Chemistry Paper 3 Practicals Flashcards
The obtaining and use of experimental data for deriving empirical formulas
from reactions involving mass changes. (Background Info)
Background Information:
The empirical formula of a compound shows the simplest ratio of atoms present.
In this experiment, we will calculate the empirical formula of the compound
magnesium oxide, formed by the combustion of magnesium in the air. We will
calculate the change in the mass of magnesium on heating it, and assume that the
increase in mass is due to the oxygen that has combined.
But when magnesium burns in air, the temperature becomes high enough for it to
combine with both the oxygen and the nitrogen in the air. The magnesium therefore
forms a mixture of magnesium oxide and magnesium nitride after combustion. So we
need to convert the magnesium nitride in this mixture to magnesium oxide, before
we can calculate the change in mass. The conversion of magnesium nitride is done in
two steps:
Obtaining and the use of experimental values to calculate the molar mass of a gas using the ideal gas equation. (Background)
Background:
The ideal gas equation can be used to calculate the molar mass (M) of a gas when P, V, T and the mass of the gas sample (m) are known:
PV = nRT
Given that n = m/M this rearranges to
M=mRT/PV
In this experiment a known mass of copper (II) carbonate was heated in a test tube. This solid will undergo thermal composition, releasing carbon dioxide and leaving copper oxide.
The carbon dioxide gas given off will be collected by displacement of water from an inverted gas tube. After heating, the test tube and contents are reweighed.
A calorimetric experiment for an enthalpy of reaction (Background)
Background:
A fuel is burnt and as much of the energy released as possible is
transferred to water by heating.
Using the fact that 4.18 J of energy are required to raise 1 g of water
by 1 °C, the amount of energy given out by the fuel can be calculated.
This can then be converted to the amount of energy given out per
mole in order to calculate the enthalpy change of combustion of the fuel.
Investigation of rates of reaction experimentally and evaluation of the results. (Background Information)
Background Information:
When we want to investigate the effect of a specific factor on the rate of a
reaction, it is sometimes convenient to choose a certain detectable ‘end
point’. By timing how long it takes the reaction to reach this point under
different conditions, we can compare the rate of reaction as the specific
variable is changed.
In this reaction we will compare the time it takes for a solution to reach a
certain level of cloudiness or turbidity as the concentration of one of the
reactants is changed.
Sodium thiosulfate, Na2S2O3, and hydrochloric acid, HCl, react together to
form sodium chloride, sulfur dioxide, and sulfur.
Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + SO2(aq) + H2O(l) + S(s)
The sulfur formed in the reaction precipitates and causes the solution to
increase in turbidity. The end point is chosen to be when a dark cross drawn
on a piece of paper that is placed under the reaction mixture is no longer
visible.
Performance of laboratory experiments involving a typical voltaic cell using
two metal/metal-ion half-cells. (Background)
A voltaic cell generates electricity from spontaneous redox change. It is composed of two electrodes or half-cells, each consisting of
a metal in contact with a solution of its ions, connected by an external circuit and by a salt
bridge. The voltage generated is measured using a voltmeter.
The obtaining and use of experimental data for deriving empirical formulas
from reactions involving mass changes. (Method)
Method:
Add water to convert magnesium nitride to magnesium
hydroxide,
releasing ammonia, NH3
Heat the magnesium hydroxide to convert it to magnesium
oxide.
Record the mass of a clean, dry crucible with its lid. (Handle the
crucible with tongs, not your fingers, to avoid moisture and oil from
your fingers being transferred.)
Use fine sand paper to scrape the oxide coating from the surface of
a strip of magnesium ribbon approximately 2 cm length. Cut the
ribbon into small pieces, place in the crucible, and weigh the
crucible, its lid, and contents.
Heat the crucible in a hot flame for 10 minutes, ensuring that the
magnesium is exposed to air but that no solid escapes. After this
time the magnesium should have been converted to a white
powder.
