Chemistry past paper questions

Physical test to determine if ethanol is pure

Heat the ethanol and a pure ethanol should boil at 78 degrees Celsius

Why is potassium more reactive than lithium?

1. Potassium has a larger atomic radius than lithium 2. This makes the outer shell less attracted to the nucleus 3. Therefore loses an electron easier

Why not use universal indicator in a titration?

UI doesn’t have a sharp colour change

How is crude oil separated into fractions by fractional distillation?

1. The crude oil is heated 2. The vapours rise up the column 3. The column is hotter at top and cooler at the bottom 4. Vapours condense at their boiling points

Why are shorter-chain alkanes more useful than longer-chain alkanes?

1. They are more flammable 2. So are more useful as fuels

Why is the reaction between ethene and bromine exothermic?

1. More energy was released when bonds were formed 2. Than was taken in when bonds were broken

Why is catalytic cracking useful?

1. There is a greater demand for smaller hydrocarbons 2. Alkenes are produced which make polymers 3. Smaller fractions are produced which can be used for petrol

Why does lead(II) bromide need to be molten for electrolysis to occur?

1. When solid, ions are in a lattice 2. No ions are free to move 3. When molten, the ions are free to move so can conduct current

Suitable material for electrodes

1. Graphite 2. Has a high melting point

Reaction of dry blue litmus paper in organic solvent vs. water with hydrogen chloride gas

1. No change in organic solvent as litmus paper stays blue 2. Because no H+ ions present 3. In aqueous solution, litmus paper turns red 4. H+ ions are formed

Alloy

A mixture of metals

Why is lithium less reactive than potassium?

1. Lithium has fewer shells than potassium 2. The electrostatic attraction between the nucleus and valence electron is stronger in lithium 3. So it loses the valence electron less easily

Why doesn't a catalyst affect the yield of methanol in a reversible reaction?

Catalyst increases the rate of forward and reverse reaction both equally

Why is an alloy harder than a pure metal?

1. In pure metals, layers of cations can easily slide over each other 2. In alloys, the larger cations disrupt the regular arrangement 3. Which prevents the cations from easily sliding over each other

Why does sodium chloride conduct electricity when molten or in solution but not when solid?

1. In solid ions are in a fixed position/lattice 2. When molten ions are free to move

How does hydrogen gas form at the negative electrode?

1. H+ ions in the water 2. Attracted to the cathode 3. Gains electrons (at the cathode) 4. Combines to form hydrogen gas

Why does the rate of reaction change during the reaction?

1. At the start of the reaction there are more reactant particles 2. Collisions between them happen more frequently 3. Rate of reaction is fast at beginning 4. As the reaction goes on, number of reactant particles decrease 5. Fewer collisions per second, so rate of reaction is decreasing

How to compare the reactivity of chlorine and bromine using solutions of chlorine and potassium bromide

1. Mix the two solutions together 2. Colour change to orange 3. As bromine is displaced 4. Chlorine is more reactive than bromine

Why is magnalium harder than aluminum?

1. Regular arrangement of atoms is disrupted 2. As aluminium and magnesium are different sizes 3. Therefore more difficult for layers to slide over each other

Why is fermentation done in the absence of air?

1. The fermentation needs to be anaerobic 2. Otherwise ethanol won't be formed

Fuel

A substance that releases thermal energy when combusted

Color change of potassium dichromate(VI) when oxidizing ethanol to ethanoic acid

From orange to green

Dynamic equilibrium

1. forward reaction and backward reaction occur at the same rate

2. concentration of reactants and products is constanrt

Why does the new equilibrium mixture contain less NO2 after increasing pressure?

1. Position of equilibrium has moved to the left 2. Because there are fewer moles on the left

Why might it be necessary to make ethene from ethanol in the future?

Crude oil is a finite resource

How to collect a sample of pure oxygen at the positive electrode

1. Fill a test tube with copper sulfate solution 2. Place it over the positive electrode

Why do metals conduct electricity?

