1
Q
Why incomplete combustion values not the same? And solutions 5
A
- Heat losses to surroundings: use a draught shield
- Incomplete combustion : burn fuel in oxygen, use bomb calorimeter
- Heat loss to calorimeter: use bomb calorimeter
- Not standard conditions
- Evaporation of fuel from wick of the burner: use cover over lighter during weighing
2
Q
Molar volume of gas from metal issues and improvements - 4
A
- Hydrogen escapes from apparatus – use a sealed apparatus with a gas syringe
- Metal covered with oxide – clean with abrasive before weighing
- Mass of metal required is too small to be measured accurately by balance available – use more precise balance/larger mass for lower percentage error
- Large measuring cylinder cannot measure volume accurately as graduations too far part – use a smaller measuring cylinder
3
Q
Why are repeats used? 2
A
- Identify anomalies and discard
- Identify precise results and use them
4
Q
Purpose of reflux 3
A
- Increases rate of reaction
- Allows boiling for a long time
- Stops loss of volatiles / products / reactants
5
Q
HCL with carbonates: independent variables 2
A
- Temp of CaCo3
- Surface area of marble chips
6
Q
What to measure for HCl with carbonates
A
Time taken in seconds to collect 100cm3 CO2
7
Q
HCl and carbonates: amount of acid used 2
A
- Moles of acid used in experiment greater than moles of CaCO3
- Excess acid should be used so reaction will finish
8
Q
Rate of reaction: what to plot
A
- Concentration x axis
- Time s-1 y axis
9
Q
Where are readings on a graph more likely to be less reliable? 2
A
- At lower values
- Higher percentage error
10
Q
Affect of mr on gas produced from carbonate 2
A
- Lower mr : more moles per mass
- More moles and therefore volume of gas produced
11
Q
Good solvent for recrystallisation 3
A
- Solvent should dissolve solute at higher temperatures
- Solvent should dissolve not much solute at room/lower temperature
- Soluble impurities remain in solution even at low temps
12
Q
Recrystallisation not full yield - 5
A
- Side reactions
- Loss of product when transferring between beakers
- Reaction doesn’t go to completion/equilibrium
- Some solid remains in solution after recrystallisation
- Loss of solid when washing final product
13
Q
Why was a carboxylic acid produced in reflux? 4
A
- Indicates that reflux/excessive heating took place
- Distillation of ethanal as it was formed did not take place
- Excess acidified dichromate was used
- Aldehyde was oxidised further
14
Q
Making methanol risks and protection - 4
A
- Gas mask because CH3OH reaction mixture is toxic
- Fire-proof clothing because CH3OH reaction mixture is flammable
- Gloves because CH3OH can be absorbed through the skin
- Eye protection because CH3OH damages eyes
15
Q
Low percentage yield - 4
A
- Loss of material on transferring between vessels
- Side reactions reducing yield of main product
- Loss during purification steps
- Reaction had not gone to completion
16
Q
Purpose of locating agent in chromatography 2
A
- Difficult to see spots as spots colourless in solution
- Iodine vapour locating agent makes it easier to see spots
17
Q
Purpose of TLC 2
A
- TLC will show products produced
- But will not give information on purity
18
Q
What affects crystallisation yield - 3
A
- Crystals damp at the end would have resulted in a higher mass/yield
- Some salt remains unreacted but it is added in excess so doesn’t affect yield
- Loss of water from crystals would reduce mass and therefore yield
19
Q
What to ensure when measuring volume of gas 2
A
- Heat until syringe stops moving/no further gas produced
- Wait until the gas has cooled before measuring the volume
20
Q
Why use pH meter instead of universal indicator?
A
- Difficult to match colour of UI with exact pH
21
Q
Does adding water reduce pH and why?
A
No pH change as ratio of acid to salt is the same
22
Q
Methyl orange colour change
A
Yellow to red
23
Q
Phenolphthalein colour change
A
Red to colourless
24
Q
Purpose of insulation in an endothermic reaction
A
Reduces heat exchange from surroundings
25
What error can be made in making a standard solution causing titre values to increase?
Flask not inverted - solution more dilute at the top
26
Experiment to calculate mr - if mr is too high 3
- Moles of compound found to be too small
- Moles of gas collected too small
- Some gas lost
27
Reasons that total energy transferred from spirit burner different for different fuels 2
- Different enthalpy changes of combustion
| - Moles of fuel burnt is different
28
How to ensure same amount of energy transferred from spirit burner between fuels and state the assumption made - 2
- Keep temperature change the same by using same start and end temperature
- Assumption: heat lost to surroundings is constant
29
What doesn't matter with catalysts?
The mass used
30
Purpose of capillary tube with bung
- Allows gas to escape, minimising pressure
| - Stops air being let in to flask - e.g stops oxidation of Fe2+
31
Why are higher concentration solutions made by dissolving crystals more accurate?
Larger mass of crystal weighed - smaller weighing error
32
What to ensure in line drawn on calibration graph
Draw through zero - zero point
33
Why isn't Fe2+ directly titrated against MnO4?
Fe2+ woud be oxidised by other oxidising agents in solution
34
Why are reactant mixtures added to larger volumes of water in titration rates of reaction experiments?
Slows down reaction so time for titration
35
Colour change of starch
Blue black to white
36
What is essential to do in TLC?
- Wear gloves to prevent contamination to sample
- Allowing plate to dry in fume cupboard as solvent evaporating could produce toxic fumes
- Adding solvent to a depth of no more than 1cm to prevent samples dissolving in stationary phase before they have travelled up it
37
What is not essential in TLC?
Allowing solvent to rise up plate to the top as Rf value can still be calculated
38
Why do amino acids have different Rf values?
- Polarity: how well amino acid dissolves in solvent by forming bonds with it
- Different solubilities in solvent
39
What colour does ninhydrin turn sample spots in paper chromotography?
Purple
40
What colour does iodine turn sample spots in chromotography?
Brown
41
Why are mixtures heated for a significant amount of time?
To ensure more molecules have E more than or equal to Ea
42
Increase significant figures of Group 2 and HCl experiment
- Greater mass of carbonate
| - More conc acid
43
How can you measure water without a balance?
Use a measuring cylinder to measure a volume
| Because 1cm3 is 1g as density of water is 1gcm-3
44
Mass of solid according to gas is lower than expected 2
- Solid lost in weighing process
| - Solid lost in gas syringe
45
What techniques should be remembered for a titration?
- Wash inside of flask with distilled water when pipetting solution in and near when end point is reached to ensure all the solution from the burette falls to the bottom of the flask and reacts
- Flush the burette with what you are using at least 3 times to minimise contaminants
46
Why does washing the flask with water in titration not affect titre value?
Doesn't change number of moles of reactant in titration flask
47
Why is it difficult to obtain a pure sample of a particular gas?
Difficult to separate gases
48
What is the problem with adding starch too soon in iodine thiosulfate titration?
- Iodine will stick to starch and won't react as expected with thiosulfate
- Only add starch when solution is pale yellow
CHEMISTRY PRACTICALS
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