Equilibrium. Flashcards
What does the term dynamic in dynamic equiibrium mean.
The term dynamic means both forward and backward reactions are occurring simultaneously.
State Le Chatelier’s Principe.
Give the little bit you sometimes forget.
Le Chatelier’s principle states that if an external condition is changed the equilibrium will shift to oppose the change (and try to reverse it).
It is the equilibrium that will shift not just “the system”.
use equilibrium system to be safe.
What are the two features of a dynamic equilibrium.
- Forward and backward reactions are occurring at
equal rates. - The concentrations of reactants and products stays constant.
Typical Exam question: What effect would increasing temperature have on the yield of ammonia?
N2 +3H2—> 2NH3. Delta H=-ve exo
Exam level answer : must include capital points…
If temperature is increased the EQUILIBRIUM WILL SHIFT TON OPOSE this and move in the ENDOTHERMIC BAACKWARDS direction to try to DECREASE TEMPERATURE. The position of equilibrium will SHIFT TOWORDS THE LEFT, giving a LOWER YIELD OF AMMONIA.
What does the term “position of equilibrium” actually mean.
We use the expression ‘position of equilibrium’ to describe the composition of the equilibrium mixture.
If the position of equilibrium favours the reactants (also described as “towards the left”) then the equilibrium mixture will contain mostly reactants.
What is the general affect of increasing temperature on the position of equilibrium.
If temperature is increased the equilibrium will shift to oppose this and move in the endothermic direction to try to reduce the temperature by absorbing heat.
What is the general affect of decreasing temperature on the position of equilibrium.
If temperature is decreased the equilibrium will shift to oppose this and move in the exothermic direction to try to increase the temperature by giving out heat.
What is the general affect of increasing pressure on an equilibrium position.
Increasing pressure will cause the equilibrium to shift towards the side with fewer moles of gas to oppose the change and thereby reduce the pressure.
What is the general affect of decreasing pressure on an equilibrium position.
Decreasing pressure will cause the equilibrium to shift towards the side with more moles of gas to oppose the change and thereby increase the pressure.
Study the following reversible reaction. What will the affect be of increasing pressure on the positron of equilibrium.
H2 + Cl —> 2HCl
If the number of moles of gas is the same on both sides of the equation then changing pressure will have no effect on the position of equilibrium
Typical Exam question: What effect would increasing pressure have on the yield of methanol?
CO +2H2 ⇌ CH3OH
If pressure is increased the EQUILIBRIUM WILL SHIFT TO OPPOSE this and move towards the side with FEWER MOLES OF GAS TO TRY TO REDUCE THE PRESSURE . The position of equilibrium will SHIFT TOWARDS THE RIGHT BECAUSE there are 3 moles of gas on the left but only 1 mole of gas on the right, giving a HIGHER YIELD OF METHANOL.
(Bits on capital are needed for all marks)
All of the reversible reactions that you must need to know are exothermic and so why are they all not done I. the freezing cold to increase the yield of desired product.
Low temperatures may give a higher yield of product but will also result in slow rates of reaction. Often a compromise temperature is used that gives a reasonable yield and rate.
CO +2H2 ⇌ CHOH
Consider the above reaction used to produce methanol in industry why is it not done at incredibly high pressure to increase the yield of methanol.
Increasing pressure may give a higher yield of product and will produce a faster rate. Industrially high pressures are expensive to produce ( high electrical energy costs for pumping the gases to make a high pressure) and the
equipment is expensive (to contain the high pressure
s).
In all cases high pressure leads to too high energy costs for pumps to produce the pressure and too high equipment costs to have equipment that can withstand high pressures.
Why does a catalyst not affect the position of equilibrium and why if not is it used In industry.
It does not effect the position of equilibrium because it speeds up the rates of the forward and backward reactions by the same amount.
In all cases catalysts speeds up the rate, allowing a lower temp to be used (and hence lower energy costs), but have no effect on position of equilibrium
How can the overall yield of all industrial processes be improved.
Recycling unreacted reactants back into the reactor can improve the overall yields of all these processes.
What are the uses of both ethanol and methanol.
Both methanol and ethanol can be used as fuels.
CO +2H2 ⇌ CH OH
How could this reaction be made carbon neutral, and define carbon neutral in your answer. What would have to happen for it top really be carbon neutral.
The term carbon neutral refers to “an activity that has no net annual carbon (greenhouse gas) emissions to the atmosphere”.
If the carbon monoxide used to make methanol in the above reaction was extracted from the atmosphere then it could be classed as carbon neutral.
It would only be carbon neutral, however, if the energy required to carry out the reaction was not made by combustion of fossil fuels .
Give the reaction and conditions for the hydration of ethene. Give also the conditions used.
CH=CH +H2O ⇌ CHCHOH H=-ve
T= 300oC, P= 70 atm, catalyst = conc H3PO4.
State why , with the hydration of ethene , excess steam and really high pressures are not used.
High pressure also, as well as too high energy costs, leads to unwanted polymerisation of ethene to poly(ethene).
Excess steam would dilute the phosphoric acid catalyst.
Give the equation for the production of methanol and give also the conditions that are used in industry.
CO +2H 2⇌ CH3 OH H=-ve exo
T= 400oC, P= 50 atm, catalyst = chromium and zinc oxides
Calculate the units for Kc for the following reactions.
1) N2 (g)+3H2 (g) 2NH3 (g)
2) H2 (g) +Cl2 (g) 2HCl (g)
1) Unit = mol-2 dm+6
2) no unit
What types of condition change will affect the value of Kc.
Why out off curiosity. (Don’t need to know)
Kc only changes with temperature. It does not change if pressure or concentration is altered.
A catalyst also has no effect on Kc
Both the equilibrium position and the value of Kc will change when you alter temperature but or all other condition (not a catalyst) only the position of equilibrium will change.
With concentration and pressure you changed them in the first place for the equilibrium to shift so the effect cancels. With temp you get a reaction without provoking the system since the temp is not in the Kc expression so can’t be compensated for.
What does the value of Kc say about the position of equilibrium.
Since Kc= Products
Reactants
What does the value of Kc say about the position of equilibrium. What value is key here.
Since Kc= Products
Reactants
The larger the Kc the greater the amount of products.
If Kc is small we say the equilibrium favours the reactants. The number 1 is the key value here and if Kc is less than 1 the reactants predominate and vice versa.
Reactions where Kc is equal to 10 to the 10 are considered to have gone to completion and reactions that have a Kc of 10 to the -10 are considered to have not taken place at all.
