Chemical Equilibria Test

1. Esters are a useful group of compounds due to their distinctive smells. One example of an ester is ethyl ethanoate, its formation is shown below.
CH3COOH(aq) + C2H5OH(aq)λCH3COO C2H5(aq) + H2O(l)
a) Systems like this are described as being a 'dynamic equilibrium'. Explain the term 'dynamic equilibrium'
b) Write down the expression for the equilibrium constant, Kc, for this reaction.
c) Calculate the value of Kc for this reaction given the equilibrium concentrations below.
[CH3COOH] = 0.08 moldm-3
[C2H5OH] = 0.08 moldm-3
[CH3COO C2H5] = 0.25 moldm-3
[H2O] = 0.1 moldm-3
d) Concentrated sulphuric acid is added to the reaction mixture as it removes water molecules. What effect would this have on the equilibrium position of this system?
(Marks available: 7)
Answer
Answer outline and marking scheme for question: 1
a)- concentrations of the species are constant
- forward and backward reactions are continually taking place but at the same rate.
b) Kc =[CH3COO C2H5] [H2O] / [CH3COOH] [C2H5OH]
c) Kc = [CH3COO C2H5] [H2O]/[CH3COOH] [C2H5OH]
= (0.25) (0.1) / (0.08)(0.08)= 3.91 (no units)
(1 mark numerical answer, 1 mark stating no units)
d) Concentrated sulphuric acid is added to the reaction mixture as it removes water molecules. What effect would this have on the equilibrium position of this system?
(Marks available: 7)
2. The Haber-Bosch process is used for the large-scale production of ammonia from nitrogen and hydrogen gas. The reaction is shown below:
N2(g) + 3H2(g)λ2NH3(g)
a) Write an expression for Kp for this reaction.
b) What are the units of Kp for this reaction? (Assume pressure is measured in kPa)
c) When the temperature is raised for this process the proportion of NH3(g) in the mixture decreases. Explain this observation.
d) What effect will an increase in the total pressure have on the equilibrium position?
(Marks available: 5)
Answer
Answer outline and marking scheme for question: 2
a) Kp = (p NH3)2 (1) (p N2)(pH2)2
b) Units = kPa2 = 1 / kPa2 = kPa-2 kPa x kPa3
c) Increase in temperature leading to decrease in NH3 means reaction towards right is exothermic.
d) An increase in pressure will lead to an increase in the proportion of NH3(g) found in the mixture.
(Marks available: 5)

Chemical equilibria

Reversible reactions
A reversible reaction is one where there is a forward and backward reaction occurring:
The double arrow signifies a reversible reaction.
If in the above reaction the concentrations of A, B, C, D do not change, although the reaction is still in progress, then the forward rate must equal the backward rate. A situation known as dynamic equilibrium has been reached.
Equilibrium constants
Any dynamic equilibrium can be described in terms of its equilibrium constant, Kc.
The equilibrium constant is the product of the molar concentrations of the products raised to the power of its coefficient in the stoichiometric equation, divided by the product of molar concentrations of the reactants, each raised to the power of its coefficient in the stoichiometric equation.
So for the reaction:
The equilibrium constant is given by:
Where [] represents the concentration of the species in moldm-3.
For gaseous systems, we use Kp instead of Kc. Here, the species are shownin the equilibrium equation in terms of their partial pressures. (In a mixture of gases, the proportion of the total pressure due to a particular gas is dependant on its mole fraction).

2. Le Chatelier Principle
Introduction
Le Chateliers principle states:
The position of the equilibrium of a system changes to minimise the effect of any imposed change in conditions.
This principle applies to any reaction that is in equilibrium.
The effect of concentration changes on equilibrium
Changing concentration of a reactant or product does not change the numerical value of the equilibrium constant, but it does change the position of the equilibrium.
In general, the position of the equilibrium is shifted towards the right if the concentration of a reactant is increased or to the left if the concentration of a product is increased.
At the start, when the change is made, the mixture is not at equilibrium, but equilibrium is eventually restored.
The effect of pressure changes on equilibrium
For a reaction involving gases, altering the pressure may cause a change in the position of the equilibrium.
For a reaction where there is an increase in the number of moles from reactants to products, increasing the pressure moves the equilibrium to the left.
Where there is a decrease in the number of moles from reactants to products, increasing the pressure moves the equilibrium to the right. The equilibrium constant remains the same.
The effect of temperature changes on equilibrium
The change that takes place when temperature is changed depends upon whether the forward reaction is exothermic or endothermic.
If the forward reaction is exothermic then the backward one is endothermic.
If the temperature is increased, the equilibrium moves to the left, since an endothermic reaction will tend to reduce the temperature.
Conversely, if the temperature is decreased then the equilibrium, moves to the right.
The effects of catalysts on equilibrium
A catalyst has no effect on the position of the equilibrium. However, it does increase the rate of both the forward and backward reactions, decreasing the time taken to reach equilibrium.

