4.1.2 Alkanes Flashcards
What’s an example of a mixture of hydrocarbons, that the majority of are alkanes
Crude oil
What are alkanes
Saturated hydrocarbons with no free spaces
What’s the general formula of alkanes
CnH2n+2
How can crude oil be separated into different fractions
Fractional distillation
How reactive are alkanes
Very unreactive due to very strong bonding between carbons (sigma bond)
3 reasons alkanes make good fuels
- release large amount of energy when burnt (exothermic)
- readily available at reasonable cost
- convenient to transport + store
- generally no toxic products
2 reasons alkanes are problematic as fuels
- fossil fuels are being consumed faster than replaced (non-renewable)
- can produce CO2, CO + C pollutants
A large quantity of what is produced when an alkane is completely combusted
Why is this bad
CO2
Is a greenhouse gas that contributes to global warming
What’s completely combusted
When plenty of oxygen is present to convert all hydrogen to H2O and all carbon to CO2
What situation may an alkane be incompletely combusted
If an engine is old/poorly maintained so oxygen is limited and less able to reach each alkane molecule
What’s produced in incomplete combustion
Carbon monoxide and carbon
Problem with carbon monoxide
Toxic - silent killer
Problem with carbon particles (soot)
Harmful by inhalation and can cause cancer, global dimming/asthma
What molecules are more susceptible to incomplete combustion
What flame do they burn with
Larger
Sooty flame
What happens to boiling point of alkanes as chain length increases
Why
Increases
Longer the chain more electrons + SA, more strength/number of IMFs, more IDDI, more surface contact
What happens to boiling point if there are branches
More branches
Less surface contact
Fewer IDDIs
Lower BP
Whats formed when an alkane reacts with a halogen
A halogenoalkane (haloalkane)
What does a chemical mechanism show
The movement of electrons during a reaction
2 ways electrons move in a mechanism
What’s the way we’re looking at
Singly/in pairs
We’re focusing on electrons moving singly
What mechanism is happening if electrons are moving singly
Free radical mechanism
What’s a free radical
An atom/molecule/ion with unpaired valence electrons
When do free radicals form
How
In homolytic fission
2 electrons in a covalent bond are returned to the atoms from which they originated during covalent bond formation
What’s formed if a Cl2 molecule is split homylitically
What’s their reactivity like and why
2 chlorine radicals
Very reactive as no longer have full outer shells and will react with many species to regain a full outer shell
What’s homolytic fission the reverse of
Covalent bond formation
What does a fishhook arrow represent
Movement of only 1 electron (singly)
What’s the mechanism for the reaction of an alkane + halogen
The free radical substitution mechanism
What does the free radical substitution mechanism involve
Free radicals
Swapping of a H atom in Alkane by a halogen atom
What are the 3 steps to this mechanism
Initiation
Propagation
Termination
What is used in the initiation to split the molecule to make free radicals
What does this provide
UV light
Provides energy to break bonds
What happens during propagation
Free radicals used up
Products are formed
More free radicals are regenerated
What’s dangerous about the propagation steps
Explosive
What steps of this mechanism are repeated explosively
2x Propagation steps
What happens in termination
2 free radicals colliding will cancel each other out and form a stable molecule
What is the result of adding up the 2 propagation Equations
The overall equation
Why can side products be made from this mechanism (further substitution)
As free radicals are so reactive
Why is free radical substitution not a reliable way of making haloalkanes
What’s the general way of making haloalkanes
Due to the many side products
By alkenes
