Acids and Bases Check 1. Define Bronsted-Lowry base; 2. Identify the acid and base; HSO 4 - (aq) + H 2 O(l) ←→ H 3 O + (aq) + SO 4 2- (aq) 3. What is a.

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Acids and Bases Check 1. Define Bronsted-Lowry base; 2. Identify the acid and base; HSO 4 - (aq) + H 2 O(l) ←→ H 3 O + (aq) + SO 4 2- (aq) 3. What is a dibasic base? Give an example. 4. What is the concentration of HCl if 0.5 moles are present in 25 cm 3 ?

Objectives 1. Define pH; 2. Explain the difference between strong and weak acids and bases; 3. Calculate pH for strong acids.

What is pH?

pH = - log 10 [H + (aq) ] pH; defining and determining [H + (aq)] means based on H + ion concentration (moldm -3 ) Hydrogen ions are usually represented as H + (aq) rather than the hydrated H 3 O + (aq)

What does pH mean? pondus hydrogenii, the power of hydrogen

pH values of a range of 0.1moldm-3 solutions of substances measured by a pH meter

Measuring pH 1.The approximate pH of a solution can be obtained by using indicators whose colour change with pH is known. 2.pH can also be measured electronically using a pH meter.

Strong Acids and Bases HCl(aq) → H + (aq) + Cl - (aq) NaOH(aq) → Na + (aq) + OH - (aq) “Virtually completely dissociated (ionised) in water.”

Weak Acids and Bases CH 3 COOH(aq) ↔ H + (aq) + CH 3 COO - (aq) NH 3 (aq) + H 2 O(l) ↔ NH 4 + (aq) + OH - (aq) “Partially dissociated (ionised) in water.”

There are a limited number of strong acids and bases in water. Hydroxide ion, OH - AcidsBases Hydrohalic acids (HCl, HBr and HI) Nitric acid, HNO 3 Sulphuric acid, H 2 SO 4 Perchloric acid, HClO 4 A strong acid does NOT have to be 100% ionised. Any ionisation above 50% entitles the acid to be classed as strong. The majority of acids are weak and are not fully ionised in water at normal concentrations.

Examples HCl(aq) + H 2 O(l) ←→ H 3 O + (aq) + Cl - (aq) KOH(aq) ←→ K + (aq) + OH - (aq) H 2 SO 4 (aq) + H 2 O(l) ←→ H 3 O + (aq) + HSO 4 - (aq) HSO 4 - (aq) + H 2 O(l) ←→ H 3 O + (aq) + SO 4 2- (aq)

Calculating pH for strong acids

[H + (aq)] = 1 moldm -3 pH ? [H + (aq)] = 2.5 moldm -3 pH ? [H + (aq)] = 10 moldm -3 pH ?

Concentration limitation pH can never have very large negative values since these would demand very concentrated acid solutions Very concentrated solutions are impossible to obtain! [H+] is limited by the solubility of the acid in water as well as by ion association in soutions

Quick recap 1.Define pH 2.Define a strong acid, with example 3.Define a weak acid, with example 4.pH can never have very large negative numbers. Explain. 5.Calculate the pH of 0.1moldm -3 HNO 3, 0.2moldm -3 KOH, 0.05moldm -3 H 2 SO 4. 6.Now, suggest a definition for a monoprotic and diprotic acid

Is pH affected by dilution? Strong acidWeak acid Concentration (moldm -3 ) pHConcentration (moldm -3 ) pH Can you explain the results? Use HCl as an example of a strong acid and CH 3 COOH as the weak acid Write dissociation eqns / reaction with water

Is pH affected by dilution? Strong acidWeak acid Concentration (moldm -3 ) pHConcentration (moldm -3 ) pH HCl + H 2 O Cl - + H 3 O + CH 3 COOH + H 2 O CH 3 COO - + H 3 O +

Dilution of a weak acid In 0.1M ethanoic acid, approximately 1.3% of the molecules are dissociated into ions. In 0.01M ethanoic acid, approximately 4.1% of the molecules are dissociated into ions. In 0.001M ethanoic acid, approximately 13% of the molecules are dissociated into ions.

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