ORGANIC 2. ALIPHATIC COMPOUNDS HALOALKANES

Physical properties

1. CH₃Cl, CH₃Br, C₂H₅Cl are all gases at room temperature and pressure. The other haloalkanes are liquids with boiling points related to molar mass.
2. They are all immiscible with water.
3. General formula: C_nH_2n+1X (where X = halogen atom)
4. Polarity: haloalkanes are polar due to the inductive effect of the halogen atom. Since the halogens are more electronegative than carbon, they have a greater share of the electrons in the C-X bond.

There are three distinct molecular environments for the halogen atom:

Exercise Which of the following haloalkanes will have the greatest polarity?

The third example (tertiary halogenoalkane) has the greatest polarity. This is because the positive carbon ion (carbocation) is stabilised by the inductive effect of the three other bound carbons.

The polarity of the C-X bond results in haloalkanes being much more reactive than their parent alkanes. Therefore they are of greater importance industrially.

Chemical properties

Types of reaction occurring at C-X bond:

1. Nucleophilic substitution reactions
2. Elimination reactions

Nucleophilic substitution

The inductive effect of the halogen atom results in a positive charge on the carbon atom to which it is attached. Hence, this carbon atom is readily attacked by nucleophiles.

a) Attack by OH- (or water): Hydrolysis

The haloalkanes are attacked only slowly by water. The rate is much faster, but a poor yield is obtained if the haloalkane is refluxed with aqueous sodium or potassium hydroxide.

b) Formation of nitriles: attack by CN^-

If a haloalkanes is refluxed with an alcoholic solution of KCN, an acid nitrile is formed: R-C=N

C₂H₅Br + CN^- → C₂H₅CN + Br^-

bromoethane propanonitrile

c) Attack by ammonia NH₃: Products are amines.

If a haloalkane is heated with an alcoholic solution of ammonia in a sealed tube, a mixture of products is formed. The mixture consists of amines and amine salts.

Mechanism for production of primary amines.

Nucleophilic attack by ammonia:

Abstraction of a proton (taken by a base)

Elimination reactions

When haloalkanes are heated with aqueous solution of potassium or sodium hydroxide, the major product is the alcohol, produced by nucleophilic displacement of the halogen by OH^-.

If the reaction conditions are changed so that the haloalkane is heated with concentrated alcoholic potassium hydroxide, the major product is an alkene due to the elimination of hydrogen halide.

ORGANIC 2. ALIPHATIC COMPOUNDS ALKENES

Physical properties

1. The simplest alkenes are gases at room temperature, then liquids, finally solids, due to increased molecular mass. This decrease in volatility is due to increasing Van der Waal's forces.
2. They have typical covalent, physical properties (i.e. almost insoluble in water, soluble in organic solvents)
3. They have the General Formula: C_nH_2n with a C=C bond.

For example: C₂H₄ - Ethene

Structure of C=C bond

In the formation of the C=C bond each carbon atom is sp² hybridised. One of the two s² electrons is promoted to the vacant 2p orbital. Then the 2s orbitals combine together to give three new orbitals that have 120^o bond angles and are planar.

Two of these orbitals then overlap with the 1s¹ orbital of a hydrogen atom to form two C-H bonds. The other overlaps with one of the sp² orbitals of the other carbon atom in the double bond.

The C-C single bond is called a sigma (σ) bond. These are formed by end-on overlap of orbitals. This still leaves a singularly occupied p orbital on each carbon atom.

These then overlap side on to produce a pi (π) bond which occupies a position above and below the plane of the molecule.

