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IQ2. How can hydrocarbons be classified based on their structure and reactivity?
IQ2. How can hydrocarbons be classified based on their structure and reactivity?
IQ3. What are the products of reactions of hydrocarbons, and how do they react?
IQ3. What are the products of reactions of hydrocarbons, and how do they react?
SYLLABUS REARRANGED
B. ALKENES and ALKYNES - THE UNSATURATED HYDROCARBONS:
2.1 construct models, identify the functional group, and write structural and molecular formulae for homologous series of organic chemical compounds, up to C8
2.3 analyse the shape of molecules formed between carbon atoms when a single, double or triple bond is formed between them - the trigonal planar alkenes and linear alkynes
2.2 conduct an investigation to compare the properties of organic chemical compounds within an homologous series, and explain these differences in terms of bonding
3.2 investigate, write equations and construct models to represent the reactions of unsaturated hydrocarbons when added to a range of chemicals, including but not limited to:
hydrogen
halogens
hydrogen halides
water
GENERAL HYDROCARBONS
2.5 describe the procedures required to safely handle and dispose of organic substances
2.6 examine the environmental, economic and sociocultural implications of obtaining and using hydrocarbons from the Earth
2.1 construct models, identify the functional group, and write structural and molecular formulae for homologous series of organic chemical compounds, up to C8
– alkenes
– alkynes
2.3 analyse the shape of molecules formed between carbon atoms when a single, double or triple bond is formed between them
The shape of the distribution of atoms around any carbon atom usually this will fit one of three types:
Task 2.1.1
Q1. Use the interactive to
a) model the homologous series alkenes and alkynes https://chemagic.org/molecules/amini.html
b) identify the shape of the molecule about the double, triple bond
c) write names and draw formulas
Q2. Write the molecular formula and draw the structural formula for the following alkenes:
a) methylbut-1-ene
b) 2,3-dimethylhex-3-ene
c) cyclohepta-1,2-diene
Q4. Write the molecular formula and draw the structural formula for the following alkynes:
a) methylbut-1-yne
b) 2,3-dimethylhex-1-yne
c) cyclooctyne
Build methane, ethene and ethyne to identify the shapes.
View Videos:
OC#10 Molecular Shapes https://www.youtube.com/watch?v=6PdKzBrheDk [10.14 mins]
TASK 2.3.1
Q1. Select the true statement concerning bonding in hydrocarbon molecules: (Thickett, 2018, p. 133)
A) Carbon-carbon single bonds are formed when 4 electrons are shared
B) Double covalent bonds between carbon atoms consist of one electron pair
C) Triple covalent bonds between carbon atoms in alkynes consist of three electron pairs
D) C-H bonds are formed when sp3 hybrid orbitals from each atom interact.
Q2. In a homologous series of alkenes, identify the property that shows the least variation as the number of carbon atoms increases:
A) Boiling point
B) Strength of intermolecular forces
C) Density at room temperature
D) Melting point
Q3. The structure of hydrocarbons is heavily dependent on the bonding within and between the carbon backbone.
a) Identify the functional group of the linear hydrocarbon with the chemical formula C4H6. Include a structural formula in your answer (2 marks)
b) Describe the different geometries around each carbon from the structural formula in part a). (Shenfield & Silove, 2018, p. 73)
Extra for experts: Intramolecular bonding of alkenes and alkynes
Extra for experts: Intramolecular bonding of alkenes and alkynes
Example 3: ethene
Example 4: HCN (same shape as for ethyne)
2.2 investigate properties and bonding of organic homologous series of alkenes and alkynes
For hydrocarbons with the same number of carbon atoms, alkynes generally have the highest boiling and melting points, followed by alkanes, and then alkenes.
General Comparison (for same C-count)
Boiling Point (BPT): Alkynes > Alkanes > Alkenes
Melting Point (MPT): Alkynes > Alkanes > Alkenes (although this is more variable due to molecular symmetry and packing)
1. Boiling Point (BPT)
The boiling point depends on the strength of intermolecular forces—primarily London dispersion forces. [1]
Ethyne (Alkyne): -84.0C
Ethane (Alkane): -88.6C
Ethene (Alkene): -103.7C
2. Melting Point (MPT)
Melting points follow the same general order as boiling points but are more sensitive to how well molecules "stack" in a solid lattice.
Alkynes: Their linear structure allows for very efficient, tight packing in a solid crystal lattice, typically resulting in the highest melting points.
Alkanes: While they pack well, their tetrahedral bond angles - 109.5degrees - make them less "streamlined" than alkynes.
