5.5 Equilibria Flashcards
(23 cards)
What is a reversible reaction
A reaction which can go backwards and forwards
What does a reversible reaction going backwards and forwards at any point depend on
Amounts of reactants and products
The conditions (such as temperature and pressure)
Enthalpy change in a reversible reaction
The enthalpy change in a reversible reaction always depends on the forwards reaction
For the backwards reaction, the enthalpy change is the same number but with the opposite sign
2 examples of reversible reactions
Dehydration of copper sulfate crystals
Thermal decomposition of ammonium chloride
Example of reversible reaction - dehydration of copper sulfate crystals
To make the reaction go forwards: heat the hydrated copper sulfate crystals (CuSO4.5H2O), the crystals turn from blue to white as they lose all their water
To make the reaction go backwards: add water to anhydrous copper sulfate (CuSO4), the white powder will absorb the water and turn blue
Example of reversible reaction - thermal decomposition of ammonium chloride
To make the reaction go forwards: heat NH4Cl, nothing appears to happen as both are colourless gases
To make the reaction go backwards: allow NH3 and HCl to mix in the absence of heat, white NH4Cl forms immediately wherever the cold gases have met
This can be demonstrated all at once:
- The NH4Cl at the bottom of the tube (hot) decomposes to form invisible NH3 and HCl
- The invisible NH3 and HCl cool as they move up the tube, recombining to form NH4Cl
What is dynamic equilibrium
When a reversible reaction is left in a sealed container, the forwards and backwards reactions will eventually ‘balance out’. From that point onwards, the mixture of reactants and products in the container is referred to as being in dynamic equilibrium
Dynamic = the forwards and backwards reactions are still happening, just at the same rate
Equilibrium = the concentrations of reactants and products remain unchanged
Analogy to remember dynamic equilibrium
Walking down the up-escalator quickly enough that you aren’t actually moving
Example to use for dynamic equilibrium
Decomposition of N2O4
Dynamic equilibrium - decomposition of N2O4 example
If you leave N2O4 in a box, some of it breaks apart forming NO2
But then, the NO2 just made could recombine to form N2O4 again
After a while, the breaking apart and recombining end up happening at the same time
So the overall concentrations of N2O4 and NO2 stay the same
The equilibrium position definition
The resulting balance of products and reactants in an equilibrium mixture
What is it called when there are more products than reactants
On the right
What is it called when there are more reactants than products
On the left
Relationship between the imbalance and the equilibrium position
The greater the imbalance of reactants and products, the further an equilibrium is to the ,eft or right
Equilibrium position - in the middle
It is possible (but rare) that the amounts of products and reactants are equal
How can you tell where an equilibrium is
By observing or measuring one of the substances
Variables impacting the position of an equilibrium shift
- Amounts of reactants or products
- Gas pressure
- Temperature
Guiding principle behind determining which way the equilibrium goes in response to a particular change
Le Chatelier’s Principle
‘The equilibrium shifts to counteract any change imposed on it
What to state when explaining an equilibrium shift
- State how the change needs to be counteracted
- Explain how the forwards/backwards reaction achieves that
Concentration changes on the position of the equilibrium
Add/remove some of the substance from the mixture
- If you add something, the equilibrium will shift to remove the extra you just added (whichever direction uses it up)
- If you remove something, the equilibrium will shift to create more of it (whichever direction creates it)
Gas pressure changes on the position of the equilibrium
Gases exert pressure on the sides of the container, and more gas means higher pressure
The equilibrium can affect the gas pressure by changing the moles of gas
- if you increase gas pressure, the equilibrium will shift to lower it back down (wherever has fewer moles gas)
- if you decrease gas pressure, the equilibrium will shift to bring it back up (wherever has most moles gas)
Temperature changes on the position of the equilibrium
Absorbing/releasing heat energy
The enthalpy change tells us whether the forwards reaction is endothermic or exothermic
- If you increase the temperature, the equilibrium will shift to bring it back down (wherever is endothermic)
- If you decrease the temperature, the equilibrium will shift to bring it back up (wherever is exothermic)
Catalyst impact on position of equilibrium
Catalysts increase rates of both forwards and backwards reactions, they balance
Therefore, adding a catalyst has no effect on the position of an equilibrium
