Topic 4- Chemical Changes

Question 1

What is oxidation/reduction?

Answer 1

Oxidation is when a substance gains oxygen

Reduction is when a substance loses oxygen

Question 2

What is the reactivity series

Answer 2

Potassium
Sodium
Lithium 
Calcium
Magnesium
Carbon
Zinc 
Iron
Hydrogen
Copper

Question 3

What is a displacement reaction?

Answer 3

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

Redox reaction
Ion is reduced
Atom is oxidised

Question 4

How can metals less reactive than carbon be extracted?

Answer 4

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

Question 5

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

Answer 5

Oxidation - loss of electrons

Reduction - gain of electrons

Question 6

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

Answer 6

Metal+ acid –> salt + hydrogen

Redox reaction, also a displacement reaction

Question 7

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

Answer 7

metal carbonate + acid –> salt+water+carbon dioxide

Question 8

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

Answer 8

Metal oxide + acid –> a salt + water

Question 9

How is a soluble salt formed?

Answer 9

Add the chosen solid insoluble substance to the acid then the solid will dissolve

You know the acid has been neutralised when excess solid sinks to the bottom, so keep adding until this happens

Filter out excess solid leaving the salt solution, the evaporate some water, then leave the rest to evaporate slowly

Question 10

What do acids and alkalis produce in aqueous solutions?

Answer 10

Acids produce hydrogen ions (H+), alkalis produce hydroxide ions (OH-)

Question 11

What are bases, acids and alkalis?

Answer 11

Bases are compounds that neutralise acids

Acids produce hydrogen ions in aqueous solutions, alkalis are soluble bases meaning they produce hydroxide ions in aqueous solutions

Question 12

What is a strong acid and weak acid?

Answer 12

Strong acid is completely ionised in aqueous solution

Weak acid is only partially ionised in aqueous solution

Question 13

What happens to the pH as concentration of H+ increases?

Answer 13

The pH decreases

Question 14

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

Answer 14

Concentrated acid has more moles of acid per unit volume than dilute

It is not the same as concentration is not the same as strength of an acid

Strength referred to whether the acid is completely ionised in water (strong) or only partially (weak)

Question 15

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

Answer 15

Increases by a factor of 10

Question 16

What is electrolysis?

Answer 16

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

Question 17

What is an electrolyte?

Answer 17

The liquid/solution which conducts electricity

Question 18

Describe the process of electrolysis

Answer 18

When an ionic substance is melted or dissolved, the ions are free to move about within the liquid or solution

Passing a current through substances that are molten or solution means that the solution can be broken down into elements.

This is electrolysis and the substance being broken down is the electrolyte

During electrolysis, positively charged ions move to the negative electrode
(cathode)

Negatively charged ions move to the positive electrode (anode)

Ions are discharged at the electrodes, producing elements

Question 19

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

Answer 19

Reduction occurs at the cathode

Oxidation occurs at the anode

Question 20

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

Answer 20

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

Question 21

How is aluminium manufactured and why is it expensive?

Answer 21

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

Question 22

Why is cryolite used in manufacturing of aluminium?

Answer 22

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

Question 23

In electrolysis, why does the positive electrode (anode) need to be continually replaced?

Answer 23

Oxygen is formed which reacts with the carbon of the positive electrodes, forming carbon dioxide, and they gradually burn away

Question 24

Why is oxygen formed at the anode?

Answer 24

It is because in the aqueous solution, water molecules break down, producing H+ ions and OH- ions that are discharged

Question 25

Why is hydrogen produced at the negative electrode (cathode)?

Answer 25

As long as the metal is less reactive than hydrogen, hydrogen will be produced and this is because more reactive ions want to stay within the solution