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Quiz about Equilibrium Reactions
Quiz about Equilibrium Reactions

Equilibrium Reactions Trivia Quiz


This quiz examines your knowledge of equilibrium reactions at the level of high school chemistry. There is no chronological order, just a mixed bunch. Hope you enjoy!

A multiple-choice quiz by pokho. Estimated time: 7 mins.
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Author
pokho
Time
7 mins
Type
Multiple Choice
Quiz #
399,232
Updated
Dec 03 21
# Qns
15
Difficulty
Tough
Avg Score
8 / 15
Plays
177
-
Question 1 of 15
1. Which of the following is UNTRUE about an equilibrium reaction? Hint


Question 2 of 15
2. Which of the following is true about the effect of a catalyst on an equilibrium reaction?

(Note: both the position of equilibrium and the reaction rates are of concern here).
Hint


Question 3 of 15
3. Methanol (CH3OH) is being produced from carbon monoxide and hydrogen gases. At equilibrium, the concentrations of methanol, carbon monoxide and hydrogen are 0.5, 1.0 and 0.1 molars respectively. What would be the value of Kc (equilibrium constant)? Hint


Question 4 of 15
4. If, at a certain temperature, the reaction rate constant for the forward reaction is 'x' and that of the backward reaction is 'y', what is the equilibrium constant? Hint


Question 5 of 15
5. How are the values of Kc and Kp related for the Haber process for the production of ammonia, at a temperature of 27 degrees Celsius?

(R is the universal gas constant.Kc is the equilibrium constant and Kp, the pressure equilibrium constant).
Hint


Question 6 of 15
6. 2X + 2Y ⇌ 4Z

Suppose this equilibrium reaction has 'x' has its equilibrium constant (Kc).
What then would be the value of Kc for the following reaction?

Z ⇌ X/2 +Y/2
Hint


Question 7 of 15
7. At a certain temperature, the equilibrium constant for the reaction involving water vapor turning to water is 'K1'. The equilibrium constant for water disassociating into its respective ions is 'K2'.

What is the equilibrium constant for the overall reaction of water vapor converting into its respective aqueous ions.
Hint


Question 8 of 15
8. An ice cube is placed in a flask half-filled with water. The system is completely closed and in equilibrium. What happens to the ice cube as time passes Hint


Question 9 of 15
9. In a packaged bottle of soft drinks, the amount of gas keeps dissolving in the drink and dissolved gas keeps escaping the drink, at equal rates. What happens when the seal cap is removed and why? Hint


Question 10 of 15
10. Take the reaction PCl5 ⇌ PCl3 + Cl2, with all the constituents being gases.

How would you shift the equilibrium to the right?
Hint


Question 11 of 15
11. The equilibrium constant of a certain reaction is 10^-4. This means that the products have been favored.


Question 12 of 15
12. Take the hypothetical equilibrium reaction:-

A + B ⇌ C

All the constituents in the mixture are gases and are sealed in a 10L container. The amounts of A,B and C at equilibrium are 4, 4 and 5 moles respectively. 1 mole of each constituent is removed. What is the value of Kc?
Hint


Question 13 of 15
13. The Contact Process involves the addition of oxygen gas to sulfur dioxide gas to form sulfur trioxide gas in an equilibrium reaction. Since the reaction is endothermic, the lower the temperature, the higher the yield. However, an optimum temperature from 673 to 723 K is used. Why not go lower? Hint


Question 14 of 15
14. Which of the following is true about the spontaneity of equilibrium reactions? Hint


Question 15 of 15
15. Take the reaction:- N2 + O2 ⇌ 2NO

The change in enthalpy for the reaction is positive and all products are gaseous. What can you do in order to decrease the yield of nitrogen monoxide?
Hint



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Quiz Answer Key and Fun Facts
1. Which of the following is UNTRUE about an equilibrium reaction?

Answer: After attaining equilibrium, the reaction ceases

Equilibrium reactions are dynamic. No change may seem to be happening, but that is just because the reverse reaction rate is equal to the forward reaction rate - keeping the amounts of the concerned substances balanced.

Another common misconception about equilibrium reactions is that, after attaining equilibrium, the concentration of reactants and products are equal and stay equal. This is a possibility, but not a necessity. The respective concentrations may be unequal (as they mostly are), but they just don't change.
2. Which of the following is true about the effect of a catalyst on an equilibrium reaction? (Note: both the position of equilibrium and the reaction rates are of concern here).

Answer: Both reactions are favored equally

The role of a catalyst is to provide an alternate pathway which requires a lower activation energy, and this is true for both the reverse and forward reactions.

