Basics/nomenclature/isomerism/alkanes/cracking 1 Flashcards
What is an organic compound/chemical
A covalent molecule based upon the element carbon.
How many covalent bonds can carbon form
Four
How many covalent bonds can oxygen form
Two
How many covalent bonds can hydrogen form
One
Define functional group
The part of the molecule that determines how it reacts
Define homologous series
A series of organic molecules that have the same functional group but each successive member has an additional -CH2
What is molecular formula
The formula that gives the actual number of atoms of each element in one molecule.
How do you write molecular formula
You first show the number of carbon atoms, then the number of hydrogen atoms, then all of the other elements are listed alphabetically.
What are the two problems with molecular formula
- A molecular formula does not give us any information on the structure of the molecule. - Different compounds have the same molecular formula.
What is empirical formula
The simplest whole number ratio of the atoms of each element in a compound.
What is displayed formula
The displayed formula shows us the relative position of each atom plus all of the covalent bonds.
What is the problem with displayed formulas
They do not show us accurate bond angles.
What is the structural formula
The structural formula of a compound shows how atoms are arranged but does not show us all of the bonds. It shows the atoms bonded to each carbon atom in turn.
What is skeletal formula
The skeletal formula shows the covalent bonds between carbon atoms plus the bonds to any functional groups. Each point in a skeletal formula represents a carbon atom.
What is the general formula
The general formula is the simplest algebraic formula for a member of a homologous series. It shows the number of atoms of each element in a substance which has n carbon atoms. Functional groups must be shown/clear in a general formula.
List the priority of functional groups when naming compounds from highest to lowest
Carboxylic acid
Nitrile
Aldehyde
Ketone
Alcohol
Amine
Alkene
Haloalkane
Summarise the method of naming an unknown compound
1) Find the longest carbon chain
2) Number the chain (remember which functional groups take priority and that some functional groups are carbon-1 by definition)
3) Put the groups on the chain in the name and write these in alphabetical order.
If there are several functional groups that are the same in a compound we are naming,which prefixes are used
Di,tri,tetra
What are isomers
Isomers are molecules that have the same molecular formula but whose atoms are arranged differently.
What are the two main types of isomerism
Structural isomerism and stereoisomerism
What is structural isomerism
Structural isomers are compounds/molecules that have the same molecular formula but whose atoms are arranged differently so they have different structures.
What is stereoisomerism (also called spatial isomerism)
Stereoisomerism is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space.
What are the three types of structural isomerism
Chain isomerism
Functional group isomerism
Positional isomerism
What is chain isomerism
A form of structural isomerism where isomers differ in the arrangement of their hydrocarbon chains.
What is functional group isomerism
A form of structural isomerism where compounds have the same molecular formula but different functional groups.
What is positional isomerism
A form of structural isomerism where isomers differ in where the functional groups are joined to the skeleton/main chain.
What are the two types of stereoisomerism
E-Z isomerism
Optical isomerism
What is E-Z isomerism (also called geometric isomerism)
A form of stereoisomerism where the groups with the highest Mr each side of the double bond are arranged differently in different molecules.
What is an E-isomer (E/Z isomerism)
The isomer where the highest priority molecules/ groups (ones with the highest Mr) are on opposite sides of the molecule.
What is a Z isomer (E-Z isomerism)
The isomer where the highest priority molecules/groups (ones with the highest Mr) are on the same side of the larger molecule.
What is the general formula of an alkane
C(n)H(2n+2)
What are the three forms that alkane molecules can take
Unbranched chains, branched chains, rings
Why are unbranched chains (often called straight chains) not actually straight
The C-C-C angle is actually 109.5 degrees.
What is the polarity of an alkane
An alkane is almost non-polar
Explain why alkanes are almost non-polar
Alkanes are almost non-polar because the electronegativities of carbon and hydrogen are very similar. As a result, the only intermolecular forces between their molecules are weak Van Der Waals forces. The larger the molecule, the larger the Van Der Waals forces.
Do branched alkanes or unbranched alkanes have lower melting points?
Alkanes with branched chains have lower melting points than straight chain alkanes with the same number of carbon atoms.
Why do branched alkanes have lower melting points than the unbranched alkane with the same number of carbon atoms
Because branched chains cannot pack together as closely as unbranched chains and so the Van de Waals forces are not as effective.
How does the boiling point of an alkane differ with chain length.
The boiling points of alkanes increases as chain length increases as the larger the molecule, the stronger the intermolecular forces (Van Der Waals forces) that need to be overcome are.
What does the complete combustion of alkanes produce
Carbon dioxide and water
When do alkanes undergo complete combustion
When there is a plentiful supply of oxygen
What does the incomplete combustion of an alkane produce
Either carbon and water or carbon monoxide and water
When does the incomplete combustion of alkanes occur
When there is a limited supply of oxygen- and not enough for complete combustion to take place.
What four things happen to the physical properties of hydrocarbons as chains length increases
1) They become more viscous
2) They become harder to ignite
3) They become less volatile
4) They have higher boiling points.
Describe the process of fractional distillation of crude oil
1) The crude oil is heated in a furnace
2) A mixture of liquid and vapour passes into a tower that is hotter at the bottom and cooler at the top
3) The vapours pass up the tower via a series of trays containing bubble caps until they arrive at a tray that is at a lower temperature than their boiling point.
