There are three ways in which to define acids and bases: the Arrhenius concept,

Question

There are three ways in which to define acids and bases: the Arrhenius concept, the Brnsted-Lowry concept, and the Lewis concept.

Arrhenius concept

Arrhenius acids are substances that, when dissolved in water, increase the concentration of the H+ ion; Arrhenius bases are substances that, when dissolved in water, increase the concentration of the OH? ion.

For example, a substance with an ionizable protons is an acid:

HNO2(aq) ? NO2?(aq) + H+(aq)

A substance that can either directly or indirectly increase hydroxide ions is a base:

NH3(aq) + H2O(l)? NH4+(aq) + OH?(aq)

However, the Arrhenius concept only applies to aqueous solution. It does not apply to reactions in other phases, which ultimately lead to the use of other definitions for such systems.

Part A

Using the Arrhenius concept of acids and bases, identify the Arrhenius acid and base in each of the following reactions:

2KOH(aq)+H2SO4(aq)?K2SO4(aq)+2H2O(l)

NH3(g)+HCl(g)?NH4Cl(s)

Drag the appropriate items to their respective bins.

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Brnsted-Lowry concept

Brnsted-Lowry acids are substances that can donate a proton (H+) to another substance; Brnsted-Lowry bases are substances that can accept a proton (H+).

A substance with transferable protons is an acid, such as HNO2 in this example:

HNO2(aq) + H2O(l) ? NO2?(aq) + H3O+(aq)

A substance that can receive a transferable proton is a base, such as water in this example:

HNO2(aq) + H2O(l) ? NO2?(aq) + H3O+(aq)

One benefit of the Brnsted-Lowry concept of acids and bases is that it is not limited to aqueous solutions and can be applied to gases, liquids, and solids.

Part B

Using the Brnsted-Lowry concept of acids and bases, identify the Brnsted-Lowry acid and base in each of the following reactions:

H2PO3?(aq)+H2O(l)?H3PO3(aq)+OH?(aq)

(CH3)2NH(g)+BF3(g)?(CH3)2NHBF3(s)

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H2PO3- H2O BF3 (CH3)2NH

Bronsted Lowry Acid Bronsted Lowry Base Neither

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Lewis concept

A Lewis acid is an electron-pair acceptor, and a Lewis base is an electron-pair donor.

A substance that accepts an electron pair is an acid, such as Fe3+ in this example where a pair of electrons on CN? is transferred to the metal:

Fe3+(aq) + 6CN?(aq) ? Fe(CN)63?(aq)

A substance that donates an electron-pair is a base, such as water in this example where a pair of electrons on the oxygen of water are transferred to the metal cation core:

[Fe(H2O)5]2+(aq) + H2O(l) ? [Fe(H2O)6]3+(aq)

The Lewis concept greatly increases the number of species that can be thought of as acids and bases. It allows cations and species with incomplete octets to be defined as an acid. You can identify a transition metal cation that act as acids by assessing whether a ligand with transferable electrons donates electrons to the metal core.

Part C

Using the Lewis concept of acids and bases, identify the Lewis acid and base in each of the following reactions:

Co(NO3)3(s)+6H2O(l)?Co(H2O)63+(aq)+3NO3?(aq)

(CH3)3N(g)+HBr(g)?(CH3)3NHBr(s)

Drag the appropriate items to their respective bins.

H2O CO(NO3)3 HBr (CH3)3N

Lewis Acid Lewis Base Neither

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Explanation / Answer

Part A: According to Arrhenius concept of acids and bases, an acid is a species that that increases H+ ion concentration in aqueous solution and a base is a species that increases OH- ion concentration in aqueous solution.

During an Arrhenius acid base neutralization reaction, H+ ion and OH- ion react to form H2O.

Here the given reactions are Arrhenius acid base neutralization reaction.

2KOH(aq)+H2SO4(aq) --------> K2SO4(aq)+2H2O(l)

Here KOH(aq) gives OH-(aq) ion and acts as Arrhenius base,

And H2SO4(aq) gives H+(aq) ion and acts as Arrhenius acid.

NH3(g)+HCl(g) ---------> NH4Cl(s)

Here HCl(g) gives H+(aq) ion and acts as Arrhenius acid,

And NH3(g) gives OH-(aq) ion and acts as Arrhenius base.

NH3(aq) + H2O(l) ----> NH4+(aq) + OH-(aq)

Part B: According to Bronsted-Lowry concept, acids are substances that can donate a proton (H+) to another substance and bases are substances that can accept a proton (H+).

H2PO3-(aq)+H2O(l) ------> H3PO3(aq)+OH- (aq)

Here H2O(l) is a Bronsted-Lowry acid as it donates H+(aq) ion to H2PO3-(aq),

And H2PO3-(aq) is a Bronsted-Lowry base as it accepts H+(aq) ion to form H3PO3(aq).

(CH3)2NH(g)+BF3(g) ----------------> (CH3)2NHBF3(s)

In the above reaction, N atom donates an electron pair (lone pair) to B atom of BF3 to form (CH3)2NHBF3(s), hence thay are neither Bronsted-Lowry acid or Bronsted-Lowry base.

Part C: According to Lewis concept, a Lewis acid is a species that accepts electron - pair, and a Lewis base is a species that donates electron pair.

The given reactions are

Co(NO3)3(s)+6H2O(l) -------------> [Co(H2O)6]3+(aq) + 3NO3-(aq)

Here Co3+ of Co(NO3)3(s) acts as Lewis acid by accepting electron pair from each of the O - atom of 6 H2O molecules,

and H2O(l) acts as Lewis base by donating electron pair to Co3+

(CH3)3N(g)+HBr(g) -----------> (CH3)3NHBr(s)

Here N atom of CH3NH2 donates lone pair to H+ ion of HBr to form CH3)3NHBr(s)

Hence CH3NH2 - Lewis base and HBr - Lewis acid.

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