Multiple Choice
Which of the following statements regarding an atom of neon-20 is correct?
a. The atomic number is 20.
b. The mass number is 20.
c. The number of neutrons is 20.
d. The number of electrons is 20.
b
Q33) Dry ice can be purchased as small pellets or larger blocks.
(c) If a single pellet of dry ice had a mass of 1.22 g, calculate the number of carbon dioxide molecules this pellet would contain. (2 marks)
1.67 x 1022 (3 SF)
Agitation (stirring) is used to make the small clumps stick together to form large clumps.
State the observation for the following reaction:
Solution of silver nitrate was added to a solution of sodium carbonate, to precipitate silver carbonate in a solution of sodium nitrate.
Two colourless solutions combined, to produce a yellow precipitate in a colourless solution.
Question 38 The Perth Desalination Plant in Kwinana supplies approximately 17% of Perth’s potable (drinkable) water.
The two (2) main sources of potable water in Perth, are groundwater and seawater.
(a) Define the term ‘desalination’ and state which of these 2 main water sources would be processed at the desalination plant. (2 marks)
The initial stages of water treatment at the desalination plant involve the processes of filtration and desalination.
(c) Give one (1) reason that the process of filtration would be used. (1 marks)
a)
c)
The surface tension of water is greater than that of methanol because
a. water has stronger intermolecular forces.
b.water has a higher molar mass.
c. water has a higher boiling point.
d. water has a greater volatility.
a
A 345 mL 1.5 M sodium chloride solution was boiled, until only 250 mL of it remained. What is the final concentration (in mol/L) of the solution?
2.07 mol/L
Question 38 Bioethanol is a widely used fuel in countries such as Brazil, the U.S. and Sweden. Bioethanol is chemically identical to ethanol, but it is produced from the fermentation of corn or sugar cane.
In the initial steps of the process, the sugars in these materials are broken down into glucose molecules. Yeast enzymes, such as zymase, are then mixed with the glucose solution to undergo fermentation. This produces ethanol and carbon dioxide gas, as shown in the chemical equation below.
C6H12O6(aq) --> 2 C2H5OH(aq) + 2 CO2(g)
(a) Justify why ethanol manufactured in this way is classed as a ‘biofuel’. (1 mark)
(b) Explain why the overall carbon emissions from bioethanol are lower than those from ethanol produced from crude oil. (2 marks)
a) It is made from biological material / biomass / plant and animal matter.
b) The carbon released when bioethanol is combusted was originally captured by photosynthesis.
Whereas the carbon released when ethanol is combusted comes from crude oil which has been stored for millions of year.
Q29) Four (4) chemical reactions were set up in separate test tubes, according to the table below.
Test tube A) Hydrochloric acid solution and cobalt metal.
Test tube B) Aqueous sodium hydroxide and nitric acid solution.
Test tube C) Aqueous potassium hydroxide and ammonium chloride powder.
Test tube D) Hydrochloric acid solution and magnesium carbonate powder.
(a) In which test tube (A, B, C or D) would a gas not be produced? (1 mark)
(b) Write a balanced ionic equation for the reaction occurring in the test tube where a pungent-smelling gas is produced. (2 marks)
a) B
b) OH-(aq) + NH4Cl(s) --> NH3(g) + H2O(l) + Cl-(aq)
Q40e) Explain why SiCl4 is classified as a non-polar molecule, despite containing polar bonds. Include the Lewis structure of SiCl4 in your answer. (3 marks)
Lewis structure correct (all valence electrons show)
The shape of the molecule is symmetrical / the molecule is a symmetrical tetrahedral shape.
Therefore the bond dipoles cancel each other out and there is no net dipole (resulting in a non-polar molecule).
Which of the following statements regarding the kinetic theory of gases shows least correlation with the behaviour of a real gas?
(a) The particles of a gas move in random straight-line motion.
(b) The particles of a gas occupy no volume.
(c) The average distance between gas particles is very large compared to the size of the particles.
(d) The average kinetic energy of gas particles increases with increasing temperature.
b
Lead (II) sulfate is obtained from the reaction between 465mL of 0.238M lead (II) nitrate solution, with excess 1.00 M sodium sulfate solution. What is the mass of lead sulfate formed?
