Chemical Equilibria and Kc Flashcards
Why is equilibrium only achieved in a closed system
Reactants or products may be lost in an open system
What are the characteristics of a dynamic equilibrium
The rate of the forwards reaction is equal to the rate of the reverse reaction.
At equilibrium the concentrations of the reactants and products remains constant
Homogenous equilibrium
When all the reactants and products are in the same phase (e.g all gases)
Heterogenous equilibrium
When all the reactants and products are in different phases
What factors affect the position of equilibrium
Concentration
Pressure
Temperature
State and explain the effect on the position of equilibrium if more reactants are added to the mixture
The position of equilibrium will move to the right to use up the added reactants. The equilibrium moves to oppose the increase of reactants.
State and explain the effect on the position of equilibrium if the products are removed
The position of equilibrium will shift to the right to produce more of the products. The equilibrium moves to oppose the loss of the products
What effect does adding dilute HCl have on the position of equilibrium
Addition of hydrochloric acid increases the concentration of H+ ions, so the position of equilibrium shifts to the right to use up the added H+ ions.
What effect does the addition of alkali have on the position of equilibrium if there are H+ ions already on the right hand side.
The OH- ions react with the H+ ions, so the position of equilibrium shifts to the right to produce more H+ ions.
How can the pressure of an equilibrium system be increased
Adding more gas into the same volume
Decreasing the volume
State with a reason, the effect of an increase in the pressure on the position of equilibrium.
The position of equilibrium will shift to the side with fewer gas molecules. The equilibrium moves in the direction that decreases the pressure.
State with a reason, the effect of a decrease in the pressure on the position of equilibrium.
The position of equilibrium will shift to the side with more gas molecules. The equilibrium moves in the direction that increases the pressure.
If the number of gaseous molecules if the same on both sides what effect does increasing/decreasing the pressure have on the position of equilibrium
The position of equilibrium does not change as there is no change in the number of gaseous molecules.
What is the advantage of increasing the pressure in an equilibrium if there are the same number of gaseous molecules on both sides
The rate of reaching equilibrium is faster
If the forwards reaction was exothermic, state with a reason the effect of an increase in temperature on the position of equilibrium
The position of equilibrium shifts to the left in the endothermic direction to absorb the added heat
If the forwards reaction was exothermic, state with a reason the effect of a decrease in temperature on the position of equilibrium
The position of equilibrium shifts to the right in the exothermic direction to give out heat
If the forwards reaction was endothermic, state with a reason the effect of an increase in temperature on the position of equlibrium
The position of equilibrium shifts to the right in the endothermic direction to absorb the added heat
Why is the rate of reaching equilibrium faster when temp is increased
At a higher temperature more particles have energy greater than the activation energy, therefore there are more successful collisions in a given time.
What effect does a catalyst have on equilibria
A catalyst increases the rate at which equilibrium is reached
State and explain the effect on the yield/position of equilibrium that a catalyst has
A catalyst has no effect on the yield as it increases the rate of the forwards and reverse reaction by the same amount
Kc =
[Concentration of products in mol dm^-3] / [Concentration of reactants in mol dm^-3]
What affects the value of the equilibrium constant
Temperature ONLY
NOT CONCENTRATION OR CATALYST
Why are solids not included in the expression for Kc
The concentration of a solid remains constant
Why might a Kc in a reaction have no units
There are the same number of moles on both sides of the equation, so the volumes cancel
How is 2HCl represented in a Kc expression
[HCl]^2
What does a Kc value of greater than 1 mean
The equilibrium lies to the right
What does a Kc value of smaller than 1 mean
The equilibrium lies to the left
What does a Kc value of greater than 1x10^10 mean
The equilibrium lies to the far right
How do you use ICE tables to calculate Kc
Draw a table with initial moles, change in moles and equilibrium moles.
Write down all the information given for each compound in the write section of the table
Unless told, the initial moles for a product is 0.
Work out the change in moles.
The sign for change of moles is opposite for products and reactants.
If products increases by +0.5 and the ratio is 1:1 , the reactants decrease by -0.5 moles.
Using the equilibrium moles and volume, find out the equilibrium conc and sub into Kc equation
If the forwards reaction was endothermic, state and explain the effect on the value of Kc and hence on the position of equilibrium if a higher temperature was used
Kc increases as temperature increases. Kc is now greater than the quotient so the quotient must increase. This happens if more products are being formed and less reactants, so the position of equilibrium moves to the right to equal the new Kc.
What affects the value of Kc
ONLY a change in temperature
4NH3 + 5O2 = 4NO + 6H2O
The forwards reaction is exothermic.
State and explain the effect on the value of Kc and hence on the position of equilibrium if NO is added.
The value of Kc does not change.
When NO is added the value of [NO]^4[H2O]^6 / [NH3]^4[O2]^5 increases.
The quotient is now greater than unchanged Kc so the quotient must decrease. This happens if more reactants are being formed and less products so the position of equilibrium moves to the left to decrease the quotient until it equals Kc again.
4NH3 + 5O2 = 4NO + 6H2O
The forwards reaction is exothermic.
State and explain the effect on the value of Kc and hence on the position of equilibrium if the temperature is decreased.
As the forwards reaction is exothermic, Kc increases as the temperature decreases.
Kc is now greater than the quotient, so the quotient must increase. This happens if more products are being formed and less reactants, so the position of equilibrium moves to the right to equal the new Kc .
4NH3 + 5O2 = 4NO + 6H2O
The forwards reaction is exothermic.
State and explain the effect on the value of Kc and hence on the position of equilibrium if the temperature is increased
As the forwards reaction is exothermic, Kc decreases as the temperature increases.
Kc is now smaller than the quotient, so the quotient must decrease. This happens if more reactants are being formed and less products, so the position of equilibrium moves to the left to equal the new Kc .
State and explain two benefits to the environment resulting from the use of catalysts in industrial processes.
Less fuel/ raw materials/ natural resources are used because the reaction has a lower activation energy.
Enables the use of an alternative process with a higher atom economy therefore less raw materials are needed.
Give two features of a reaction in dynamic equilibrium
Rate of the forwards reaction is equal to the rate of the reverse reaction.
Concentrations of products and reactants remain constant.