Both reactions being favored equally means that equilibrium is reached more quickly, but the equilibrium concentrations of the reactants or products stays the same; i.e. the position of equilibrium stays the same.
3. Methanol (CH3OH) is being produced from carbon monoxide and hydrogen gases. At equilibrium, the concentrations of methanol, carbon monoxide and hydrogen are 0.5, 1.0 and 0.1 molars respectively. What would be the value of Kc (equilibrium constant)?

Answer: 50

It is necessary to write down the balanced equation of the reaction first.

CO + 2H2 ⇌ CH3OH

Thus the equilibrium expression would be: (Concentration of Methanol)/(Concentration of Carbon Monoxide) * (Concentration of Hydrogen)^2

You will get the correct answer after plugging in the values.

Although the units are mostly not needed for Kc, it would be moles^-2 dm^6.
4. If, at a certain temperature, the reaction rate constant for the forward reaction is 'x' and that of the backward reaction is 'y', what is the equilibrium constant?

Answer: x/y

Suppose your reaction is:-

aX + bY ⇌ cZ + dW, where X,Y,Z and W are the reactants and products while a, b, c and d are respectively the stoichiometric coefficients in the formula of the reaction.

The rate of forward reaction = x *[Z]^c * [W]^d and the rate of backward reaction = y * [X]^a * [Y]^b

Since the rate of forward reaction is equal to that of the backward reaction ->
(x *[Z]^c * [W]^d )= (y * [X]^a * [Y]^b)

Transposing the equilibrium expression on one side would be equal to x/y.
5. How are the values of Kc and Kp related for the Haber process for the production of ammonia, at a temperature of 27 degrees Celsius? (R is the universal gas constant.Kc is the equilibrium constant and Kp, the pressure equilibrium constant).

Answer: Kp = Kc/90000R^2

The general term for the relation between the two is:-

Kc = Kp (RT)^Δn , where R is the universal gas constant, T is the temperature, Δn is the change in gaseous moles.

In order to know the latter, you may have to write the equation down:-
N2 + 3H2 ⇌ 2NH3

There are four gaseous moles on the reactants' side, two on the products'.
Hence, Δn = -2.

Plug the values into the formula, without forgetting to change the unit of temperature to Kelvin, which will be 300 K.

Although R is a constant with a fixed value, I didn't enter its value in the answer because the units used may have to be clarified upon (lazy me!), and so that it is easier to do your calculations manually.
6. 2X + 2Y ⇌ 4Z Suppose this equilibrium reaction has 'x' has its equilibrium constant (Kc). What then would be the value of Kc for the following reaction? Z ⇌ X/2 +Y/2

Answer: x^(-1/4)

Often, diatomic gases are written with 1/2 as their stoichiometric coefficients. Since all the coefficients are being divided by 4, the entire equilibrium expression will have its powers divided by 4.

Notice that we are concerned with the reverse reaction of our original reaction. The equilibrium constant of the forward reaction is the inverse of the equilibrium constant for the backward reaction.
7. At a certain temperature, the equilibrium constant for the reaction involving water vapor turning to water is 'K1'. The equilibrium constant for water disassociating into its respective ions is 'K2'. What is the equilibrium constant for the overall reaction of water vapor converting into its respective aqueous ions.

Answer: K1 * K2

Temperature is not needed when the equilibrium constants are already given.

K1 = Concentration of water / Concentration of water vapor

K2 = Product of the concentrations of the ions / Concentration of water

If you multiply them, through cancellation you get {K1 * K2 =} product of the concentrations of the ions / Concentration of water vapor, which will be the equilibrium constant for the overall reaction.
8. An ice cube is placed in a flask half-filled with water. The system is completely closed and in equilibrium. What happens to the ice cube as time passes

Answer: The shape changes, but the mass remains the same

Remember that this is dynamic equilibrium. The reactants and products continue to react, although they completely negate each other. This means that the rate of melting is equal to the rate of freezing of the sugar molecules, so the mass doesn't change. But since melting still continues, the shape of the cube will be worn down.
9. In a packaged bottle of soft drinks, the amount of gas keeps dissolving in the drink and dissolved gas keeps escaping the drink, at equal rates. What happens when the seal cap is removed and why?

Answer: Gas fizzes out, equilibrium shifts to the side with dissolved air

This is an example of equilibrium in physical processes. The equilibrium can be represented as:-

CO2 (aq) ⇌ CO2 (g)

When the seal is opened, the pressure has decreased. In order to make up for the lost pressure, the equilibrium responds by producing more gaseous molecules. Hence, the equilibrium shifts to the right and further carbon dioxide is released.
10. Take the reaction PCl5 ⇌ PCl3 + Cl2, with all the constituents being gases. How would you shift the equilibrium to the right?