33.7g
Question 29 Consider the two (2) gas chromatograms below. Both analyses, A and B, were performed on identical gas samples. All chromatography conditions were controlled, except the temperature of the gas chromatograph oven was different in each case.
(a) Name an appropriate gas which could have been used as the mobile phase. (1 mark)
c) Why can’t the sample be cooled any lower than -273.15 °C? (2 marks)
a) Any of the following: helium, nitrogen, hydrogen, argon
c) This temperature is absolute zero
Sample cannot be cooled any lower because; (any one of following justifications)
Question 35 (8 marks)
Hydrogen peroxide is a colourless solution which decomposes into water and oxygen gas under standard laboratory conditions.
The enthalpy change for this reaction is given as; DH = -196 kJ mol-1 of hydrogen peroxide.
The rate of this reaction can be greatly enhanced by adding some solid manganese(IV) oxide catalyst to the hydrogen peroxide solution. The activation energy, in the presence of the catalyst, is 23 kJ mol-1.
(c) sketch an energy profile diagram for the catalysed reaction. Label the enthalpy change and the activation energy. (3 marks)
Enthalpy curve needs to be roughly to scale based on the data provided (much smaller Ea compared to enthalpy change).
Exothermic shape. Enthalpy and activation both labelled correctly with values/units.
Q39) Analysis has shown that vehicles emit most of their pollution within the first 5 minutes of driving.
(c) Suggest a reason for this. (2 marks)
A sample of juice was being analysed by high-performance liquid chromatography (HPLC) to confirm whether citric acid was present. Subsequently, the results of the analysis were compared to a calibration curve to determine the concentration of citric acid in the juice.
In order for the data to be reliable, the HPLC conditions used for the citric acid analysis must be the same as those used to produce the citric acid calibration curve. These are referred to as ‘controlled variables’.
Which of the following is not a variable that needs to be controlled in this investigation?
a. The stationary phase.
b.The mobile phase.
c.The pressure applied.
d.The amount of sample loaded.
d
Q36c) The Sabatier process is used by NASA on board the International Space Station, to produce water for the crew. The chemical equation for the Sabatier process is provided again below, for convenience.
CO2(g) + 4 H2(g) --> CH4(g) + 2 H2O(g)
The carbon dioxide exhaled by the astronauts is collected and reacted with hydrogen gas that forms as a by-product of a different on-board reaction. The water vapour is cooled and condensed into a liquid. This produces 2495 kg of water each year.
(e) Calculate the total mass of carbon dioxide that must be exhaled by the astronauts each year, in order to produce this mass of water. State your answer to the appropriate number of significant figures. (5 marks)
3.047 x 106 g or 3.047 t (4SF)
Question 33 Dry ice is solid carbon dioxide. Under atmospheric pressure, it can only form at a temperature below -78.5 °C. It is used for transporting fresh meat, poultry, fish and pre-made meals. At temperatures above -78.5 °C, dry ice undergoes sublimation, which means it converts directly from a solid to a gas.
(a) State whether the process of sublimation is endothermic or exothermic. Justify your answer, including a discussion of the movement of heat between system and surroundings. (4 marks)
b) Write a thermochemical equation, including state symbols, representing the sublimation of carbon dioxide. Your equation should incorporate information regarding the associated enthalpy change. (2 marks)
a)
a)
b)
CO2(s) + heat --> CO2(g)
(1m reactants and products, 1m heat)
Question 33 (8 marks)
Phosphorus pentachloride reacts with water to produce a mixture of phosphoric and hydrochloric acids, as shown in the chemical equation below.
PCl5(s) + 4 H2O(l) --> H3PO4(aq) + 5 HCl(aq)
(b) Define a ‘weak’ acid, and identify which of the products is classified as weak. (2 marks)
(c) Define a ‘monoprotic’ acid, and identify which of the products is classified as monoprotic. (2 marks)
b) A weak acid is one that partially ionises in solution. H3PO4
c) A monoprotic acid has one ionisable / acidic hydrogen per molecule. HCl
Q40) Both the NaCl and SO2 products are water-soluble.
(e) Explain, in terms of intermolecular forces, why each of these products is soluble in water. You may use labelled diagrams as part of your answer if you choose. (5 marks)
A labelled diagram could replace certain points (use marker's discretion):
Q25) A sample of powdered copper(II) sulfide was placed in a beaker and 0.5 mol L-1 nitric acid was then poured in. The chemicals react according to the equation below.
