Rates and equilibria
- Minimum amount of kinetic energy particles need to react is activation enthalpy.
- Catalysts speed up reactions by lowering Ea by providing an alternative pathway for bonds to break and remake. Means more particles will have enough energy to react.
- Dynamic equilibrium occurs when the rate of forward reaction is equal to the rate of back reaction. The concentration of reactants and products remain constant.
- Le Chatelier's principle says: if there's a change in pressure or temperature, the POE will move to help counteract the change.
- Catalysts have NO EFFECT on POE - they can't increase yield, only rate of reaction.
Rates and equilibria - conditions of an industrial
Consider: rate of reaction, product yield, cost, right reaction conditions.
- Need to make lots of product cheaply and quickly.
- TEMP - high = faster, but expensive.
- PRESSURE - high = faster, expensive and DANGEROUS (reaction vessels must be made of stron material and incorporate safety systems = expensive)
- CATALYSTS - can be expensive, but will make reaction faster at lower temp. DON'T GET USED UP.
Haber process (don't need to know)
POE affects how economical a reaction is.
N2(g) + 3H2(g) <----> 2NH3(g) Enthalpy change = -92kJmol^-1
- Carried out at 400C and 200atm.
- High pressure moves POE to right as fewer molecules, so increasing yield. Also increases rate of reaction.
- Higher temp makes reaction go faster, but lowers product yield as POE would shift to left.
- 400C is compromise - greater yield at lower temp, but too slow to be economical.
- Kc is equilibrium constant.
- Need to know MOLAR CONCENTRATION of each substance at equilibrium.
- Kc value only true for that particular temp.
- With gases, can use PARTIAL PRESSURES instead of concentrations.
- Total pressure of a gas mixture is the sum of all partial pressures of individual gases.
- Partial pressure of A = (number of moles of A/total number of moles of gas) * total pressure
- If number of moles of gas increases, so does its partial pressure.
- Only changes in TEMPERATURE causes Kc and Kp to change.
- Catalysts don't have an effect, as they affect forward and back reactions equally, so no overall change to equilibrium.
- Occurs naturally as diatomic molecule and has triple bond.
- N2 very unreactive as triple bond difficult to break.
- In ammonia, N forms covalent bonds with H leaving a LP.
- NH3 forms H-bonds = very soluble in water.
- LP means can form dative covalent bonds - LIGAND.
- LP also means acts as a BASE - NH3 + H+ forms NH4+.
- 3 oxides need to learn about:
- NO - nitrogen monoxide - colourless gas.
- N2O - dinitrogen monoxide - colourless gas, sweet smell.
- NO2 - nitrogen dioxide - brown gas, sharp odour, toxic.
- The purpose of the nitrogen cycle is to convert nitrogen into more accessible forms. It converts nitrogen through its different oxidation states.
- LEARN NITROGEN REDOX REACTIONS.
- Industrial chemical production impacts on the environment by:
- Using raw materials and energy
- releasing waste products
- % ATOM ECONOMY = (mass of desired product/total mass of products) x 100
- % YIELD = (actual product yield/theoretical yield) x 100
- Costs involved running a chemical plant
- Raw materials - cheap, widely available ones best.
- Lots of E needed for high temp and pressure and to transport chemicals.
- Fixed costs - staff wages, rent, insurance, bills,
- Disposal costs - unwanted products disposed of safely.
- Risks of producing a chemical
- Some chemicals highly flammable - must be stored and handled correctly.
- Some chemicals harmful to health - Cl toxic if inhaled - irritate eyes and lungs. Spill during transportation could expose danger to public.
- Some chemicals damage environment - SO2 contributed to acid rain.
Chemistry industry can improve food production. There are three ways:
1. FERTILISTING THE SOIL
- Provides crops with nutrients = increased crop yield.
- May result in leaching, therefore eutrophication.
- Nitrate (V) ions get into drinking water - health risks to children.
2. ACID NEUTRALISATION
- Each crop has optimum soil pH.
- Too acidic = poor plant growth, so add limestone to neutralise acid.
3. KILLING PESTS
- Pesticides kill pests, which reduce crop yield.
- Pesticides may accumulate in food chain and build up being harmful to birds etc.
- Kill non-target organisms, which may have been beneficial to crop.
- May be washed from soil, end up in drinking water.