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1) the reaction quotient, Q, expresses the relative ratio expresses the relative ratio of product to reactant at a given instant.
2) Q<Kc there are more products than reactant thus, it would shift to left hand side.
3) Kc is Equilibrium constant at a fixed temperature can be calculated using values for conc. at equilibrium at Qc

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1. Reaction quotient measures the relative amounts of reactants and products at a point in time.

2. Q = 42.7
Kc = 170
Q < Kc
Hence the reaction mixture is not in equilibrium. The direction in which the reaction will progress is towards the products (towards the right)

3. Kc is calculated using the equilibrium concentrations of reactants and products, and Q is calculated using concentrations of reactants and products at any given point in time.

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1) the reaction quotient expresses the relative ratio of products to reactants at a given instance

2) Q < Kc, the reactants are more than the products therefore the reaction shifts to the right to convert more reactants to products to achieve equilibrium.

3) Kc is the equilibrium constant which is calculated using  the values for concentration at equilibrium  while the Qc, the reaction quotient does not specify the instance at which the concentrations are calculated.

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1) The reaction quotient Q measures the relative amounts of reactants and products at a point of time. This is expressed with non-equilibrium concentrations.
2) K= [(NH₃)² ]/ [(N₂ ) * (H₂)³]
= [(8.00)² ]/ [(1.50 ) * (1.00)³]
=42.7
As the Q<Kc there are more reactants than products thus to decrease the amount of reactants, the reaction will shift to the right and produce more products.
3) Kc the concentration of reactants and products are at equilibrium. whereas, Q is measured at any point of time.

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1] A reaction quotient is the proportion of the concentrations of reactants and products within a system. This can be measured at all times in the reactionfrom the initial stage to the very end.

2] Kc = 1.7*10(2) @ 500K
Q =   (8.00)^2/[(1.00)^3*(1.50)^]
Q = 42.7
Q<Kc hence the reaction is moving net forward

3] Q is the proportion of reaction to products at any given point in the reaction but Kc can be measured only at equilibrium.

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Q1
Reaction quotient expresses the relation between the relative amount of products and reactants in an given instant.
Q2
Q<Kc, therefore the reaction shifts right, moves forward
Q3
Kc helps in calculating the final concentration at equilibrium for a reaction while Qc helps us understand the relative amounts of products and reactant in a reaction in a given instant

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Kindly answer the questions attached with this Pad-let after going through the link. You can use the attached video for reference.

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Kindly answer the questions attached with this Pad-let after going through the link. You can use the attached video for reference.

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Sometimes it is necessary to determine in which direction a reaction will progress based on initial activities or concentrations. In these situations, the relationship between the reaction quotient, \(Q_c\), and the equilibrium constant, \(K_c\), is essential in solving for the net change. With this relationship, the direction in which a reaction will shift to achieve chemical equilibrium, whether to the left or the right, can be easily calculated.

Introduction

\(K_c\) can be used calculate the final concentrations at equilibrium for a reaction using an ICE table and the natural progression of the reaction, from left to right or from right to left. However, what if you do not know which way the reaction will progress? A simple relationship between \(K_c\) and the reaction quotient, known as \(Q_c\), can help. The reaction quotient, \(Q\), expresses the relative ratio of products to reactants at a given instant. Using either the initial concentrations or initial activities of all the components of the reaction, the progression of an reaction can easily be determined.

Given the general chemical reaction

\[ aA +bB \rightleftharpoons gG +hH \]

\(Q\) may be expressed as the following equations:

\[ Q= \dfrac{a_{init}^ga_{init}^h}{a_{init}^a a_{init}^b} \]

or

\[ Q_{c}= \dfrac{[G]_{init}^g[H]_{init}^h}{[A]_{init}^a[B]_{init}^b} \]

Remember that the concentrations of liquids and solids do not change, so they are excluded from the expression. As shown above, the value of \(Q\) can be found by raising the products to the power of their coefficients, or stoichiometric factors, divided by the reactants raised to their coefficients. If the concentration of products in the numerator is much larger than that of reactants in the denominator, \(Q\) will be a large value. On the other hand, a small amount of products (small numerator) divided by a large value for the concentration of reactants (large denominator) would result in a small value for Q. The expressions for Q are very similar to those for \(K\):

\[ K= \dfrac{a_G^g a_{H}^h}{a_{A}^a a_{B}^b} \]

or

\[ K_c = \dfrac{[G]^g [H]^h}{[A]^a [B]^b} \]

To determine which direction a reaction will go towards, simply compare \(Q_c\), the initial concentration ratio, to \(K_c\), the equilibrium constant, and evaluate the results.

Q vs. K: What Does It Mean?

When you set \(Q\) against \(K\), there are five possible relationships:

\(Q=K\)

\(Q=0\)

\(Q<K\)

\(Q= \infty\) and

\(Q>K\).

To properly predict which way a reaction will progress, you must know these relationships.

\[ CO_{(g)}+2H_{2 \; (g)} \rightleftharpoons CH_{3}OH_{(g)} \]

Consider again:

\[ CO_{(g)}+2H_{2 \; (g)} \rightleftharpoons CH_{3}OH_{(g)} \]

For Q<K:

\[ CO_{(g)}+2H_{2 \; (g)} \longrightarrow CH_{3}OH_{(g)} \]

\[ CO_{(g)}+2H_{2 \; (g)} \longleftarrow CH_{3}OH_{(g)} \]

Remembering the Relationship Between K and Q

An easy way to remember these relationships is by thinking of the > or < as the mouth of an alligator. The alligator will "eat" in the direction that the reaction shifts as long as \(Q\) is written before \(K\).

