Acids and Bases

• Properties

• pH values

• Indicators

• Reactions - bases, metal oxides, carbonates, metals

(Only the reactions in die textbook have to be memorised.)

Properties

Acids feel smooth and taste sour.

Acids all contain hydrogen (H-).

Formulas of examples: HCℓ , H₂SO₄ , HNO₃ , CH₃COOH (vinegar)

Bases feel soapy and taste bitter.

Bases mostly contain hydroxide (-OH).

Formulas of examples: NaOH, KOH, Ca(OH)₂, NH₄OH

(NH₃ dissolved in H₂O)

pH scale

The pH scale actually indicates the quantity of hydrogen in a solution.

A low pH value (0 to 7) means an acidic solution.

A pH value equal to 7 means a neutral solution.

A high pH value (7 to 14) means a basic solution.

Indicators

In grade 9 we mostly use bromothymol blue, which …

is BLUE in a BASE and

is YELLOW in an ACID.

In a NEUTRAL solution it is GREEN.

A mixture of different indicators forms a universal indicator, which can have many more colours.

How do acids react with bases?

Acids neutralize bases and bases neutralize acids.

In practice a good example is: Caustic soda can be used to neutralize any excess acid in a swimming-pool and to increase the pH.

swimming-pool acid + caustic soda → salt + water

HCℓ + NaOH → NaCℓ + H₂O

How do acids react with bases?

In practice another example is: Vinegar (acetic acid) can be used to remove the accumulated lime deposits in a kettle.

acetic acid + calcium hydroxide (lime) → salt + water

2CH₃COOH + Ca(OH)₂ → Ca(CH₃COO)₂ + 2H₂O

The salt that is formed, is soluble and can easily be rinsed out.

General reaction

acid + base → salt + water

Example:

HCℓ + NaOH → NaCℓ + H₂O

↓ ↓

NaCℓ

The H- from the acid combines with the -OH from the base and forms water.

Practical: Pour acid into a flask, with a few drops of indicator → yellow solution;

add base until the colour changes → turns green → neutral solution.

If too much base is added → blue solution.

How do acids react with metal oxides?

Acids are also neutralized by metal oxides.

In practice a good example is: Burnt lime is used in farming to increase the pH of acidic soil, formed due to acid rain and rotting compost.

acid rain + burnt lime → salt + water

H₂SO₃ + CaO → CaSO₃ + H₂O

Because the metal oxide neutralizes the acid like a base does, we call it a basic oxide.

How do acids react with metal oxides?

In practice another example is: Oxalic acid in cleaning agents can be used to remove rust stains and to clean dull zinc plates.

oxalic acid + iron oxide (rust) → salt + water

(COOH)₂ + Fe₂O₃ → (new iron salt) + H₂O

Lemon juice (containing citric acid) or diluted white vinegar (acetic acid) is also recommended for removing rust stains on smooth concrete.

General reaction

acid + metal oxide → salt + water

Example:

2HCℓ + CuO → CuCℓ₂ + H₂O

↓ ↓

CuCℓ₂

Copper salts and their solutions usually have a blue colour.

The black copper oxide can therefore be used to form a blue copper solution.

How do acids react with carbonates?

Acids neutralize carbonates and release carbon dioxide (gas bubbles).

In practice a good example is: Bicarbonate of soda is used to remove the crystals formed around the terminals of a motor car battery.

battery acid + bicarbonate of soda → salt + water + carbon dioxide

H₂SO₄ + 2NaHCO₃ → Na₂SO₄ + 2H₂O + 2CO₂(g)

How do acids react with carbonates?

In practice another example is: Antacids can be used to alleviate heartburn. (One is very aware of the fact that gas is formed.)

gastric acid + bicarbonate of soda → salt + water + carbon dioxide

HCℓ + NaHCO₃ → NaCℓ + H₂O + CO₂(g)

Previously Mg(OH)₂ was also commonly used to treat heartburn. This so-called “milk of magnesia” is then a base which neutralizes the gastric acid.

