4. Chemical changes (Paper 1) Flashcards
This question is about electrolysis.
Aluminium is produced by electrolysing a molten mixture of aluminium oxide and cryolite.
Explain why a mixture is used as the electrolyte instead of using only aluminium oxide. (2 Marks)
Mixture has a lower melting point (than aluminium oxide)
allow cryolite lowers melting point (of aluminium oxide)
ignore boiling point
do not accept cryolite is a catalyst
1
(so) less energy needed
ignore cost
1
Oxygen is produced at the positive electrode.
Complete the balanced half-equation for the process at the positive electrode. (2 Marks)
→ O2 +
2 O2- → O2 + 4 e-
Sodium metal and chlorine gas are produced by the electrolysis of molten sodium chloride.
Explain why sodium chloride solution cannot be used as the electrolyte to produce sodium metal. (2 Marks)
hydrogen (gas) would be produced (instead of sodium)
1
(because) sodium is more reactive than hydrogen
1
This question is about acids and alkalis.
Which ion do all acids produce in aqueous solution?
H+
Calcium hydroxide solution reacts with an acid to form calcium chloride.
Complete the word equation for the reaction.
calcium hydroxide + ___________________ acid → calcium chloride + ___________________
hydrochloric (acid)
allow HCl
1
water
allow H2O
1
The higher the concentration of a sample of dilute sulfuric acid, the greater the volume of sodium hydroxide needed to neutralise the acid.
The student tested two samples of dilute sulfuric acid, P and Q.
Describe how the student could use titrations to find which sample, P or Q, is more concentrated.
Level 3: The design/plan would lead to the production of a valid outcome. All key steps are identified and logically sequenced.
5–6
Level 2: The design/plan would not necessarily lead to a valid outcome. Most steps are identified, but the plan is not fully logically sequenced.
3–4
Level 1: The design/plan would not lead to a valid outcome. Some relevant steps are identified, but links are not made clear.
1–2
No relevant content
0
Indicative content
allow converse using acid added to alkali
Key steps
- measure the volume of acid
- add indicator to the acid
- add sodium hydroxide solution
- until the colour changes
- record volume of sodium hydroxide solution added
- repeat procedure with the other acid
Use of results
• compare the two volumes of sodium hydroxide solution to find which sample P or Q is more concentrated
Other points
- pipette to measure volume of acid
- use a few drops of indicator
- swirl
- use a white tile
- rough titration to find approximate end point
- add dropwise near the endpoint
- read volume from bottom of meniscus
- repeat and take a mean
Soluble salts are formed by reacting metal oxides with acids.
Give one other type of substance that can react with an acid to form a soluble salt.
any one from:
- metal
- (metal) hydroxide
allow ammonium hydroxide
• (metal) carbonate
allow ammonium carbonate
• alkali
Calcium nitrate contains the ions Ca2+ and NO3−
Give the formula of calcium nitrate.
Ca(NO3)2
allow Ca2+(NO3−)2
Describe a method to make pure, dry crystals of magnesium sulfate from a metal oxide and a dilute acid.
Level 3: The method would lead to the production of a valid outcome. All key steps are identified and logically sequenced.
5−6
Level 2: The method would not necessarily lead to a valid outcome. Most steps are identified, but the method is not fully logically sequenced.
3−4
Level 1: The method would not lead to a valid outcome. Some relevant steps are identified, but links are not made clear.
1−2
No relevant content
0
Indicative content
- use magnesium oxide and sulfuric acid
- add sulfuric acid to a beaker
- warm sulfuric acid
- add magnesium oxide
- stir
- continue adding until magnesium oxide is in excess
- filter
- using a filter paper and funnel
- to remove excess magnesium oxide
- heat solution in an evaporating basin
- to crystallisation point
- leave to crystallise
- pat dry with filter paper
credit may be given for diagrams
An atom of iron is represented as Fe. (56 - Atomic Mass, 26 Atomic number)
Give the number of protons, neutrons and electrons in this atom of iron. (3 marks)
Number of protons _______________________
Number of neutrons _______________________
Number of electrons _______________________
26
1
30
1
26
1
