Chemical Changes

Question 1

Describe a method to test for metal ions using sodium hydroxide.

Answer 1

• Measure 1cm³ of the metal ion in solution and add to a test tube.
• Measure 2cm³ of 0.1M sodium hydroxide and add to the test tube.
• Observe any colour change.
• Add another 5cm³ of 1M sodium hydroxide (in excess).
• Observe any further changes.

Question 2

What is a 1M solution of NaOH?

Answer 2

It’s a 1 molar solution of NaOH; it contains 1 mole of NaOH per litre (it has a concentration of 1mol/L).

Question 3

What happens when you add HCl then BaCl₂ to sodium sulfate?

Answer 3

A bright white precipitate is formed.

Question 4

Test for halide ions.

Answer 4

Add dilute nitric acid then silver nitrate solution.

Question 5

Ionic equation for a carbonate ion test.

Answer 5

CO₃^2-_(aq) + 2H⁺_(aq) -> CO_2(g) + H₂O_(l)

Question 6

Ionic equation for a halide ion test.

Answer 6

Ag⁺_(aq) + X^-_(aq) -> AgX_(s)

where X = halide ion

Question 7

Ionic equation for a sulfate ion test.

Answer 7

SO₄^2-_(aq) + Ba²⁺_(aq) -> BaSO_4(s)

Question 8

Write and balance an ionic equation for adding sodium hydroxide to copper to produce an insoluble hydroxide.

Answer 8

Cu²⁺_(aq) + OH^-_(aq) -> Cu(OH)_2(s)

Question 9

Aluminium also reduces silicon dioxide.
Carbon is used rather than aluminium to reduce silicon dioxide because carbon is cheaper than aluminium.
Carbon can be obtained by heating coal.
Aluminium is obtained from aluminium oxide.
Explain why aluminium is more expensive than carbon. [2 marks]

Got 0/2 on March 23rd.

Answer 9

More energy is needed (to obtain aluminium) (because) aluminium is obtained (from aluminium oxide) by electrolysis.

Question 10

Explain why the pH of an acid depends on:
* the strength of the acid
* the concentration of the acid. [4 marks]

Got 3/4 on March 23rd.

Answer 10

General principle
* pH depends on H⁺ ion concentration
* the higher the concentration of H⁺ ions the lower the pH

Strength
* the stronger an acid the greater the ionisation / dissociation (in aqueous solution)
* (so) the stronger the acid the lower the pH

Concentration
* the higher the concentration of an acid the more acid / solute in the same volume (of solution)
* (so) the higher the concentration of the acid the lower the pH

Level-marked - 3 levels.

Question 11

A student titrated sulfuric acid with barium hydroxide solution.
The equation for the reaction is:
H2SO4(aq) + Ba(OH)2(aq) → BaSO4(s) + 2H2O(l)
The student measured the electrical conductivity of the mixture during the titration.
The better a conductor, the higher the electrical conductivity value.

https://cdn.discordapp.com/attachments/1062056354874937405/1224834120346108027/image.png?ex=661eeeaa&is=660c79aa&hm=3f889a2085a99610f8745873da2f94d678aba1ece1298a5f427f521c587c4c49&

Explain why the electrical conductivity of the mixture was zero when the sulfuric acid had just been neutralised.
Use the equation for the reaction.
Refer to ions in your answer. [3 marks]

Got 1/3 on March 23rd.

Answer 11

there are no ions that are free to move
(because) barium sulfate is solid / insoluble
(and) hydrogen ions have reacted with hydroxide ions to produce water