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2H 2 O( l ) H 3 O + ( a q ) + OH ( a q )

The ion product of water, K w is the equilibrium constant for the autoionization reaction:

K w = [ H 3 O + ] [ OH ] = 1.0 × 1 0 −14 at 25 °C

Key equations

  • K w = [H 3 O + ][OH ] = 1.0 × 10 −14 (at 25 °C)

Chemistry end of chapter exercises

Write equations that show NH 3 as both a conjugate acid and a conjugate base.

One example for NH 3 as a conjugate acid: NH 2 + H + NH 3 ; as a conjugate base: NH 4 + ( a q ) + OH ( a q ) NH 3 ( a q ) + H 2 O ( l )

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Write equations that show H 2 PO 4 acting both as an acid and as a base.

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Show by suitable net ionic equations that each of the following species can act as a Brønsted-Lowry acid:

(a) H 3 O +

(b) HCl

(c) NH 3

(d) CH 3 CO 2 H

(e) NH 4 +

(f) HSO 4

(a) H 3 O + ( a q ) H + ( a q ) + H 2 O ( l ) ; (b) HCl ( l ) H + ( a q ) + Cl ( a q ) ; (c) NH 3 ( a q ) H + ( a q ) + NH 2 ( a q ) ; (d) CH 3 CO 2 H ( a q ) H + ( a q ) + CH 3 CO 2 ( a q ) ; (e) NH 4 + ( a q ) H + ( a q ) + NH 3 ( a q ) ; (f) HSO 4 ( a q ) H + ( a q ) + SO 4 2− ( a q )

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Show by suitable net ionic equations that each of the following species can act as a Brønsted-Lowry acid:

(a) HNO 3

(b) PH 4 +

(c) H 2 S

(d) CH 3 CH 2 COOH

(e) H 2 PO 4

(f) HS

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Show by suitable net ionic equations that each of the following species can act as a Brønsted-Lowry base:

(a) H 2 O

(b) OH

(c) NH 3

(d) CN

(e) S 2−

(f) H 2 PO 4

(a) H 2 O ( l ) + H + ( a q ) H 3 O + ( a q ) ; (b) OH ( a q ) + H + ( a q ) H 2 O ( l ) ; (c) NH 3 ( a q ) + H + ( a q ) NH 4 + ( a q ) ; (d) CN ( a q ) + H + ( a q ) HCN ( a q ) ; (e) S 2− ( a q ) + H + ( a q ) HS ( a q ) ; (f) H 2 PO 4 ( a q ) + H + ( a q ) H 3 PO 4 ( a q )

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Show by suitable net ionic equations that each of the following species can act as a Brønsted-Lowry base:

(a) HS

(b) PO 4 3−

(c) NH 2

(d) C 2 H 5 OH

(e) O 2−

(f) H 2 PO 4

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What is the conjugate acid of each of the following? What is the conjugate base of each?

(a) OH

(b) H 2 O

(c) HCO 3

(d) NH 3

(e) HSO 4

(f) H 2 O 2

(g) HS

(h) H 5 N 2 +

(a) H 2 O, O 2− ; (b) H 3 O + , OH ; (c) H 2 CO 3 , CO 3 2− ; (d) NH 4 + , NH 2 ; (e) H 2 SO 4 , SO 4 2− ; (f) H 3 O 2 + , HO 2 ; (g) H 2 S; S 2− ; (h) H 6 N 2 2+ , H 4 N 2

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What is the conjugate acid of each of the following? What is the conjugate base of each?

(a) H 2 S

(b) H 2 PO 4

(c) PH 3

(d) HS

(e) HSO 3

(f) H 3 O 2 +

(g) H 4 N 2

(h) CH 3 OH

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Identify and label the Brønsted-Lowry acid, its conjugate base, the Brønsted-Lowry base, and its conjugate acid in each of the following equations:

(a) HNO 3 + H 2 O H 3 O + + NO 3

(b) CN + H 2 O HCN + OH

(c) H 2 SO 4 + Cl HCl + HSO 4

(d) HSO 4 + OH SO 4 2− + H 2 O

(e) O 2− + H 2 O 2 OH

(f) [ Cu ( H 2 O ) 3 ( OH ) ] + + [ Al ( H 2 O ) 6 ] 3+ [ Cu ( H 2 O ) 4 ] 2+ + [ Al ( H 2 O ) 5 ( OH ) ] 2+

(g) H 2 S + NH 2 HS + NH 3

The labels are Brønsted-Lowry acid = BA; its conjugate base = CB; Brønsted-Lowry base = BB; its conjugate acid = CA. (a) HNO 3 (BA), H 2 O(BB), H 3 O + (CA), NO 3 ( CB ) ; (b) CN (BB), H 2 O(BA), HCN(CA), OH (CB); (c) H 2 SO 4 (BA), Cl (BB), HCl(CA), HSO 4 ( CB ) ; (d) HSO 4 ( BA ) , OH (BB), SO 4 2− (CB), H 2 O(CA); (e) O 2− (BB), H 2 O(BA) OH (CB and CA); (f) [Cu(H 2 O) 3 (OH)] + (BB), [Al(H 2 O) 6 ] 3+ (BA), [Cu(H 2 O) 4 ] 2+ (CA), [Al(H 2 O) 5 (OH)] 2+ (CB); (g) H 2 S(BA), NH 2 ( BB ) , HS (CB), NH 3 (CA)

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Identify and label the Brønsted-Lowry acid, its conjugate base, the Brønsted-Lowry base, and its conjugate acid in each of the following equations:

(a) NO 2 + H 2 O HNO 2 + OH

(b) HBr + H 2 O H 3 O + + Br

(c) HS + H 2 O H 2 S + OH

(d) H 2 PO 4 + OH HPO 4 2− + H 2 O

(e) H 2 PO 4 + HCl H 3 PO 4 + Cl

(f) [ Fe ( H 2 O ) 5 ( OH ) ] 2+ + [ Al ( H 2 O ) 6 ] 3+ [Fe ( H 2 O ) 6 ] 3+ + [ Al ( H 2 O ) 5 ( OH ) ] 2+

(g) CH 3 OH + H CH 3 O + H 2

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What are amphiprotic species? Illustrate with suitable equations.

Amphiprotic species may either gain or lose a proton in a chemical reaction, thus acting as a base or an acid. An example is H 2 O. As an acid:
H 2 O ( a q ) + NH 3 ( a q ) NH 4 + ( a q ) + OH ( a q ) . As a base: H 2 O ( a q ) + HCl ( a q ) H 3 O + ( a q ) + Cl ( a q )

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State which of the following species are amphiprotic and write chemical equations illustrating the amphiprotic character of these species:

(a) H 2 O

(b) H 2 PO 4

(c) S 2−

(d) CO 3 2−

(e) HSO 4

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State which of the following species are amphiprotic and write chemical equations illustrating the amphiprotic character of these species.

(a) NH 3

(b) HPO 4

(c) Br

(d) NH 4 +

(e) ASO 4 3−

amphiprotic: (a) NH 3 + H 3 O + NH 4 OH + H 2 O , NH 3 + OCH 3 NH 2 + CH 3 OH ; (b) HPO 4 2− + OH PO 4 3− + H 2 O , HPO 4 2− + HClO 4 H 2 PO 4 + ClO 4 ; not amphiprotic: (c) Br ; (d) NH 4 + ; (e) AsO 4 3−

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Is the self ionization of water endothermic or exothermic? The ionization constant for water ( K w ) is 2.9 × 10 −14 at 40 °C and 9.3 × 10 −14 at 60 °C.

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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