It is this π bond that is responsible for the majority of reactions of alkenes and also geometric isomerism that occurs in alkenes.

Geometric isomerism occurs due to there being no rotation around the C=C bond. Groups bonded to the C=C bond are held in fixed positions in the same plane at an angle of 120^o.

This means that if two different groups/atoms are bonded onto the same sides of the double bond (cis), then two isomers exist. One in which the same group is on the same side of the double bond, and one where they are on opposite sides (trans).

Chemical properties

The electron charge cloud of the C=C is the centre of attraction for electrophiles. In most reactions involving alkenes, electrophiles are added to the C=C to give saturated compounds. This means that the characteristic reactions of alkenes are electrophilic addition reactions.

The general mechanism is:

Addition of Bromine

This is a test for C=C. The orange bromine decolourises if C=C is present.

Mechanism:

Reaction with Hydrogen Bromide

Reaction with Hydrogen

Reaction with Steam

Oxidation by Manganate (VII) ions

Polymerisation

ORGANIC 2. ALIPHATIC COMPOUNDS ALKANES

Physical properties

1. At room temperature and pressure:

• C1 - C4 are gases
• C5 - C15 are liquids
• C16 - are solids.

2. As the number of carbon atoms in a straight chain alkane increases, the boiling points rise in a uniform and predictable manner.

This is because each molecule is identical in terms of types of atom and bonds present, the only difference being the regular increase in mass.

3. Compounds with branched chains always have boiling points below that of the related straight chain compound.

Examples:

hexane: boiling point = 69^oC

2,2 dimethylbutane: boiling point = 49^oC.

This is because the straight chain molecules have greater areas of contact between them and hence have stronger forces of attraction.

4. The melting point also increases with the number of carbon atoms in the chain. However, when plotted on a graph, two curves are obtained due to the fact that 'even' carbon molecules pack tighter together, and therefore van der Waal's forces are greater than 'odd' carbon molecules.

5. The alkanes are non-polar and are therefore immiscible in water and other polar solvents. Methane is the most soluble.

Chemical properties

The C-C and C-H bonds in alkanes are very strong since they are non-polar and almost totally covalent in character. Therefore, alkanes are relatively inert with regard to most chemical reagents.

They have three main properties:

1. Combustion
2. Substitution reactions
3. Catalytic (or thermal cracking)

Combustion

The alkanes burn in a plentiful supply of oxygen to produce CO₂ and H₂O.

Example: 2C₂H_6(g) + 7O_2(g) 4CO_2(g) + 6H₂O_(g)

A gaseous mixture of an alkane and oxygen are extremely explosive. These reactions are used commercially when fuels such as natural gas, petrol and oil are burnt in air.

Substitution reactions involving chlorine and methane

A mixture of chlorine and methane:

• a) Does not react if kept in the dark at room temp.
• b) Does not react if kept in the dark at 300^oC
• c) Reacts at room temperature if exposed to sunlight or U.V. light.
• d) Explodes if exposed to bright sunlight or sparked.

So, energy is required to initiate the reaction.

Four products are formed from this reaction:

These products are explained in terms of a stepwise reaction:

• CH_4(g) + Cl_2(g) → CH₃Cl_(g) + HCl_(g)
• CH₃Cl_(g) + Cl_2(g) → CH₂Cl_2(g) + HCl_(g)
• CH₂Cl_2(g) + Cl_2(g) → CHCl_3(g) + HCl_(g)
• CHCl_3(g) + Cl_2(g) → CCl_4(g) + HCl_(g)

Each of these reactions is an example of a substitution reaction (a Cl atom is substituted for a H atom).

Mechanism: Free radical substitution

It has been shown that the reaction will proceed in the presence of a free radical.

There are 3 stages:

a) Initiation:

Light or heat will cause a few chlorine molecules to split homolytically into chlorine radicals having unpaired electrons.

Cl:Cl_(g) → 2Cl^o_(g) (chlorine radical)

b) Propagation:

The radicals are very reactive and will react with the first particle they meet - most probably a methane molecule because the formation of a H-Cl bond is more exothermic than the formation of a C-Cl bond.

