direction
For exo - Kc decrease as temperature increase
For endo - Kc increases as the temperature increase
