AS Chemistry Term 2 Flashcards
Describe the physical properties of alkanes
- Smaller alkanes (C1-4) are gases, medium sized (C5-16) are liquids and the larger ones are waxy solids.
- They are non-polar molecules as their charge is evenly spread due to similar electronegativities of H and C. This means they float on top of water as they are less dense. However, they do dissolve in non-polar solvents.
Describe the chemical properties of alkanes
- They are fairly unreactive so their reactions require high temperatures and/or ultra-violet light, or peroxides.
- Halogenation: Alkanes react with halogens in substitution reactions. Requires light or high temperature.
- Combustion: Alkanes burn completely to form CO2 + H2O or incompletely to form CO or C + H2O
Describe the mechanism of free radical substitution
Initiation: Producing free radicals which is done by homolytic bond breaking. e.g.
Cl-Cl → Cl • + Cl •
Propogation: Free radicals attack normally unreactive alkanes forming a product and another free radical e.g.
CH4 + Cl • → CH3• + HCl
CH3• + Cl2 → CH3Cl + Cl•
Termination: Free radicals react with each other, terminating the propogation and forming a single product e.g.
CH3• + CH3• → CH3CH3
CH3• + Cl• → CH3Cl
Describe how hydrocarbons are derived from crude oil
- Crude oil consists of a mixture of hydrocarbons - alkanes, cycloalkanes (aliphatic hydrocarbons) and aromatic compounds (aromatic hydrocarbons).
- The hydrocarbons in crude oil are separated by fractional distillation which takes place in a fractionating column.
- The top of the fractionating column is cooler than the bottom.
- Crude oil enters as vapour and liquid. The liquid components are tapped off at the bottom while more volatile hydrocarbons move up the column.
- The gases condense at different levels as the temperature gradually falls and are collected as liquids.
- The most volatile hydrocarbons leave at the top of the fractionating column as gases.
Describe the process of cracking
- Long alkanes such as kerosene and diesel fractions of crude oil are passed over a strong heating catalyst for a short period of time in the absence of air.
- This results in hydrocarbon chains producing a mixture of products with shorter hydrocarbon chains, some of which are alkenes.
Describe the different types of addition reactions that occur with alkenes
- Hydrogenation: Uses nickel catalyst at 140°C, forms alkane.
- Hydration (using steam): Used to make alcohols. Requires concentrated phosphoric acid as a catalyst, a temp. of 330°C and a pressure of 6 MPA.
- Halogenation: Can be used to test if a molecule is an alkene. If an alkene is added to bromine water, bromine water is decolourised.
- Addition of hydrogen halides: Halogenoalkane is formed when alkene is bubbled through a concentrated solution of hydrogen halide. With an asymmetric alkene, two products are formed. The major product is the one in which the carbocation intermediate is the most stable.
Describe the oxidation of alkenes
- In a cold, dilute, acidified solution of potassium permanganate, alkenes are converted to diols.
- In a hot, concentrated acidic solution of potassium manganate(VII), the diol is split into two fragments as the carbon-carbon double bond ruptures and these are further oxidised.
- if double bond carbon is: CH2= , then oxidised product will be CO2
- if it is RCH=, then oxidised product will be RCOOH
- if it is RCR=, then oxidised product will be RCRO
Define addition polymerisation
It is the process where unsaturated compounds, called monomers, join together to form a very large molecule called a polymer. The section of a polymer chain inside the brackets is the repeat unit.
Describe the mechanism of electrophilic addition in alkenes
- The double bond consists of sigma and pi bonding which means there is a high electron density around the double bond, making it open to attack by electrophiles.
- For example, HBr is a polar molecule, so the Br atom carries a partial negative charge and the H atom has a partial positive charge. This means the H atom acts as an electrophile, and accepts a pair of electrons from the C=C bond.
- Another example is Br2 which is non polar. As the bromine molecule approaches the alkene, the area of high electron density repels the pair of electrons in the Br-Br bond away from the nearer bromine atom making it act as the electrophile.
Describe the inductive effects of alkyl groups on the stability of carbocations
Alkyl groups tend to release electrons and have a positive inductive effect on a carbocation. This energetically stabilises the carbocation as the charge is spread over several atoms instead of concentrated over one atom. This means that a tertiary carbocation is the most stable, while secondary is less stable and primary is the least stable.
Describe the uses of polythene and polyvinylchloride
Polythene (poly(ethene)): Used as an insulator and packaging.
Polyvinylchloride (poly(chloroethene)): Used as electrical insulation, plastic bottles, clothing.
Describe how to deduce the repeat unit of a given monomer, as well as how to identify the monomer present in a section of a polymer
Deducing repeat unit: Turn the double bond into a single bond and show the bonds on either side of the two carbon atoms.
Identifying monomers present: Split the polymer into repeat units and put the C=C double bond back into the monomer.
Why is it difficult to dispose of polyalkenes
- Their lack of reactivity mean that they are resistant to chemical attack and they take hundreds of years to decompose, causing visual pollution.
- A solution to this is to burn the polyalkenes and using the energy released to generate electricity. However, this adds CO2, meaning it would not help to combat global warming. CO may also be released from incomplete combustion of hydrocarbons.
- Another problem is the difficulty of separating plastic waste as if poly(chloroethene) is burnt, HCl is released as well as toxic dioxins.
State the colours of the halogens, describe the trend in volatility and explain this trend.
Fluorine: A pale, yellow gas with the lowest boiling point and melting point of all the halogens.
Chlorine: Pale green gas with a higher boiling point than fluorine.