Alkenes: The double bond creates a rigid "kink" in the chain.
Key Trends Across All Groups
Molecular Weight: Within any homologous series (e.g., just alkanes), BPT and MPT increase as the number of carbon atoms increases.
Branching: Increased branching decreases BPT because it reduces the surface area available for molecular contact.
Symmetry: More symmetrical molecules generally have higher melting points because they fit into a crystal lattice more easily
Investigation 2.2.1
On one graph, plot the information from Table 10.1 for melting points for alkanes, alkenes, alkynes
2. On one graph, plot the information from Table 10.1for boiling points for alkanes, alkenes, alkynes
3. Explain the graphs in terms of number of carbons in the series for state at room temperature
4. From the data in the second table below
a) explain the change in enthalpy of combustion for methane to hexane
b) predict whether the complementary alkene/alkyne would have a lower or higher enthalpy. Justify your answer.
3.1 investigate, write equations and construct models to represent the reactions of unsaturated hydrocarbons when added to a range of chemicals, including but not limited to hydrogen (H2), halogens (X2), hydrogen halides (HX) and water (H2O)
Addition Reactions
Addition Reactions
Alkenes are more chemically reactive than alkanes. The double bond is a site of high electron density and will readily combine with species with high electronegativities (eg N, O, F, Cl, Br, I).
Ethene (ethylene) is a small molecule with a highly reactive double bond. This means it can be readily transformed into many useful products.
1. Hydrogenation – Production of ethane
Hydrogenation is the process of adding hydrogen atoms across a double bond in an alkene (or across a triple bond in an alkyne). Ethene can be hydrogenated to produce ethane. A metal catalyst, eg Ni or Pt, is often used.
CH2=CH2 + H2 (Pt catalyst) → CH3-CH3
2. Halogenation - Production of dichloroethane
Halogenation is the process of adding halogen atoms (F, Cl, Br, I) across a double bond in an alkene (or across a triple bond in an alkyne). Ethene can be halogenated with chlorine to produce 1,2-dichloroethane. No catalyst is needed.
CH2=CH2 + Cl2 → CH2Cl-CH2Cl
3. Hydrohalogenation - Production of chloroethane
Hydrohalogenation is the process of adding one hydrogen and one halogen atom (H-F, H-Cl, H-Br, H-I) across a double bond in an alkene (or across a triple bond in an alkyne).
Example: ethene can be hydrohalogenated with hydrogen chloride to produce chloroethane.
CH2=CH2 + HCl → CH3-CH2Cl
NB To work out which carbon will take the hydrogen and which will take the halogen, we use Markovnikov’s Rule. This rule states that the hydrogen atom from the hydrogen halide will bond with the carbon which has the greater number of hydrogens prior to the addition reaction.
4. Hydration - Production of Industrial Alcohol (ethanol)
Hydration is the process of adding water molecules, or the equivalent of water molecules, to a substance. Ethylene can be hydrated to produce ethanol when heated with a dilute sulfuric acid (H2SO4) catalyst.
eg CH2=CH2 + H-OH (H+ catalyst) → CH3-CH2OH
ATAR Notes Key Point: Unsaturated hydrocarbons undergo addition reactions by ‘opening’ double bonds (like a bridge opening) and incorporating the additional molecule. (Silove, 2018, p. 61)
View videos:
OC#14 https://www.youtube.com/watch?v=MtRdeqv7jRw&list=UUkdi7YOXBGAapx_dR40UoFQ&index=18 [11.50]
High reactivity https://youtu.be/G3QMD0t-YRw 11.15
Reactions of alkenes https://www.youtube.com/watch?v=Fn51x8_RM9c [6.30 mins]
Practical 3.1 Properties of Hydrocarbons
Introduction
There is a distinct difference between the chemistry of alkenes and that of alkanes.
Alkenes are unsaturated hydrocarbons that react readily by the process of addition.
Alkanes are saturated hydrocarbons that react slowly by the process of substitution.
This difference in reactivity is used as the chemical basis for this experiment.
Compare the reactivities of alkenes with alkanes in bromine water. Write chemical equations for all reactions observed.
Materials
1x bromine water (dropper bottle)
1x safety glasses
test tubes + rack + labels
hydrocarbon test samples (e.g. cyclohexene, cyclohexane/hexane*)
waste container for the safe disposal of test samples
Construct a table to show the name of each compound tested, whether it is an alkane or alkene and what you observed after the addition of bromine water.
Write a conclusion based on your observations.
Background
Bromine water has a distinctive brown colour.
Saturated: Containing only single carbon-to-carbon bonds.
