R3.1 Acid And Bases

Brønsted-Lowry acid

A Brønsted-Lowry acid is a species that gives away a proton (H⁺).

Brønsted-Lowry base

A Brønsted-Lowry base is a species that accepts a proton (H⁺).

HCl (aq) in reaction with H₂O (l)

In the reaction, HCl (aq) is acting as a Brønsted-Lowry acid.

H₂O (l) in reaction with HCl (aq)

In the reaction, H₂O (l) is acting as a Brønsted-Lowry base.

True or False: All bases are alkalis

False. Not all bases are alkalis.

True or False: An alkali is a soluble base

True. An alkali is a soluble base.

HCl (g) in reaction with NH₃ (g)

In the reaction, HCl (g) is acting as a Brønsted-Lowry acid.

NH₃ (g) in reaction with HCl (g)

In the reaction, NH₃ (g) is acting as a Brønsted-Lowry base.

H₂O (l) in reaction with H₃PO₄ (aq)

In the reaction, H₂O (l) is acting as a Brønsted-Lowry base.

H₃PO₄ (aq) in reaction with H₂O (l)

In the reaction, H₃PO₄ (aq) is acting as a Brønsted-Lowry acid.

True or False: A proton in aqueous solution can be represented as H⁺ (aq) only

False. A proton in aqueous solution can be represented as both H⁺ (aq) and H₃O⁺ (aq).

Conjugate acid-base pair

A conjugate acid-base pair is two species that are different from each other by a H⁺ ion.

Acid and conjugate base pair in reaction

In the reaction, the acid and conjugate base pair is H₂CO₃ (aq) and HCO₃⁻ (aq).

Base and conjugate acid pair in reaction

In the reaction, the base and conjugate acid pair is CN⁻ (aq) and HCN (aq).

Conjugate Acid

The conjugate acid in the reaction is CH₃COOH.

Conjugate Base

The conjugate base of NH₄⁺ is NH₃.

Strong Acid

The conjugate base of a strong acid is a weak base.

Acid-Base Strength Relationship

The stronger the acid, the weaker its conjugate base.

Amphiprotic

Amphiprotic is a term describing species that can act both as proton donors and acceptors.

Amphiprotic Species Example

Water is an example of an amphiprotic species.

Amphoteric Compound

An amphoteric compound has both basic and acidic character.

Amphiprotic vs Amphoteric

Not all amphoteric substances are amphiprotic.

Example of Amphoteric Substance

For example, aluminium oxide is amphoteric but not amphiprotic.

Al₂O₃ in Reaction

In the following equation, Al₂O₃ is acting as an acid.

HCO₃⁻

HCO₃⁻ is amphiprotic.

pH Definition

pH is the negative logarithm of the hydrogen ion concentration in mol/dm³.

pH Scale Measurement

The pH scale measures how acidic or basic a substance is.

pH Scale Type

The pH scale is logarithmic.

pH Range for Acidic Solutions

The pH range for acidic solutions is 0−6.

pH Range for Alkaline Solutions

The pH range for alkaline solutions is 8−14.

Accurate pH Measurement

The most accurate way of measuring pH is using a pH meter.

pH of Neutral Solution

The pH of a neutral solution at 298K is 7.

pH Change with [H⁺]

As [H⁺] increases, pH decreases.

Acidity Comparison

A solution with pH 4 is 10 times more acidic than a solution with pH 5.

pH Calculation Example

The pH of HCl with a concentration of 0.43 mol/dm³ is 0.37.

Equilibrium in Water

An equation for the equilibrium established in water is: H₂O (l) ⇌ H⁺ (aq) + OH⁻ (aq).

Ion Product of Water

The equation for the ion product of water is: K_w = [H⁺][OH⁻].

K_w Temperature Dependence

The value of K_w changes with temperature.

Temperature Effect on K_w

As temperature increases, K_w increases.

[H⁺] and [OH⁻] Relationship

In pure water at 298K, [H⁺] = [OH⁻].

temperature

How does increasing temperature affect K ? As temperature increases, K increases.

[H+] and [OH-] in pure water at 298 K

In pure water at 298K, [H+] = [OH-].

Acidic solutions

In acidic solutions, [H+] > [OH-].

Alkaline solutions

In alkaline solutions, [H+] < [OH-].

pH of water with increasing temperature

As temperature increases, the pH of water decreases.

Ionisation of water

The ionisation of water is endothermic.

pH of sodium hydroxide solution

The pH of a solution of sodium hydroxide, NaOH (aq) with a concentration of 1.0 × 10^-4 mol dm^-3 is 10.

[H+] calculation

Rearranging gives [H+] = Kw ÷ [OH-].

pH calculation

pH = - log[1.0 × 10^-10 (aq)] so the pH = 10.

[H2O(l)] in Kw expression

[H2O(l)] not included in the expression for Kw as the concentration of water molecules remains constant.

weak acid

A weak acid is an acid that partially (or incompletely) dissociates in aqueous solutions.

