Introduction to Hydrocarbons Flashcards
Define organic chemistry
a branch of chemistry which deals with carbon compounds
Covalent bond
Sharing of valence electrons between non-metals to gain a stable electron arrangement.
What is the simplest type of organic compounds
Hydrocarbons
Examples of carbon compounds with localised and delocalised electrons
In diamond, all 4 valence electrons of C are localised between the atoms in covalent bonding. Because the movement of electrons is restricted, an electric current cannot be conducted.
In graphite, each C atom uses 3 of out 4 valence electrons to covalently bond with 3 other C in a plane. Each C atom contributes one electron to the delocalised system of electrons that is also part of the chemical bonding. The delocalised electrons are free to move throughout the plane, meaning that graphite can conduct electricity along the planes of C atoms, but not in a direction perpendicular to the plane
The electrons in CH4 are localised between the hydrogen and carbon nuclei.
What is the simplest form of hydrocarbons?
Alkanes
What are alkanes
A homologous series where the molecules have a single covalent bond between the carbon atoms. This means that the molecules are saturated as all the bond sites are filled with single bonds.
How to find the name of an alkane
The name is based on the longest chain (so branches are disregarded). Ends in “ane”
Prefixes of carbon compounds
- Meth
- Eth
- Prop
- But
- Pent
- Hex
- Hept
- Oct
General formula of alkanes
C(n)H(2n+2)
eg octane = 8
so C8H18
Ways of expressing the formula of hydrocarbons and all molecules
Molecular: Number of each atom
Structural: Graphical representation. Shows how atoms are arranged and bonded.
Condensed structural: eg ethane = CH3CH3
Each C atom is written separately and following each C atom are listed the other atoms that are bonded to that C atom.
What are alkenes
A homologous series of hydrocarbons that contain one or more double bonds between two adjacent carbon atoms.
Why are alkenes unsaturated
Because it contains at least one double bond, the two pairs of electrons are being shared between two carbon atoms. This means that these carbon atoms do not contain the maximum number of hydrogen atoms, and that the double bonds can be broken and more atoms can be added.
What is the general formula of alkenes
C(2)H(2n)
Why does methene not exist
Because two carbon atoms are required to form a double bond.
How to tell the difference between alkanes and alkenes?
Add bromine water. The alkane should stay red-brown. The alkene should turn colourless.
This is because the double bond within an alkene is more reactive than the single bonds between the C atoms in an alkane. One of the bonds breaks and the C atoms of the double bond remain bonded with a single bond. This means that they have a spare electron to covalently bond with the bromine atoms. So, when a reactive substance, such as bromine, is added to an alkene, the double bond opens and a new type of molecule forms.
What are alcohols
A homologous series of organic molecules with an alkane chain and a hydroxyl (OH) group attached to one of the carbon atoms. The hydroxyl group replaces one of the hydrogen atoms.
Are alcohols acidic, basic or neutral
NEUTRAL.
General formula for alcohols
C(n)H(2n+1)OH
Eg Condensed structural formula for Octane
CH3(CH2)6CH3
Physical states of alkanes at 20°C
Gases: Methane, Ethane, Propane, Butane
Liquids: Pentane to Octane
Solids: more than 20 C
Explanation of the melting and boiling point trends
The melting and boiling points of hydrocarbons increases as the carbon chain length increases. This is because as the size of the chain of the hydrocarbon molecules increase, the stronger the force of attraction between the molecules so more energy is required to separate the molecules, hence it has a higher melting and boiling point.
Neighbouring molecules are held together by forces of attraction between the molecules. In order to push the molecules further apart, and therefore change their state, these forces of attraction must be overcome. So the longer the carbon chain, the greater the force of attraction so more E is needed to overcome these forces to change the substance’s state.
Immiscible
when two insoluble liquids are formed.
Usually layers will form
Tow liquids that do mix are miscible
Solubility of alkanes, alkenes and alcohols in water
There is very little attraction between alkane and alkene molecules and water molecules. This means alkanes and alkenes are all insoluble in water.
Methanol and ethanol are are soluble in water in all proportions because water molecules and alcohol molecules are strongly attracted to each other.
State of ethene and propene at room temperature
Both are gases at room temp
State of methanol and ethanol at room temp
colourless liquids. Because both of their boiling points are below 100°C, these liquids are described as volatile. (easily vaporised)
Explain solubility
A solute will dissolve in a solvent if the forces of attraction between the solvent and the solute are great enough to overcome the forces of attraction between the solute molecules.
A solute will not dissolve if the forces of attraction between the solute and solvent are weaker than the force of attraction between the solute molecules.
Do alcohols have higher melting and boiling points compared to corresponding alkanes?
Yes. The attractive forces between the alcohol molecules are stronger than those between alkane molecules, and more energy is required to break those forces.
How to separate alcohol from water?
Distillation. Methanol has a lower boiling point than water. by heating the solution gently, the methanol will evaporate first, turning into a gas and leaving the liquid behind. The gaseous methanol can be collected and cooled back into a liquid.
