Topic 4: Chemical Change

What is oxidation/reduction?

Oxidation - When a substance gains oxygen Reduction - When a substance loses oxygen

What is the reactivity series of metals? What are the trends in reactivities of metals in reactions with acids/water?

The series shows the metals in order of their reactivity. Metals above H2 in reactivity series react with acid to produce H2 . The more reactive the metal is, the quicker and more violent reaction with acid occurs. Metals below H2 don't react with acids. Not all metals above H2 react with water - mostly Group I and II metals. Aluminium is the borderline case.

What is a displacement reaction?

A reaction where a more reactive metal displaces a less reactive metal from a compound

How are unreactive metals found in Earth?

In their natural state (well, they are unreactive...)

How can metals less reactive than carbon be extracted?

Reduction with carbon. Carbon displaces the metal in a metal oxide - gets oxidised to carbon oxides. Metal from the metal oxide gets reduced to the pure metal.

How are metals more reactive than carbon extracted?

By electrolysis

How are oxidation and reduction defined in terms of electron transfer?

Oxidation - loss of electrons Reduction - gain of electrons

What is the general equation for a reaction between metals and acids? What type of reaction is this?

Metal + acid → salt + hydrogen Redox reaction, also a displacement reaction

Which metals in the reactivity series will react with acid?

Those above hydrogen

What is the general equation for a neutralisation reaction?

Base + acid → salt + water

What is the general equation for the reaction between metal carbonate and acid?

Metal carbonate + acid → salt + water + carbon dioxide

What is the general equation for the reaction between metal oxides and acids?

Metal oxide + acid → a salt + water

What is a redox reaction?

A reaction where both oxidation and reduction occurs

Explain in terms of gain or loss of electrons which species has been oxidised and which species has been reduced when magnesium reacts with hydrochloric acid

Magnesium has lost electrons and thus has been oxidised (Mg to Mg2+) The hydrogen in HCl has gained electrons and thus has been reduced (H+ to H2 )

How is a soluble salt formed?

a) React the excess acid with some insoluble chemical (e.g. metal oxide) b) Filter off the leftovers c) Crystallise the product

What do acids and alkalis produce in aqueous solutions?

Acids produce hydrogen ions, alkalis produce hydroxide ions

What are bases, acids and alkalis?

Bases are compounds that neutralise acids, acids produce hydrogen ions in aqueous solutions, alkalis are soluble bases - produce hydroxide ions in aqueous solutions

What is the pH scale and what does a pH of 7 show?

The measure of acidity/alkalinity of a solution; neutral solution

State the general equation for a neutralisation reaction in a short, ionic form.

H+ + OH− → H2O

What is a strong acid and weak acid?

Strong acid is completely ionised in aqueous solution; weak acid is only partially ionised in aqueous solution

What happens to pH as concentration of H+ increases?

The pH decreases

What is a concentrated acid and what is a diluted acid? Is this the same as a strong and weak acid?

- Concentrated acid has more moles of acid per unit volume than dilute (dilute refers to solutions of low concentrations) - It is not the same - concentration is not the same thing as strength of an acid. - Strength refers to whether the acid is completely ionised in water (strong) or only partially (weak).

As the pH is decreased by one unit, what change is seen in the hydrogen ion concentration?

Increases by a factor of 10

Name the following salts: LiNO3 , K2CO3 , MgBr2 , BaSO4

Lithium nitrate Potassium carbonate Magnesium bromide Barium sulfate

What is electrolysis?

The passing of an electric current through ionic substances that are molten or in solution to break them down into elements; ions are discharged (they lose/gain electrons) at electrodes to produce these

What is an electrolyte?

The liquid/solution which conducts electricity

What is a cathode and what is an anode?

Cathode is the negative electrode, anode is the positive electrode

What occurs at the cathode and what occurs at the anode during electrolysis?

Reduction occurs at the cathode Oxidation occurs at the anode

In aqueous electrolysis, which element is discharged at the cathode? Oxygen is produced at the anode unless what?

The less reactive element discharges at the cathode. Hydrogen is produced unless there is a less reactive metal, in which case the said metal is produced. Oxygen is produced at the anode unless the solution contains halide ions, in which case halogen molecules are produced.

How is aluminium manufactured? Why is it expensive?

Aluminium is made through the electrolysis of aluminium oxide and cryolite. Lots of energy is needed to produce the current in electrolysis which makes this process expensive.

What are the half equations in the extraction of aluminium?

Al3+ + 3 e− → Al (cathode) 2 O2− → O2 + 4 e− (anode) Oxygen reacts with C of the anode producing CO2 .

Why is cryolite used in manufacturing of aluminium?

It lowers the melting point of aluminium oxide, reducing energy costs

What are the half equations in electrolysis of the aqueous Na2SO4 ?

2 H+ + 2 e− → H2 (cathode) 4 OH− → 2 H2O + O2 + 4 e− (anode)

What are the half equations in electrolysis of the molten and aqueous KCl?

K + + e− → K (cathode) 2 Cl− → Cl2 + 2 e− (anode) 2 H+ + 2 e− → H2 (cathode) 2 O2− → O2 + 4 e− (anode), respectively

What are the half equations in electrolysis of the aqueous CuBr2 ?

Cu2+ + 2 e− → Cu (cathode) 2 Br− → Br2 + 2 e− (anode)