Topic 6: Equilibrium Flashcards
True or false - for any given reaction , the proportions of reactants and products will be the same
true
dynamic
forwards and reverse reactions are continuing but at the same rate in the opposite direction
equilibrium
overall composition of the mixture undergoes no net change
true or false - during equilibrium the pressure of the system will remain constant
true
state two features/characteristics of a system which has reached dynamic equilibrium
-concentrations of products and reactants are constant
-rate of forward and backwards reaction are equal
If the reactants were pink and products were blue, how could you make the reaction more pink (the forward reaction is endothermic)
Decrease the temperature so the exothermic reaction is favoured causing equilibrium position to shift to the left making reactants yield increase
How does the equilbrium yield change of temp is increased at constant pressure or pressure increased at constant temp
temp increase - decrease yield (favour endothermic)
pressure increase - increase yield (favour least moles)
what is a closed system
no chemicals can get in or out
define dynamic equilibrium
-The rates of the forward and backwards reactions are the same, concentrations of the reactants and products must remain constant
-Reaction must take place in a closed system
can equilibrium be reached from either side of the reaction
yes
Equilibrium graph
-For the reactants over time, the forward reaction would decrease than plateaus
-For the products over time the backwards reaction would increase than plateaus
-The rates of forward and backward reactions are equal
Le Chatiler’s principle
-Use to determine the effect a change will have on a mixture at equilibrium
-Changes –> temperature, pressure, concentration
-This principle states that if a factor affecting the position of equilibrium is altered, the position of the equilibrium shifts to oppose the effect of the change
Increase temperature
-The forward reaction is exothermic so backwards reaction must be endothermic
-If we increase temperature the position of equilibrium would favor the endothermic reaction to oppose the increase in temperature. This means that the yield of reactants increase but yield of products decrease
Decrease temperature
-If we decrease temperature the exothermic reaction is favored and the system shifts to the right in order to oppose the change. The yield of products increases and reactants decreases
increase reactant concentration
If the concentration of one reactant is increased, equilibrium will shift to the right to reduce amount of reactant
decrease reactant concentration
-If the concentration of one reactant is reduced, the position will shift left to increase amount of reactant
pressure
-Pressure only affects reactions involving gases
-the reaction with a greater pressure will have more moles of gaseous reactants
-If pressure of the system is increased the position of equilibrium would shift right as it favors the side of the reaction with the fewest moles of gaseous molecules.
When bismuth chloride is added to water, a reaction occurs and a white precipitate forms. What would be the effect on the amount of precipitate if sodium hydroxide solution is added to the mixture
-NaOH reacts with HCl and removes it
-equilibrium shifts right to replace lost HCl
-more BiOCl white precipitate formed
to increase the rate of the reaction, a vanadium pentoxide catalyst is used. Explain what effect this has on the overall percentage yield o sulfur trioxide
-there is no effect on the percentage yield of sulfur trioxide
-forward and backwards reaction are increase by the same rate
A chemist recommends lowering the temperature to 800K to save energy. Explain two disadvanatges of making this change
-decrease in temp causes a decrease in rate of reaction due to fewer frequent and successful collisions (less energy than Ea)
-decrease in temperature causes a shift in equilibrium position so yield decreases
Explain the main disadvantage of making a slight increase in pressure
increase in pressure causes shift to left so yield decreases bc of fewer moles on reactant side
effect of catalysts on equilibrium yield
-When a catalyst is added to a reversible reaction, it will increase the rate of both the forward and reverse reactions equally
-catalysts have no change to the position of equilibrium and therefore do not affect equilibrium yield
reasons why catalysts are beneficial in industry
-cheaper (reduced cost of production)
-increased rate of reaction
-reusable
-temperatures and pressures don’t need to be raised so high in order to increase rate
Industrially, ethanol is manufactured by hydration of ethene, which is a product of cracking crude oil. The forward reaction is exothermic and is catalyzed by phosphoric acid. State the best reaction conditions for this process
-Low temp
-high pressure
-excess ethene
-remove product as formed
temp, catalyst, pressure
-high temperatures increase rate of reaction but can reduce equilibrium yield
-catalysts increase rate of reaction but have no effect on equilibrium
-high pressure increases equilibrium yield but also costs more money
what does the position of equilibrium indicate
The position of equilibrium indicates the ratio of products to reactants
If the graph shows an equilibrium position that lies to the right (products more than the reactants) then the concentration of products is greater than the concentration of reactants. This means there is a high equilibrium yield
If the graph shows an equilibrium yield that lies to the left (reactants greater than products) then the concentration of reactants is always higher than the products)
when a reversible reaction is at equilibrium
When a reversible reaction is at equilibrium then the ratio of concentrations of products to reactants is constant
Kc = ratio of products to reactants = position of equilibrium
what happens if Kc is greater than 1
If Kc is greater than 1, then the concentration of products is greater than the concentration of reactants. This means that the equilibrium lies to the right.
