C8 - Acids, Bases and Salts Flashcards
Properties of Acids (7)
- pH values of below 7
- have a sour taste (when edible)
- are corrosive
- are proton donors
- substances that can neutralise a base, forming a salt and water
- when added to water, they form positively charged hydrogen ions (H+)
- presence of H+ ions is what makes a solution acidic
Properties of bases (5)
- Have pH values of above 7
- A base which is water-soluble is referred to as an alkali
- Bases are substances which can neutralise an acid, forming a salt and water
- Bases are usually oxides or hydroxides of metals
- When alkalis are added to water, they form negative hydroxide ions (OH–)
The reactivity series
What does an acid metal reaction form?
Acid + Metal –› Salt + Hydrogen
What does the name of the salt depend on in an acid metal reaction?
The anion within the acid
What are the three main strong acids and what salt do they form?
- Hydrochloric acid (HCl) - chloride salt
- Nitric acid (HNO3) - nitrate salt
- Sulphuric acid (H2SO4) - sulphate salt
How is hydorchloric acid formed?
HCl (aq) → H+ (aq) + Cl- (aq)
How is nitric acid formed?
HNO3 (aq) → H+ (aq) (NO3)-
How is sulhuric acid formed?
H2SO4 (aq) → 2H+ (aq) + (SO4)-2
How partial dissociation shown in the reaction of weak acids and bases?
With a double layered arrow pointing in either direction
Neutralisation reaction
Acid + Base → Salt + Water
Acid and metal carbonate reaction
Acid + Metal Carbonate → Salt + Carbon Dioxide + Water
Blue litmus paper
- red: acidic
- blue: basic
Red litmus paper
- red: acidic
- blue: basic
Universal indicator
- red: acidic
- green: neutral
- blue/purple: basic
Methyl orange
- yellow: basic
- pink: acidic
Penolphalein
- pink: basic
- colourless: acidic
Tymolphtalein
- blue: basic
- colourless: acidic
How do ammonium salts react with alkalis?
- Ammonium salts undergo decomposition when warmed with an alkali
- Even though ammonia is itself a weak base, it is very volatile and can easily be displaced from the salt by another alkali
- A salt, water and ammonia are produced
Volatile
a substance easily evaporated at normal temperatures
What ions are produced when acids are added to water?
positively charged hydrogen ions (H+)
What ions are produced when alkalis are added to water?
negative hydroxide ions (OH–)
What happens during a neutralisation reaction?
- A neutralisation reaction occurs when an acid reacts with an alkali
- When these substances react together in a neutralisation reaction, the H+ ions react with the OH– ions to produce water
pH scale indication
Acids
Acids are proton donors as they ionise in solution producing protons, which are H+ ions (these make the aqueous solution acidic)
Bases
Bases are proton acceptors as they accept the protons which are donated by the acid
Strong acids
Completely dissociate (or ionise) in water, producing solutions of a very low pH
* include HCl and H2SO4
Weak acids
Partially dissociate (or ionise) in water and produce pH values which are closer to the middle of the pH scale, whilst still being below 7
* include organic acids such as ethanoic acid, CH3COOH
What does it mean to have a concentrated solution of acid?
A concentrated solution of an acid is one that contains a higher number of acid molecules per dm3 of solution
* not necessarily strong acid
Oxides
Compounds made from one or more atoms of oxygen combined with one other element
Acidic oxides
Acidic oxides are formed when a non-metal element combines with oxygen
They react with bases to form a salt and water
When dissolved in water they produce an acidic solution with a low pH
* Common examples include CO2, SO2, NO2 and SiO2
Basic oxides
Basic oxides are formed when a metal element combines with oxygen
They react with acids to form a salt and water
When dissolved in water they produce a basic solution with a high pH
* Common examples include CuO and CaO
Neutral oxides
Do not react with either acids or bases
* Examples include N2O, NO and CO
Amphoteric oxides
Amphoteric oxides can behave as both acidic and basic, depending on whether the other reactant is an acid or a base
In both cases salt and water are formed
* Two of the most common amphoteric oxides are zinc oxide, ZnO and aluminum oxide, Al2O3
Salt
A salt is a compound that is formed when the hydrogen atom in an acid is replaced by a metal
How are salts used in real life?