4) They condense
5) The mixture of liquids that condenses on each tray is drained off
6) The shorter chain hydrocarbons condense in the upper trays and the longer chain hydrocarbons condense in the lower trays.
7) Any excess liquid is drained off from the bottom and this is called tar or bitumen.
8) This process separates the crude oil into fractions- mixtures with similiar length hydrocarbons.
What type of substance is petrol
Petrol is a mixture of mainly alkanes containing between four and twelve carbon atoms.
Which fraction of crude oil is most desired
Naphtha
What are two reasons why cracking is useful
- it allows us to meet demands for shorter chain hydrocarbons which are more economically valuable
- some of the products are alkenes which are more reactive than alkanes
What are the two types of cracking
Thermal cracking and catalytic cracking
What conditions are needed for thermal cracking
Temperature of 900 degrees Celsius
Pressure of 70 atmospheres
No catalyst
What products does thermal cracking produce
Alkenes
What are the conditions needed for catalytic cracking
Temperature of 450 degrees Celsius
Pressure of 1-2 atmospheres
A zeolite catalyst
What products does catalytic cracking produce
Motor fuels (aromatics,cyclic alkanes,branched alkanes)
What is a suitable catalyst for catalytic cracking in the labatory
Aluminium oxide
How are the C-C bonds broken in catalytic cracking
- One electron from the pair in the covalent bond goes to each carbon atom and makes it a free radical.
- The free radical acts as a very reactive intermediate
- as there are not enough hydrogen atoms to make two alkanes, one molecule produced must be an alkene with a double C=C bond.
What type of enthalpy does combustion have
A negative enthalpy
Name five alkane fuels
1) Methane (main component of natural gas)
2) propane (camping gas)
3) Butane (calor gas)
4) petrol (mixture of hydrocarbons with chain length around C(8)
5) Paraffin (mixture of hydrocarbons with chain length C(10)-C(18)
Describe Carbon Monoxide as an atmospheric pollutant
- Created when fuels undergo incomplete combustion.
- It is toxic to humans.
- fuels should be burnt in a plentiful supply of oxygen.
Describe Nitrous oxides as atmospheric pollutants
- Created when nitrogen and oxygen from the air react at high temperatures within an engine.
- They react with water vapour and oxygen in the air to form acid rain and photochemical smog.
- We should use catalytic converters in cars.
Describe sulphur dioxide as an atmospheric pollutant
- Produced from the combustion of sulphur containing impurities in fuel.
- Contributes to acid rain and photochemical smog.
- We should remove Sulphur from fuel before burning (flue gas desulferisation)
Describe carbon (soot) as an atmospheric pollutant
- Created by the incomplete combustion of fuels.
- It blackens buildings, causes respiratory issues and global dimming.
- We should ensure there is a good supply of oxygen when burning fuels.
Describe unburnt hydrocarbons as an atmospheric pollutant
- They are a waste of fuel and greenhouse gases.
- They contribute to photochemical fog which causes health problems.
- We should ensure engines are well-tuned and there is a good supply of oxygen so we use all the fuel.
Describe carbon dioxide and water vapour as atmospheric pollutants
They are greenhouse gases to contribute to global warming. We should burn less fossil fuels.
What is the equation for the reaction that takes place in catalytic converters and converts nitrogen-oxide and carbon monoxide into less harmful products.
2CO (g) + 2NO (g) —> N2 (g) +2CO2 (g)
What is the general word equation for the reaction in catalytic converters that removes nitrogen-oxide
Hydrocarbons + nitrogen oxide —> Nitrogen + carbon dioxide + water
What are the characteristics of a homologous series
- The compounds have the same general formula
- Members are chemically similar
- All members have the same functional group/s
- Each successive member contains an additional CH2
- There are trends in physical properties
What is the molecular formula of ring alkanes
CnH2n because the end hydrogens are not required.
Which alkanes have no isomers
Methane, ethane and propane.
What is the link between heat output of an alkane during combustion and number of carbon atoms present
The more carbon atoms (ie the longer the hydrocarbon chain), the bigger the heat output during combustion.
What two conditions must a molecule satisfy in order to be named by the Cis-Trans system
1) The carbon atoms on the double bond must be attached to two different groups.
2) At least one of these groups must be the same for both carbon atoms.
What is the Cis isomers in Cis-Trans isomerism
The molecule in which the same group is on the same side of the double bond.
What is the trans isomer in Cis-Trans isomerism
When the same group is on opposite side of the double bond in the molecule.
What is the advantage of the E/Z isomerism system
The four groups connected to the two carbon atoms joined by a double bond can all be different from each other and still be named by this system.
Are alkanes saturated or unsaturated
Saturated as they contain no double bonds
What type of C-C bond is present in alkanes
Sigma bond
Describe the key properties of a sigma bond
- Sigma bonds form when electron orbitals from adjacent carbon atoms directly overlap.
- A sigma bond contains a pair of electrons: one from each atom either side of the bond.
- The pair of electrons in a sigma bond lie directly between the bonding atoms.
- A sigma bond is fully rotational.
What is a problem with free radical substitution
A large range of side products are produced and it is a chain reaction so can continue for a very long time.