Answer: Remove some of the chlorine from the reaction

Adding a catalyst, as has been mentioned in the quiz, has no effect whatsoever on the position of equilibrium as both the forward and backward reactions are boosted equally.

When the pressure is increased, the equilibrium shifts to reduce the pressure - which is to the left, since the reactants' side has only one gaseous molecule compared to two in the products' side.

When the volume of the system is decreased, the respective concentrations of the substances increase. Since there are more molecules on the products' side, the value of Kc would increase were it not for the Le Chatelier principle. In order to resist any change in the value of Kc, the reverse reaction increases and equilibrium shifts to the left.

If chlorine is removed from the reaction, the forward reaction increases to replenish the lost chlorine and equilibrium shifts to the right.
11. The equilibrium constant of a certain reaction is 10^-4. This means that the products have been favored.

Answer: False

The expression for the equilibrium constant is (Reaction's products powered to their stoichiometric coefficients multiplied)/ (Reaction's reactants powered to their stoichiometric coefficients multiplied with each other).

If the value of the constant is below 1, quite clearly the denominator is greater and the reactants are favored to some extent. If the value is below 10^-2, the reactants are considered to be dominant in the equilibrium mixture and the reaction has hardly proceeded. If the value is above 10^2, the products are dominant and little reverse reaction has taken place. Anything between these values means that both are present in noticeable amounts.
12. Take the hypothetical equilibrium reaction:- A + B ⇌ C All the constituents in the mixture are gases and are sealed in a 10L container. The amounts of A,B and C at equilibrium are 4, 4 and 5 moles respectively. 1 mole of each constituent is removed. What is the value of Kc?

Answer: 5/16

Did you fall for the trick? Adding or removing reactants/products has no results on the value of Kc or Kp - it is after all the CONSTANT of equilibrium. The position of equilibrium is shifting precisely to minimise the effect of the change on Kc, as has been noted by the Le Chatelier principle. Don't be distracted by the information about 1 mole of each constituent being removed.

Remember that the amounts are not equal to the concentration. The concentrations of A, B and C in a 10L container would respectively be 0.4, 0.4 and 0.5 molars respectively.

The concentration of the product of the reaction divided by the the concentrations of the reactants multiplied would give you the correct answer.
13. The Contact Process involves the addition of oxygen gas to sulfur dioxide gas to form sulfur trioxide gas in an equilibrium reaction. Since the reaction is endothermic, the lower the temperature, the higher the yield. However, an optimum temperature from 673 to 723 K is used. Why not go lower?

Answer: The reaction rate may be too slow

The Contact Process is an important step in the production of sulphuric acid, a substance with manifold applications. Remember that the Kc of a reaction only tells you the position of an equilibrium reaction, not how quickly you are going to attain it.

As we may have learnt in chemical kinetics, the lower the temperature, the lower the rate of reaction. Although the concentration of sulfur trioxide will be higher, the slower rate of reaction is unfeasible for industrial mass production.
14. Which of the following is true about the spontaneity of equilibrium reactions?

Answer: The equilibrium point respresents the lowest value of Gibbs Free energy

The relation between Gibbs free energy and the equilibrium constant is given by the van' t Hoff Isotherm :-

ΔG = -RT * natural log of Kc

When the ΔG for a reaction is negative, the ΔG for the reverse reaction would be positive. This may seem to imply that only the forward reaction is spontaneous, and hence proceeds to completion. However, the equilibrium point represents an intermediate stage with the lowest Gibbs free energy. This means that both the reactions remain spontaneous, as should be for an equilibrium reaction.
15. Take the reaction:- N2 + O2 ⇌ 2NO The change in enthalpy for the reaction is positive and all products are gaseous. What can you do in order to decrease the yield of nitrogen monoxide?

Answer: Just decrease temperature

Since there are two gaseous molecules on both sides of the equilibrium, there is no change in the position of equilibrium when the pressure of the system is altered. If the enthalpy is positive, the reaction is endothermic. The equilibrium reacts to the change by producing more heat, according to the Le Chatalier principle. This is done by increasing the rate of backward reaction.

Nitrogen monoxide is a highly toxic gas produced from the exhaust fumes of a vehicle, which is responsible for both the greenhouse effect and acid rain; which is why automobile manufacturers try to set up conditions which reduce their yield.
Source: Author pokho

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