units for Kc
Units for concentration = mol dm-3
-To find the units for equilibrium constant expand and then cancel e.g
heterogenous Kc
-Kc can only be calculated for gases and aqueous substances and all liquids except H2O
-no water (l) and no solid
-For a liquid in a sealed onctainer = [H2O (g)] / [H2O (l)]
-[H2O (l)] –> constant depending on density and mr
State whether the forward reaction in this equillibrium results in an increase, decrease or no change in the amount of moles of gas
increase –> lower yield at higher pressure so shifts left favouring backwards reaction
At 745K 0.150dm3 of an equilibrium mixture contains 0.0285mol of hydrogen and 0.0870 mol of nitrogen. Calculate the amount of moles of ammonia present
1) Write equation and substitute values
2) rearrange equation to make ammonia the subject
2) Calculate (mol x vol)
3) square root answer to get units of mol/dm3
4) multiply answer by concentration to get moles
What is the equation for equilibrium constant
Kc = [c] [d] / [a] [b]
constant concentrations
solids (s) and water (l) have constant concentrations so cannot be used and are ignored when calculating equilibrium constant
what is the effect of increasing pressure on the value of Kc
no effect on Kc
deduce in terms of X the amount in moles of R and calculate its concentration
mol R = 2x
3.6 = (2x)2 / (1-X)2
what factors affect Kc
temperature e.g if equilibrium shifts right, temp will increase
concentration and catalyst have no effect on value of Kc
Explain in terms of equilibrium yield and cost why a temperature of 800 degrees is used, pressure of 300pKa and a nickel catalyst
-reaction is endothermic so high temperature cause equilibrium position to shift right and favor forward reaction thus increasing equilibrium yield. However high temp is more costly
-More moles of gas on RHS so equilibrium shifts right to increase yield due to low pressure. Lower pressure increases yield and has a lower cost
-catalyst has not effect on yield but lowers cost
Describe the stages of this alternative route
-reactant absorbed onto platinum surface
-bond making occurs on the surface
-desorption of the product
the pool manager maintains the water at a pH slightly greater than 7.0 (alkali)
Explain how this affects the equillibrium established when chlorine is added to water
-alkali ions react with acid
-equilibrium position shifts right
suggest a reason why the catalyst must be hot
to overcome the activiation energy
suggest a reason why the catalyst remains hot during the reaction
the reaction is exothermic
dynamic
forward and backward reactions occur simultaneously
Kc
[c] [d] / [a] [b]
calculating moles at equillibrium
initial moles - moles reacted –> reactants
initial moles + moles formed –> products
effect of changing conditions of value of Kc
-larger Kc = greater number of products (favour products)
-Kc only changes with temperature
-pressure, concentration and catalyst has no effect on Kc value
e.g if temperature increases and forward reaction is exothermic then Kc would be smaller as reactants favoured
when one mole of ammonia is heated to a give temperature, 50% dissociates and the following equilibrium is etablished, what is tht eotal amont of gases in moles
1.5 mol
which change leads to a higher concentration og SO3 in this equilibrium mixture
higher concentration of O2
define Kc
-shows the relationship between reactants and products of a reaction at dynamic equillibrium
-calculated in homogenous solutions (aq and g)
-larger Kc = favours products
1 litre =
1 dm 3
true or false when calculating units you square if the power is 2
false = multiply by 2
how do you calculate units of Kc
input moldm-3 into the Kc expression you have calculated e.g [moldm-3]^3
Kc =
products / reactants
Kc definition
Kc = at equilibrium the ratio of the concentrations of products to reactants is constant. Kc is the ratio of products to reactants.
-Kc is always the same for a particular reaction for a given temp, pressure, concentration
why are H2O (l) and solids not included
-H2O (l) and solids are removed from equilibrium constant is bc their concentrations are constant
Kc = greater than 1
-If the concentration of products is higher than the concentration of reactants, Kc is greater than 1. This means the position of equilibrium lies to the right
steps for ICE table
1) draw ICE table and fill in from question
2) deduce concentrations at equilibrium (mol / volume)
3) write out Kc expression (always check if it is homogenous or heterogeous). If homogenous then liquid water can still be included in equation
4) work out units and subsitute
True or false -> you must include mol to mol ratio when calculating equilibrium moles
true
Make sure to include mol to mol ratio e.g if 2 moles of NO2 =2.5 then one mol of O2 is 1.25
state and explain the effect of a decrease in overall pressure on yield of products
-yield decreases
-equilibrium shifts the left side (more moles) to increase pressure