Unsaturated: Containing at least one double carbon-to-carbon bond.
Alkenes react spontaneously with bromine water due to their unsaturated nature.
The reaction between an alkene and bromine involves the breaking open of the alkene’s double bond and the inclusion of bromine in its structure (an addition reaction).
This results in the production of an alkane with two bromine functional groups.
Therefore, when alkenes come into contact with bromine water, they cause it to decolourise.
Alkanes do not react spontaneously with bromine water due to their saturated nature.
If placed in ultraviolet light, a substitution reaction may occur between an alkane and bromine water.
This results in the production of hydrogen bromide and an alkane with a single bromine functional group.
Therefore, when alkanes come into contact with bromine water and are exposed to ultraviolet light, they decolourise bromine water.
Experiment
1. Place 5 mL of cyclohexane and cyclohexene in separate, labelled 10 mL test tubes.
2. Add 2 mL of bromine water to each test tube using a dropper.
3. Stopper and gently agitate each test tube to the same extent.
4. Allow the test tubes to rest for 30 seconds.
5. Observe and record any colour changes.
The hydrocarbon forms an immiscible layer at the top of the bromine water.
TASK 3.1.1
In the margarine industry, alkenes are often hydrogenated to convert unsaturated oils into solid fats that have a greater proportion of saturated molecules.
a) Using ethene as an example, write an equation for this reaction and state the type of reaction this represents. (2 marks)
b) Describe a test that could be used to confirm that all the ethene has been converted. (2 marks)
2. (2011, Q11)
Which compound can form when bromine water reacts with propene?
(A) 1-bromopropane
(B) 2-bromopropane
(C) 1,1-dibromopropane
(D) 1,2-dibromopropane
(2009, Q6)
Bromine, Br2, dissolves in unsaturated hydrocarbons and reacts immediately. Which of the following is the best description of this process?
(A) Bromine is polar and reacts by adding bromine atoms across the double bond.
(B) Bromine is polar and reacts by substituting hydrogen atoms with bromine atoms.
(C) Bromine is non-polar and reacts by substituting hydrogen atoms with bromine atoms.
(D) Bromine is non-polar and reacts by adding bromine atoms across the double bond.
4. (2011, Q1)
Which of the following industrial processes is used to produce ethanol from ethylene?
(A) Hydration
(B) Dehydration
(C) Addition polymerisation
(D) Condensation polymerisation
5. Use an example to demonstrate your understanding of Markovnikov’s rule.
6. PAST HSC QS (IF NO MARKS ARE INDICATED, THE QS ARE MULTIPLE CHOICE. THERE ARE CURRENTLY 20 MC QS PER PAPER, BUT IN PAST YEARS THERE WERE 15).
2016 Q15
2015 Q25A 3
2013 Q18
2011 Q11
2010 Q24A 2
2006 Q20
2017 Q7
2016 Q15
2011 Q11
2010 Q24B 2
2009 Q6
2008 Q16B 3
2005 Q16A 1; 16B 2; 16C 2
7. VIEW and complete Bromine Water HSC Qs answers (see below)
TASK 3.2.1
1. Excess fluorine is added to methane in the presence of UV light. What would be the expected products? Justify your response. (3 marks)
2. Intense conditions are required to undergo halogenation of an alkane. Which of the following do not contribute to these conditions?
A) High temperatures (400oC)
B) UV light
C) High halogen concentration
D) Concentrated acid catalyst
3. Explain why the chlorination of methane, if not carefully controlled, can produce tetrachloromethane. In your answer, include relevant reactions. (3 marks)
ANS
The chlorination of methane does not necessarily stop after one substitution (chlorination). It may actually be very hard to get a mono-substituted chloromethane. Instead di-, tri- and even tetra-chloromethanes are formed.
One way to avoid this problem is to use a much higher concentration of methane in comparison to chlorine. This reduces the chance of a chlorine radical running into a chloromethane, starting the mechanism over again and forming dichloromethane. Through this method of controlling product ratios one is able to have a relative amount of control over the product.
dichloromethane CH3—CH2Cl + Cl2 → CH2Cl—CH2Cl + HCl
TASK 3.2.2
1. In general, the alkene homologous series with increasing number of carbons show little variation in what? (Shenfield & Silove, 2018, p. 72)
A) Freezing point
B) Strength of inter-molecular forces
C) Density at room temperature
D) Flammability
2. (2006, Q1)
Which is the main industrial source of ethylene?
A) Ethanol
B) Glucose
C) Petroleum
D) Polyethylene
3. (08, Q16, 5 marks) (modified).