Allow the crucible to cool and then add 10 drops of water to
convert the magnesium nitride to magnesium hydroxide.
Heat the crucible gently to drive off any water, then strongly for 3
minutes to convert the magnesium hydroxide to magnesium oxide.
Allow the crucible to cool. Weigh the crucible, its lid, and the
product of the reaction.
Obtaining and the use of experimental values to calculate the molar mass of a gas using the ideal gas equation. (Method)
Method:
Set up the apparatus as shown below. Try to ensure that the
inverted glass tube is full of water with no air trapped.
Put two spatula loads of copper carbonate, CuCO3, into the boiling
tube and weigh the tube with contents carefully.
Gently heat the sample until the graduated tube is about 3/4 full of
gas.
Be very careful to avoid suck-back by taking the delivery tube out
of the trough of water as soon as you stop heating. Allow time for
the boiling tube to cool.
Measure the volume of gas in the inverted tube.
Reweigh the boiling tube and contents.
Investigation of rates of reaction experimentally and evaluation of the results. (Method)
Method:
From the 1.0 mol dm–3 HCl supplied, prepare a serial dilution of the following concentrations: 0.5
mol dm–3, 0.25 mol dm–3, 0.1 mol dm–3, 0.05 mol dm–3.
Fill a burette with the 0.5 mol dm–3 sodium thiosulfate solution, Na2S2O3(aq). Measure 25.00 cm3
into a conical flask.
Place the conical flask on a piece of white paper with a dark cross marked on it.
Pipette 25.00 cm3 of 1.0 mol dm–3 hydrochloric acid into the conical flask and time the reaction
from when the reactants mix. Stop timing when the dark cross is no longer visible.
Repeat the process with the four solutions of HCl of different concentration.
Measure the rate using the equation:
Equation: r = 1/time(s)
A calorimetric experiment for an enthalpy of reaction (Method)
Method
Put 200 cm3 of cold water into the copper can. Record the temperature of the water.
Support the copper can over a spirit burner containing the fuel that you are going to burn. It should be arranged that when the wick is lit the flame just touches the bottom of the can.
Weigh the spirit burner and fuel.
Replace the burner under the can and light the wick. Use the thermometer to stir
the water all the time that the fuel is being burnt. Don’t leave the thermometer resting on the base of the can otherwise it will not register the correct temperature of the water.
Continue heating and stirring until the temperature of the water has risen by about 20 °C.
Extinguish the flame and weigh the burner.
Continue stirring the water until there is no more rise in temperature. Note the final reading on the thermometer.
Performance of laboratory experiments involving a typical voltaic cell using
two metal/metal-ion half-cells. (Method)
Method
Construct two different half-cells by immersing a different metal in a solution of its ions in
a beaker, e.g. a strip of Mg(s) in a solution of 1.0 mol dm–3 Mg(NO3)2(aq) and a strip of
Cu(s) in a solution of 1.0 mol dm–3 Cu(NO3)2(aq).
Use crocodile clips to attach wires from each metal to a voltmeter.
Take a strip of filter paper and immerse it in a solution of 1.0 mol dm–3 KNO3. Place the
soaked filter paper so that it connects the two beakers, as shown above.
Record the voltage generated.
Repeat using different combinations of half-cells and record your results.
Construction of 3D models (real or virtual) of organic molecules. (Background and Method)
Background
In organic chemistry we commonly represent structural formulas in two
dimensions on paper for convenience. For example:
But in order to understand several aspects of organic structure and
reactions, we need to consider the three-dimensional shape. This can be
visualized by building real molecular models, and also virtually using
molecular software programs. In this experiment you will have the chance
to explore some of these ways of representing the three-dimensional
shapes of organic molecules.
Method:
There are many software programs available to build and explore molecular models. Some free ones include:
- Chemsketch (not for Mac users)
- Avogadro
- Phet Colorado
