Predicting The Direction Of A Reaction: Definition, Prediction, Experimental Examples

When working with reversible reactions, it's crucial to know which direction the reaction is going at any given time. For example, in order to produce ammonia on a commercial scale from nitrogen and hydrogen, we must optimize the process. 

Key Terms: Reversible reactions, direction, Reaction Quotient, Equilibrium Constant, Chemical Reaction

Also Read: Cis-Trans Isomerism

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Calculating Reaction Quotient & Equilibrium Constant

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Let us consider a chemical reaction given as mentioned below,

a A + b B ⇆ c C + d D

The reaction quotient may be computed in terms of partial pressure (Qp) and molar concentration (Qc), much as the equilibrium constant can be calculated in terms of partial pressure (Kp) and molar concentration (Kc) as shown below.

Also Read: Carboxylic Acid

Calculating the Equilibrium Constant

Although all reversible reactions have a direction, certain irreversible reactions exist if the yield of reactants or products is favored. 

In a reversible reaction to produce nitrogen dioxide from dinitrogen tetroxide, suppose a little amount of colorless dinitrogen tetroxide is introduced.

After a given amount of time, you'll notice that the gas turns a yellowish-orange color and steadily darkens until it becomes continuous.

The concentration of NO₂ in the container is initially 0 mole. As N₂O₄ is converted to NO₂, the concentration of NO₂ increases to a certain point and then stays there.

Similarly, the concentration of N₂O₄ falls until it reaches equilibrium. The reaction is considered to have reached equilibrium when both NO₂ and N₂O₄ concentrations remain constant.

Also Read: Preparation of Alcohols 

It's important to note, though, that even if a response is constant at equilibrium, it still happens. As a result, it's also known as dynamic equilibrium.

The equilibrium constant is used to express the connection between the products and reactants in an equilibrium process (denoted by K). The Kc for the reaction in terms of the molar concentration of the components may be written as when the reaction is at equilibrium.

\(K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}\)

And Kp of the reaction in terms of the partial pressure of the components can be given as,

\(K_p = \frac{P_C^c P_D^d}{P_A^a P_B^b}\)

Also Read: States of Matter

Calculating Reaction Quotient

The reaction quotient, abbreviated Q, is a measurement of the quantity of reactant and product present at a specific stage in a chemical process. We write the reaction quotient in terms of their molar concentration when the reaction is not at equilibrium: 

\(Q_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}\)

And the reaction quotient in terms of the partial pressure of the components as,

\(Q_p = \frac{P_C^c P_D^d}{P_A^a P_B^b}\)

The amount of the reaction quotient determines whether or not a reaction container exists. But, exactly, what does this imply? 

Consider a reaction in which the product concentrations are 0 and only the starting ingredients are present. The reaction quotient is also zero because the numerator is 0. When the denominator of an equation contains just products, A = B = 0, Q grows infinitely large. 

Some of the other product and reactant mixtures will be present a maximum number of times, but keep in mind that extremely tiny Q values indicate the majority of reactants are present. 

Extremely high Q values, on the other hand, indicate that the reaction container is mostly filled with products.

Also Read: Oxidation and Reduction

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Predicting The Direction Of A Chemical Reaction

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Qc > Kc

In this case, the reaction proceeds in the reverse or the backward direction or in the direction of the reactants.

Qc < Kc

In this case, the reaction proceeds in the forward direction or the direction of the products.

Also Read: Molecular Orbital Theory Postulates 

Qc = Kc

In this case, the reaction is said to be at equilibrium and no net reaction occurs in any direction.

The above result has been clearly shown in the figure given here. For any chemical reaction,

Also Read: Dioxygen

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Experimental Examples

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An experiment example below demonstrates the creation of ammonia when hydrogen and nitrogen react.

At 350° Celsius, a reaction between hydrogen (H₂) and nitrogen (N₂) produces ammonia gas (NH₃). The chemical equation for this equilibrium reaction is:

N₂ (g) + 3H₂ (g) ⇄ 2NH₃ (g)

Consider an experiment in which 1.00 moles of nitrogen and 1.00 moles of hydrogen are combined in a 1.00 liter sealed container and heated to 350° Celsius. The yield of ammonia, as well as the consumption of hydrogen and nitrogen, are then monitored over time by measuring the concentration of each element in the container.

The observations of the experiment are represented in the table below:

The concentrations of nitrogen and hydrogen (reactants) fall while the concentration of ammonia (product) increases, as predicted until equilibrium is attained at time t = 4. The concentration of each chemical component remains unchanged at this moment. 

So, when the reaction progresses toward equilibrium in a forward direction, what will happen to the mass-action expression (Q or reaction quotient) value?

Also Read: Hybridization

Calculation of the mass-action expression value at each time is done and represented in the following table:

The mass-action expression Q value is growing as the response moves closer to equilibrium in the advancing direction.

The reaction quotient Q value is constant and similar to the equilibrium constant K value for the aforementioned reaction when it reaches equilibrium around time = 4.

At the point of equilibrium: Q = 6.09 = K

Another way to think about it is that the reaction favors forward motion as long as the reaction quotient Q is less than the equilibrium constant K:

If Q is greater than K, the reaction advances forward.

Consider adding 1.00 moles of ammonia to this mixture at more than 350 degrees Celsius at time = 6, and then monitoring the concentration of each ingredient.

The observations are given in the following table:

Le Chatelier's Principle states that if more ammonia is given to an equilibrium system, it will conduct the reaction in the opposite direction, consuming part of the new ammonia to generate more nitrogen and more hydrogen until the time equilibrium state is achieved around time t = 10.

You can observe that as additional ammonia (product) is added to the mixture at equilibrium, the Q value rises.

The reaction quotient Q value increases as the reverse reaction progress, consuming ammonia and creating nitrogen and hydrogen, but it decreases until the equilibrium constant equals the reaction constant.

If Q > K, the reaction moves in the reverse direction.

Also Read: Polarity

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Things To Remember

• By calculating the reaction quotient and equilibrium constant, one can determine the direction of a chemical reaction.
• A reaction quotient Q is a measure of how many reactants and products are involved in a reaction at a specific time.
• If Qc>Kc, the reaction goes from right to left.
• If Qc<Kc, the reaction goes from left to right.
• If Qc=Kc, the reaction is at equilibrium.

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