7. Acids, Bases and Salts Flashcards
Acids - Properties
- Contain H⁺ ions (protons)
- Soluble in water
- Sour to the taste
- Corrosive
Bases - Properties
- Metal oxides/carbonates/hydroxides
- Some bases are soluble in water (these are called alkalis)
- Soapy to the touch
- Corrosive
Alkalis
A soluble base. (also accept protons from acids)
Acids - What are they
An acid is a substance that gives away (donates) hydrogen (cat)ions (H⁺).
This only happens in a solution with water. H⁺ ions are known as protons.
Acids are therefore called proton donors.
(The donating of protons is what makes a solution acidic –> before being added to water compounds aren’t acidic)
Bases - What are they
A substance that IS ABLE TO recieve /accept hydrogen ions (H⁺) (also known as protons)
Bases contain OH⁻ ions (hydroxides)/ O²⁻ ions (oxides)/ CO3²⁻ (carbonates)
By accepting a H⁺ ion from an acidic solution, the base is neutralised.
Before accepting the protons, the base is a base –> and is corrosive. Afterwards it is neutralised and is thus no longer corrosive.
Indicator
A chemical substance that changes colour depending if it is in the presence of acid or base.
–> Used to identify if a substance is acidic or basic.
Litmus paper –> Use as an indicator
Colour in acid - Red
Colour in neutral solution - Original colour of the litmus paper –> blue/red
Colour in base - Blue
(Litmus paper can either be originally blue or red)
Methyl orange - Use as an indicator
Colour in acid - Red
Colour in neutral substance - Orange
Colour in base - Orange
Thymolphtalein - Use as an indicator
Colour in acid - Colourless
Colour in neutral substance - Colourless
Colour in base - Blue
Universal indicator - Use as an indicator
Strong acid - Red
Weak acid - Orange/yellow
Strong base - Purple
Weak base - Blue
pH scale
A measure of the concentration of hydrogen ions present (used to determine if a substance is acidic or alkaline (basic)
0-6 - Acidic
7 - Neutral
8-14 - Alkaline
(The further out towards the edges, the stronger the acid/alkaline is)
Reaction of (hydrochloric) acid with metal
- Can occur with any metal above hydrogen in the reactivity series.
Acid + metal –> salt + hydrogen
Magnesium + hydrochloric acid –> Magnesium chloride + hydrogen
Reaction of (hydrochloric) acid with a base (Copper (II) oxide)
acid + metal oxide (base) –> salt + water
Copper (II) oxide + hydrochloric acid –> copper chloride + water
(Neutralisation reaction bc any metal oxide is basic, protons in HCl are donated to CuO anion)
Reaction of (hydrochloric) acid with a base (Sodium hydroxide)
acid + base –> salt + water
Sodium hydroxide + hydrochloric acid –> sodium chloride + water
(Neutralisation reaction bc HCl donates protons to OH ions)
Reaction of (hydrochloric) acid with a base (Sodium carbonate)
acid + metal carbonate –> salt + carbon dioxide + water
Sodium carbonate + hydrochloric acid –> sodium chloride + carbon dioxide + water
(Neutralisation reaction bc HCl donates protons to anion base)
Reaction of (hydrochloric) acid with base (ammonia solution)
ammonia solution + hydrochloric acid –> ammonium chloride
Reaction of a (sulphuric) acid with metal
Acid + metal –> salt + hydrogen
Magnesium + sulfuric acid –> magnesium sulfate + hydrogen
Reaction of a (sulfuric) acid with a metal oxide (Copper (II) oxide)
acid + metal oxide (base) –> salt + water
Copper (II) oxide + sulfuric acid –> Copper sulfate + water
(Neutralisation reaction bc metal oxides are basic –> protons in H2SO4 / sulfuric acid are donated to anions in CuO)
Reaction of a (sulphuric) acid with a metal hydroxide
acid + base –> salt + water
Sodium hydroxide + sulfuric acid –> sodium sulfate + water
(Neutralisation reaction –> protons in H2SO4 / sulfuric acid are donated to hydroxide anion)
Reaction of a (sulphuric) acid with metal carbonate
acid + metal carbonate –> salt + carbon dioxide + water
Sodium carbonate + sulfuric acid –> sodium sulfate + carbon dioxide + water
(Neutralisation reaction bc metal carbonates are basic –> protons in H2SO4 / sulfuric acid are donated to anions in Na2CO3)
Reaction of ammonia solution (base) with an acid (sulphuric)
Acid + ammonia –> salt
Ammonia solution + sulfuric acid –> ammonium sulfate
(Neutralisation reaction bc ammonia solution is basic and sulfuric acid is acidic)
NO WATER PRODUCED
How to neutralise an acid
React an acid with a base neautralises it.
eg.
Ammonia + hydrogen chloride –> ammonium chloride
Acid rain (how farmers neutralise the soil)
Acid rain causes many soils to be too acidic for crops to grow.
Farmers must neutralise the soil using lime (calcium oxide - a metal oxide) in a process called liming. The products of this are salt and water, which has a neutral pH.
Amphoteric substances
Can ACT as both an acid and a base by BEING ABLE TO react with both acid and bases.
Conjugate base
In Brønsted–Lowry theory, a conjugate base is formed by an acid donating a proton.
For example, the chlorine ion from hydrochloric acid.
