5 Salts and electrolytes Flashcards
What are alkalis?
soluble hydroxides which are alkaline solutions
produce OH- ions when we add them to water
e.g.sodium hydroxide solution
What are bases?
include alkalis, substances which can neutralise acids
metal oxides and metal hydroxides are bases
e.g.iron oxide and copper hydroxide
What pH is pure/distilled water?
neutral
pH 7
What are acids?
substances which produce H+ ions when we add them to water
e.g. citric (in citrus fruit and fizzy drinks) acid, sulfuric acid, ethanoic (in vinegar) acid
How do we know from the pH scale what is an alkali and what is an acid?
less than 7 = acid
7 = neutral
more than 7 = alkali
What is the Bronsted-Lowry theory of acids and alkalis?
Acids are normally called ‘something’ acid
Alkalis are normally called ‘something’ oxide
What are indicators?
substances which change colour when we add them to acids and alkalis
e.g. litmus paper
What are examples of products on the pH scale?
0
1 hydrochloric acid very acidic
2 citric acid (lemon juice)
3 orange juice and vinegar
4
5 black coffee slightly acidic
6 rainwater
7 pure water neutral
8 seawater and baking soda
9 milk of magnesia and soap slightly alkaline
10
11
12 washing soda
13 very alkaline
14 oven cleaner and sodium hydroxide
Explain how neutralisation takes place between acids and alkalis in terms of ions….
Acids dissolve in water to form hydrogen ions (H+)
Soluble bases (alkalis) dissolve in water to form hydroxide ions (OH-)
These ions then combine to form water
If there are the same number of these ions each then the solution will become neutral
What does an acid and a metal react together to make?
a salt + hydrogen
if the metal is very reactive e.g. sodium or potassium, the reaction with acid is too violent to carry out safely
What does an acid and an insoluble base react together to make?
a salt + water
What does the salt formed from a reaction depend on?
the metal or base that we use as well as the acid
so bases containing sodium ions will a;ways make sodium salts etc.
In terms of the acid used, what type of salt will be formed from a reaction with hydrochloric, sulfuric, nitric and ethanoic acid?
hydrochloric = chlorides sulfuric = sulfates nitric = nitrates ethanoic = ethanoates
What happens when ammonia reacts with water?
forms a weakly alkaline solution
NH3 (aq) + H2O (l) –> NH4+ (aq) + OH- (aq)
What happens when ammonia solution reacts with an acid (e.g.nitric acid)?
acid + ammonia solution = an ammonium salt + water
HNO3 (aq) + NH4+ (aq) + OH- (aq) –> NH4NO3 (aq) + H2O (l)
What is the reaction called when we react an acid with an alkali to make a salt and water?
a neutralisation reaction
Why do we need to use an indicator?
to tell us when the reaction is complete and that the acid and alkali have completely reacted
How do we make insoluble salts?
by reacting two soluble salts together
What is the name given to the reaction making an insoluble salt called and why is this?
a precipitation reaction
because the salt is formed as a solid
Why is the a precipitation reaction important?
when we want to remove metal ions from polluted water (the salt formed is a solid making it easier to extract)
In a precipitation reaction, what does each reactant contain?
one of the ions of the insoluble salt
e.g. LEAD nitrate solution + potassium IODIDE solution –> LEAD IODIDE precipitate + potassium nitrate solution
(Pb2+ + 2I- –> PbI2) key ions separated from spectator ions
What is electrolysis?
the splitting up of a compound/breaking down of a substance using electricity
What must happen for electrolysis to work?
