Chemistry ATP
→ Apparatus:
And a glass
rod for
mixing
Measuring mass:
★​ Use digital balance:
○​ Balance should be tared (set to zero) to avoid zero error
○​ Balance should be allowed time to settle before reading recorded
Measuring volume:
★​ Burette (most accurate)
○​ Read burette scale from top to bottom
★​ Volumetric pipettes
○​ Most accurate way of measuring fixed volume
○​ 10cm3 – 25cm3
★​ Measuring cylinder
○​ Not very accurate (used for approximation)
○​ 10cm3 – 1 litre (1 dm3 = 1000cm3)
Measuring gas:
★​ Gas cylinder
○​ Most precise and accurate way
★​ Downward displacement of water
○​ Measuring cylinder is inverted in water to collect gas
○​ Does not work if gas is soluble in water
○​ If gas is heavier than air and coloured cylinder doesn't have to be inverted
○​ First few bubbles are ignored as it is air
★​ To ensure that no gas escapes: use a divided/partitioned flask and tip flask to start reaction
Measuring temperature:
★​ Traditional thermometer
○​ More accurate
★​ Digital temperature probe
○​ More precise
○​ Can be corroded by some reagents and expensive
Indicators:
Indicators are used to show endpoint in reactions
Indicator
Colour in Acid
Colour in Base
Color in Neutral
Litmus solution
Red
Blue
Purple
Red litmus paper
Stays red
Turns blue
No change
Blue litmus paper
Turns red
Stays blue
No change
Phenolphthalein
Colorless
Pink
Colorless
Thymolphthalein
Colorless
Blue
Colorless
Methyl orange
Red
Yellow
Orange
Substance Safety:
Substance
Precaution
Safety Measure
Ammonia
Is toxic/ poisonous
Fume vent/cupboard & mask
Chlorine gas
Is toxic
Fume vent/cupboard & mask
Ethanol
Is toxic/ very flammable
Keep away from heat
Lead salts
Is toxic
Take care
Sodium hydroxide
Is hazardous/irritant/caustic
Boiling will cause mixture to
spit/blow out so warm carefully
Other important colors:
Substance
Colour
Fluorine gas
Pale-yellow
Chlorine gas
Pale-green
Bromine liquid
Red-brown (orange)
Iodine solid
Grey-black (brown in aq)
Copper metal
Brown
Silver metal
Metallic gray
Color of salts in solid form:
Color of salt
Cation present
White solids (form colorless solutions)
Na+, Ca2+, Mg2+, Al3+, Zn2+, Pb2+, Ba2+, Ag+, NH4+
Pale green
Fe2+
Green
Ni2+ and Cr3+
Yellow/Brown
Fe3+
Blue/ pale green (depends on compound)
Cu2+
Common errors and improvements:
Errors
Improvements
No repeats
Repeat at least thrice and take average
Hard to judge end point
Use colorimeter
Measuring cylinder inaccurate
Use burette
Adding past endpoint
Add volume in smaller quantities
Temperature/heat loss
Use a thermostatically regulated water bath
→ Graphs
Independent variable on x-axis and dependent variable on y-axis
Questions:
Why does the graph level off?
★​ The reaction is finished and all of [named reactant] has reacted/been used up
Why must graph pass through origin
★​ It is because when no [named reactant] is added, there is no reaction
→ Question Four
Specify the volume/mass/etc.
State apparatus in which reaction takes place (e.g. beaker, conical flask)
State measuring instruments (e.g. burette, volumetric pipette, measuring cylinder)
State constant variables (fair tests) if necessary (e.g. in titration, neutralisation)
State a conclusion
→ Frequently asked questions:
What is a saturated solution?
★​ A solution with the maximum concentration of solute dissolved in the solvent at a given
temperature (or solution that can dissolve no more solute)
Explain why it would be an improvement to measure using a burette.
★​ More accurate
Explain why it is not possible to use a volumetric pipette to measure volumes
★​ The volume is not constant/fixed/not the same in every run
How to check if liquid is pure water?
★​ Check boiling point (exactly 100℃) or melting point (exactly 0℃)
Why is it important to immediately put a bung in the conical flask where gas is produced?
★​ To minimise gas loss/escape before bung is replaced
Describe additional measurements that need to be taken to determine whether the reaction is endothermic
or exothermic
★​ Measure the temperature before and after the reaction
Why use a partition flask?
★​ So the reaction can be started by tipping container
★​ Do not have to replace/remove bung
★​ So no gas is lost/escaped when bung is removed
Describe how a Bunsen burner is adjusted to make the flame as hot as possible
★​ Air hole must be fully open
State the advantage of measuring volume of gas collected every 10 seconds rather than every 20 seconds
★​ More points/data so better/smoother graph
Which flask is used in distillation?
★​ Round bottom flask
Why are anti-bumping granules used in distillation?
★​ To reduce size of bubbles formed when liquid boils
★​ Used for safety to stop flask from shaking
What is the purpose of water or condenser in fractional distillation (of mixture of alcohols)?
★​ To condense the vapours from gaseous to liquid state
In distillation, why is the thermometer placed in that way?
★​ To measure the temperature of the vapors as the temperature of the liquid is not constant
Why would carrying out an experiment in a polystyrene cup rather than a boiling tube or glass beaker
improve accuracy of results?
★​ Polystyrene is a good insulator so it reduces heat loss
Why is it not a good idea to heat something in a polystyrene cup?
★​ It will melt
Advantage of measuring cylinder
★​ Quick, easy to use, convenient
Disadvantage
★​ Not accurate measurement
What will happen if gas continues to enter a closed conical flask with liquid inside of it?