Conjugate acid
In Brønsted–Lowry theory, a conjugate acid is formed by a base accepting a proton. For example, the ammonium ion from ammonia.
Strong acids
Fully ionised or dissociated in aqueous solution.
For example, hydrochloric acid. The more H⁺ ions and Cl⁻ ions dissociated –> the stronger the acid.
Weak acids
Partially ionised or dissociated in aqueous solution.
Different pHs of oxides
Metal reacting with oxygen –> metal oxide (basic)
Eg. 2Mg(s) + O2(g) → MgO(s)
Non-metal reacting with oxygen –> acidic covalent oxides (acidic)
Eg. S(g) + O2(g) → SO2(g)
Change in metal oxide pH while going across a period.
As you go across a period there is a change in the behaviour of the oxide from basic to acidic.
What type of compound do metals combusting in oxygen form?
Basic ionic oxides
Reaction of basic oxides with an acid
Basic oxide (metal oxide) + acids –> Salt + Water
Acid + metal
Salt + hydrogen
Acid + alkali
Salt + water
Acid + metal oxide
Salt + water
Weak vs Strong acid, example
Strong acid - Hydrochloric acid
Weak acid - Ethanoic acid
Acid + metal carbonate
Salt + Water + Carbon Dioxide
How an alkaline solution is formed using basic oxides.
Group 1 and Group 2 metals react with water to produce an alkaline metal hydroxide.
Adding water to sodium oxide –> creates sodium hydroxide (which is an alkaline solution)
How an acidic solution is formed using an acidic oxide
A non-metal oxide (acidic oxide) eg. sulfur dioxide, reacts with water to form sulfuric acid (an acidic solution).
Amphoteric oxides
- Some oxides show properties of both acidic and basic oxides –> amphoteric.
–> Can act as both acidic or basic.
–> Amphoteric oxides can react with either acids or bases to form salt and water
eg.
Aluminium oxide + Hydrochloric acid –> Aluminium chloride + Water
or
Aluminium oxide + Sodium hydroxide –> Sodium aluminate + Water
(Zinc oxide is also amphoteric)
Which salts are ALL soluble and have no insoluble salts?
- Nitrates
- Sodium salts
- Potassium salts
- Ammonium salts
Which chlorides are insoluble?
- Lead chloride
- Silver chloride
Which sulfates are insoluble?
- Barium sulfate
- Calcium sulfate
- Lead sulfate
Which hydroxides are soluble?
- Sodium hydroxide
- Potassium hydoxide
- Ammonium hydroxide
- Calcium hydroxide (partially soluble)
Which carbonates are soluble?
- Sodium carbonate
- Potassium carbonate
- Ammonium carbonate
Which salts have a few insolubles and most common salts are soluble?
- Chlorides
- Sulfates
Which salts have a few solubles and most common salts are insoluble?
- Hydroxides
- Carbonates
What is a salt?
Salts are ionic compounds made up of cations and anions.
Cation - Metal ion
Anion - Non-metal ion
What type of reaction is a neutralisation reaction?
Exothermic
3 ways to prepare salts
acid + metal → salt + hydrogen
acid + metal oxide (insoluble base) → salt + water
acid + metal carbonate → salt + carbon dioxide + water
acid + alkali –> salt + water
How salts are prepared
1- Warm some acid gently (to speed up the reaction)
2. Add to the acid an excess metal/ metal oxide/ metal carbonate. (adding excess ensures all the acid is neutralised)
3. When reacting metals or metal carbonates, leave aside the solution to let all the effervescence escape.
4. Filter the mixture to remove the excess solid. (metal/ metal oxide/ metal carbonate that hasn’t reacted)
5. Transfer to an evaporating dish and evaporate some of the water (not needed in the acid + metal reaction) off over a Bunsen burner until crystals start to form.
6. Leave to cool.
7. Filter to get the crystals.
8. Dry the crystals on filter paper.
Hydrated salts
In some salts, water molecules can get inside the crystal structure + chemically bond to the metal ion (cation).
When this occurs we say taht this form of the salt is a hydrated salt.
Anhydrous salt
When the salt crystals exist without water molecules bonded into the crystal structure, they are called anhydrous salts.
Water of crystallisation
The water molecules chemically combind within hydrated salt crystals, are call water of crystallisation
Water of crystallisation formula
Salt·Water
Eg. CuSO⁴·5H²O
Preparing salts through a neatralisation reaction
Using a titration, add alkali to an acid.
Add thymolphthalien to the conical flask containing acid.
When the thymolphalein turns blue, the solution has been neutralised (just a bit more alkali).
Record the volumes used + repeat.
- Then crystallise through evaporation + cooling + filtering + drying crystals.
What are soluble salts.
When adding soluble salts to a solution, the ions will dissociate from each other.
How are insoluble salts formed?
2 soluble salts in solution –> ions dissociate.
–> Electrostatic forces of attraction between two of the ions in solution is greater than electrostatic force keeping them apart
–> ions join together to form insoluble salt.
–> Creates a precipitate
(The other ions which don’t form the insoluble salt, react to form a soluble salt …. but immediately dissolve bc its soluble.)
What is needed for an insoluble salt to form?
2 soluble salts (one contatins the cation and one contains the anion of the insoluble salt being prepared)
eg.
Barium Nitrate + Sodium Sulfate –> Barium Sulfate + Sodium Nitrate
Ba²⁺ (aq) + SO4²⁻ (aq) → BaSO4 (s)