How do acids react with carbonates?

In practice another example is: Learners usually use vinegar and bicarbonate of soda to represent the eruption of a volcano in a project.

acetic acid + bicarbonate of soda → salt + water + carbon dioxide

CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂(g)

The released gas forms the foam that symbolizes the eruption. Food colouring in the vinegar makes it even more spectacular.

General reaction

acid + carbonate → salt + water + carbon dioxide

Example:

2HCℓ + CaCO₃ → CaCℓ₂ + H₂O + CO₂(g)

↓ ↓

CaCℓ₂

Carbonates (-CO₃) usually release CO₂(g) during chemical reactions. The gas turns clear lime water milky.

The reaction is completed when the fizzing (release of gas) stops.

Tasty application at home

Make your own “sherbet”:

Sift and mix 450 g icing sugar, 30 g tartaric acid and 30 g bicarbonate of soda.

When the powder lands on your tongue, the moisture allows the acid (tartaric acid) and carbonate (bicarbonate of soda) to dissolve and react. Fortunately the icing sugar is enough to disguise any strange tastes. The effervescence on your tongue is only due to the carbon dioxide that is released.

The chemical reaction (inside your mouth!) is:

C₄H₆O₆ + NaHCO₃ → (new sodium salt) + H₂O + CO₂(g)

Baking powder consists of a similar mixture, and the release of carbon dioxide allows the dough to rise. (Lactic acid causes muscle stiffness after exercise, and can be neutralized by a similar chemical reaction.)

How do acids react with metals?

Most acids react with most metals and release hydrogen.

In practice a good example is: Children sometimes make rockets from 2ℓ cold drink bottles in which they allow zinc to react with hydrochloric acid.

hydrochloric acid + zinc → salt + hydrogen

2HCℓ + Zn → ZnCℓ₂ + H₂(g)

Because the hydrogen has a low density, a container filled with hydrogen gas will always tend to move upwards. The more gas captured in the container, the faster it will move!

How do acids react with metals?

In practice another example is: When the sulphuric acid in a motor car battery is replenished, great care has to be taken not to spill the acid on any iron or steel components or tools.

sulphuric acid + iron → salt + hydrogen

H₂SO₄ + Fe → FeSO₄ + H₂(g)

The acid can cause huge damage, added to the danger of the released hydrogen gas being very explosive.

How do acids react with metals?

Only as a point of interest! In practice there are a few metals that only react

with certain acids and then do not release hydrogen, but other gases.

If a mercury thermometer were to fall and break, nitric acid can be used to remove the poisonous drops of mercury from between the floor tiles. Soluble mercury nitrate forms, which can be washed away more easily than mercury balls can be picked up. Thus: nitric acid (HNO₃) + mercury (Hg) → Hg(NO₃)₂ + (another gas)

Electronic circuits were previously made by using acid to etch a path (circuit) from

the laminated copper foil on a non-conductor (“printed circuit boards” or PCB),

before the components were soldered to the remaining copper path (circuit).

Thus: “etching acid” + copper (Cu) → copper salt + (another gas)

General reaction

acid + metal → salt + hydrogen

Example:

2HCℓ + Ca → CaCℓ₂ + H₂(g)

↓ ↓

CaCℓ₂

The reaction between an acid and a metal usually releases H₂(g). The gas ignites with a popping sound.

The reaction is completed when the fizzing (release of gas) stops.

Specific reactants form specific salts

All the above-mentioned reactions with acids form salts.

Specific reactants are needed to form specific salts.

Remember:

Hydrochloric acid (HCℓ) … always forms chloride salts.

Sulphuric acid (H₂SO₄) …… always forms sulphate salts.

Nitric acid (HNO₃) ……….... always forms nitrate salts.

Summary of general reactions

Look closely at the similarities and differences; it will help with studies.

(1) acid + base → salt + water

(2) acid + metal hydroxide → salt + water

(3) acid + metal oxide → salt + water

(4) acid + metal carbonate → salt + water + CO₂

(5) acid + metal → salt + H₂