Acids and Bases s2

GRADE 11 SCIENCE ACIDS AND BASES

RECAP

Arrhenius’ theory (First theory of acids and bases)

ACID: substance which produces hydrated oxonium (or hydrogen) ions in aqueous solution.

HCl + H2O ® H3O+ + Cl- (ionisation)

ALKALIS: substance which produces hydrated hydroxide ions in aqueous solution.

NaOH(s) ® Na+(aq) + OH-(aq) (dissociation)

- Strong acid or strong base: highly (fully) ionised in solution

e.g. HCl, H2SO4, HNO3

H2SO4 + 2H2O ® 2H3O+ + SO42-

all soluble metal hydroxides e.g. NaOH, KOH, Ca(OH)2

Ca(OH)2 ® Ca2+(aq) + 2OH-(aq)

- Weak acid or weak base: slightly ionised in solution

e.g. all organic acids e.g. ethanoic acid CH3COOH

CH3COOH(aq) + H2O(l) ⇌ CH3COO-(aq) + H3O+(aq)

base : ammonia NH3

NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq)

BUT Arrhenius doesn’t explain how acids/bases still react when not in aqueous solution

Bronsted – Lowry Theory (40 yrs after Arrhenius)

ACID IS A PROTON (H+) DONOR

BASE IS A PROTON (H+) ACCEPTOR

HCl + NH3 ® NH4+ + Cl-

acid base

Conjugate acid – base pairs

The above reaction is reversible so: NH4+ + Cl- ® HCl + NH3

acid base

Every base has a conjugate acid and every acid a conjugate base.

HCl + NH3 ⇄ NH4+ + Cl-

acid base acid base

Note: the conjugate acid always has an extra H+ compared to its conjugate base.

e.g. the conjugate acid of OH- is H2O

the conjugate base of OH- is O2-

Ampholytes

Some substances can act as acids or base and these are known as ampholytes or ampholytic.

Water is a common example – it can donate or gain a proton

HCl + H2O à H3O+ + Cl-

base

Other commonly asked examples:

HCO3- HSO4-

a) Identify the conjugate acid-base pairs in the following reversible reactions:

HSO4- + OH- ⇄ SO42- + H2O

CH3COOH + H2O ⇄ CH3COO- + H3O+

b) The conjugate acid of H2O is ______

c) The conjugate base of H2PO4- is ______

d) Why can CO2 not be an acid? ______

e) What is the conjugate acid of SO42-? ______

Reactions

1.  ACID + REACTIVE METAL ® SALT + HYDROGEN

(Zn, Fe, Mg, Ca)

2HCl + Mg ® MgCl2 + H2

2.  ACID + BASE ® SALT + WATER

(note: a base is a metal hydroxide or metal oxide)

H2SO4 + 2NaOH ® Na2SO4 + 2H2O

3.  ACID + METAL CARBONATES ® SALT + WATER+ CARBON DIOXIDE (also for hydrogen carbonates)

2HNO3 + CaCO3 ® Ca(NO3)2 + H2O + CO2

Note: - the salt of hydrochloric acid is a chloride

-  the salt of nitric acid is a nitrate

-  the salt of sulphuric acid is a sulphate

-  the salt of phosphoric acid is a phosphate

Write the balanced chemical equation for the reaction between the following:

a)  aluminium and hydrochloric acid

b)  copper oxide and nitric acid

c)  sodium carbonate and sulphuric acid

d)  magnesium hydroxide and phosphoric acid

e)  sodium hydroxide and ethanoic acid

f)  potassium hydrogen carbonate and sulphuric acid

Neutralisation

The reaction between an acid and a base to form salt.

If the base is a solution of a hydroxide the actual reaction that happens is:

H+ + OH- ® H2O or H3O+ + OH- ® 2H2O

If the base is a carbonate or hydrogen carbonate then the actual reaction is:

CO32- + 2H3O+ ® CO2 + 3H2O

HCO3- + H3O+ ® CO2 + 2H2O

Preparation of salts

Salts are ionic substances consisting of anions and cations – produced by reacting acids and bases.