1. Delocalised electrons 2. That are free to move throughout the structure

Chemical test (other than indicator) to show ethanoic acid

1. Place magnesium inside 2. Effervescence

Why is the molar enthalpy change 0 kJ/mol for reversible condensation polymerization?

1. Same bonds broken and formed 2. So the same amount of energy needed to break the bonds in the reactants is given off when the bonds are made in the products

Why is neon unreactive (in terms of electronic configuration)?

1. Full outer shell 2. Does not need to lose or gain electrons

Gas with the lowest percentage by volume in the atmosphere

Carbon dioxide

Why swirl the flask continuously?

To mix the solutions more thoroughly

Why is the OH- ion a proton acceptor in the reaction H+(aq) + OH-(aq) -> H2O(l)?

1. H+ ions are protons 2. OH- bonds with H+ to create water

How does copper metal form at the negative electrode?

1. Cu2+ ions 2. Travel to the cathode 3. Where they accept electrons 4. And become copper atoms

Why is ethene described as an unsaturated hydrocarbon?

1. Double bond between carbon 2. Ethene is a hydrocarbon because it contains hydrogens and carbons 3. Only

Describe the steps to complete a titration.

1. Rinse burette with sodium hydroxide 2. Fill burette with sodium hydroxide 3. Ensure that the space between tap and tip contains liquid 4. Add a few drops of phenolphthalein indicator 5. Add sodium hydroxide from the burette until indicator changes from colourless to pink 6. Swirl the conical flask 7. Add sodium hydroxide dropwise closer to endpoint 8. Record initial and final volume on burette 9. Repeat until concordant results

What is the purpose of yeast in ethanol production from glucose?

Has the enzyme that acts as a catalyst that speeds up the reaction

Why do some countries manufacture ethanol by fermentation?

1. No crude oil 2. Plenty of land to grow sugar cane

Reagent and condition for converting ethanoic acid to ethyl ethanoate

Reagent: ethanol, Condition: concentrated sulfuric acid

Why is a burette used in a titration instead of a pipette for adding acid?

Addition of acid can be controlled

Reasons why the volume of gas collected was less than expected

1. Gas was lost between adding acid and replacing bung 2. Leaks in apparatus

Why do sodium and sodium chloride have different melting points?

1. Cations and delocalised electrons 2. Electrostatic attraction 3. Cations and anions 4. Electrostatic attraction 5. Forces in Na are weaker than NaCl

why do we add sulphuric acid or any dilute acid before water is electrolysed?

to increase its electrical conductivity

in electrolysis, why is the volume of oxygen less than expected even though there are no leaks in the apparatus?

oxygen is soluble in water so some dissolves in it

The formula of a calcium ion is Ca²⁺. Explain why the formula of calcium phosphide is Ca₃P₂.

1. charge of phosphide ion is P^3-

2. charges on three calcium ions cancel out charges on two phosphide ions

Explain why calcium phosphide has a high melting point. Refer to structure and bonding in your answer.

1. giant ionic lattice

2. strong electrostatic forces of attraction between cation and anion

3. requires a lot of energy to overcome

What is the formula of the cation in the electrolyte (sodium chloride)?

A - H⁺

B - Cl^-

C - Na⁺

D - OH^-

C

During the production of sodium, small explosions can be heard. Explain why using sodium chloride that is not completely dry could cause these small explosions.

1. water reacts with sodium

2. to produce hydrogen that causes small explosions

The student wants to obtain an accurate value for the volume of NaOH needed to neutralise 25.0cm³ of HNO₃. Describe all the further steps the student should take. (he four steps from before VAGUELY described a full titration so ADD DETAILS)

1. rinse conical flask with distilled water

2. repeat experiment by adding sodium hydroxide dropwise near end point

3. swirl flask

4. record initial and final volume of burette

5. repeat until obtain concordant results

6. find average

catalyst and condition for haber process

1. catalyst = iron

2. 200 atm

3. 450C