Question

a) Use the following reactions to explain the meaning of conjugate acid and base:

i) CH3COOH + H2OCH3COO- + H3O+ ii)CO -2 +H OHCO - +OH-

Write the order of Lowry-Bronsted acid and base in the above equations. b) Using the following equations, explain the statement “Water is amphoteric”

i) HCl + H2OH3O+ + Cl- ii) NH3 + H2ONH4+ + OH-

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Answer 1

a) The Lowry-Bronsted theory involves transfer of proton (H+ ion) when acid and base comes into picture. Lowry-Bronsted acid is proton donor and Lowry-Bronsted base is proton acceptor.

In reaction (i) Lowry-Bronsted acid is acetic acid because it donated a proton (H+) to Lowry-Bronsted base which is water because it accepted the proton (H+). According to given reaction acetate ion is formed when acetic acid released a proton, the acetate ion (CH3COO-) is conjugate base which means when a Lowry-Bronsted acid releases a proton the resulting species is its conjugate base. And hydronium ion (H2O+) is a conjugate acid as it is formed when water accepted the hydrogen ion or a proton from acetic acid which means when Lowry-Bronsted base accepts a proton the resulting species is its conjugate acid. Acetic acid and acetic ion is a conjugate pair and the other conjugate pair of this reaction is water and its hydronium ion.

In reaction (ii) Lowry-Bronsted acid is water because it donated a proton (H+) to Lowry-Bronsted base which is CO2- because it accepted the proton (H+). According to given reaction OH- ion is formed when water released a proton, the OH- ion is conjugate base which means when a Lowry-Bronsted acid releases a proton the resulting species is its conjugate base. And HCO- ion is a conjugate acid as it is formed when CO2 accepted the hydrogen ion or a proton from water which means when Lowry-Bronsted base accepts a proton the resulting species is its conjugate acid. water and OH- ion is a conjugate pair and the other conjugate pair of this reaction is CO2- and HCO- ion.

b) Amphoteric compound is a species which react both as acid and as a base as well.

in both of the reaction (i) & (ii) we can clearly see that water acted as a base because it accepted a proton to make it as H3O+ ion. And in second reaction we can see that water have donated proton or H+ ion to ammonia so it means that it is acting as an acid. So, it clearly shows that water is amphoteric in nature because it can react as a base and it can also react as an acid.