Question

In each reaction, identify the Bronsted-Lowry acid, the BronstedLowry base, the conjugate acid, and the conjugate base. a. \(\mathrm{HI}(a q)+\mathrm{H}_{2} \mathrm{O}(I) \longrightarrow \mathrm{H}_{3} \mathrm{O}^{+}(a q)+\mathrm{I}^{-}(a q)\) b. \(\mathrm{CH}_{3} \mathrm{NH}_{2}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{CH}_{3} \mathrm{NH}_{3}{ }^{+}(a q)+\mathrm{OH}^{-}(a q)\) c. \(\mathrm{CO}_{3}^{2-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{HCO}_{3}^{-}(a q)+\mathrm{OH}^{-}(a q)\) d. \(\mathrm{HBr}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{H}_{3} \mathrm{O}^{+}(a q)+\mathrm{Br}^{-}(a q)\)

Short answer

a. Acid: HI, Base: H2O, Conjugate acid: H3O+, Conjugate base: I-. b. Acid: H2O, Base: CH3NH2, Conjugate acid: CH3NH3+, Conjugate base: OH-. c. Acid: H2O, Base: CO32-, Conjugate acid: HCO3-, Conjugate base: OH-. d. Acid: HBr, Base: H2O, Conjugate acid: H3O+, Conjugate base: Br-.

Step-by-step solution

Identifying the Roles in Reaction a

In reaction a, the Bronsted-Lowry acid is HI because it donates a proton (H+) to H2O. H2O acts as the Bronsted-Lowry base as it accepts the proton, forming H3O+. After donating the proton, HI becomes I-, which is the conjugate base. H3O+ is the conjugate acid, as it can potentially donate a proton in the reverse reaction.

Identifying the Roles in Reaction b

For reaction b, CH3NH2 is the Bronsted-Lowry base, because it accepts an H+ from H2O. The H2O, therefore, is the Bronsted-Lowry acid as it donates an H+. CH3NH3+ is the conjugate acid formed after CH3NH2 gains a proton, and OH- is the conjugate base formed from the H2O after losing a proton.

Identifying the Roles in Reaction c

In reaction c, the CO32- ion is the Bronsted-Lowry base, as it accepts an H+ from water. H2O is the Bronsted-Lowry acid because it donates the H+ ion. HCO3- is the conjugate acid after CO32- gains an H+, and OH- remains as the conjugate base after H2O gives up an H+.

Identifying the Roles in Reaction d

Reaction d's Bronsted-Lowry acid is HBr, since it donates an H+ to water. Water acts as the Bronsted-Lowry base because it accepts the H+. The ion H3O+ is the conjugate acid which has received the proton, and Br- is the conjugate base, left after HBr loses the proton.

Key concepts

Acid-Base Reactions

Understanding acid-base reactions is fundamental to grasping the concepts in chemistry, especially when discussing the Bronsted-Lowry acid-base theory. In this theory, an acid is defined as a substance that can donate a proton (H+), whereas a base is a substance that can accept a proton. These reactions involve the transfer of a proton from the acid to the base.

Consider the reaction between hydroiodic acid (HI) and water (H2O), where HI donates a proton to H2O, forming hydronium ion (H3O+) and iodide ion (I-). Here, we see a clear example of an acid-base reaction, with the proton donor and acceptor participating in a transformation that leads to the formation of new substances – the conjugate acid and base.

Conjugate Acid-Base Pairs

Digging a bit deeper into Bronsted-Lowry theory, every acid-base reaction consists of two conjugate acid-base pairs. A conjugate base is what remains of the acid after it has donated a proton, and a conjugate acid is the species formed when a base accepts a proton.

Referring to our initial example, after HI donates a proton, it becomes I-, the conjugate base. Water, which accepts the proton, forms H3O+, the conjugate acid. In essence, a conjugate acid-base pair is simply the acid or base and its corresponding partner formed after a proton has been transferred. Recognizing these pairs is crucial for predicting the direction and extent of chemical reactions.

Proton Donors and Acceptors

In Bronsted-Lowry theory, the terms 'proton donors' and 'proton acceptors' describe the roles substances play during acid-base reactions. A proton donor is an acid that loses a hydrogen ion in the form of a proton. Similarly, a proton acceptor is a base that gains a proton during the reaction.

For instance, in the reaction involving methylamine (CH3NH2) and water, CH3NH2 accepts a proton becoming CH3NH3+. Therefore, methylamine is the proton acceptor and water which donates the proton is the proton donor. Identifying these roles helps elucidate the function of different substances in acid-base chemistry.

Chemical Equilibrium

Chemical equilibrium is the state reached when the forward and reverse reactions occur at the same rate, resulting in no overall change in the concentrations of the reactants and products. It's a dynamic balance, not a static one, as reactants and products continue to interconvert, but their concentrations remain constant over time.

For example, in the reaction between carbon dioxide (CO3^2-) and water, the system can reach equilibrium, meaning the rate at which CO3^2- and H2O form bicarbonate (HCO3^-) and hydroxide ion (OH^-) is equal to the rate at which the bicarbonate and hydroxide ions revert back to CO3^2- and water. Understanding equilibrium is important for predicting the concentrations of substances in a reaction mixture at any given time.