CuS(s) +2 HNO3(aq) --> Cu(NO3)2(aq) +H2S(g)
The mass and temperature of the reaction mixture was recorded at the start (i.e. as soon as the chemicals were mixed) and then again after 5 minutes had passed (i.e. once all sign of reaction had stopped).
The results are shown in the table below.
Initial After 5 minutes
Mass (g) 128 123
Temperature (°C) 21 18
Which of the following conclusions regarding this data is least likely to be correct?
a) The decrease in mass is a result of the H2S gas being produced.
b) The decrease in temperature is a result of the reaction being endothermic.
c) The decrease in temperature is a result of the energy required to break the bonds being greater than the energy released when the new bonds form.
d) The decrease in mass and temperature are a result of mass and energy not being conserved.
d
Question 37 Floatation tanks (float tanks, sensory deprivation tanks) are lightless, soundproof tanks containing a salt solution which is dense enough to enable the user to float. Float tanks are said to provide benefits such as relaxation and pain relief, as well as improved sleep, circulation and immunity.
Float tanks contain around 1000 L of salt solution. This solution contains a very high concentration of Epsom salts, MgSO4.7H2O(s), which is maintained at 35 °C. The resulting MgSO4(aq) solution is designed to be just under saturation point. If too little salt is added, the solution will not be dense enough to allow the user to float. If too much salt is added, crystals of solute form and may block water filtering equipment.
A particular float tank company prepared a tank containing 1000 L of salt solution. This solution had a density of 1.27 kg L-1 and was known to be composed of 30.0% MgSO4 by mass.
(b) Calculate the concentration of the MgSO4(aq) solution in moles per litre. (5 marks)
(c) Calculate the mass of Epsom salts, MgSO4.7H2O(s), that would have been dissolved to produce this solution. (2 marks)
b) 3.165 mol L-1
c) 780175 g (780 kg)
Question 36
Analysis of various hair products such as dyes, sprays and serums can be of great benefit to forensic investigators. Since hair samples are often found at crime scenes, identification of the hair products which are coating the hair can provide valuable information to investigators.
One common and effective method used for the analysis of hair products is thin layer chromatography (TLC). The TLC plate below shows the analysis of six (6) different popular brands of hair spray.
The plate, which is the stationary phase, is made of glass coated with silica. A small amount of each hair spray was spotted onto the sample line. The plate was then placed into a solution which acted as the mobile phase.
(a) Briefly describe how the technique of TLC is able to separate the various components of a sample. Your answer should make reference to the role of both the stationary and mobile phases. (4 marks)
Question 28 Consider the information in the table below, regarding the boiling points of two silicon-containing species.
Compound Boiling point Silane, SiH4 -111.9
Silica, SiO2 2950
Explain, in terms of structure and bonding, the difference in boiling points of these two substances. (5 marks)
SiH4 is covalent molecular / consists of discrete molecules.
Therefore it has only weak intermolecular (dispersion) forces.
SiO2 is covalent network.
Therefore it has strong covalent bonds extending throughout the 3D lattice structure.
Thus a much larger amount of heat (energy) is required to disrupt the bonding in SiO2 compared to SiH4 (resulting in a higher boiling point).
Question 39
Hexane is a colourless, odourless organic solvent. It is a significant component of petrol and is also used as a cleaning agent in furniture and shoe factories.
The term ‘hexanes’ refers to a mixture of hexane isomers. Isomers are substances that have the same molecular formula but different structures. A mixture of hexanes is predominantly composed of hexane, but may also contain isomers such as 2-methylpentane and 2,3-dimethylbutane. Hexanes is much cheaper and is used in situations where pure hexane is not necessary.
Exposure to high levels of hexane has been known to cause vertigo, fatigue, headaches and drowsiness. Therefore, the concentration of hexane in air must be monitored in order to ensure the health and safety of factory workers who may inhale this compound. The recommended exposure limit for hexane is 50 ppm over an 8 hour work day.
The air in a particular furniture factory was known to contain 50 ppm hexane. On average, a factory worker takes 15 breaths per minute, and each breath has a volume of 0.50 L. The density of air was measured to be 1.225 g L-1.
(f) Calculate the number of hexane molecules that would be inhaled by a factory worker during an 8 hour work day. (8 marks)
1.54 x 1021 molecules