• i) Cl^o_(g) + CH_4(g) → ^oCH_3(g) + HCl_(g) (formation of a methyl radical is more likely)

Or:

• Cl^o_(g) + CH_4(g) → CH₃Cl_(g) + H^o_(g)

The methyl radical produced initiates further propagation steps:

• ii) ^oCH_3(g) + Cl_2(g) → CH₃Cl_(g) + Cl^o_(g)
• iii) CH₃Cl_(g) + Cl^o_(g) → ^oCH₂Cl_(g) + HCl_(g)
• iv) ^oCH₂Cl_(g) + Cl_2(g) → CH₂Cl_2(g) + Cl^o_(g)
• v) CH₂Cl_2(g) + Cl^o_(g) → ^oCHCl_2(g) + HCl_(g)

c) Termination:

Some reactions occur in which atoms or radicals combine together to produce a molecule without a new radical being formed:

• Cl^o_(g) + Cl^o_(g) → Cl_2(g)
• ^oCH_3(g) + Cl^o_(g) → CH₃Cl_(g)
• ^oCH₂Cl_(g) + Cl^o_(g) → CH₂Cl_3(g)

Radicals are removed from the system thus preventing the chain reaction going to completion.

Bromination of methane occurs by a similar mechanism but requires more energy. Iodination does not take place.

Thermo-cracking is used to break down high molecular mass alkanes into low molecular mass alkanes as well as alkenes using heat and a catalyst. As bond breaking is a random process, a variety of products can be formed.

For example:

ORGANIC

1. General properties

I. General Principles

A. principles Properties

1. Bonding of the carbon atom.

Electronic structure of the carbon atom is 1s² 2s² 2px¹ 2py¹.

The outer electrons are located in orbitals (volumes of electron probability) having the following shapes:

Carbon is unable to take part in ionic bonding since it is not energetically possible to form either C4⁺ or C4^-. It must therefore bond covalently by sharing electrons.

To achieve its maximum valency of 4. The 2s electrons must become uncoupled to give the electronic structure: 2s¹ 2px¹ 2py¹

During bonding the 2s and 2p orbitals blend together to form four identical orbitals. This process is known as hybridisation.

2. Reactions of covalent bonds

For any organic reaction to take place, covalent bonds must be broken. The factors influencing bond breaking are:

a) Kinetic factors

For bond breaking to occur, the molecules must possess a certain amount of energy (activation energy). Sufficient energy may be obtained through collisions with other molecules and transfer of kinetic energy to the bond.

Increasing the temperature of the reaction increases the number of molecules with energies in excess of the activation energy, a rise of about 10^oC doubles the rate of reaction.

b) Equilibrium of reactions

The reaction must have a favourable equilibrium constant.

3. Mechanism of bond breaking and making

A covalent bond can be broken in two ways:

a) Homolytic fission

• A:B → A^o + B^o

Produces free radicals which have an unpaired electron and are very reactive.

b) Heterolytic fission

• A:B → A⁺ + B^-
• A:B → A^- + B⁺

Produces ions.

4. Bond polarity

Many organic molecules possess a dipole moment, due to an unequal distribution of electrons in the molecule. This occurs because some atoms tend to attractor repel electrons.

Consider the molecule, CH₃Cl.

Chlorine has a greater share of the electrons due to the electronegativity it possesses:

This kind of bond polarisation is known as the inductive effect.

The inductive effect is the power of an atom or group of atoms to attract electrons compared to the power of a hydrogen atom.

NO₂ > Cl > I > NH₃ > C-H <>3 C₂H₅ < (CH₃)₂CH

Groups which are attracting electrons have a negative inductive (-I) effect. They give rise to electron deficient atoms C^δ+.

Groups which are electron repelling have a positive inductive (+I) effect. They give rise to electron rich carbon atoms C^δ-.

Organic reagents

Organic reagents can be classified as either:

1. Nucleophiles: Attack centres of low electron density (nucleus loving). They possess a lone pair of electrons and are usually negatively charged.

Examples include: H₂O, ROH, OH^-, RO^-, Br^-, NH₃, RNH₂, CN^-.

2. Electrophiles: Attack centres of high electron density (electron loving) The are capable of accepting a lone pair of electrons and are usually positively charged.

Examples include: H⁺, Br⁺, R-N=N, CN^-.

