Equilibria Quiz

Dynamic Equilibrium: A situation in which the [blank_start]composition[blank_end] of a constant [blank_start]concentration[blank_end] reaction mixture does not [blank_start]change[blank_end] because both forward and backward reactions are proceeding at the same [blank_start]rate[blank_end].

Which of the following are conditions which apply to all equilibria?

Once equilibrium is reached the concentrations of the reactants and the products do not change.

What can be said about the rates of the forwards and backwards reactions when equilibrium has been reached?

A + B <---> C + D At the [blank_start]start[blank_end] of the reaction the forward rate is fast, because A and B are [blank_start]plentiful[blank_end]. There is no [blank_start]reverse reaction[blank_end] because there is no C and D. Then as the [blank_start]concentrations[blank_end] of C and D build up, the reverse reaction speeds up. At the same time the concentrations of A and B [blank_start]decrease[blank_end] so the forward reaction [blank_start]slows down[blank_end]. A point is reached where exactly the same [blank_start]number of particles[blank_end] are changing from A and B to C and D as are changing from C and D to A and B. [blank_start]Equilibrium[blank_end] has been reached. NB. An equilibrium [blank_start]mixture[blank_end] can have any proportions of reactants and products. The proportions may be changed depending on the [blank_start]conditions[blank_end] of the reaction, such as temperature, [blank_start]pressure[blank_end], and concentration. But at any given constant conditions the proportions of reactants and products [blank_start]do not[blank_end] change.

Le Chatelier's Principle: For a closed system in dynamic equilibrium, if a change is made the equilibrium will shift to increase that change.

If you [blank_start]increase[blank_end] the concentration of one of the reactants, Le Chatelier's principle says that the [blank_start]equilibrium[blank_end] will shift in the direction what tends to reduce the concentration of this reactant. A(aq) + B(aq) <---> C(aq) + D(aq) If you add some extra A (increase its [blank_start]concentration[blank_end]), the only way that the system can reduce the concentration of A is by [blank_start]reacting[blank_end] it with B to form more C and D. So, adding more A uses up more B, [blank_start]produces[blank_end] more C and D, and moves the equilibrium to the [blank_start]right[blank_end]. You end up with a [blank_start]greater[blank_end] proportion of products in the reaction mixture than before.

Pressure changes only affect reactions involving [blank_start]gases[blank_end]. Changing the overall pressure will only change the position of equilibrium of a gaseous reaction if there are a [blank_start]different[blank_end] number of molecules on either side of the equation. N204(g) <---> 2NO2 [blank_start]Increasing[blank_end] the pressure of a gas means that there are more molecules of it in a given volume - it is equivalent to increasing the [blank_start]concentration[blank_end] of a solution. If you increase the pressure on this system, Le Chatelier's principle tells us that the position of equilibrium will move to [blank_start]decrease[blank_end] the pressure. This meas it will more to the [blank_start]left[blank_end] because fewer molecules exert less pressure.

Increasing the pressure or decreasing the volume of a mixture of gases decreases the concentration of all the reactants and products by the same amount, not just one of them.

The rate at which equilibrium is reached will be speeded up by increasing the pressure, as there will be more collisions in a given time.

Increasing the temperature of a reaction will cause the equilibrium to shift in which direction?

What effect would adding a catalyst have on a reversible reaction in a closed system? (Choose TWO)

80% of ammonia produced is used to make which product?

N2(g) + [blank_start]3[blank_end]H2(g) <---> [blank_start]2[blank_end]NH3(g) [delta H = -92kJmol^-1] Almost all ammonia is made by the [blank_start]Haber[blank_end] process, in which the reaction above is the key step. A low temperature and high [blank_start]pressure[blank_end] would give close to 100% conversion whereas low pressure and high temperature would give almost no [blank_start]ammonia[blank_end]. However, a compromise is used to ensure the rate of reaction is [blank_start]economically[blank_end] viable and the conditions safe. Most plants run at a pressure of around 20000kPa (c. [blank_start]200[blank_end] atmospheres) and a temperature of about [blank_start]670[blank_end]K (c. 450 celsius). The raw materials for the Haber process are air (providing the nitrogen), water, and natural gas (methane). These provide the hydrogen by the following reaction: [blank_start]CH4[blank_end](g) + H2O(g) ---> [blank_start]CO[blank_end](g) + 3H2(g) The nitrogen and hydrogen are fed into a converter in the ratio [blank_start]1:3[blank_end] and passed over an [blank_start]iron[blank_end] catalyst. The conversion is not at maximum as the conditions are not optimum (a [blank_start]compromise[blank_end] is used) and the gases flow continuously over the catalyst so they do not spend long enough in contact with the catalyst to reach [blank_start]equilibrium[blank_end]. Any unconverted reactants are [blank_start]recycled[blank_end] through the converter to increase the yield.

The most common source of ethanol for industrial use is the fermentation of sugars such as glucose.

Ethanol is the alcohol in alcoholic drinks, but also has many [blank_start]industrial[blank_end] uses for example making cosmetics, drugs, detergents, inks, and as a motor [blank_start]fuel[blank_end]. At present the main source of ethanol for industrial use is [blank_start]ethene[blank_end] from crude oil. Ethanol is made by the hydration of ethene. The reaction is [blank_start]reversible[blank_end] and is speeded up by a catalyst of [blank_start]phosphoric[blank_end] acid absorbed on silica. The equation is: C2H4(g) + [blank_start]H2O[blank_end](g) <---> C2H5OH(g) [delta H = -46kJmol^-1] Applying Le Chatelier's principle to this predicts the maximum yield will be produced with [blank_start]high[blank_end] pressure, low temperature, and excess steam. However, there are practical problems: high pressure can cause ethene to polymerise; high pressure increases [blank_start]costs[blank_end] and danger; low temperature reduces the rate of reaction; too much steam dilutes the catalyst. Therefore in practise, conditions of about 570K and [blank_start]6500[blank_end]kPa are used. These give a conversion to ethanol of only around 5%, but the unreacted reactants are [blank_start]recycled[blank_end] until about 95% conversion is obtained.

Le Chatelier's principle tells us that the methanol synthesis reaction will give the highest yield at low temperature and high pressure.

Which of the following is true? The Kc of a reaction is...

Kc > 1 --> [blank_start]equilibrium lies towards the products[blank_end] Kc = 1 --> [blank_start]equal concentrations[blank_end] Kc < 1 --> [blank_start]equilibrium lies towards the reactants[blank_end]

The equilibrium constant (Kc) does not change with concentration or pressure, but it does when the temperature is changed.

When can the number of moles of products and reactants be used to calculate the Kc instead of the concentrations?

The Kc of the reaction between nitrogen monoxide (NO) and oxygen (O2) to form nitrogen dioxide (NO2) has no units.