- there must be ions to have a charge to attract to an electrode(not covalent molecules)
- the ions must be able to move (to the electrode)
- the electrodes must be inert (unreactive)
Why is it hard to predict what will form at the electrodes when we electrolyse a solution?
water can also form ions
- the less reactive element, between hydrogen (H+) ions and the metal will usually be produced at the cathode
- at the anode, we often get oxygen gas given off from discharged hydroxide ions
What happens during electrolysis?
positively charged ions move to the negative electrode (cathode) and at the same time the negative ions move to the positive electrode (anode)
What is oxidisation?
a loss of electrons
at the anode the ions give away electrons
impure copper dissolves
e.g. Cu(s) –> Cu2+(aq) + 2e-
What is reduction?
a gain of electrons
at the cathode the ions pick up electrons
pure copper is plated
e.g. Cu2+(aq) +2e- –> Cu(s)
What are the uses of electrolysis?
- to make pure metal
- to split up elements e.g. aluminium oxide, lead bromide
What is important about the electrolysis of sodium chloride solution, brine?
Produces 3 very useful products: Sodium chloride(aq) -->(electrolysis) hydrogen(g) + chlorine(g) + sodium hydroxide(aq)
What are the half equations in the electrolysis of brine?
at the cathode:
2H+ + 2e- –> H2 (hydrogen ions are less reactive than sodium ions)
at the anode:
2Cl- –> Cl2 + 2e-
What are the uses of hydrogen?
- pure so is useful in the food industry to manufacture margarine by reacting hydrogen with vegetable oils
- can also be used to make hydrochloric acid…
What are the uses of chlorine?
- used in water and sewage treatment
- used in the manufacture of: PVC, textiles, solvents, titanium dioxide, pesticides
- used in drugs to treat leukaemia, heart disease and arthritis
- can also be used to make hydrochloric acid and bleach…
What are the uses of sodium hydroxide?
- used for purifying petroleum and natural gas
- used for degreasing metals
- used in water and sewage treatment
- used in the manufacture of: paper, cellulose, wrapping, artificial fibres, soaps, detergents, aluminium and titanium dioxide
- can also be used to make bleach…
What are the uses of hydrochloric acid?
- used in cleaning materials
- used for water and efficient treatment
- used in the manufacture of: medicine, food products (e.g. soy sauce, gelatin), cosmetics, synthetic rubber, precious metals (e.g. gold)
What are the uses of bleach?
- used for treating water in swimming pools
- used for household bleach
What are the 3 useful products made from the electrolysis of brine?
- chlorine) used to make bleach which kills bacteria and to make plastics
- hydrogen) used to make margarine
- sodium hydroxide) used to make bleach, paper and soap
What are the uses of aluminium?
- pans
- overhead power cables
- aeroplanes
- cooking foil
- drink cans
- window and patio door frames
- bicycle frames and car bodies
What makes aluminium useful?
- low density
- very strong
- oxidised layer prevents it from rusting so does not corrode
- shiny so it reflects heat
How is aluminium extracted?
electrolysis
What is bauxite ore?
ore mined by open cast mining containing mainly aluminium oxide with a few other rocky impurities such as iron oxide
How is aluminium produced?
- aluminium oxide is dissolved in molten cryolite in order to use less energy to melt it (from 2000 degrees C to 850 degrees C)
- during electrolysis, the aluminium metal is collected at the negative electrode in the cells whilst oxygen is formed at the positive electrode
During the electrolysis of aluminium oxide, why do the carbon anodes need regular replacement?
because they burn away as they react with oxygen to form carbon dioxide
What is electroplating?
when an object is coated with a thin layer of metal by electrolysis (an electroplated object)
Why do we electroplate objects?
- to protect the metal beneath from corroding
- to make the object look more attractive
- to increase the hardness of the surface and its resistance to scratching
- to save money by using a thin layer of precious metal instead of the pure expensive metal (also helps people who are allergic to metals such as nickel used in cheap jewellery)
How does electroplating work?
- the positive electrode (anode) is made from the plating metal (they are oxidised and lose electron(s) to form ions which go into the solution)
- the metal to be plated is used as the negative electrode (cathode) (the plating ions from the solution are reduced, gaining electron(s) to form atoms deposited on the cathode)
- the electrolysis takes place in a solution (electrolyte) containing ions of the plating metal