Functional groups

The properties of an organic molecule are predominately determined by the properties of the functional group in that compound. Functional groups are atoms or combinations of atoms such as OH^-, -COOH.

Once the properties of the functional groups are known then the properties of any molecule containing a functional group maybe predicted.

The common functional groups are listed below:

Homologus series: This is a series of compounds in which all the members are similar in constitution (i.e. they contain the same functional group, if any) and therefore in chemical properties.

Each homologous series has a general formula (e.g. C_nH_2n+2 for the alkanes) and each member of the series differs from the next by CH₂ unit in all cases.

There is a regular change in physical properties as one ascends the series for example: methane - gas, octane - liquid, and higher alkanes - solid.

B. ISOMERISME

Types of isomerism

Isomers are compounds with the same molecular formula but with a different structural formula. In some cases it is only the configuration of the structural formula that differs. This type of isomerism is called stereo isomerism.

The types of isomerism can be summarised as below:

RATE REACTION

1. The rate of a chemical reaction

What is rate?

Rate is a measure of how fast or slow something is. In chemistry, we speak of a rate of reaction, this tells us how fast or slow a reaction is.

Why do chemists want to know the rate of a reaction?

If you are making a product, it is important to know how long the reaction takes to complete, before the product is produced.

Rate is a measure of a change that happens over a single unit time. That unit time is most often a second, a minute, or an hour.

Reaction between zinc and dilute hydrochloric acid

What we observe over time is that gradually the zinc disappears and bubbles of gas appear. After a few minutes the bubbles of gas form less and less quickly until finally no bubbles appear because all the acid has been used up, some zinc remains.

To summarise, during this reaction zinc chloride and hydrogen gas are been formed at the same time as zinc and hydrochloric acid react.

How to measure rate

Using the reaction between zinc and hydrochloric acid as an example, the following are methods by which you could measure the rate of that reaction.

1. Measure that amount of zinc used up per minute

2. Measure the amount of hydrochloric acid used up per minute

3. Measure the amount of zinc chloride been formed per minute

4. Measure the amount of hydrogen been produced per minute

When choosing which method to measure rate always choose the most straightforward.

In the example above, by far the easiest would be to collect the bubbles of hydrogen and measure its volume.

Methods Used for Measuring Rate

Measuring volume of gas evolved:

To measure the hydrogen gas released in the above reaction we use the apparatus as shown. As the bubbles of gas are given off, the plunger in the syringe moves out as hydrogen gas fills it. After, say every 20 seconds we read the volume of gas in the syringe. The reaction is complete when the syringe no longer moves.

To find the actual rate we plot a graph of volume of hydrogen (cm³) against time (seconds).

Note:

1. The rate is not a constant throughout the reaction - it changes!

2. The reaction is fastest at the start, gradually becoming slower as the reaction proceeds.

3. From the graph, the fastest part of the reaction is shown by the steepest curve.

4. The curve on the graph goes flat when the reaction is complete. This is because, as time goes on the volume of the gas evolved does not change.

Measuring the Rate of Loss of a Gaseous Product:

In the reaction between calcium carbonate (marble chips) and hydrochloric acid we can use the apparatus below to find the rate of reaction.

Marble chips and acid are placed in the flask but separated by a piece of card - preventing the reaction from proceeding. This apparatus is placed on a balance and the mass of the flask and its contents is read.

To start the reaction, the flask is gently lent to one side, causing the card to fall and the marble chips and acid to mix.

A piece of cotton wool is placed in the neck of the flask to allow carbon dioxide gas to escape. As the gas escapes the mass of the flask reduces. Take readings of mass loss over a time interval, e.g. 30 seconds.

To find the actual rate we plot the loss in mass (grams) against time (seconds)

As with the previous experiment, the steepest part of the curve is at the start, hence the fastest part of the reaction is at the start.

Gradually the curve becomes less and less steep as the reaction slows down. Eventually a flat curve appears indicating the end of the reaction.

2. Changing the rate of a reaction

How to change the rate of a reaction

There are 4 methods by which you can increase the rate of a reaction:

1. Increase the concentration of a reactant.

2. Increase the temperature of the reactants.

3. Increase the surface area of a reactant.

4. Add a catalyst to the reaction.

Before, we discover the reasons for the above causing an increase in rate, we must first look at what is needed to cause a reaction to occur!

