Predicting the Direction of A Reaction

When dealing with reactions which are reversible, it is necessary to rule out the direction of any reaction at a given point. For instance, when ammonia is produced commercially by mixing nitrogen and hydrogen, the whole process needs to be optimised for efficient yield. Therefore, prediction direction of reaction is immensely vital.

It is known that direction of a chemical reaction can be figured out by the reaction quotient and equilibrium constant. So, in the following section, you will get to know about equilibrium constant, prediction of direction of reaction and reaction quotient.

Let’s start!

Equilibrium Constant

All reversible reactions have a direction, but some irreversible reactions also exist if they favour the yield of reactants or products. Suppose some amount of colourless dinitrogen tetroxide is added in a reversible reaction of production of nitrogen dioxide from dinitrogen tetroxide. After a certain point in time, you would observe that the gas will change to yellowish-orange colour and will get darker gradually until it becomes constant.

Initially, the concentration of NO2 in the container is 0 mole. As N2O4 gets transformed into NO2, the concentration of NO2 rises to a specific level and then remains fixed.

In the same manner, N2O4 concentration decreases until it approaches equilibrium. When both NO2 and N2O4 concentrations stay constant, the reaction is said to have reached equilibrium. But you must remember that even if a reaction is constant at the state of equilibrium, the reaction still occurs. Therefore, it is also referred to as dynamic equilibrium.

In order to calculate equilibrium constant Kc, consider a balanced reversible equation aA + bB ⇋ cC + dD. If the molar concentration of each species is known, Kc can be evaluated by the following equation:

Kc = [C]c [D]d / [A]a [B]b

Next, let us proceed to what is reaction quotient?

Reaction Quotient

The measure of amounts of reactants and products involved in a reaction at a specific time is known as the reaction quotient Q.

Suppose a reversible reaction aA + bB ⇋ cC + dD, where all these variables are the stoichiometric coefficients of a balanced reaction, Q can be evaluated by the below-given equation:

Q = [C]c [D]d / [A]a [B]b

The above equation may seem familiar to you as the concept of Q is closely associated with equilibrium constant K. The reaction quotient Q can be evaluated for both cases whether a reaction is in equilibrium or not, but K is based on equilibrium concentrations.

Reaction quotient magnitude determines that what is there is a reaction container. But what does it mean? Consider a reaction that holds only starting substances, and the product concentrations are zero. As the numerator is zero, reaction quotient is also zero. If a reaction contains only products, [A] = [B] = 0 in denominator of the equation, Q becomes infinitely huge.

Maximum number of times, some or the other mixture of products and reactants will be there, but you can keep it in mind that extremely small Q values show that mostly reactants are there. On the other hand, when extremely Q values are there, it shows that products are mostly present in the reaction container.

Prediction of the Direction of a Reaction

The direction of a chemical reaction is explained through an experiment example below, which shows the production of ammonia when hydrogen and nitrogen undergo a reaction.

Take a look!

At the temperature of 350° Celsius hydrogen gas (H2) and nitrogen gas (N2) will undergo a reaction to yield ammonia gas (NH3). This equilibrium reaction can be expressed by the below-mentioned chemical equation:

N2 (g) + 3H2 (g) ⇋ 2NH3 (g)

For instance, consider an experiment where 1.00 moles of nitrogen and 1.00 moles of hydrogen is added to a 1.00 Litre sealed container and heated at a temperature of 350° Celsius. After that, the yield of ammonia is monitored, and consumption of hydrogen and nitrogen with time by measuring concentration of each element in the container.

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

As expected, it is seen that concentration of nitrogen and hydrogen (reactants) decreases, whereas the concentration of ammonia (product) increases, till equilibrium when time t = 4 is reached. At this point, concentration of each chemical substance does not change. So, what will happen to the mass-action expression (Q or reaction quotient) value as the whole reaction is moving towards equilibrium in a forward direction?

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

Since the reaction is moving towards equilibrium in forward direction, the mass-action expression Q value is increasing.

At the state of equilibrium around time = 4, the reaction quotient Q value is constant and is similar to the equilibrium constant K value for the above reaction.

At the point of equilibrium: Q = 6.09 = K

The other way to illustrate this is that as long as the reaction quotient Q value is lesser than equilibrium constant K value, the reaction favours forward direction:

If Q < K, reaction moves in forward direction.

Now consider that at time = 6, 1.00 moles of ammonia are added to this mixture at more than 350 degree Celsius, then concentration of each substance is monitored. The observations are given in the following graph and table:

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The Le Chatelier’s Principle predicts that if more ammonia is added to a system which was at equilibrium, it will carry the reaction in reverse direction, by consuming some amount of added ammonia to yield more nitrogen and more hydrogen till the time equilibrium state is reached somewhere near time t = 10

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Now again, the reaction quotient value can be calculated and graphically represented at each time:

You can see that Q value is increasing as soon as more ammonia (product) is put to the mixture at equilibrium.

As this reverse reaction proceeds, by consuming ammonia and producing nitrogen and hydrogen, reaction quotient Q value becomes high but reduces until when equilibrium constant is equal to reaction constant.

As long as reaction quotient value is more than equilibrium constant, the reaction will move in the reverse direction until the time equilibrium is created.

If Q > K, reaction moves in reverse direction.

So, concerning the above example, it can be summarised that when:

• Q = K, the reaction is in equilibrium, and there is no net reaction in any direction.

• Q > K, the reaction moves in reverse direction or reactants’ direction, which is from right to left.

• Q < K, the reaction moves in forward direction or products’ direction, which is from left to right.

Predicting reaction direction in Chemistry is an important concept which you need to understand well as a lot of questions come from this portion in exams. If you want to know more about reaction quotient and equilibrium constant, install the Vedantu app today.