Weak acids as proton donors

Weak acids are proton donors and their solutions are poor conductors.

strong base

A strong base is a base that dissociates almost completely in aqueous solutions.

strong acids examples

HCl, HBr, HI, HNO3 and H2SO4 are all strong acids.

strong base examples

Examples of strong bases include Group 1 hydroxides (e.g. NaOH, KOH).

stronger acid comparison

HI is a stronger acid than HBr because the HI bond is a longer bond than HBr and is weaker.

conductivity of acids

Strong acids conduct electricity better than weak acids.

strong acid conductivity reason

A stronger acid has a higher concentration of H+ so it conducts electricity better.

pH comparison

CH3CH2COOH has a higher pH than HNO3.

strong acids reactivity

Strong acids react more vigorously than weak acids because the concentration of H+ is greater in strong acids compared to weak acids.

ionisation of ethanoic acid

The equation for the ionisation of ethanoic acid is: CH3COOH (aq) ⇌ CH3COO- (aq) + H+ (aq).

strong acid conductivity

A strong acid has a higher concentration of H+ ions so it conducts electricity better than a weak acid.

weak acids acidity

Weak acids are less acidic than strong acids as they contain fewer H+ in solution.

weak base example

An example of a weak base is NH3.

strong acids produce weak conjugate bases

In general, strong acids produce weak conjugate bases and weak acids produce strong conjugate bases.

ionic equation for base accepting hydrogen ions

The ionic equation to show how a base accepts hydrogen ions is: OH- (aq) + H+ (aq) ⇌ H2O (l).

HNO3 dissociation

HNO3 dissociates to form H+ and NO3- ions.

NO3- ion strength

The NO3- ion is a weak conjugate base.

HCN concentration

A solution of 10 mol dm^-3 HCN is a concentrated weak acid.

neutralisation

A neutralisation reaction.

Acid-Base Reaction

A reaction in which an acid and a base/alkali react together to form water and a salt.

Sulfate Salt

A sulfate salt is produced when sulfuric acid is reacted with an alkali.

Nitrate Salt

A nitrate salt is produced when nitric acid is reacted with an alkali.

Chloride Salt

A chloride salt is produced when hydrochloric acid is reacted with an alkali.

Ethanoic Acid

Ethanoic acid is used to make ethanoate salts.

General Equation for Acid and Metal Hydroxide Reaction

The general equation for the reaction between an acid and a metal hydroxide is: acid + metal hydroxide → salt + water.

Symbol Equation for Hydrochloric Acid and Potassium Hydrogencarbonate

HCl (aq) + KHCO₃ (s) → KCl (aq) + H₂O (l) + CO₂ (g).

Equation for Nitric Acid and Copper Carbonate Reaction

2HNO₃ (aq) + CuCO₃ (s) → Cu(NO₃)₂ (aq) + H₂O (l) + CO₂ (g).

Equation for Sulfuric Acid and Barium Hydroxide Reaction

H₂SO₄ (aq) + Ba(OH)₂ (aq) → BaSO₄ (s) + 2H₂O (l).

Ammonium Sulfate

Ammonium sulfate is formed when sulfuric acid reacts with ammonia.

Copper(II) Chloride Formation

Bases that could be used to form copper(II) chloride are copper(II) oxide, copper(II) hydroxide, copper(II) carbonate.

pH Meter

During a titration, a pH meter can be used and a pH curve plotted.

Equivalence Point

X is the equivalence or stoic point.

pH at Equivalence Point for HCl and NaOH Titration

The pH at the equivalence point of a titration between HCl and NaOH is 7.

Information from pH Curve

From a pH curve, you can determine: the initial pH of the acid, the pH at the equivalence point, the volume of base at the equivalence point, the range of pH at the vertical section of the curve.

pH at Start of Titration

The pH at the start of the titration is pH 3.

pH at Equivalence Point

The pH at equivalence point is pH 8.

Volume of NaOH Added to Reach Equivalence Point

The volume of NaOH (aq) that was added to reach the equivalence point is 30 cm³.

Equivalence Point in Strong Acid and Strong Base Titration

The equivalence point for a strong acid and strong base titration is pH 7.

Equivalence Point for Weak Acid and Strong Base Titration

The equivalence point for a weak acid and strong base titration is above 7.

Equivalence Point for Strong Acid and Weak Base Titration

The equivalence point for a strong acid and weak base titration is below 7.

Half Equivalence Point

The half equivalence point is the stage of the titration at which exactly half the amount of weak acid has been neutralised.

Buffer Region

The region that is labelled X is the buffer region.

Relationship Between Half Equivalence Point and pH

pKₐ = pH at half equivalence point and pKₐ = pOH at half equivalence point.

Concentration at Half Equivalence Point

[NH₄⁺ (aq)] = [NH₃ (aq)] at the half equivalence point during a titration involving NH₄⁺ (aq) and HCl (aq).

Weak Acid and Weak Base pH Curve

A weak acid and weak base pH curve is shown.

Weak Base and Strong Acid pH Curve

A weak base and strong acid pH curve is shown.

pH at Y-Intercept

The pH at the y-intercept is pH 11.

pH

pH = -log[H+] where [H+] is the concentration of hydrogen ions.

Hydrogen ion concentration

[H+] = 2.80 x 10^-3 mol dm^-3 at 298 K.