C2.5

What are acids and alkalis?

Substances that produce H+ ions when they are added to water with pH values of less than 7. Alkalis are bases that dissolve in water to = alkaline solutions. Produce hydroxide ions OH- (aq) in solution with pH values greater than 7

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What are aqueous solutions, bases and indicators?

Aqueous solutions: substance dissolved in water. Bases: react with acids + neutralise them. Indicators have different colours in acidic + alkaline solutions – Universal indicators.

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What metals will acids react with?

Those above hydrogen in the reactivity series but when very reactive metals (i.e. Na, K etc.) are involved, the reactions are too violent to be done safely. When metals react with acids they produce a salt + hydrogen gas H2SO4 + Zn -> ZnSO4 + H2

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What happens when metals reacts with bases?

Metal oxides/hydroxides = bases. Acid + base -> neutralisation takes place-> salt + water produced. (2HCL + MgO --> MgCl2 + H2O). Insoluble metal/base added a little at a time until all acid is reacted, mixture filtered leaving salt solution.

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How can we make a soluble salt?

By reacting acid and an alkali. (i.e. HCl(aq) + NaOH(aq)--> NaCl(aq) + H2O(l)). Neutralisation reaction: H+(aq) + OH-(aq) --> H2O(l)) No visible change when acids react with akalis --> need to use pH meter/indicator. Obtain salt by crystallisation.

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How can we make insoluble salts?

By mixing solutions of soluble salts that contain ions needed i.e. we can make lead iodide by mixing solutions of lead nitrate + potassium iodide - forms a precipitate that can be filtered from the solution, washed with distilled water + dried.

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How can some pollutants i.e. metals ions be removed from water?

By precipitation- water treated by adding substance that react with the pollutant metal ions dissolved in the water to form insoluble salts.

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What is electrolysis?

Process that uses electricity to break down ionic compounds into elements -when electricity is passed through a molten ionic compound/solution containing ions. Substance broken down called electrolyte.

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What is reduction?

Gaining electrons (i.e. when positive ions reach neg electrode they gain to become neutral atoms). Ions with 1+ charge gain 1 electron, 2+ charge gain 2 electrons.

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What is oxidation?

Losing electrons (i.e. at positive electrode, neg ions lose electrons to become neutral atoms). Some non-metal atoms combine to for molecules i.e. Br.

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How can we represent half equations for changes at electrodes i.e. lead bromide?

At neg electrode: Pb 2+ + 2e- --> Pb. At pos electrode 2Br- --> Br2 + 2e-.

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Water contains O and H ions. What happens when solutions of ions in water are electrolysed?

H produced at neg electrode if other pos ions in the solution are those of a metal more reactive than H. O2 usually produced from aqueous solutions at pos electrode, but if solution contains high concentration of halide ion then a halogen will be pro

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Why must aluminium be extracted from its core by electrolysis?

More reactive than carbon. Ore contains Al oxide-must be purified, then melted to be electrolysed. Al2O3 melts at over 2000oC – requires a lot of energy. Mix with ionic compound (cryolite)-->melts at 850oC. Electrolysed at lower temp, produces Al+O2

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What is the overall reaction in an electrolysis cell (extraction of aluminium)?

Aluminium Oxide --> Aluminium + Oxygen. 2Al2O3(l) --> 4Al(l) + 3O2(g). The cryolite remains in the cell + fresh aluminium oxide is added as aluminium and oxygen are produced.

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Why do the positive electrodes used in the cell (for the extraction of aluminium) need to be replaced regularly?

As they’re made of carbon. At high temperatures of the cell the O2 reacts with the Carbon electrodes to produce CO2. The carbon electrodes gradually burn away.

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What happens during electrolysis of aluminium oxide (using cryolite)?

At neg electrode, Al ions are reduced to Al atoms by gaining electrons. The molten aluminium metal is collected from the bottom of the cell. At pos electrode, oxide ions are oxidised to O atoms by losing electrons + the O atoms form O2 molecules.

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What are the half equations for electrolysis) extraction of aluminium?

At neg electrode: Al3+¬(l) + 3e- --> Al(l). At pos electrode: 2O2-(l) --> 2O2(g) + 4e-v

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What is brine?

A solution of sodium chloride in water. The solution contains sodium ions Na+(aq), Chloride ions Cl-(aq), hydrogen ions H+(aq) and hydroxide ions, OH-(aq).

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What is produced when brine is electrolysed?

Hydrogen is produced at the neg electrode from the hydrogen ions (2H+ + 2e- --> H2). Chlorine is produced at the pos electrode from the Chloride ions (2Cl--->Cl2 + 2e-) – leaving a solution of sodium ions + hydroxide ions, NaOH(aq).

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Why is the electrolysis of brine an important industrial process?

Sodium hydroxide = strong alkali (making soap, paper, bleach, neutralising acids + controlling pH). Cl2 used to kill bacteria in drinking water + swimming pools + to make bleach, disinfectants+plastics. H2 used to make margarine + HCl.

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What is electroplating and what are the half equations for nickel electroplating?

Electroplating uses electrolysis to put a thin coating of metal onto an object, Gold, silver + chromium plating are often used. At positive nickel electrode: Ni(s)-->Ni2+(aq) + 2e-. At negative nickel electrode to be plated: Ni2+(aq) + 2e--->Ni(s).

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Name 4 reasons why electroplating is used?

To: make the object look more attractive, protect the metal object from corroding, increase the hardness of a surface + reduce costs by using a thin layer of metal instead of the pure metal.

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