fertilisers, batteries, cleaning products, healthcare products and fungicides
General method of preparing soluble salts by adding acid to a solid metal, insoluble base or insoluble carbonate
- Add dilute acid into a beaker and heat using a bunsen burner flame
- Add the insoluble metal, base or carbonate, a little at a time, to the warm dilute acid and stir until the base is in excess (i.e. until the base stops disappearing and a suspension of the base forms in the acid)
- Filter the mixture into an evaporating basin to remove the excess base
- Heat the solution to evaporate water and to make the solution saturated. Check the solution is saturated by dipping a cold, glass rod into the solution and seeing if crystals form on the end
- Leave the filtrate in a warm place to dry and crystallize
- Decant excess solution and allow crystals to dry or blot to dry with filter paper
preparation of pure, hydrated copper(II) sulfate crystals by adding acid to a solid metal, insoluble base or insoluble carbonate
- Add dilute sulfuric acid into a beaker and heat using a bunsen burner flame
- Add copper(II) oxide (insoluble base), a little at a time to the warm dilute sulfuric acid and stir until the copper (II) oxide is in excess (stops disappearing)
- Filter the mixture into an evaporating basin to remove the excess copper(II) oxide
- Leave the filtrate in a warm place to dry and crystallize
- Decant excess solution
- Blot crystals dry with filter paper
Equation for sulphuric acid and copper (II) oxide reaction
CuO (s) + H2SO4 (aq) → CuSO4 (aq) + H2O (l)
copper(II) oxide + sulfuric acid → copper(II) sulphate + water
Preparing salts by reacting a dilute acid and alkali (soluble base) (7)
- Use a pipette to measure the alkali into a conical flask and add a few drops of indicator (thymolphthalein or methyl orange)
- Add the acid into the burette and note the starting volume
- Add the acid very slowly from the burette to the conical flask until the indicator changes to the appropriate colour
- Note and record the final volume of acid in the burette and calculate the volume of acid added (final volume of acid - initial volume of acid)
- Add this same volume of acid into the same volume of alkali without the indicator
- Heat the resulting solution in an evaporating basin to partially evaporate, leaving a saturated solution (crystals just forming on the sides of the basin or on a glass rod dipped in and then removed)
- Leave to crystallise, decant excess solution and allow crystals to dry
Percipitation reaction
A reaction in which a solid product is formed after two solutions are mixed together
When can a percipitation reaction take place?
Insoluble salts can be prepared using a precipitation reaction
* The solid salt obtained is the precipitate, thus in order to successfully use this method the solid salt being formed must be insoluble in water, and the reactants must be soluble
General method for preparing insoluble salts
- Dissolve soluble salts in water and mix together using a stirring rod in a beaker
- Filter to remove precipitate from mixture
- Wash filtrate with distilled water to remove traces of other solutions
- Leave in an oven to dry
Preparation of pure, dry lead(II) sulfate crystals using a precipitation reaction
- Dissolve lead(II) nitrate and potassium sulfate in water and mix together using a stirring rod in a beaker
- Filter to remove precipitate from mixture
- Wash precipitate with distilled water to remove traces of potassium nitrate solution
- Leave in an oven to dry
Equation of reaction between lead (II) nitrate and potassium sulphate
lead(II) nitrate + potassium sulfate → lead(II) sulfate + potassium nitrate
Pb(NO3)2 (aq) + K2SO4 (aq) → PbSO4 (s) + 2KNO3 (aq)
Hydrated salts
Hydrated means when there is water within a substance. Hydrated salts contain water of crystallisation which affects the molecular shape and colour.
Anhydrous salts
A substance is anhydrous when it contains absolutely no water.
Water of crystallisation
Water molecules included in the structure of some salts during the crystallisation process are known as water of crystallisation
* A compound that contains water of crystallisation is called a hydrated compound
