Chapter 7 Chemical Equilibrium Flashcards
What is Equilibrium ?
•When confined to a closed system, reversible reactions can reach a point where they are said to be in
equilibrium.
•For a chemical reaction to be at equilibrium, two equal but opposing reactions are proceeding at the same rate.
How does a reaction reach Equilibrium ?
- The rate of the forward reaction must equal the rate of the reverse reaction.
- Consequently the effects of reaction 1 and reaction 2 cancel each other out.
- So at equilibrium the concentration of both reaction remains constant over time.
- This constancy of concentrations for all the reactants and products is the hallmark of a chemical system that has reached Equilibrium.
Why is a chemical system at equilibrium said to be dynamic as opposed to static ?
This is because it is still active as both the forward and reverse reactions despite the static appearance of a system at equilibrium.
What are some points to remember about the equilibrium constant K?
- Only gases and aqueous species appear in K. Solids and liquids have a fixed concentration and so they are not included in the expression for K.
- While K has a constant value for all conditions of concentration and pressure its value does change if temperature changes.
What does the different sizes of K indicate?
- large values of K imply the equilibrium favours products
- small values of K imply equilibrium favours reactants
- values of K Close to 1 imply significant concentrations of both reactants and products are present at equilibrium.
Define Impose Changes.
It is the ability of a chemical equilibrium system to return to a state of equilibrium after alterations.
List the imposed changes that will affect the equilibrium:
- Changing the Concentration of any one species in the system by selectively adding it to the system or removing it from the system.
- Changing the Total Pressure in a Gaseous equilibrium by reducing its volume.
- Changing the Temperature of the system by adding or removing Heat from the system.
What happens when the concentration in an equilibrium system is altered by a change imposed on the system?
- A new equilibrium will form in such a way that partially counteracts this imposed change.
- In doing this a system will consume some of the added reagent or replace some of the consumed reagent.
What is happen during these changes (system being altered by a an imposed change)?
-The system is temporarily out of equilibrium and the rate of the forward and reverse reactions won’t be equal.
When does the system partially minimise the original imposed change in concentration?
-At some point the system returns to equilibrium (when the rates of forward and reverse reactions are again equal) but when it does, the concentrations and amounts of all the reagents will have altered in such a way that has partially minimised the original imposed change in concentration.
Explain the Concentration Time Graph.
- A concentration time graph shows how an imposed change results in a new equilibrium.
- In this example the Dichromate/Chromate system is at equilibrium up until time (T1).
- At T1 a small amount of concentrated NaOH(aq) is added to the system (the imposed change).
- This imposed change means the (OH-) has been selectively raised.
- The system is now temporarily out of equilibrium as the increased (OH-) means the forward reaction rate, increases while the reverse reaction rate, remains unchanged (remember rate rises with concentration).
- This temporarily favours the formation of products and consumption of reactants.
- Consequently all concentrations change during the interval T1 to T2 until a new equilibrium is established (at T2) as predicted by Le Chatelier’s principle.
How does an equilibrium system involving gases achieve an equilibrium?
It must be in a sealed container.
What happens if the pressure in the system is altered?
The equilibrium will change in a way that minimises the pressure change.
In relation to Le Chateler’s principle explain what happens when the pressure of an equilibrium system is altered.
- Increasing the pressure by reducing volume will favour the side of the equilibrium reaction with fewer moles of gas. Fewer moles of gas mean less pressure within the system thus partially counteracting the imposed change.
- Decreasing the pressure by increasing volume will favour the side of the equilibrium reaction with the greater moles of gas. More moles of gas means more pressure thus partially counteracting the imposed change.
List situations in which the pressure has no effect in the equilibrium system.
- If both sides of the equilibrium reaction have the same number of moles of gas, then changing the pressure has no effect on the equilibrium position.
- Increasing pressure by adding an inert gas has no effect on the equilibrium position.
- Also, changing the pressure by adding or removing one of the gaseous reactants or products is equivalent to selectively altering the concentration of that substance only.
What is an inert gas?
It is one that does not react with the substances in the equilibrium system.
In relation to Le Chateler’s principle explain what happens when the temperature of an equilibrium system is altered.
- Raised: A new equilibrium is established favouring the endothermic process. By doing this, the system converts some of the added heat to chemical potential energy (bond energy), thus causing its temperature to fall, ie partially counteracting the imposed change.
- Lowered: A new equilibrium is established favouring the exothermic process. By doing this, the system converts some of the chemical potential energy (bond energy) to heat, thus causing its temperature to rise, ie partially counteracting the imposed change.
Define Heat.
Energy that naturally flows from a hot system (high temp) to a cooler one.
Define Chemical Potential Energy.
Energy stored by the chemical bonds in a substance.
Define Enthalpy.
A measure of the total energy in a chemical system. It includes potential energy (bond energy) and particle kinetic energy.
What happens in an exothermic reaction?
In an exothermic reaction temperature rises as some chemical potential energy (bond energy) is converted into particle kinetic energy.
- As a result, heat inevitably flows out of the hot reaction mixture and into the cooler surroundings.
- The formation of heat in an exothermic reaction and its eventual loss to the surroundings means the enthalpy of the reacting system must decrease and hence -ve.
What happens in an endothermic reaction?
- In an endothermic reaction temperature falls as particle kinetic energy is converted to chemical potential energy.
- As the reaction mixture becomes cool, heat then flows into the mixture from the warmer surroundings and so its enthalpy increases, ie. +ve.
What does a Catalyst do?
The presence of a catalyst in an equilibrium system has no effect on the equilibrium position; however, the catalyst will speed up the rate of attainment of equilibrium.
Why doesn’t a Catalyst affect an equilibrium system?
This happens as the catalyst increases the rate of both the forwards and reverse reactions equally.