The process of fractional distillation is used to separate crude oil into different fractions. One of the compounds obtained from fractional distillation is C10H22. This compound undergoes catalytic cracking as follows:
C10H22 → C8H18 + X
a) Identify X.
b) What is the functional group for X?
c) To which homologous series does C8H18 belong?
Task 3.2.2
Complete Revision Hydrocarbons and Associated Reactions Q A
12C M7 Hydrocarbons and Reactions HSC Q A.pdf2.5 describe the procedures required to safely handle and dispose of organic substances
THIS IS REQUIRED LEARNING:
The most critical aspect to the safe handling of hydrocarbons is their flammability. Most hydrocarbons easily combust and so must be kept well away from a naked flame.
The alkanes from methane to octane are gases or volatile liquids at SLC and have low flash points.
Petrol is a mixture of hydrocarbons and other components which lower its flash point and make it potentially dangerous to store and use. There are warnings in petrol stations regarding smoking and the use of mobile phones to ensure there is no opportunity to ignite the volatile liquid.
Weak dispersion forces between the smaller alkanes lowers their boiling points and increases their volatility. They have low flash points (see definition below) and can be readily ignited even during the lower temperatures in winter.
Volatility is a term used to describe substances which vapourise at room temperature to produce a high vapour concentration above the solid or liquid phase. This high concentration is known as the ‘equilibrium vapour pressure’.
Some hydrocarbons vapourise readily at standard laboratory conditions. In general, the lower the molecular weight, the greater the rate of evaporation and the higher the volatility, due to weaker dispersion forces between molecules.
Boiling point is a good indicator of volatility. Volatility increases with increasing temperature.
Flash Point is a term used to describe the minimum temperature at which the vapour pressure of a hydrocarbon is sufficient to combust in air. Combustible fuel-air mixtures can be dangerous because a spark or flame can ignite them. Flash points can vary, but generally the higher the boiling point of a hydrocarbon, the higher its flash point.
Solid waxes have higher boiling points and flash points than liquid hydrocarbons because stronger dispersion forces makes them less volatile. This lowers the vapour pressure and raises the flash point.
Liquids like octane have weaker dispersion forces between the molecules and are more volatile with lower flash points. This makes them potentially dangerous to store and use.
Safety Precautions:
One common safety precaution in cars is to locate the fuel tank as far from the engine as possible. This ensures any volatile fuel/air mixture has little chance of coming in contact with a spark which might ignite it.
Gases are stored in air tight cylinders which have been carefully sealed and should be checked regularly for leaks.
Liquids should be stored in clearly labelled metal containers with a narrow opening and close-fitting lids.
Fuels should be stored in cool, well ventilated spaces to prevent the build-up of a volatile fuel/air mixture.
Liquid transfers should be done outside in a well-ventilated area to prevent any build-up of combustible mixtures.
Flammable liquids have specific labels (HazChem Codes) which should always be displayed to warn of the dangers.
Keep a regularly serviced fire extinguisher near any storage area used for volatile compounds.
In a laboratory, laws regarding the safe handling of hydrocarbons apply. Riskassess produces safety information (Safety Data Sheet, SDS https://www.safeworkaustralia.gov.au/sds) for all chemicals used in a laboratory. The information on the following page relates to the alkane hexane (Crisp, 2019).
View Videos:
TASK 2.5.1
1. Explain the terms ‘flash point’, ‘ignition temperature’ and ‘vapour pressure’ and why they are relevant when discussing safety in relation to handling of hydrocarbons. (3 marks)
2. Look at the two RiskAssess labels in the information box above. Evaluate the choice of cyclohexane as a safer option over hexane for experiments involving hydrocarbons. (3 marks)
3. What is the safest way to dispose of hydrocarbons once you have concluded your experiments? Justify your response (2 marks)
4. Explain why each of the following safety precautions is taken when handling or storing hydrocarbon fuels:
a) LPG gas bottles should be inspected and tested regularly
b) Lawn mower fuel should be poured into the mower’s fuel tank out in the open.
c) The use of mobile phones is not permitted around petrol stations.
d) Large quantities of petrol should not be stored at home.
e) Petrol tankers are grounded through attachment to steel chains.
f) The petrol tank in a car is usually at the opposite end to the engine.
2.6 examine the environmental, economic and sociocultural implications of obtaining and using hydrocarbons from the Earth
Hydrocarbons are commonly found in high concentrations in fossil fuels. Two of the simplest hydrocarbons are methane and ethane, the primary components of natural gas.
Petroleum is a critical commodity in the modern world. What sorts of compounds can be obtained from a barrel of crude oil? How are they obtained?
