Chemistry Past Paper Questions Flashcards
What is meant by an acid?
Produces H+ ions in solution
What is meant by an acid that is weak?
Only partially dissociates when dissolved in solution
Write an equation for the reaction of strontium with water
Sr + 2H2O —> Sr(OH)2 + H2
Using oxidation numbers, explain why the reaction of strontium with water is a redox reaction
Sr —> Sr 2+
0 —> 2+ which has lost electrons and is therefore oxidation
H —>H2
1+ —> 0 which is gain of electrons therefore reduction
Explain why calcium reacts more slowly with water than strontium does
- smaller atomic radius
- less shielding
- greater nuclear attraction on outer electrons
- loss less easily
Barium oxide is added to water, producing a colourless solution. A student adds a few drops of dilute sulphuric acid to the colourless solution with a pH of 13. Describe what the student would observe and give the formula of the compound produced.
Observation: white precipitate
Formula of compound: BaSO4
A graph showing mass and time shows that a reaction gets slower over time and eventually stops. Explain why in terms of collision theory. Reaction is between HCl and ZnCO3
- Acid particles used up and reaction stops
- ZnCO3 in excess and so HCl is the limiting reactant
- Once all acid is used up there is no further change in mass
A student investigates he reaction between zinc and dilute sulfuric acid. Copper sulfate is a catalyst.
- The student adds zinc to each test tube
- The student adds a few drops of aqueous copper sulfate to one of the test tubes, forming a pale blue solution
- The student adds an excess of dilute sulfuric acid to each test tube
Describe two differences the student would observe between the test tubes
- More fizzing/bubbling with the catalyst
- Zn would dissolve
In industry, hydrogen is manufactured from methane:
CH4 + H20 reversible CO + 3H2
The industrial process is carried out at 15atm and 800 degrees Celsius using an excess of steam. A nickel catalyst is used.
Explain why these conditions are used industrially.
- 15 atm is cheap and a higher pressure would require expensive equipment
- Increasing pressure shifts equilibrium to the side with fewer moles to minimise the effect. In this case, this is the reactants so using a high pressure would decrease yield and reaction rate
- The forward reaction is endothermic so increasing the temperature shift equilibrium to the right, increasing yield
- Nickel catalyst doesn’t effect equilibrium position but speeds up the rate by providing an alternative pathway and lowering activation energy
- Excess steam ensures it is not the limiting reactant and all methane is reacted, resulting in a higher yield
CO2 and H2O have different shapes. State the bond angles in both molecules and explain, in terms of electron pair repulsion, why the bond angles are different.
Carbon dioxide is linear with 180 degree bond angles. It has 4 bonding pairs, 2 around each double bond with no lone pairs of electrons.
Water is an angular/bent molecule with 104.5 degree bind angles. This is because it has 2 bonding pairs and 2 lone pairs of electrons. The lone pairs repel more than bonding pairs.
In free radical substitution, suggest 2 reasons why the yield of CH3CH2CHBrCH3 is low
- In the carbocation stage, it is not guaranteed that the Br free radical with attach to the molecule in that specific place, it may attach elsewhere
- Free radicals of the same species could react with each other to form Br2 or a longer unwanted alkane chain
A student mixes butan-2-ol, sodium bromide and sulfuric acid in a pear shaped flask and refluxes the mixture. After an hour the mixture has separated into 2 layers: an aqueous and an organic. Describe the procedures the student would need to carry our to obtain a pure, dry sample of 2-bromobutane from this mixture
- Separate out the 2 layers using a separating funnel and collect top organic layer
- Add drying agent until no change
- Filter
- Distill
