Green chemical processes
atom economy = (Mr useful product / total Mr reactants) x 100
Rearrangement and Addition reactions also have an atom economy of 100%
CH3CH2CH2CH2CH2CH3 --------> CH3C(CH3)2CH2CH3
Mr = 86.0 Mr = 86.0 =100% atom economy
CH2CH2 + H20 ---------> CH3CH2OH
Mr = 28.0 Mr = 18.0 Mr = 46.0 =100% atom economy
Substitution and Elimination reactions have and atom economy of less than 100%
The principles of green chemistry are:
- minimise waste
- reduce feedstock consumption
- reduce energy consumption.
Bonding and structure properties
The properties of materials are determined by:
- The types of particles in the material - e.g. atoms, ions or molecules
- The bonding in the material - e.g. covalent, ionic, metallic or intermolecular bonds
- The structure present in the material - e.g. giant lattice, molecular or macromolecular
Trends in properties
- Giant lattice structures generally have high - very high meting and boiling points.
- Covalent molecular structures usually have low - moderate meting and boiling points.
- Giant lattice structures are usually insoluble in non-polar solvents
- Covalent molecular structures are mostly soluble in non-polar solvents
Equilibria and Concentrations
The law of equilibria states:
Kc = [C]^c[D]^d / [A]^a[B]^b (Products divided by the reactants)
The Kc is the equilibrium constant for the reaction at a specified temperature.
Kc is a measure of how far a reaction proceeds. If an equilibrium mixture is composed largely of reactants, then the value of Kc is small, and vice versa.
Exothermic reactions Endothermic reactions
Temperature increases Value of Kc decreases Value of Kc increases
Temperature decreases Value of Kc increases Value of Kc decreases
Equilibrium, rates and industry
Manufacture of ammonia (Haber Process)
The raw materials are air and natural gas, from these, a feedstock of nitrogen and hydrogen, ratio 1:3, is made.
N2 (g) + 3H2 <------> 2NH3 (g)
The usual conditions are:
- iron catalyst
- temp 450 degrees Celsius
- 200 atm pressure
Increasing the temperature speeds up the rate of the equilibrium, but decreases the yield. The position of the equilibrium moves to the left (exothermic reaction)
Increasing the pressure increases yield (position of equilibrium moves to side with fewer moles) and the rate. It is expensive (capital costs and electricity costs). Using a catalyst speeds up the rate at which equilibrium is achieved.
Nitrogen gas consists of diatomic molecules, which are very unreactive because of the extremely high activation energy needed to start breaking bonds.
Nitrogen species Formula Oxidation state Action producing the species
Nitrogen gas N2 (g) 0 Denitrifying bacteria in the soil
Nitrate(v) ion NO3- (aq) +5 Nitrifying bacteria in the soil
Nitrate(iii) ion NO2- (aq) +3 Nitrifying bacteria in the soil
Ammonium ion NH4+ (aq) -3 Root nodules in legumes, bacteria and microorganisms in soil
Dinitrogen(i) oxide N2O (g) +1 Denitrifying bacteria in soil
Nitrogen(ii) oxide NO (g) +2 Car engines, thunderstorms, denitrifying bacteria in soil
Nitrogen(iv) oxide NO2 (g) +2 Oxidation of NO in the atmosphere