Acid-base properties of HCO_3^1-: a) Write two equations for the reaction of HCO

Question

Acid-base properties of HCO_3^1-: a) Write two equations for the reaction of HCO_3^1- with water: in one equation HCO_3^1- is reacting like an acid and in the other equation HCO_3^1- is reacting like a base. Identify the conjugate acid-base pairs in each equation. b) Explain how the reactions you did in Experiment D show that the HCO_3^1- ion is both an acid and a base. c) Based on the pH of the NaHCO_3 solution is the HCO_3^1- ion a stronger acid or a stronger base? Explain your reasoning. d) HCO_3^1- is the conjugate base of H2CO_3 (seltzer) and HCO_3^1- is the conjugate acid of CO_3^2-. i. Based on your pH data which is the stronger acid, H_2CO_3 or HCO_3^1-? ii. Based on your pH data which is the stronger base, HCO_3^1- or CO_3^2-? iii. Are your results consistent with the Bronsted-Lowry acid-base model? Use your data to support your answer. e) Write an equation for the reaction of H_2CO_3 with CO_3^2-. i. Based on your answer to the previous question is the equilibrium more favorable going to the right or to the left? ii. Which species would you expect to find in solution if you mixed equal quantities of H_2CO_3 and CO_3^2-? iii What would you predict the pH of the resulting solution to be?

Explanation / Answer

6. Acid-base properties of HCO3-

a) Equation of HCO3- acting as an acid,

HCO3- + H2O <===> CO3^2-    +       H3O+

acid       base          conjugate         conjugate

                                    base                 acid

HCO3-/CO3^2- are acid/conjugate base pair

H2O/H3O+ are base/conjugate acid pair

Equation of HCO3- acting as a base,

HCO3- + H2O <===> H2CO3    +       OH-

base       acid          conjugate         conjugate

                                    acid                 base

HCO3-/H2CO3 are base/conjugate acid pair

H2O/OH- are acid/conjugate base pair

b) Thus as seen from above, HCO3- acts both as an acid (donates H+ to H2O) or acts as a base (accepts H+ from H2O).

HCO3- also reacts in a similar fashion with acid HCl to form H2CO3 (conjugate acid) or with NaOH to form CO3^2- (conjugate base).

HCl in water gives H+ and acts as an acid, whereas, NaOH in water gives OH- and acts as a base.

c) pH of HCO3- is greater than 7, thus NaHCO3 (or HCO3-) is a stronger base gives OH- in solution and acts as a base.

d) acid-base pair

i. H2CO3 is a stronger acid than HCO3-

ii. CO3^2- is a stronger base than HCO3-

iii. Yes, the results are consistent with the Bronsted-Lowry acid-base model.

e) Reaction,

H2CO3 + CO3^2- <==> HCO3- + HCO3-

i. The equilibrium is more favourable going to the right.

ii. We expect to find HCO3- in solution when equal quantities of H2CO3 is mixed with CO3^2-.

iii. The pH of the resulting solution would be basic, HCO3- acts as a base.

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