19 — hydrocarbons Flashcards
Homologous series
Consists of a family of compounds with the same general formula, and similar chemical properties because they have the same functional group
Organic compounds in the same homologous series:
1. Have the same GENERAL formula (specify)
2. Each successive members differ by a -CH2- unit
3. Have the same functional group
4. Have similar chemical properties
Describe a trend down the homologous series
1. Melting and boiling points increases as the size and mass of the molecules increases. More energy is required to overcome the stronger intermolecular orcas of attraction.
2. Viscosity of compounds increases as the size and mass of the molecules increases, resulting in stronger intermolecular forces of attraction.
3. Flammability decreases.
Alkanes
Alkanes are hydrocarbons that contain only carbon-carbon single bonds and carbon-hydrogen bonds.
General formula: CnH2n+2
- No functional group. There r only C—C and C—H bonds.
- suffix: —ane
Alkenes
Alkenes are unsaturated hydrocarbons that contain carbon — carbon double bonds (C=C).
General formula: CnH2n
Functional group:
Carbon — carbon double bond (C=C)
Suffix: —ene
Number of carbon atoms and their prefixes
1: meth—/methan—
2: eth—/ethan—
3. Prop—/propan—
4. But—/butan—
(Note: Alkenes homologous series dont have ‘methene’ as alkene’s functional group is carbon—carbon double bond thus u nd 2 carbons)
saturated hydrocarbons
Saturated hydrocarbons r hydrocarbons that contains only single covalent bonds betw the carbon atoms and between carbon and hydrogen atoms
Physical properties of alkanes
- Low melting and boiling points.
- Alkanes r simple molecular substances w weak intermolecular forces of attraction.
- generally increases down the homologous series
——> down the homologous series, size of the molecule increases,resulting in stronger intermolecular forces of attraction thus more energy is needed to overcome it. - Viscosity
Becomes more viscous as their molecular sizes increase due to stronger intermolecular forces of attraction - Solubility
Alkanes are insoluble in water but soluble in most organic solvents such as tetrachloromethane (CCl4). Liquid alkanes r also often used as solvents for other organic compounds.
Chemical properties of alkanes
Generally is reactive as they only consist of strong carbon-carbon single bonds and strong carbon-hydrogen bonds which r hard to break. However, they undergo combustion and substitution reaction.
Combustion of alkanes
In excess oxygen, complete combustion of alkanes occur.
Alkane + oxygen -> carbon dioxide + water
(Water is in gaseous state during combustion and liquid when cooled)
In insufficient oxygen, incomplete combustion takes place. Instead of carbon dioxide, carbon monoxide and soot is formed.
Alkane + oxygen -> carbon monoxide + carbon + water
Substitution reaction in alkanes
Alkanes react w halogens in presence of ultraviolet light.
[A hydrogen atom is substituted by a x atom to form y compound.]
Unsaturated hydrocarbons
Are hydrocarbons that contain double covalent bonds between the carbon atoms
Physical properties of alkenes
- Low melting and boiling points
- as alkenes r simple molecular substances w weak intermolecular forces of attraction.
- There is a gradual increase down the homologous series. - Viscosity
- more viscous as their molecular sizes increases, exulting in stronger intermolecular forces of attraction - Solubility
Insoluble in water but soluble in most organic solvents like tetrachloromethane (CCl4)
- they’re more reactive than alkanes and might interfere in reactions thus not usually used as solvents
- more often used as starting materials in the manufacture of various products eg plastics and detergents
Combustion of alkenes
In excess oxygen, complete combustion occurs
CnH2n + O2 -> carbon dioxide (g) + H2O (g)
(Chemical Eqn is not balanced, just general terms)
Percentage by mass of carbon in Alkenes molecules is higher than in alkanes thus more likely to undergo incomplete combustion, thus producing carbon monoxide and carbon. Hence, they burn w a sootier flame than alkanes.
Alkanes + oxygen -> carbon monoxide + carbon + water
Addition of hydrogen to alkenes (hydrogenation)
Reagent: H2
Conditions: 150dgC, nickel catalyst
Application: manufacture of margarine by the hydrogenation of vegetable oil
As more hydrogen molecules r added into the oil molecule, there r fewer carbon-carbon double bonds and the melting point of the oil increases. Trans-fat may be formed due to the side reaction of the hydrocarbon chain w the catalyst.
Polyunsaturated fats
Contain hydrocarbon chains w 2 or more carbon-carbon double bonds in each chain.
The greater the level of saturation, (saturation is correct) the higher the melting point of the substance. (Packed closer tgt than unsaturated fats)
Fats vs oil
Fats: solids at rtp
Oils: liquid at rtp
Fats: contain mainly saturated fat molecules
Oils: higher percentage of unsaturated fat molecules
Addition of bromine to alkenes (bromination)
Alkenes can react w halogens. When ethane is bubbled thru liquid bromine, the red-brown colour of bromine disappears (decolourisation). A colourless liquid (1,2-dibromoethane) is formed.
Addition of steam (hydration)
Alkenes can react w steam to produce alcohols.
Reagent: steam H2O
Conditions: 300dgC, 60atm, phosphoric(V) acid (H3PO4) catalyst.
Addition polymerisation
Is the process where unsaturated short-chain monomers [1] join covalently together to form a long-chain molecule without the loss of any small molecules or atoms [1]
Conditions: high tempt & pressure, catalyst
Used to manufacture materials like plastic.