ENVIRONMENTAL:
Obtaining:
Obtaining petroleum and natural gas from the Earth involves drilling through rocks deep in the Earth’s crust. Hydrocarbons from the drill machine’s lubricants can be dispersed into surrounding water polluting the surrounding seawater or ocean. These hydrocarbons are toxic to aquatic organisms that reside in the sea.
These rocks, which often contain barium ions from the traces of lubricant that remain, are also returned into the ocean. These barium ions interfere with enzyme activities, which can result in death of living organisms.
Potassium ions in the machine lubricants in extracting the hydrocarbons can result in uncontrollable algae growth, leading to eutrophication. This allows algae to grow on the water surface, blocking the sunlight reaching to the plants beneath the water as well as oxygen gas that is dissolved in the water. This can result in the death of plants, which decreases the oxygen availability in the water for aquatic animals like fish, as well as bacteria that further use more oxygen to decompose dead plants. It destroys the aquatic ecosystem, turning a habitable environment into a toxic one for the original species.
Another environmental implication would involve the noise pollution through the sending of sound waves to detect potential hydrocarbon deposits for drilling. This would disturb local aquatic organisms, such as whales, as well as any humans that are near the area. The sound waves can disorientate whales, resulting in large scale whale stranding on beaches and can cause death.
Hydrocarbons must be transported by ship from the sea to land to oil refinery so that petroleum can be split into components. Oil spillages have also caused environmental damage similar to those already mentioned, but hydrocarbons can be washed ashore polluting beaches.
Using:
Hydrocarbons that enter the human body could cause severe respiratory irritation. The long term effects of hydrocarbon exposure is also currently not fully understood.
Along with the expansion of the petrochemical industry from energy generation to polymer production and the associated economic benefits of both industries, environmental considerations have become more critical, especially as scientists link climate change to our reliance on the burning of fossil fuels. This has spawned new industries devoted to greener energy production and investigations into alternate fuel sources which might reduce the impact on the environment.
Burning of hydrocarbons release carbon-dioxide in the atmosphere which is in fact, a greenhouse gas. Thus extensive use of hydrocarbons increases the risks of greenhouse effect.
Incomplete burning of hydrocarbons results in the formation of carbon monoxide, a fatal gas that can cause death within minutes to exposure.
CFC, a widely used hydrocarbon in refrigerators and spray cans when exposed to the environment causes harm to the ozone layer and thus, is a contributing factor to the greenhouse effect. It also causes exposure of humans and animals to harmful UV rays.
Aldehydes and other toxic materials released from burning plywood inhibit photosynthesis in plants, cause eye and lung irritations, and even possibly cause cancer.
Benzene molecules have been found to deplete red blood cells, cause cancer in mammals and damage bone marrow.
Through potential implications such as leaking toxic hydrocarbons, potassium ions and eutrophication as we have discussed earlier as an environmental implication, they could all result in the death and reduction in biodiversity of aquatic organisms. It is important to preserve biodiversity for many countries as aquatic organisms serves a major source of economic revenue.
Economic:
Potential implications such as leaking toxic hydrocarbons, potassium ions and eutrophication discussed earlier could all result in the death and reduction in biodiversity of aquatic organisms. It is important to preserve biodiversity as aquatic organisms serve as a major source of economic revenue in many countries.
The world's reliance on a small number of sources of petroleum puts their financial viability at risk should there be shortages from natural causes, embargoes or war.
Sociocultural:
Workers involved in the drilling process to obtain hydrocarbons will be exposed to drilled rocks that are covered in toxic hydrocarbon lubricants, as well as the lubricants in the machinery itself, which can be accidentally or voluntarily inhaled as oil mists in the air.
The leaking of toxic hydrocarbons, potassium ions and eutrophication during the extraction of hydrocarbons can result in a decline in biodiversity of aquatic organisms. This has implications in removing or limiting the range of available food which humans can enjoy. As the supply of aquatic organisms decreases due to hydrocarbon pollution, the price of seafood would increase, it would be less affordable for the global population in general.
The presence of toxic hydrocarbons would result in our everyday potable (drinkable) water derived from the sea being toxic. The treatment of water would need to be more extensive, which would incur additional cost to consumers.
View video:
TASK 2.6.1
From the information box above, compile a table of implications of obtaining and using hydrocarbons from the Earth. Use headings: environmental, economic and sociocultural
REVISION TASK 2.5/2.6
Complete past HSC Qs: Safe handling and Environment Q A
12C M7 Safe handling Environment Q A.pdfGoogle Sites
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