★​ Liquid will be pushed out of flask
In a reaction where gas is produced, how does the results show that the reaction is
over?
★​ Gas volume constant/ stay at {final volume of gas}
Suggest why the mixture in the beaker was stirred as it was heated.
★​ to speed up dissolving.
Explain why the conical flask was swirled as acid was added from the burette.
★​ To mix the content
Explain why the conical flask was not rinsed with solution L
★​ It will add an unknown volume of solution L in the flask.
State why using 50 cm3 of solution L would cause a problem.
★​ It is more than can fit into burette
State the temperature of the mixture after two hours?
★​ Room temperature, heat lost to surroundings until mixture solution is equal to environmental
temperature
Why was the residue rinsed?
★​ To make sure all the filtrate goes through. no filtrate left behind in the filter paper. (This will
increase yield).
★​ To wash-out / dissolve / remove salt
Explain, in terms of particles, why the rate of reaction was greatest in this experiment
★​ More particles per unit volume
★​ Greater chance of collisions so more frequent collisions.
Suggest and explain a disadvantage of using a graduated pipette instead of a measuring cylinder to
measure solution
★​ Too slow / slower addition of solution / takes longer to add
★​ Measuring time taken less accurate / results less accurate
Suggest how a sample of copper could be obtained from the solution of copper(II) nitrate.
★​ Add more reactive metal like magnesium
★​ Magnesium displaces copper and solid copper is formed and can be filtered.
Why is it necessary to gently heat the solid in the tube first?
★​ Solid spits out of tube/ the tube might crack
Why is the burette washed with (i) distilled water and (ii) acid added?
★​ (i) to remove residue/impurities
★​ (ii) to remove traces of water/ so acid is not diluted
Why is solid in excess (preparation of salt)?
★​ To ensure that all acid has reacted – This is done to avoid leaving any unreacted acid which
would become dangerously concentrated during evaporation and crystallization.
Define ‘in excess’:
★​ Unreacted solid that has not dissolved (insoluble)
Describe how to conduct a flame test:
★​ Heat nichrome wire
★​ Use concentrated hydrochloric acid to wash nichrome wire before and after use to remove residue
★​ Dip the wire into the salt to be tested
★​ Hold sample into hot flame
★​ Describe flame color
Explain why a yellow flame is not suitable for a flame test.
★​ It is not hot enough
★​ Masks the colour caused by metal ion
When a solid substance has no obvious cation it is
★​ Probably a transition metal
★​ Is a catalyst for the other reactions occurred
How would results be improved by taking repeated measurements?
★​ It can spot anomalies and an average/mean can be taken which is more reliable
Suggest how reliability of results could be checked
★​ By repeating experiment and comparing results or by taking an average
Why must electrolosing object be very clean and free of grease
★​ So that coating will not stick/be even
★​ Dirt or grease will be trapped
For an experiment with DV of time taken for text to become visible: (i) what would be the change in
results if narrower and taller beaker was used and (ii) explanation:
★​ (i) time taken longer
★​ (ii) due to greater depth of liquid
Explain why cotton wool is placed in the neck of a conical flask
★​ To prevent splashing/escape of acid
★​ To allow gas to escape
Suggest why small glass beads are used in fractionating column
★​ Larger surface area so it causes higher boiling point liquids to condense and fall back into flask
more effectively
→ To test for presence of water
★​ Anhydrous copper sulfate + water
○​ Exothermic (temperature increase)
○​ Color change – white to blue
★​ Cobalt chloride + water
○​ Endothermic (heat is applied for color change)
○​ Color change – blue to pink
→ Chromatography
Why is the baseline drawn in pencil?
★​ Pencil is insoluble in solvent
Possible solvent
★​ Water or organic solvent (e.g. ethanol)
Why does the mixture on the baseline of chromatography not move sometimes?
★​ Compound is insoluble in given solvent
Why are there only two spots seen from a mixture of <2 dyes?
★​ Due to same Rf value
What happens when the initial level of solvent is above baseline?
★​ The solvent will dissolve the compounds and the compound will not migrate effectively
→ Electrolysis
Mention (for question four):
★​ Inert electrodes of graphite/carbon/platinum
★​ Product at each electrode
★​ Electrolyte
★​ Definition of electrolysis
How to carry out titration?
★​ 1. measure known volume of alkali into a conical flask
★​ 2. using pipette
★​ 3. methyl orange indicator added
★​ 4. add hydrochloric acid
★​ 5. from a burette in small portions, and stir as each portion is added
★​ 6. mix/swirl
★​ 7. until indicator changes colour
★​ 8. record / calculate volume acid added
★​ 9. repeat with same volume acid without indicator
Errors in titration:
★​ Going past end-point
★​ Using measuring cylinder
★​ Only carrying out experiment once
Improvements:
★​ Use burette (in place of measuring cylinder)
★​ Add more slowly
★​ Repeat (and calculate mean)
→ Preparing salts
How to form hydrated crystals from aqueous form:
★​ 1. If reactant is in excess, filter to obtain only the liquid product
★​ 2. Heat to saturation/crystallisation point
★​ 3. Allow to cool
★​ 4. Dry between filter paper/absorbent paper
How to form anhydrous powder from aqueous form:
★​ 1. If reactant is in excess, filter to obtain only the liquid product
★​ 2. Heat
★​ 3. Weigh solution
★​ 4. Reheat, reweight and repeat until mass is constant
How to prepare insoluble salt:
★​ Formed by a precipitation reaction (also called double decomposition)
★​ 1. Filter solution
★​ 2. Wash the residue with distilled water to remove any residue (e.g. soluble salt)
★​ 3. Dry between filter paper