Chemical changes

Question 1

Titration definition

Answer 1

It is a method or procedure in which acids react with alkalis to determine unknown volumes or unknown concentrations of alkali or acid.

Question 2

neutralisation definition

Answer 2

Neutralisation is a chemical reaction in which an acid reacts chemically with an alkali to produce a salt and water.

Question 3

neutralisation ionic equation

Answer 3

H+ (aq) + OH- (aq) –> H20

Question 4

equipment for titration practical

Answer 4

conical flask, white tile, indicator, biurette

Question 5

Most biurettes at school have a capacity of?

Answer 5

50 milliletres

Question 6

What is the purpose of a white tile in a titration practical?

Answer 6

To provide contrast in colour to know when the acid is neutralised by alkali or vice versa.

Question 7

Give differences between a burette and pipette

Answer 7

burette: It is a high resolution instrument that can be used to measure fixed/ varying volumes of a liquid (controlled by a tap/ stop cock). It has a resolution of 0.1 cm3/ ml. The zero value is at the top.

Pipette: It is measuring instrument that could measure a fixed volume of a liquid depending on it’s capacity.

Question 8

Q: Describe an investigation to determine the concentration of an acid using titration.

Answer 8

1, Transfer a volume of acid into a burette and bring its level to the zero mark (avoid parallax error by measuring at eye level).
2, Using a pipette, transfer a fixed volume of alkali into a conical flask. Place the conical flask on a white tile to see the colour contrast shown by an indicator.
3, Add 2/3 drops of a suitable indicator once. Note the characteristic colour. You are now ready to perform titration.
4, Open the tap of the burette and allow an acid to chemically react with the alkali. Close the tap once the colour changes and record the volume in a results table.
5, Repeat step 4 for at least three more times and calculate the average.
6, Only use concordant values/ results (results that are PRECISE). These are results that differ from each other by 0.1 cm3/ 0.1 ml.
Endpoint: When the indicator changes colour to give a compromise colour. The acid/ alkali will have fully reacted with the alkali/ acid. There is no excess acid or alkali. (eg: thymol blue will show a turquoise colour).
7. Using a calculator and a balanced equation, the unknown concentration can be determined.

Question 9

Concentrated acid (2)

Answer 9

A concentrated acid is an acid that has a high ratio of moles of a substance to volume of water ( dm3).

Question 10

strong acid (2)

Answer 10

A strong acid is a chemical substance that dissociates/ ionises completely (100%) in aqueous solution to produce a high concentration of hydrogen ions in aqueous solution.

Question 11

oxidAtion

Answer 11

It is a process during which oxygen chemically reacts to a compound or electrons are lost.

Question 12

REduction

Answer 12

It a process during which oxygen is removed. It is the process during which electrons are gained.

Question 13

Why are the electrodes made of graphite?

Answer 13

• unreactive
• conduct electricity
• high mpb

Question 14

Electrolysis

Answer 14

The decomposition/splitting of an ionic (aqueous) solution using electricity to make chemicals/ electrical energy to convert to chemical energy.

Question 15

anode

Answer 15

The electrode/rod at which oxidation always takes place (OXAN)

Question 16

Cathode

Answer 16

The electrode/rod at which reduction always takes place.

Question 17

ore

Answer 17

a rock which contains enough metal to make it economically viable to be extracted.

Question 18

Aluminium oxide can also be called

Answer 18

Bauxite

Question 19

Spectator ion

Answer 19

An ion that undergoes no chemical reaction before and after. No chemical change.

Question 20

Aluminium oxide electrolysis- molten

Answer 20

1, Aluminium oxide–> Bauxite
2, Add cryolite- lowers mp
3, Cathode- –>cation: 3e- + Al3+ –> Al (s)
4, Anode+ –>anion : 202- –> O2(g) + 4e-
5, Oxygen chemically reacts with graphite electrode at a high temperature to produce CO2 so the anode wears away over time and needs to be replaced which is very expensive.

Question 21

PANIC

Answer 21

Positive anode negative is cathode

Question 22

OXAN

Answer 22

oxidation always takes place at the anode

Question 23

REDCAT

Answer 23

Reduction always takes place at the cathode

Question 24

What extra ions are in aqueous electrolysis and why?

Answer 24

H+ and OH- because water is present.

Question 25

Cathode rule

Answer 25

It is always the less reactive ion that migrates towards the cathode to become reduced.

Question 26

Why does H+ migrate towards the cathode and not the sodium ion?

Answer 26

Sodium is more reactive than hydrogen. It is always the less reactive ion that migrates towards the cathode to become reduced.

Question 27

NACL aqueous electrolysis

Answer 27

1, Na+, H+,OH-, Cl-
2, cathode rule: 2e- + 2H+ –> H2(g)
3,anode rule: Halide ion takes priority–> if not, hydroxide ion –> if not, other anion would react: 2Cl- - 2e- –> Cl2 (g)
4, H2 can be used in hydrogenation, source of energy.
5, Cl2 is a very good bleaching agent, regulates pH in swimming pool water, used to kill bacteria (prevents the growth of algae), used on window and door frames if they are PVC (polyvinylchloride).

Question 28

Hydroxide half equation

Answer 28

4OH- (aq) –> O2 (g) + 2H20(l) + 4e-