Cracking
A process in which larger and longer straight-chain hydrocarbon molecules, usually alkanes, which are less useful and lower in demand r broken down into smaller hydrocarbon molecules that are more useful and higher in demand, to be used as raw materials in the production of alcohol or plastics.
Conditions:
1. High temperature of 500dgC to 700dgC, pressure of 1atm and finely divided catalyst (Al2O3, SiO2 or porous pot)
- More useful shorter-chain alkenes such as ethane and propene, which r raw materials for many important industrial processes
- Higher demand short-chain alkanes such as petrol and fuel
- Produces hydrogen as a byproduct used in impt industrial reactions eg production of ammonia
Isomers
Isomers are molecules with the same molecular formula but diff structural formula.
- Molecular formula: constant
- Structural formula: varies
- Boiling point: varies
- Combustion products: same moles same products
- Hydrogenation: same molecule produced (if alkane)
- Polymerisation: different product
- Chemical properties: mostly similar
- Physical properties such as melting boiling points: different
Isomers can be due to
1. Different ways C atoms are joined (straight vs branched)
2. Different positions of the functional groups
3. Different functional groups
Test for unsaturation (alkenes)
Aq bromine is a red-brown solution formed when bromine dissolves in water
Aq bromine remains red-brown when shaken w alkanes, decolourises when shaken w alkenes.
X is a gas at rtp that burns to produce carbon dioxide and water. What can you tell?
X is a hydrocarbon with small carbon chain that has less than 5 carbon atoms for it to be a gas at rtp. Therefore, X is methane.
Nylon vs Terylene
Nylon is a macromolecule that has small units joined by amide linkages. Terylene is a macromolecule that has small units joined by ester linkages.
Nylon is made from dicarboxylic acid and diamine but terylene is made from dicarboxylic acid and diol.
Are products x and y isomers of h? Explain your reasoning. [2]
Product X is not an isomer as it has a different molecular formula than h. Product 2 is an isomer as product 2 has the same molecular formula as y but different structural formula.
Isomers are compounds with the same molecular formula but different structural formula
Processes used to manufacture alkene
Fractional distillation of crude oil followed by catalytic cracking
Process used to manufacture polymer
Addition polymerisation of monomer (alkene or alkane)
Process used to manufacture ethanol
Process 1: catalytic addition of steam to ethene
Process 2: fermentation of glucose
Process used to manufacture carboxylic acid
Oxidation of corresponding alcohol
Use the info given in table to give 2 pieces of evidence that suggests that the aldehydes are a homologous series. [2]
Each member of the aldehyde homologous series differs from the next by CH2. The aldehydes have the same functional group, –CHO. Their molecular formulas can be reduced to the same general formula.
Give a similarity and a difference between the bonds in these vegetable oils and alkenes. [2]
S: both contains carbon-carbon double bonds
D: Alkenes have 1 carbon-carbon double bond in each molecule but polyunsaturated vegetable oils have 2 or more.
Describe the manufacture of margarine from vegetable oils [2]
Hydrogenation of vegetable oil where hydrogen as is passed through vegetable oils at 150dgC with nickel catalyst
Explain why propane and butane leave the column in the same fraction. [1]
They have a similar number of carbon atoms and a similar range of boiling points
Suggest reasons why the enthalpy change of combustion of octane is more negative than the enthalpy change of combustion of butane. [3]
More energy is released to form C-O and O-H bonds in CO2 and water after the combustion of one mole of octane as compared to amount of energy released to form bonds in CO2 and H2O after the combustion of one mole of butane.
Octane contains a larger carbon composition than butane and thus has more C-H bonds to be broken.
Explain why carboxylic acid turns Universal Indicator solution orange. [1]
Carboxylic acid is an organic acid and thus is a weak acid with pH 3.
Why the enthalpy change of combustion in the 2 isomers are different.
Different bonds are broken in the isomers and the energy absorbed to break bonds in isomer 1 is greater than in isomer 2 thus enthalpy change of combustion in the 2 isomers are different.
How isomerism affects melting boiling points
More branches there are in the alkane, lower the melting boiling point, as molecular size decreases, lesser energy needed to overcome weaker intermolecular forces of attraction.
Explain whether A B D and C are products of isomerisation, cracking, substitution or neither.
A is isomerisation. Pentane has the same molecular formula (C5H12) as the product formed but they have different structural formula.
B is neither isomerisation nor cracking as they have the same number of C atoms but different molecular formula.
C is cracking as the straight long-chain heptane molecule is broken down into smaller ethene molecule.
D is formed from substitution reaction as one hydrogen atom of octane is replaced by a bromine atom [1]
Evaluate the use of H2 and octane as fuels via ease of storage and energy content of the fuels [3]
At ROOM TEMPERATURE AND PRESSURE, octane is a LIQUID (QUOTE) WHILE H2 IS A GAS (QUOTE). Thus, easier to store octane than H2 gas as LIQUID OCCUPIES LESS SPACE
DENSITY OF OCTANE IS HIGHER THUS MORE OCTANE CAN BE STORED PER UNIT VOLUME COMPARED TO H2
1kg of H2 gives off more energy than 1kg of octane.
Polymers
Polymers is a large organic molecule which contains a large number of small units, monomers, joined together by covalent bonds through a process called polymerisation.
Condensation polymerisation
Occurs when monomers with 2 reactive functional groups join together covalently to form condensation polymers, with the loss of small molecules or atoms such as water.
Uses of poly(ethene)
Cling wraps, plastic bags and plastic toys
Uses of nylon and terylene
Nylon: clothing, curtain materials, fishing line
Terylene: clothing, curtain materials, parachutes, sleeping bags