Predicting the Shift of a Reaction Without Calculations

Depending on what a problem asks of you, sometimes it is unnecessary to make any calculations at all. Take, for example, the now familiar reversible reaction listed below:

\[ CO_{(g)}+2H_{2 \; (g)} \rightleftharpoons CH_{3}OH_{(g)} \]

What do you think will happen if more of the product, methanol (CH3OH), is added? Equilibrium will be disrupted, and the increase in products mean that Q>K. In order to re-establish equilibrium, the reaction will progress to the left, towards the reactants. This means some of the added methanol will break down into carbon monoxide and hydrogen gas.

\[ CO_{(g)}+2H_{2 \; (g)} \longleftarrow CH_{3}OH_{(g)} \]

\[ CO_{(g)}+2H_{2 \; (g)} \longrightarrow CH_{3}OH_{(g)} \]

The ideas illustrated above show Le Châtelier's Principle whereby when an equilibrated system is subjected to a change in temperature, pressure, or concentration of a species in the reaction, the system responds by achieving a new equilibrium that partially offsets the impact of the change. Predicting which way a reaction will go can be the easiest thing that you will ever do in chemistry!

Example: Putting It All Together

To properly use the relationship between Q and K, you must know how to set it up. Take, for example, the reaction below:

If you start with 4.00M CH ₄, 2.00M C ₂H ₂, and 3.00M H ₂, which direction will the reaction progress to reach equilibrium?

Problems

1) Consider this reaction:

\(2NOBr_{(g)} \leftrightharpoons 2NO_{(g)}+Br_{2}\)

If K c= 0.0142 and the initial concentrations are 1.0 M NOBr, 0.2M NO, and 0.8M Br ₂, which way will the reaction progress to reach equilibrium?

2) What is Q and its purpose?

3) Consider the reaction following in equilibrium:

\(N_{2}O_{4 \; (g)} \leftrightharpoons 2NO_{2 \; (g)}\)

If more N ₂O ₄ is added, which way will the reaction proceed?

4) Consider the following reaction:

\(CO_{(g)}+Cl_{2 \; (g)} \leftrightharpoons COCl_{2 \; (g)}\)

With a K c of 1.2 x 10 ³ at 668 K, is the reaction in equilibrium when there are 5.00 mol CO(g), 2.00 mol Cl ₂(g), and 6.00 mol of COCl ₂(g) in a 3.00L flask? If not, which direction will the reaction progress to reach equilibrium?

5) Consider the following reaction:

\(H_{2 \; (g)}+I_{2 \; (g)} \leftrightharpoons 2HI_{(g)}\)

If K c=50.2 at 718 K and the initial concentrations are 0.5 M H ₂, 0.15M I 2, and 0.05M HI, which way will the reaction progress?

6) Consider the following reaction:

\(2COF_{2 \; (g)} \leftrightharpoons CO_{2 \; (g)}+CF_{4 \; (g)}\)

If K c= 2.00 at 473 K and the initial concentrations are 2.0 M CO ₂, 4.0 M CF ₄, and 0.5 M COF ₂, which way will the reaction progress?

7) Consider the following reaction:

\(2SO_{2 \; (g)}+O_{2 \: (g)} \leftrightharpoons 2SO_{3 \; (g)}\)

K c=100. With the initial masses of 20 g SO ₂, 13 g O ₂, and 25 g SO 3 in a 5.0 L container, which way will the reaction progress.

Solutions

1) The reactions shifts to the left, towards the reactants.

NOBr= 1M, NO= 0.2M, Br ₂= 0.8M

\(Q_c= \dfrac {[0.2]^2 [0.8]}{[1]^2}\)

\(Q_c= 0.032\)

Therefore, Q c> K c and the reactions shifts towards the reactants.

2) Q is a reaction quotient, which helps determine if a reaction will shift forward or backwards. As a system approaches towards equilibrium, Q approaches towards K.

3) The reaction will proceed to the right.

4) No, it is not at equilibrium. Since Q<K, the reaction will shift to the right to reach equilibrium.

5) Q = 0.033, so Q<K. The reaction will shift to the right.

6) Q = 32.0, so Q>K. The reaction will shift to the left.

7) Q = 12, so Q<K. The reaction will shift to the right.

References

1. Alberty, R., A. Cornish-Bowden, et al. (1994). "Recommendations for nomenclature and tables in biochemical thermodynamics." Pure Appl. Chem 66: 1641-1666.

2. Gold, J. and V. Gold (1985). "Le Chatelier's Principle and the Laws of van't Hoff." Education in Chemistry 22: 82-85.

3. Petrucci, Harwood, Herring, Madura. General Chemistry: Principles & MOdern Application. Ninth Edition. Pages 636-638.

Contributors

Rubi Medrano (UCD), Irene Ly (UCD)

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Sometimes it is necessary to determine in which direction a reaction will progress based on initial activities or concentrations. In these situations, the relationship between the reaction quotient, \(Q_c\), and the equilibrium constant, \(K_c\), is essential in solving for the net change.

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7.1 Reaction quotient (Q) and equilibrium constant (Kc)