Developing fuels Flashcards
What does exothermic mean?
Energy transferred to the surroundings.
Bond making (-)
What does endothermic mean?
Absorbs energy from surroundings.
Bond breaking (+)
What is enthalpy change?
Energy to and from surroundings is measured as enthalpy change.
formula for enthalpy change
Hproducts- Hreactants
What are enthalpy changes measured in?
kilojoules per mole
system
refers to the bonds in the reactants and products of the systen
surroundings
the rest of the world eg air and test tube
What are standard conditions?
To compare enthalpy changes standard conditions are chosen.
What’s the standard temperature?
298k
What is the standard pressure?
100 kPa
What is the standard concentration of solutions?
1 mol/dm3
How to convert Celsius to Kelvin?
Add 273
Standard enthalpy change of a reaction
The enthalpy change when molar quantities of reactants react together under standard conditions.
Standard enthalpy change of combustion
Enthalpy change that occurs when one mole of a substance is burnt in oxygen. Impossible under standard conditions.
Standard enthalpy change of formation
Enthalpy change that occurs when one mole of a compound is formed from its elements. When you make one mole of water from hydrogen and oxygen , 286kj are transferred to surroundings.
Enthalpy cycles (Hesse’s law)
Provide an alternative route to find the enthalpy change for a difficult to measure reaction.
Aromatic compounds
If the carbon atoms contain a benzene ring they are called aromatic compounds (historical) and called arenes.
Aliphatic compounds
They do not contain a benzene ring
homologous series
methane, ethane, propane and butane as they all differ by CH2.
General formula for alkanes
CnH2n+2
Why are alkanes saturated?
They have single bonds
What is the alkyl group?
the alkane with a hydrogen removed
examples of alkyl groups
methyl (CH3)
ethyl (CH3 CH2)
propyl (CH3 CH2 CH2)
How to name cycloalkanes?
- Count the no of C’s in the cyclic structure and find the correct prefix
- Add -ane to the end of the cyclo prefix.
formula for overall energy change
energy from bond breaking - energy for bond making
Why are alkenes unsaturated?
Has a double bond
Heterogenous catalyst
At a diff state to species in reaction.
Adsorbing molecules onto an active site on the surface of the catalyst. These increase the proximity of molecules and weaken the covalent bonds in the molecules allowing reactions to occur more easily. Faster rate of reaction.
Ideal gas equation
Pressure x volume = amount of gas (moles) x gas constant (8.31) x temp
Sigma bond
Single c-c bond electrons are arranged between atoms in an area of increased electron density
Pi bond
C=C bond there’s 1 sigma and second is pi
Moles of a gas
Volume/ 24
Solid line
Bond is in plane of paper
Wedged line
Bond comes out plane of paper
Dotted line
Bond goes into plane of paper
Overall enthalpy change
Energy to break bonds (+ve) + energy to make bonds (-ve)
Hess law enthalpies of formation
Arrows point up from central product C.
Hess law enthalpies of combustion
Arrows point down. Products always water and CO2.
Catalyst poisoning
Impurities in a reaction may bind to a heterogenous catalysts surface and block reactants from being adsorbed.
Thermal cracking
High temp and high pressure
Catalytic cracking
Produces aromatic compounds with carbon rings. Lower temp w normal pressure but a catalyst used to compensate. Hydrocarbon vapour passed over the heated catalyst.
Test for alkenes
Bromine water colour change from orange to colourless.
Electrophiles
Electron acceptors.
Electrophilic addition
Reaction mechanism that shows how electrophiles attack the double bond in alkenes. Carbocation forms. A carbon atom with only 3 ions making it pos ion.
Alkenes undergo addition reactions with hydrogen to form alkanes. Conditions depend on catalyst used:
- Nickel: high temp and pressure
- Platinum: room temp and pressure
Alkenes undergo addition reactions with catalysts to form alcohols
- Sulphuric acid then add water
- Steam, phosphoric acid, high temp and pressure
Stereoisomers
Same structural formula but diff spatial arrangement of atoms and bonds
E isomer
Hydrogen atoms are apart of
Z isomer
Hydrogen atoms together on the same side.
