ACIDS, ALKALIS AND SALTS

REVISION BY JAMES EZARD 10GP

WHAT IS AN ACID, BASE AND ALKALI

• Acid – A substance with a pH of less than 7. Acids form H+ ions in water.

• For example, hydrochloric acid (HCl) produces H+ ions and Cl- ions in water.

• Base – A substance with a pH of greater than 7.

• Alkali – A base that dissolves in water (a soluble base). Alkalis form OH- ions in water.

• For example, potassium hydroxide (KOH) produces K+ ions and OH- ions in water.

• So H+ ions make solutions acidic and OH- ions make them alkaline.

THE PH SCALE• On the pH scale, 7 is neutral, <7 is acidic and >7 is alkaline.

EXAMPLES OF SUBSTANCES ON THE PH SCALE

BASES

• Bases will neutralise acids.

• All metal oxides and metal hydroxides are bases.

• A metal hydroxide is compound of a metal with the hydroxide ion OH−

• Example of a metal oxide = copper oxide = CuO

• Example of a metal hydroxide = sodium hydroxide = NaOH

STATE SYMBOLS

• State symbols tell you what physical state it’s in

• (s) – Solid

• (l) – Liquid

• (g) – Gas

• (aq) – Dissolved in water (aqueous)

NEUTRALISATION REACTIONS

• When we react an acid and a base, a neutralisation reaction occurs.• Here is the equation for a neutralisation reaction = Acid + Base = Salt + Water.

• This gives us a neutral solution of pH 7.

• The water is formed when H+ ions from the acid react with OH- ions from the alkali (base).

• Here is the equation for this = H+(aq) + OH-(aq) = H₂O(l)

• The salt is formed from the other ions in the acid and base.

• For example, Hydrochloric acid + Copper Oxide = Copper Chloride + Water

• The symbol equation for this reaction; 2HCl(aq) + CuO(aq) = CuCl₂(aq) + H₂O(l)

MORE EXAMPLES OF NEUTRALISATION REACTIONS

• Sulfuric Acid + Zinc Oxide = Zinc Sulfate + Water

• H₂SO₄(aq) + ZnO(s) = ZnSO₄(aq) + H₂O(l)

• Nitric Acid + Potassium Hydroxide = Potassium Nitrate + Water

• HNO₃(aq) + KOH(aq) = KNO₃(aq) +H₂O(l)

WHAT ARE “H+” AND “OH-” IONS

• H+ ions are made of one atom of hydrogen which has lost 1 electron so it is positively charged.

• These are made by acids in water.

• OH- ions are made of one molecule of hydrogen and water covalently bonded together, which have gained 1 electron and therefore are negatively charged.

• These are made by alkalis in water.

INDICATORS

• An indicator is just a dye that changes colour depending on whether the solution is above or below a certain pH.

• Universal Indicator (UI) is a combination of dyes which gives the colours shown below.

• UI is very useful for estimating the pH of a solution (pH 7, green, is neutral).

SOLUBLE AND INSOLUBLE SALTSSoluble Insoluble

All nitrates No insoluble nitrates

Most sulfates Lead sulfate, barium sulfate

Most chlorides Silver chloride, lead chloride

Most bromides Silver bromide, lead bromide

Most iodides Silver iodide, lead iodide

Sodium carbonate, potassium carbonate Most other carbonates

Sodium hydroxide, potassium hydroxide Most other hydroxides

MAKING SOLUBLE SALTS USING AN ALKALI• Salt solutions can be crystallised to produce solid salts.

• Here is the method for producing pure solid crystals of sodium chloride from sodium hydroxide and hydrochloric acid;

• Add 10cm³ of Sodium Hydroxide to a small beaker (this is the alkali)

• Add 2 drops of Universal Indicator (so we know what the pH of the solution is)

• Slowly add drops of hydrochloric acid until neutral (the solution will turn green due to the presence of the UI)

• Add activated carbon with a spatula (in excess) and stir with a glass rod until the solution is colourless (this removes the UI so pure crystals of sodium chloride can be formed)

• Filter out the carbon

• Evaporate ½ with a methane burner and leave to crystallise on its own (to gain pure crystals)

• This produces crystals of the soluble salt sodium chloride (note that water is also produced in this reaction)

• Word equation; Sodium Hydroxide + Hydrochloric Acid = Sodium Chloride + Water

• Symbol equation; NaOH(aq) + HCl(aq) = NaCl(aq) + H₂O(l)

MAKING SOLUBLE SALTS USING A METAL OR INSOLUBLE BASE• Here is the method which is very similar to the previous method;

• Pick the right acid, plus a metal or insoluble base to produce to produce the soluble salt you want.

• For example, mix hydrochloric acid and copper oxide to make copper chloride.

• Mix the acid and metal together.

• You know when all the acid has been used up as the metal or insoluble base will sink to bottom of the flask.

• Filter out the excess metal, metal oxide or metal hydroxide to get the salt solution.

• Crystallise like before using evaporation to gain pure crystals of the soluble salt.

MAKING INSOLUBLE SALTS – PRECIPITATION REACTIONS

• If the salt you want is insoluble, you can use a precipitation reaction;

• Pick two solutions that contain the ions you need.

• For example, to make lead chloride mix lead nitrate solution and sodium chloride solution.

• Once the salt has precipitated out (and is lying at the bottom of the flask), all you need to do is filter it from the solution, wash it and then dry it on filter paper.

• This method gives you a pure precipitate of your insoluble salt (e.g. lead chloride).

USES OF PRECIPITATION REACTIONS

• To remove poisonous ions (e.g. lead) from drinking water.

• To remove unwanted ions in sewage.

• To remove calcium and magnesium ions from water as they make it hard.

AMMONIUM AS A FERTILISER

• Ammonia dissolves in water (is soluble) wo make an alkaline solution.

• Then, to get a neutral salt, ammonium nitrate, you can react it with nitric acid.

• This is a neutralisation reaction although no water is produced, just the salt.• Ammonia + Nitric Acid = Ammonium nitrate

• NH₃(aq) + HNO₃(aq) = NH₄NO₃(aq)

• Ammonium nitrate is especially good as a fertiliser for two reasons –

• It is very soluble in water.

• It contains a high proportion of nitrogen (from two sources) which plants need to make proteins.

METALS REACT WITH ACIDS TO GIVE SALTS

• Here is the main equation;

• Metal + Acid = Salt + Hydrogen (MASH)

• And here are the main key points of the experiment;• The more reactive the metal, the faster the reaction will go – very reactive metals react explosively.

• Copper does not react with dilute acids at all as it is less reactive than hydrogen.

• The speed of the reaction (rate of reaction) is determined by how quickly bubbles are given off (hydrogen).

• The hydrogen’s presence is confirmed by the squeaky pop test (using a burning splint).

• The name of the salt produces depends on the metal and acid used.

EXAMPLES OF METALS WITH ACIDS

• Hydrochloric acid will always produce chloride salts;• For example, Hydrochloric Acid + Magnesium = Magnesium Chloride + Hydrogen

• 2HCl + Mg = MgCl₂ + H₂

• Sulfuric acid will always produce sulfate salts;

• For example, Sulfuric Acid + Magnesium = Magnesium Sulfate + Hydrogen

• H₂SO₄ + Mg = MgSO₄ + H₂

• Nitric Acid produces Nitrate salts when neutralised but it can produce nitrogen oxides instead.

END OF ACIDS, ALKALIS AND SALTS REVISION