Bronsted-Lowry theory of acids and bases

Hydrogen chloride and ammonia both ionise in water:

`HCl + H_2O ⇌ H_3O^+ + Cl^– " " "equation 1 "`

`NH_3 + H_2O ⇌ NH""_4^+ + OH^– " ""equation 2"`

i. State the name of the ion `H _3O^+`

ii. Identify the acid and the base on the left-hand side of each equation

iii. By referring to equation 1 and equation 2, explain why water is described as being amphoteric

i. The name of the ion `H_3O^+` could be oxonium or hydronium or  hydroxonium

ii. As for equation 1: HCl is acid and H₂O is base

As for equation 2: NH₃ is the base and H₂O is acid

iii. Amphoteric means a substance can act as either acids or bases. Here, in equation 1, HCl donates a proton to water, making it become `H_3O^+`, which accepts a proton, whereas in equation 2, `NH_3` accepts a proton from water to become an `NH""_4^+` ion.

When dissolved in an organic solvent, hydrogen chloride reacts with hydrogen iodide as follows:

`HCl + HI ⇌ H_2Cl^+ + I^–`
i. Use the Brønsted–Lowry theory of acids and bases to explain which reactant is the acid and which reactant is the base.

ii. Identify which of the products is the conjugate acid and which is the conjugate base of the substances you have identified in part i.

i. According to Brønsted–Lowry theory, acid is a substance which donates an H⁺ ion or a proton and creates its conjugate base and the base is a substance which accepts an H⁺ ion or a proton and forms its conjugate acid. 

Thus, with this reaction, HI donates H⁺ ion and HCl accepts that H⁺ ion, leading HI to act as an acid and HCl to act as a base.

ii. H₂Cl⁺ is the conjugate acid of HCl, and I⁻ is the conjugate base of HI.

Hydrochloric acid is a strong acid but ethanoic acid, CH₃COOH, is a weak acid. 

i. Explain the difference between a strong acid and a weak acid.

i.. Suggest a value of the pH for a 0.1 mol dm^–3 solution of ethanoic acid in water.

iii. Write a chemical equation to show the reaction when ethanoic acid donates a proton to water.

i. Strong acid is virtually completely ionised in water, whereas weak acid is only slightly ionised in water.

ii. A value of the pH for a 0.1 mol dm^–3 solution of ethanoic acid in water is 0.

iii. `CH_3COOH(l) + H_2O(l) ⇌ CH_3COO^–(aq) + H_3O^+(aq)`

i. Which species in the following reaction acts as a Brønsted-Lowry base.

`HSO""_4^(-) (aq) + H_3O^+ (aq) ⇌ H_2SO_4 (aq) + H_2O (l)`

ii. Which species in the following equation is acting as a Brønsted-Lowry acid. 

`CO""_3^(2-) (aq) + H^+ (aq) ⇌HCO""_3^(-) (aq)`

i. The species acting as a Brønsted-Lowry base are `HSO""_4^-` and `H_2O`, in which Brønsted-Lowy definition states that bases are proton acceptors.

ii. The species acting as a Brønsted-Lowry acid is `HCO""_3^-`in which Brønsted-Lowy definition states that acids are proton donors.

A titration was performed between ethanoic acid, CH₃CO2H (aq) and sodium hydroxide, NaOH (aq). Using the information in Table below, suggest which indicator would be suitable for this titration.

i. Write an equation for the reaction between ethanoic acid and sodium hydroxide

ii. Using the information in Table above, suggest which indicator would be suitable for this titration

i. The equation is:

`CH_3CO2H (aq) + NaOH (aq) → CH_3CO2Na (aq) + H_2O (l)`

ii. A suitable indicator for a strong base weak acid titration is Cresol purple.

Which type of titration is indicated by the following pH curve?

A. Strong acid - strong base

B. Weak acid - weak base

C. Strong acid - weak base

D. Weak acid - strong base

The answer is D.

The pH starts high and then decreases. The equivalence point, which is halfway down the vertical section, is above pH 7, as such, this indicates strong base and a weak acid.

A is incorrect because the equivalence point is at pH 7

B is incorrect because the pH curve has a very short vertical section

C is incorrect because the equivalence point is below pH 7

Using your knowledge of the Brønsted-Lowry theory, which of the following correctly describes ammonia?

A. Neutral

B. Acid

C. Base

D. Amphoteric

The answer is C.

The Brønsted–Lowry theory describes acids as proton donors and bases as proton acceptors.

A is incorrect because ammonia is not neutral

B is incorrect because it cannot donate a proton

D is incorrect because it does not act as an acid

In the following reaction, identify which two species are acting as Brønsted–Lowry acids

`H_3PO_4 (aq) + OH^− (aq) ⇋ H_2PO""_4^(−) (aq) + H_2O (l)`

A. `H_2PO""_4^-` (aq) and `OH^-` (aq)

B. `H_3PO_4` (aq) and `H_2PO""_4^-` (aq)

C. `H_2PO""_4^-` (aq) and `H_2O` (l)

D. `H_3PO_4` (aq) and `H_2O` (l)

The answer is D.

Brønsted–Lowry acids are proton donors. `H_3PO_4` loses a proton to become `H_2PO""_4^-` and `OH^-` gains a proton to become `H_2O`, as such, `H_2O` loses a proton so behaves as an acid in the backward reaction.

The pH range of four different indicators are:

Which indicator would be suitable for a titration involving hydrochloric acid and ammonia?

A. Methyl orange

B. Phenolphthalein

C. Thymol blue

D. Bromothymol blue

The answer is A.

This titration is between a strong acid and a weak base, as such, the equivalence point is below pH 7.00, making methyl orange become the best indicator.

B is incorrect because this indicator is suitable for above pH 7.0

C is incorrect because this indicator is too low for the strong acid – weak base titration

D is incorrect because it is still too high or a strong acid - weak base titration

The following reaction occurs between concentrated sulfuric and nitric acids

`H_2 SO_4 + HNO_3 ⇋ H_2NO""_3^+ + HSO""_4^-`

Identify the two species which are acting as Brønsted–Lowry bases.

A. `H_2NO""_3^+` and `HSO""_4^-`

B. `H_2NO""_3^+` and `H_2SO_4`

C. `H_2SO_4` and `HSO""_4^-`

D. `HNO_3` and `HSO""_4^-`

The answer is D.

Brønsted–Lowry bases are proton acceptors. 

A is incorrect because `H_2NO""_3^+` acts as a Brønsted–Lowry acid because it donates a proton to `HSO""_4^-`

B is incorrect because `H_2NO""_3^+` acts as a Brønsted–Lowry acid because it donates a proton to `HSO""_4^-`

C is incorrect because `H_2SO_4`acts as a Brønsted–Lowry acid because it donates a proton to `HNO_3`