If we take the reaction between magnesium and hydrochloric acid, in order for them to react together:

1. They must collide with each other

2. The collision must be with sufficient energy.

The rate of a reaction depends on how many successful collisions there are in a given unit of time.

The Effect of Concentration

If the concentration of acid (a reactant) is increased, the reaction proceeds at a quicker rate.

In dilute acid there are less acid particles. This means there is less chance of an acid particle hitting a magnesium particle as compared with acid of a higher concentration.

In concentrated acid there are more acid particles, therefore there is a greater chance of an acid particle hitting a magnesium particle.

Remember: the more successful collisions there are the faster the reaction.

The graph below shows results from two experiments. Experiment A was with concentrated acid and experiment B used dilute acid.

As you can see, the greater the concentration of the acid used in a reaction the steeper the curve and the shorter the reaction time. Hence, these results show that an increase in concentration increases the rate of a reaction.

The Effect Of Temperature

At low temperatures the reacting particles have less energy. When particles are heated they gain energy. The gaining of energy enables the particles to move around quicker, this increases their chance of colliding but also, the increase in energy increases the possibility of a collision occurring with sufficient energy. Therefore rate of reaction increases with increasing temperature.

The Effect of Surface Area

The rate of reaction between magnesium and hydrochloric acid increases as you increase the surface area of the magnesium.

For example: powdered metal (greater surface area) reacts quicker with acid than strips of metal (lower surface area).

The greater the surface area of the metal the more of its particles are exposed to the acid. This increase in exposure increases the frequency of successful collisions.

The Effect of a Catalyst

Some reactions may be speeded up by using a catalyst. A catalyst reduces the energy required for the reactants to successfully collide. The result is more collisions become successful, hence the rate of a reaction increases.

Products from crude oil

1. Chemicals from oil

How oil is formed

Oil is thought to have formed over millions of years from the break down of tiny dead creatures. Natural gas is formed alongside oil.

The dead organisms sank to the bottom of lakes or seas and became trapped in muddy sediments. As the sediments built up, the lower layers were under pressure. They eventually turned to rock. If there was no oxygen in the sediments, heat and pressure turned the remains of the organisms into oil and natural gas.

Some rocks are porous - they have a network of tiny holes in them.Sandstone and limestone are examples. Oil is a liquid so it seeps into porous rocks. Gas also diffuses into these rocks.

Porous rocks may also contain water. Gas and oil do not mix with water. They are less dense than water. This means they form layers above the water.

Sometimes the rock layers form so that the oil and gas are trapped under the rock such as shale that is not porous. Large amounts of oil and gas may collect in a porous rock. The pressure on the oil may build up so much that when a hole is drilled through the rock cap, oil gushes out.

Fractional distillation of crude oil

Crude oil is a mixture of many thousands of different compounds with different properties. They are called hydrocarbons because they only contain the elements hydrogen and carbon.

To make crude oil useful, batches of similar compounds with similar properties need to be sorted. These batches are called fractions and they are separated by fractional distillation.

The theory behind this technique is that some of the compounds in crude oil are easily vaporised, for example, they are volatile due to their low boiling points. Others are less volatile and have higher boiling points.

In fractional distillation, the crude oil is heated to make it vaporise. The vapour is then cooled. Different fractions of the oil are collected at different temperatures.

Fraction:	No. of carbon atoms:	Colour:	Boiling point range oC:	Uses:
Refinery gas	1 - 4	Colourless	Below room temp.	Gaseous fuel, making chemicals.
Gasoline (petrol)	4 - 12	Colourless to pale yellow	32-160oC	Motor car fuel, making chemicals.
Kerosine (paraffin)	11 - 15	Colourless to yellow	160-250oC	Heating fuel, jet fuel.
Diesel oil	15 - 19	Brown	220-350oC	Diesel fuel for lorries, trains, etc. and heating fuel.
Residue lubricating oil heavy fuel oil bitumen	C	Dark brown	Above 350oC	Fuels for power stations, ships etc. Some is distilled further to give lubricating oils, waxes, etc.
	20 - 30
	30 - 40
	50 and above

As the hydrocarbon molecule chain increases its boiling point increases, it becomes more viscous, becomes more difficult to light, the flame becomes sootier and it develops a stronger smell.

