Topic 4 - Chemical Changes

Question 1

What is the pH scale?

Answer 1

A measure of how alkaline or acidic a solution is.

Question 2

How can you measure the pH of a solution?

Answer 2

Use an indicator. A dye that changes colour depending on the pH. Eg universal indicator.

Use a pH probe which is attached to a pH meter and measures the pH manually.

Question 3

Whats an acid?

Answer 3

A substance that forms aqueous solutions with pH less than 7. They form H+ ions in water.

Question 4

Whats an alkali?

Answer 4

A base that dissolves in water to form a solution with pH more than 7. Creates OH- ions in water,

Question 5

What is a base?

Answer 5

A substance with pH greater than 7.

Question 6

Whats the reaction for neutralisation?

Answer 6

Acid + base —-> salt + water

H+(aq) + OH-(aq) —-> H20(l)

Question 7

Whats the features of strong acids?

Answer 7

Ionise completely in water. (All ions dissociate to release H+ ions).
Faster rate of reaction since high concentration H+ ions to react with other substances.

Question 8

Whats the features of weak acids?

Answer 8

Dont ionise fully in water.

The ionisation of a weak acid is a reversible reaction.

Question 9

How much more does H+ ion concentration increase per 1pH that you go down?

Answer 9

x 10
ph 4 has 10 times the concentration of H+ than pH 5.

pH 3 has 100 times the concentration of H+ ions than pH 5

Question 10

Whats the difference between a strong acid and concentrated acid?

Answer 10

Acid strength tells you what proportion of the acid molecules ionis in water.

Concentration measures how much acid there is in a certain volume of water. (How watered down your acid is)

Question 11

What is formed when a metal hydroxide or metal oxide reacts with acid?

Answer 11

Salt and water

Question 12

Whats formed when a metal carbonate reacts with water?

Answer 12

Salt
Water
Carbon dioxide

Question 13

What does a metals reactivity depend on?

Answer 13

How easily they lose electrons.

Question 14

List the metal reactivity series from most reactive to least.

Answer 14

Potassium
Sodium
Lithium
Calcium
Magnesium
Carbon
Zinc
Iron
Hydrogen
Copper

Question 15

What does a metal reaction with an acid tell you about the metal reactivity?

Answer 15

There will be a faster reaction the more reactive it is.

So vigorous reaction and lots of bubbles produced

Also the greater the temperature change, the more reactive

Question 16

Whats formed when a metal reacts with water?

Answer 16

Metal hydroxide
Hydrogen

Note: less reactive metals like zinc iron and copper dont react with water.

Question 17

Whats the reaction called that separates a metal from its oxide?

Answer 17

Reduction reaction

Question 18

What is oxidation?

Answer 18

Metal reacts with oxygen and forms an oxide

Question 19

Whats reduction?

Answer 19

When oxygen is lost

Question 20

How can you extract metals from their ores with carbon? Can all metals be extracted by reduction from carbon?

Answer 20

The ore is reduced as oxygen is removed from it. The carbon gains the oxygen removed from the ore.

The metal has to be lower down the reactivity series than carbon to be able to be reduced

Question 21

Why are some metals mined it its element form already?

Answer 21

Theyre just so unreactive. Eg gold

Question 22

Whats a redox reaction?

Answer 22

Oxidation and reduction but in terms of electrons being transferred

Question 23

Is oxidation loss or gain of electrons? And reduction?

Answer 23

Oxidation is loss of electrons.

Reduction is gain of electrons

Question 24

Whats a displacement reaction

Answer 24

When a more reactive metal displaces a less reactive one from its compound.

Remember its always the atom that loses electrons and is oxidised and the ion gains the electrons to be reduced

Question 25

What happens during electrolysis?

Answer 25

During electrolysis, an electric current is passed through an electrolyte. Ions move towards the electrodes, where they react, and the compound decomposes.

Question 26

Which ions move to which electrode during electrolysis?

Answer 26

Negative ions move towards the anode. (+ve electrode). They lose electrons (oxidation)

Positive ions move towards the cathode. (-ve electrode). They gain electrons (reduction)

Question 27

Why cant an ionic solid be electrolysed? Why can molten ionic solids electrolyse?

Answer 27

The ions are in fixed positions and cant move in the solid.

When molten, it can be electrolyses as the ions can move round freely and conduct electricity.

Question 28

In electrolysis in aqueous solutions, what is formed at the anode and cathode, different to normal electrolysis?

Answer 28

At the cathode, hydrogen gas will be produced if the metal ions form a metal element more reactive than hydrogen (eg sodium ions). If less reactive then a solid layer of the pure metal will be produced.

At the anode, if halide ions are present (CL-, BR-, I-) molecules of chlorine, bromine, or iodine will form. If no halide ions present then oxygen is formed.

Question 29

What is in aqueous solutions in electrolysis alongside ions from the ionic compound?

Answer 29

H+ and OH- ions.

The H ions are found at the cathode.
The OH ions are found at the anode.