Problem 110 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{B}(\mathrm{OH})_{3}+\mathrm{NaOH} \rightleftharpoons \mathrm{NaBO}_{2}+\mathrm{Na}\left[\mathrm{B}(\mathrm{OH})_{4}\right]+\mathrm{H}_{2} \mathrm{O}\) How can this reaction be made to proceed in forward direction? (a) addition of cis- 1,2 -diol (b) addition of borax (c) addition of trans- 1,2 -diol (d) addition of \(\mathrm{Na}_{2} \mathrm{HPO}_{4}\)

Step-by-Step Solution

Answer

Add cis-1,2-diol to push the reaction forward.

Step 1: Understanding the Reaction

The reaction given is a reversible reaction between boric acid ( B(OH)_3 and sodium hydroxide. It produces sodium borate ( NaBO_2 ), sodium tetraborate ( Na[B(OH)_4] ), and water ( H_2O ). It indicates an equilibrium process where the forward and reverse reactions can be influenced by various factors.

Step 2: Identifying the Affecting Components

The four options given suggest adding different substances: (a) cis-1,2-diol, (b) borax, (c) trans-1,2-diol, (d) Na_2HPO_4 . Each of these could influence the equilibrium of the reaction by either shifting it towards the products or the reactants.

Step 3: Analyzing the Role of Diols

Cis-1,2-diols are known to form complexes with boric acid, which can stabilize its structure in the solution, preventing the reverse reaction from occurring readily. This stabilization would favor the formation of Na[B(OH)_4] by providing more of the boric acid structure available for continuing the forward reaction.

Step 4: Evaluating the Effect of Borax and Phosphate

Borax ( Na_2B_4O_7 ) and sodium phosphate ( Na_2HPO_4 ) are both known to interact with boron compounds in different ways. However, they typically do not affect the formation of Na[B(OH)_4] or destabilize B(OH)_3 to push the reaction forward efficiently compared to cis-1,2-diol complex formation.

Step 5: Conclusion

Based on the analysis, the addition of cis-1,2-diol will most likely make the reaction proceed in the forward direction efficiently by stabilizing boric acid as a complex, shifting the equilibrium towards the production of the sodium borate complex.

Key Concepts

Reversible Reactions

Chemical reactions that can proceed in both directions are known as reversible reactions. In the reaction you are studying, boric acid reacts with sodium hydroxide to form sodium borate, sodium tetraborate, and water. This is a typical example of a reversible reaction because the end products can react to regenerate the reactants.

The presence of reversible reactions means that the system can achieve a state of balance known as equilibrium. At equilibrium, the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of the reactants and products remain constant. Understanding how a reaction can move forward or backward is key to finding ways to manipulate it toward a desired direction.

• Reversible reactions can achieve dynamic balance.
• Products can turn back into reactants.
• The system in balance shows no net change in concentrations.

By identifying the conditions that favor the forward reaction, you can understand what factors might push the reaction to form more products, as desired in many chemical processing scenarios.

Equilibrium Shift

In a reversible reaction, you can influence the direction of the reaction by manipulating conditions to shift the equilibrium. This is effectively changing the concentrations of reactants and products.

If you want to produce more products, such as sodium borate and sodium tetraborate in this case, increasing the concentration of reactants or removing products can help. One way to achieve this is by adding a component that specifically stabilizes one of the products, thus reducing the reverse reaction.

• Adding reactants can push the reaction forward.
• Removing products from the system can also drive the reaction towards the products.
• Using stabilizers to change the energetics of formation aids the desired shift.

In your specific reaction, the addition of a cis-1,2-diol can help shift equilibrium toward more product formation. This addition would enhance complex stability and suppress backward reactions. Understanding equilibrium shifts is crucial in optimizing chemical reactions for maximum yield.

Complex Formation

Complex formation involves the creation of more stable structures through strong interactions, often by coordination bonds. In the realm of chemical equilibrium, forming such complexes can have significant implications on the reaction pathways and their outcomes.

For instance, when cis-1,2-diol is added to your reversible reaction of boric acid and sodium hydroxide, it forms complexes with boric acid, facilitating the forward reaction. This results in the formation of a more tightly-bound structure that does not readily revert back to the starting materials.

• Complexes are often stronger than individual reactants.
• Complex formation can alter the direction and rate of chemical reactions.
• Stabilization of intermediates via complex formation can prevent reverse reactions.

In essence, by understanding the concept of complex formation, you can identify potential additives like cis-1,2-diol that will not only stabilize desired products but will also optimize the reaction to proceed mostly in the forward direction, enhancing your yields and process efficiency.