2.Products from crude oil

Alkanes

Physical properties:

The chemistry of carbon compounds is called organic chemistry. There are millions of organic chemicals, but they can be divided into groups called homologous series. All members of a particular series will have similar chemical properties and can be represented by a general formula.

The alkane series is the simplest homologous series. The main source of alkanes is from crude oil.

Alkanes are covalent compounds. They are hydrocarbons, which means they contain hydrogen and carbon. The general formula for an alkane is C_nH_2n+2.

Properties and uses of alkanes:

Name of alkane:	Melting point oC:	Boiling point oC:	Density g/cm3:	State at room temperature:
Methane CH4	-182	-162	0.42	Gas
Ethane C2H6	-183	-88	0.55	Gas
Propane C3H8	-188	-42	0.58	Gas
Octane C8H18	-57	126	0.72	Liquid

The first four alkanes are gases at room temperature.

Alkanes with 5-17 carbon atoms are liquids.

Alkanes with 18 or more carbon atoms are solids.

As the number of carbon atoms increases, the melting points, boiling points and densities increases.

They are insoluble in water but dissolve in organic solvents such as benzene.

Their chemical reactivity is poor. The C-C bond and C-H bond are very strong so alkanes are not very reactive.

They will carry out combustion. Burning alkanes in air (oxygen) produces water and carbon dioxide. The reactions are very exothermic (give out heat energy), so alkanes in crude oil and natural gas are widely used as heating fuels.

For example:

If alkanes combust in too little air, carbon monoxide may form. This is dangerous and can cause death.

Cracking alkanes

The lighter fractions (for example, petrol) are in large demand. The heavier fractions are not so useful but unfortunately chemists have to be able to convert these heavier fractions into petrol and other useful products, due to supply and demand, by a method known as cracking.

Cracking breaks down molecules into smaller ones. Catalysts or heat may be used to crack the alkane chain into smaller ones.

Note, that one of the products that is formed when we crack naphtha contains a double bond between two carbon atoms. A hydrocarbon that possesses one double bond belongs to the next homologous series called alkenes.

Another reaction that often occurs after fractional distillation is reforming. Hydrocarbons of the same formula have different boiling points. Straight-chained alkanes have greater boiling points than the branched version. This means they catch light more easily - but this can be too much for the hot cylinder of the car engine. Reforming converts straight-chained alkanes to branched.

Alkenes

The members of this series contain a double bond. They are hydrocarbons.

The general formula of the alkenes is C_nH_2n Most alkenes are formed when fractions from the fractional distillation of crude oil are cracked.

Properties of alkenes:

Like alkanes, the boiling point, melting point and densities increase with larger size molecules.

They are insoluble in water.

They combust like alkanes to produce carbon dioxide and water. However, they burn with sootier flames due to their higher percentage of carbon content to hydrogen.

Chemically, alkenes are more reactive than alkanes. This is because they possess a double bond that can be broken open and added to in a reaction.

For example:

These reactions are called addition reactions.

Saturated and unsaturated:

Organic compounds, like alkanes, which have four single covalent bonds to all their carbon atoms are described as saturated.

Alkenes are hydrocarbons with a double bond between two carbon atoms and are described as unsaturated. This is because they do not have the maximum number of atoms attached to their four bonds, as one is double!

Polyunsaturated margarines and vegetable oils contain many C=C bonds.

3. Polymerisation

Making plastics

Facts about plastics:

Polythene (polyethene) is made by forming a long chain of ethene molecules. Many other compounds are made in a similar way. A compound made like this is called a polymer.

Polymers are long chains of monomers. A monomer is the building block or in other words the repeating unit that is used to make the polymer. In the above example, ethene is the monomer and polythene the polymer.

Polystyrene (many styrene molecules) is another well-known polymer.

Many polymers can be easily moulded into many shapes - these are called plastics.

Polymerisation is the name given to the reaction that produces polymers.

Remember: alkenes can become polymers but alkanes cannot. This is because alkanes are saturated whereas alkenes are unsaturated which means that they can carry out addition reactions, required for polymerisation.

This type of polymerisation is called addition polymerisation.