Question
Question: Among \(\,N{H_3}\,\),\(\,P{H_3}\,\) ,\(\,As{H_3}\,\) and \(\,Sb{H_3}\,\), which one is a strongest r...
Among NH3,PH3 ,AsH3 and SbH3, which one is a strongest reducing agent?
A. NH3
B. PH3
C. AsH3
D. SbH3
Solution
A compound is differentiated as reducing agent or oxidizing agent while it is taking part in a redox reaction. Strong reducing agents will easily lose an electron without much energy. There are various factors that determine a strong reducing agent.
Complete step by step answer:
Let us first understand what is a reducing agent and its requirements;
-When the compound loses or donate an electron (undergoing oxidation) they are called reducing agents. They help in the reduction of another molecule in the reaction.
-As the bond energy gets increased, the bond length gets decreased and therefore they are inversely proportional.
-As the size of the molecule increases, the bond length between the metal and hydride increases. So, the increase in the bond length will decrease the bond energy down the group.
-Therefore, we understand that the reducing agents are also dependent upon the minimum bond energy.
-S-block elements are good reducing agents.
-All the good reducing agents have low electronegativity atoms and a good potential of an atom or molecule to attract the bonding electrons and very small ionisation energies of the species.
Let us analyse each option now;
-While analysing NH3, nitrogen is the smallest element compared to the other elements mentioned in the question as it lies in the top of the group ( atomic size increases as we move from top to bottom in a group). All of the elements mentioned in the question belong to the group 15 in the periodic table. So as its bond length with hydrogen is smaller as well. As a result it will have higher bond energy and hence not a good reducing agent.
-While analysing PH3, phosphor is the second element in the respective group. Even though it is larger than nitrogen, it is still smaller when compared to arsenic(As). Hence, this is also not a good reducing agent due to smaller size and higher bond energy with hydrogen.
-Now, in AsH3, arsenic (As) is the third element in the group 15. Therefore, it is larger than nitrogen and phosphorus and has a larger bond length and lower bond enthalpy when compared to them. Therefore, it is a stronger reducing agent than ammonia (NH3) and phosphine (PH3). However, there is one more element underneath this. So this cannot be the right option here.
-In all the options SbH3 is the one having the largest size, so the Sb and H will have the highest bond length. Therefore it will have less bond energy indicating that it is a strong reducing agent. Also, In its respective group, sb5+ is the most stable, so it is readily oxidised, making it the best reduction agent.
Therefore, the correct option is option D.
Additional information:
-In the case of molecular weight, as it increases the intermolecular force of attraction also increases which results in the increasing of the boiling point.
-The trend of high boiling point is SbH3>NH3>AsH3>PH3
Note: Note that a reducing agent is something that actually gets oxidized in a reaction and oxidizing agent is something that gets reduced in a reaction. If, in a reaction, the reducing agent does not transfer electrons to another substance, so the reducing phase cannot occur. Reducing agents are also responsible for certain phenomena like corrosion, which is the oxidation of metals due to electrochemical reactions.