SALT = CATION + ANION

From Base From Acid

Write Equations for the preparation of the following:

a)  sodium sulphate from an oxide

b)  magnesium chloride from a hydroxide

c)  zinc phosphate from a metal

d)  ammonium nitrate

Ionization constant of an acid, Ka

HCl(aq) + H2O(l) ⇄ H3O+(aq) + Cl-(aq)

Ka = [H3O+].[ Cl-]

[HCl]

The Ka value indicates the strength of an acid (i.e. degree of ionisation)

e.g. Ka for CH3COOH is 1,8 x 10-5 at 25oC (low value therefore weak acid)

Ionization constant of a base, Kb

NH3(aq) + H2O(l) ⇄ NH4+(aq) + OH-(aq)

Kb = [NH4+].[ OH-] = 1,8 x 10-5 ( NH3 weak base)

[NH3]

Ionic Product of a water, Kw

Water undergoes autoprotolysis. This can be written as either

H2O ⇄ H+ + OH- or H2O(l) + H2O(l) ⇄ H3O+(aq) + OH-(aq)

In any aqueous solution at 25 oC: Kw = 10-14 = [H+].[ OH-]

In a neutral solution [H+] = [ OH-] = 10-7 mol.dm-3

This means that ALL aqueous solutions have both H+ ions and OH- ions in them.

If [H+] > [ OH-] then the solution is acidic.

If [H3O+]<[ OH-] then the solution is alkaline.

If [H3O+] = [ OH-] then the solution is neutral.

a)  What is the concentration of hydroxide ions in an aqueous solution of a salt where the concentration of hydrogen ions is 3 x 10-4 mol.dm-3 at 25 oC?

b)  What is the concentration of hydrogen ions in a 0,02 mol.dm-3 solution of calcium hydroxide (Ca(OH)2) at 25oC ? Ca(OH)2 à Ca2+ + 2OH-

c)  If a solution of sulphuric acid contains OH- ions with a concentration 2,4 x 10-14 mol.dm-3, what was the concentration of the acid. [Hint: remember sulphuric acid is a diprotic acid].

pH

This indicates the acidity or alkalinity of a solution.

It is a logarithmic measure of the concentration of hydrogen (oxonium) ions.

pH = -log[H+]

pH 7 is acidic, >7 alkaline, 7 neutral (at 25oC)

e.g What is the pH of a 0,01 mol.dm-3 of HNO3?

HNO3 à H+ + NO3-

pH = -log[H+] = -log(0,01) = 2

Note: we only work out pH for fully ionised acids and alkalis i.e. strong ones

pH of acids

pH = -log[H+] c = n/V n = m/M

a)  What is the pH of a solution of 0,004 mol.dm-3 hydrochloric acid?

b)  What is the concentration of a H2SO4 solution with pH 0,22? [Hint: diprotic acid]

c)  What is the pH of a solution of nitric acid if 16,4 g of HNO3 is mixed with water to make 400cm3 of solution?

d)  If all the following solutions are the same concentration, place in order of pH. Lowest pH first:

ethanoic acid; sodium hydroxide; ammonium hydroxide; hydrochloric acid; sulphuric acid.

pH of alkalis

When working out the pH of alkalis one has to us the ionic product of water at 25oC.

Kw = [H+].[OH-] = 10-14 or pH + pOH = 14

Eg What is the pH of a 2,0 x 10-3 mol.dm-3 solution of NaOH

[OH-] = 2 x 10-3 mol.dm-3

pOH = - log[OH-] = -log(2 x 10-3) = 2,70

pH = 14 – pOH = 14 – 2,70 = 11,30

a)  What is the pH of a solution if 2,2 g of Ca(OH)2 is dissolved in water to make 500 cm3 of solution?

b)  What is the concentration of a solution of KOH with a pH of 8,7?

c)  What mass of sodium hydroxide is needed to make 250 cm3 of solution with a pH of 12,5?

pH of salts in solution (Hydrolysis)

a)  Salts derived from a strong acid and a strong base give neutral solutions.

b)  When a salt derived from a strong base and a weak acid dissolves in water, the solution becomes alkali. This occurs as a result of the hydrolysis of the negative ions to form hydroxide ions.

