Molecules and Energy
Photons of electromagnetic radiation hit matter and transfer energy to it. The energy absorbed causes changes to happen in the matter. The photons of different energy cause different changes in the matter.
These energy levels are quantised as only certain fixed energy levels are possible. By absorbing photons of a specific frequency or energy the molecule is able to move to a higher energy level.
Energy of 1 bond
Energy of 1 bond = Bond enthalpy x 1000/ Avogadro constant
Avogadro's constant= 6.02x1023
Sunscreens are chemicals that absorb harmful UV radiation from the sunlight to protect the skin
desirable properties include;
- absorb the harmful uv radiation
- easy to apply
- long lasting, waterproof, nice smell....
The Atmosphere is a good sun screen as gasses absorb the uv radiation strongly, preventing much of the suns harmful radiation from reachinghte earths surface.
Ozone or O3 is the most important at doing this.
However is ozone was present at ground levelit would be an ashma trigger and also very corrosive of rubber and plastic.
Radiation and Radicals
There are two ways in which a covalent bond can be broken.
Heterolytic fission and Homolytic fission
- Heterolytic fission- The covalent bond brake unevenly with both bonding electrons going to one of the atoms to produce ions.
- Homolytic fission- The covalent bond breaks evenly with each atom keeping one of the bonding electrons. This produces an uncharged species with an unpaired electron called a radical.
Radicals have certain characteristics;
- An atom or group of atoms with an unpaired electron
- Very reactive--> so short lived.
- However some like NO are fairly stable and exist as molecules.
Radical reactions involve 3 stages:
Homolytic fission of a covalent bond by a light photon to produce 2 radicals. This can also be called photodissociation.
for every radical used up andother is produced as there is no net destruction of the radicals the reaction can occur over and over and result in a chain reaction.
two radicals combine and are removed from the reaction. Such reactions are not common as the concentration of radicals is low.
These reactions are very fast, initiated by light or heat and often occur in the gas phase.
Formation of ozone
1. photodissociation of O2 to produce two Oxygen radicals.
2. The 2 radicals can recombine to form O2 or one of the radicals can combine with O2 to form O3
Once produced it can be destroyed in 2 ways
1. an ozone molecule can combine with an O radical to form 2 O2 molecules
2. Ozone molecules can photodissociate when they absorb uv radiation with frequencies in the rangy 10-14x1014 HZ
Factors affecting reaction rates
Factors affecting reaction rates
Temperature- 10°c rise approximately doubles rate
Pressure- Gases= double pressure-> doubles concentration
Concentration- Double concentration-> doubles rate
Surface area- increasing the surface area increases the places for the reaction to occur therefore increasing rate.
Catalyst- provides an alternative route with a lower activation enthalpy
Light intensity- increase the light intensity- more photons allowing more bonds to break- generates more radicals.
How do catalysts work
Catalysts speed up reactions by provinding a new pathway for the reaction with a lower activation enthalpy. Therefore at a given temperature, more colliding reactant molecules have sufficient energy to overcome this lower energy barrier and rate is faster.
homogeneous catalysts normally work by forming an intermediate compound with the reactants. this compound then breaks down to give the product and reform the catalyst.
uses of CFC's
- solvents in dry cleaning
- blowing agents for expanding plastics
- propellants in aerosol cans
Properties making them suitable
- non toxic
- non flammable
- low boiling point
- relatively cheap
Why they destroy the Ozone layer
- stable in the troposphere so don't break down
- CFC's migrate to the stratosphere
- High energy UV light in the stratosphere breaks it down forming Cl radicals which catalyse the destruction of ozone.
HCFC's HCF's Alkanes
Advantages of replacing CFC's with HCFC's HCF's Alkanes
- Don't produce as many Cl radicals
- highly flammable
- Green house gasses
- some take up to 10 years to decompose.
Green House effect
The suns energy reaches the earth as mainly vsible and Infra red light. The earth absorbs some of this energy and heats up the earth's surface which emits the IR radiation into the troposphere. Greenhouse gasses in the troposphere absorb some of the IR radiation. This absorption causes the bonds to vibrate more so the molecule moves to a higher vibrational level this energy is then transferred to other molecules. The kinetic energy of the gas molecules increase. So the temperature of the gas increases so heats up the atmosphere and the earth.
Increasing the concentration on the greenhouse gasses increase the greenhouse effect.
Reversible reactions can reach a state of dynamic equilibrium.
A chemical equilibrium is a state of balance in which the concentration of reactants and products do not change.
Chemical Equilibria are always dynamic because the rate of the forward reaction equals the rate of the reverse.
Positions of the equilibria
Higher concentrations of reactiants than products the equilibria lies on the Left Hand Side of the reaction
Higher concentrations of products than reactants then the equilibria lies on the Right Hand Side of the reaction.
Le Chatelier's Principle
Le Chatelier's Principle- "If a system is at equilibrium, and a change is made in any of the conditions, then the system responds to counteract the change as much as possible.
Increasing pressure shifts the equilibrium to the side with the less gas molecules as this tends to reduce the pressure
Decreasing the pressure shifts the equilibrium to the side with more gas molecule as this tends to increase the pressure.
If temperature is increased the reaction counteracts this by moving in the direction that uses the heat up-> in the endothermic direction
If temperature is decreased the reaction counteracts this by moving in the direction that produces heat-> in the exothermic direction.