P5 Flashcards
To ensure that drying process for a solid is complete
heat to constant mass. This ensures all water has
been evaporated.
since alcohols are highly flammable,
it is dangerous to heat them on a naked flame. Always use a water
bath to heat alcohols and mixtures containing alcohols
CO2 and O2 are
water soluble gases (oxygen is only slightly soluble)
To increase reliability,
repeat experiment trial multiple times and take average reading
Give burette readings to the nearest 0.05 cm3
since burette can only measure up to ± 0.05 cm3. It cannot
take readings more accurate than this.
In electrolysis, some deposited metal could possibly
fall off the electrode while washing it, leading to a
lower reading of mass than expected.
While weighing a mass to prepare standard solution
weigh a larger mass to reduce percentage error and
make experiment more accurate
Always use distilled water in experiments since
tap water may contain dissolved ions which alter its boiling
point and may interfere with experimental results
To keep temperature of a solution constant,
place it in a thermostatically controlled water bath
If a heated solution is left without a lid
water may evaporate, changing the concentration of reactants
In any condenser
cold water always enters at the bottom
To control exothermic reactions
add reactants dropwise.
Certain reaction enthalpies cannot be measured directly since
it is impossible to know when the reaction is
complete
Before starting an experiment where temperature is measured,
allow system to equilibrate and reach the
same temperature
Initial trial experiments are often done to
calibrate instruments
When asked how one variable varies with respect to another
answer in terms of directly/inversely
proportional
Soda lime
(mixture of CaO and Ca(OH)2) is alkaline in nature
When a hot solid is being made to react with a gas,
the gas may be passed over it before the solid is begun
to be heated and after it has finished reacting. This is to prevent the solid from reacting with other gases in
air.
Instruments must be
wiped to ensure they are clean and dry to prevent water/dirt does not interfere with
readings
During titration, ensure that
titre readings are concordant. If they are not consistent/close to each other,
repeat titration until concordant readings are obtained.
If indicator and one of the reactants used is the same colour
change the indicator to one which displays a
different colour to improve the accuracy of results.
When taking volume measurements use
lower
meniscus for liquids with concave meniscus, and
upper meniscus for liquids with convex meniscus
To eliminate anomalous readings,
repeat the
experiment for those values
While doing enthalpy change experiments, it is
important to
measure the temperature before and
after the reaction.
In a decomposition reaction, if heating is very
strong
time measurement will be very small, increasing the percentage error and reducing accuracy of
reaction. At the same time, underheating may not ensure that all of the solid has decomposed.
FILTRATION UNDER REDUCED PRESSURE
FRACTIONAL DISTILLATION
COLLECTING GAS OVER WATER
COLLECTING GAS IN A GAS SYRINGE
SETUP FOR ELECTROLYSIS
PERFORMING A REACTION UNDER REFLUX
Measuring temperature change of a reaction
COLLECTING A CONDENSED GAS FROM A REACTION
Steps to make a standard solution:
- Add known mass of solid to a beaker and dissolve it in distilled water (if starting compound is liquid,
transfer known volume of liquid to volumetric flask directly using funnel, then add washings of
funnel to flask) - Transfer solution to a volumetric flask (100 cm3, 250cm3, 500cm3 as appropriate)
- Wash beaker with some distilled water and add washings to the flask
- Fill flask up to the mark with distilled water and shake thoroughly
Steps to accurately measure the mass of a solid transferred to another container:
- Add some mass of a substance to a weighing boat
- Weigh the mass of substance + weighing boat (W1)
- Transfer the substance to another container/beaker
- Weigh the mass of weighing boat after transfer is done (W2)
- Calculate mass of substance transferred using W1 – W2
Note: don’t reverse the order of weighing since some of the substance might remain in the boat as a
residue after the transfer, giving an inaccurate reading of mass transferred
Justifying why results are reliable:
Because measurements are close to each other
Because all points lie either on or very close to the line of best fit
Because there are no anomalous points
While filling a burette:
- Run some distilled water through the burette to ensure it is clean
- Run some of the liquid you intend to fill through the burette, to remove any of the water in it
- Fill burette using a funnel
- Open the tap and run some liquid through the tap to eliminate any air bubbles in the liquid column
How to know when reaction is complete:
- Colour change of
indicator/reactants/products - No more bubbles seen
- No more mass increase seen
- Temperature reaches its max and then
starts to fall (for exothermic reactions) - Temperature reaches its minimum and then
starts to rise (for endothermic reactions)
Terms used to describe observations:
Colourless (not transparent or clear): used
to describe a solution that looks like water
Precipitate: describes small pieces of solid
mixed with a liquid or solution
Gelatinous precipitate: a precipitate that
looks like jelly
Effervescence: describes the formation of
bubbles
Possible observations of reactions:
Burette:
to measure volume accurately
Digital probe thermometer:
to show accurate temperature changes (more accurate than lab
thermometer)
Glass rod stirrer
to stir solutions (for example, while being heated to ensure uniform heating)
Thermostatically controlled water bath
to control/maintain the temperature of a sample or a
reaction
Systematic error
measurement errors caused by
measurements differing from the true value by a
consistent amount for each measurement that is
made.
Random error:
measurement error caused by
measurements varying unpredictably from one
measurement to the next
Sources of error (examples):
- Heat loss in experiments where enthalpy is being measured
(not all heat from heater is transferred to reaction
mixture/some heat produced in reaction is lost to
surroundings) - In some titrations, it is difficult to judge the end point of
the reaction since colour changes may be
subjective/difficult to discern - Insufficient capacity of apparatus may cause reactants to
splash out/escape (e.g: filling a beaker to the very brim)
o make readings more precise/accurate:
- Changes in apparatus: use burette instead of measuring cylinder, use thermocouple instead of lab
thermometer, use pH meter instead of pH paper - Repeating readings to identify and eliminate anomalies and make results more reliable
- Performing experiments with larger quantities of reactants, to reduce percentage error
- Changes in method: using weighing boat method to measure mass transferred, using lower
meniscus for accurate volume measurements
If the least count of a burette is
0.1 cm3 then the uncertainty/error in measurements could be ±0.05 cm3
Uncertainty is
± half of the smallest scale division of an instrument.
f this burette is used to measure a change in volume of 20 cm3,
that means that volume is being read
twice – initial volume and final volume. Thus, to find percentage uncertainty:
Precautions and Safety Measures
. Wear gloves while handling chemicals that can cause skin irritation or while handling heated
apparatus
2. Do not dispose solutions which are toxic to aquatic life/pose environmental hazards down the sink
3. Keep alcohols and flammable compounds away from naked flames
4. In experiments in which toxic gases are evolved that pose a health hazard, perform experiment in
fume cupboard
5. Wear a mask while performing experiments where compounds are harmful by inhalation/if
swallowed
6. Use insulation while performing an experiment involving temperature changes being measured
7. Ensure that instruments have no zero error. If a zero error exists, be sure to add/subtract the error
as appropriate to avoid systematic error