e.g. Na2CO3: CO32- + H2O ⇄ HCO3- + OH-

c)  When a salt derived from a strong acid and a weak base dissolves in water, the solution becomes acidic. This occurs as a result of the hydrolysis of the positive ions to form hydronium ions.

e.g. NH4Cl: NH4+ + H2O ⇄ NH3 + H3O+

d)  Salts derived from a weak base and a weak acid may be acidic or basic. To determine whether a salt will give an acidic or basic solution the Ka and Kb values of the weak acid and weak base have to be compared. If Ka > Kb it will give an acidic solution

Assign the following salts as acidic, basic or neutral:

a)  calcium chloride

b)  ammonium nitrate

c)  potassium sulphate

d)  sodium ethanoate

e)  ammonium ethanoate

pH indicators

An indicator is a dye which changes colour with a certain change in the pH of its surroundings.

Some indicators are natural e.g Beetroot – red in acid – purple in alkali

Commonly used laboratory indicators – see titrations

Universal indicator is a mixture of dyes which changes colour as the pH changes.

Titrations

A titration is used to determine the concentration of a solution using a standard solution.

Definition: A standard solution is one where the concentration is known accurately and remains constant for a long time.

A standard solution is made in a volumetric flask, usually by dissolving the required mass of solid.

Selecting an Indicator

Different indicators change colour over different ranges. One chooses ones indicator according to the type of titration (strong acid/ weak base etc).
You must learn the colours and types of titration for the 3 indicators below:

End Point

The end-point of an acid-base titration results when the number of moles of acid added to base, or visa-versa, is in the same ratio as that in the chemical equation. This does not necessarily mean that the solution is neutral. It will be in a strong acid/strong base titration but not in the other two examples in the table.

At the end-point a drop of acid or alkali will cause a large swing in pH and this causes the colour of the solution to change (see titration curves).

e.g. 25,0 cm3 of a standard solution of 0,05 mol.dm-3 calcium hydroxide was placed in a conical flask. Hydrochloric acid was run into this and the end-point was reached after 14,8 cm3 was added. Calculate the concentration of the acid.

2HCl + Ca(OH)2 à CaCl2 + 2H2O

Exercise

2,4 g of NaOH was weighed out and dissolved in some water. This was then made up to 100 cm3 of volume with further water. 25,0 cm3 of this NaOH solution was then titrated against oxalic acid and it took 13,8 cm3 of the acid solution to effect the correct colour change in the indicator.

HOOCCOOH + 2NaOH à (OOCCOO)Na2 + 2H2O

a) What would be a good indicator for this titration and what colour would it be in alkali?

b) Was the acid in the burette?

C) Calculate the concentration of the acid solution.

INDUSTRIAL APPLICATIONS OF ACIDS & BASES#[1]

The chemistry of hair and hair products

Hair is primarily made up of a protein calledkeratin. Proteins give hair a natural acidic pH of between 4 and 5.

The following hair products alter this pH and do controlled damageto get the desired effect.

Permanent waving application

Another name for a permanent waving application is aperm.

There are four common steps in the perming process:

1.  The basic ammonia swells the hair and makes it permeable.

2.  A reducing agent reduces the proteinmolecules in your hair which breaks bonds and allows the hair structure to be changed. This reaction occurs faster at higher temperatures, and the hair is generally heated at this time (e.g., on a hot curling iron).

3.  The reducing solution is washed out of the hair after the desired bond breaking has occurred.

4.  The hair is treated with an oxidising agent (hydrogen peroxide H2O2), which acts as a neutraliser. The protiens are then oxidised and the broken bonds are reformed in new positions which fix the hair in this new structure.

Hair relaxers

Hair relaxers work by doingcontrolled damageto your hair in much the same way as perms do. They also reduce the protiens and allow the hair structure to be changed.

There are two main types of hair relaxers:lyeandno-lye. Thelyehair relaxers contain sodium hydroxide (NaOH) and have a very high pH (in the range 12 - 14). These types of hair relaxers break bonds and strip the hair of its natural oils. Often heating is also used and this damages the hair even more.