Bromine: Orange/Brown Liquid
Iodine: Grey/Black Solid
- Volatility of the halogens decrease down the group. The boiling point values of all the halogens are relatively low because there are only weak Van Der Waals forces between their molecules. As the number of electrons increase, the greater the opportunities for instantaneous dipoles arising within molecules, and for induced dipoles on neighbouring molecules. Hence, iodine has the greatest boiling point as it has the greatest number of electrons as the largest molecule.
Explain why halogens are strong oxidising agents, with fluorine being the strongest oxidising agent
- Halogen atoms gain an electron to achieve a stable electron configuration meaning they oxidise other substances, becoming oxidising agents and reducing themselves.
- Due to the small atomic radii of fluorine, an electron entering its outer shell will be much closer to the nucleus and will experience less shielding meaning that fluorine has the highest electronegativity.
- This makes fluorine the strongest oxidising agent
Describe the reactions of halogens with hydrogen
- The halogens all form hydrogen halides with hydrogen. However, fluorine reacts explosively, even in cool, dark conditions, while iodine forms an equilibrium mixture when heated.
- Chlorine reacts explosively in sunlight, while bromine reacts slowly on heating.
Explain the relative thermal stabilities of hydrides in terms of bond energies
- HCl and HF do not decompose in temperatures up to 1500°C.
- HBr decomposes slightly more readily than HCl and HF, however, HI is the least thermally stable and thermally decomposes readily.
- This trend in thermal stability is due to the relative bond energies of the hydrides, with the H-I bond having the lowest bond energy and H-F having the highest.
- This is because iodine is the largest atom meaning the overlap of its outer shell with a hydrogen atom gives a longer bond length, meaning it is weaker and requires less energy to break it.
Describe the reactions of halide ions with silver ions followed by NH3
- All silver halides, except for silver fluoride, forms a precipitate, meaning that adding silver ions to a halide solution can be used to distinguish which halide is present.
- AgCl is a white precipitate, AgBr is a creamy white precipitate, and AgI is a pale yellow precipitate.
- AgCl dissolves in dilute ammonia, AgBr dissolves in concentrated ammonia, and AgI does not dissolve.
Describe the reactions of sodium or hydrogen halides with concentrated H2SO4
- NaCl(s) + H2SO4(l) → NaHSO4(s) + HCl(g)
- NaBr(s) + H2SO4(l) → NaHSO4(s) + HBr(g)
HBr is a strong enough reducing agent to reduce H2SO4 - 2HBr(g) + H2SO4(l) → SO2(g) + Br2(g) + 2H2O(l)
- NaI(s)+ H2SO4(l) → NaHSO4 + HI(g)
HI is a strong reducing agent - HI(g) + H2SO4(l) → SO2(g) + I2(g) + 2H2O(l)
HI also reduces SO2 - 6HI(g) + SO2(g) → H2S(g) + 3I2 + 2H2O(l)
Define the term disproportionation and describe the reaction of chlorine with hot and cold NaOH
- When an atom is both oxidised and reduced in a reaction
In cold alkali (15°C):
Cl2 (aq) + 2NaOH (aq) → NaCl(aq) + NaClO(aq) + H2O(l)
In hot alkali (70°C):
3Cl2(aq) + 6NaOH(aq) → 5NaCl(aq) + NaClO3(aq) + 3H2O(l)
Discuss the uses of chlorine
- Chlorine is used as a water purifier as the reaction with water forms HClO, or chloric(I) acid, which decomposes slowly in solution. This releases reactive oxygen atoms that kill bacteria in water.
- Chlorine is used in bleach as NaCl and NaClO which bleaches colours and stains because oxygen atoms oxidise dye and other coloured molecules.
- Chlorine is found in organic compounds which are used as solvents, refrigerants and aerosols.
Describe the substitution reactions of halogenoalkanes
- Are all nucleophilic substitution
Hydrolysis: - Reagent is NaOH(aq). Conditions are reflux in aqueous solution. OH- is the nucleophile. Water will also hydrolyse the haloalkane but more slowly as water is a poor nucleophile.
Formation of Nitriles:
- Reagent is alcoholic potassium cyanide. Reflux in alcoholic solution. Nucleophile is CN-
Formation of primary amines by reaction with ammonia:
- Reagent is alcoholic ammonia. Reflux in alcoholic solution under pressure. Product is an amine. Nucleophile is NH3
- Amine produced is also a nucleophile and can attack another haloalkane forming a secondary amine, then a tertiary amine and finally as ionic quaternary ammonium salt.
Describe elimination reactions in halogenoalkanes
- elimination reactions convert halogenoalkanes to alkenes. It involves the loss of a small molecule from the original organic molecule.
- The reagent used is ethanolic sodium hydroxide.
- If aqueous NaOH is used, a nucleophilic substitution reaction occurs instead.
Describe the different mechanisms of nucleophilic substitution in halogenoalkanes
SN2: S stands for substitution, N for nucleophilic and 2 is the rate of reaction.
- A nucleophile approaches the haloalkane and donates a pair of electrons to the slightly positive carbon atom. At the same time the halogen-C bond is breaking and the halogen takes both electrons in the bond (heterolytic fission) and leaves as a halide ion.
SN1: The halogen-C bond breaks and a halide ion leaves forming a carbocation. A nucleophile bonds to the carbocation.
- Tertiary halogenoalkanes favour SN1 as their carbocations are more stable due to the inductive effect of alkyl groups. Primary halogenoalkanes favour SN2 as their carbocations are less stable. Secondary halogenoalkanes exhibit both SN1 and SN2 fairly equally.
