chemistry ocr b: ozone

how do chemical reactions occur?

when reactant particles collide with energy greater than or equal to the activation enthalpy and they must collide in the correct orientation

what is activation enthalpy?

the minimum enthalpy required for a reaction to occur between 2 colliding particles

what effect does concentration/ pressure have on the rate of reaction?

when the concentration or pressure of a species increase, there are more molecules of the substance in the same volume, so they are packed closer. Therefore collisions between molecules become more likely. The probability of collisions occurring with energy greater than the/equal to activation enthalpy increases, so rate of reaction increases

what effect does temperature have on the rate of reaction?

when a substance is heated, thermal energy is transferred. This energy is converted into kinetic energy and molecules of the substance move faster and further. Increased molecule movement means more frequent collisions with greater energy. As a result, more collisions have greater than the activation enthalpy, causing more reactions. Therefore, increasing the reaction temperature, increases the rate of reaction

boltzmann distribution

the distribution of energies of molecules at a particular temperature, usually shown as a graph

what does the peak of the boltzmann curve show?

the most probable energy

what happens to the boltzmann distribution curve when the gas is heated?

the most probable energy increases, but fewer molecules have this energy. More particles have greater than the activation enthalpy

what happens to the boltzmann distribution curve when a catalyst is added?

the curve remains unchanged in shape but the position of the activation enthalpy is shifted to the left, so that a greater proportion of molecules have sufficient energy to react

rate of reaction equation

change in property/ change in time

how to measure rate of reaction?

volume of gas evolved

mass changes

pH measurement

colorimetry

chemical analysis

how to use colorimetry?

measure the change in colour in an experiment

what is chemical analysis?

involves taking samples at regular intervals and quenching the reaction before analysis

how do catalysts affect the rate of reaction?

provide an alternate reaction pathway with a lower activation enthalpy, increasing the rate of reaction without being used up

what is homolytic fission?

the breaking of a covalent bond with one of the bonded electrons going to each atom, forming two radicals.

What is heterolytic fission?

covalent bond breaks and both electrons move to one atom, two oppositely charged ions are formed

what are the three steps in the radical chain reation?

initiation, propagation, termination

what happens in initiation?

one or more molecules react to form radicals

a covalent bond breaks homolytically, producing 2 radicals (endothermic- photodissociation)

UV light provides energy to break bond

what happens in propagation?

highly reactive radicals react to form new molecules

a bond breaks and a new bond is formed

usually occurs in pairs

new radicals are created to go and react with non-radicals, so propagate the reaction

what happens in termination?

occasionally, 2 radicals react collide and the 2 unpaired electrons pair to make a covalent bond

this stops the reaction = termination (endothermic)

is termination exothermic or endothermic?

endothermic

is initiation exothermic or endothermic?

exothermic

how are radicals formed?

alkanes react with halogens in the presence of UV light to produce haloalkanes

the UV light breaks down halogen bonds by homolytic fission, producing reactive intermediates called free radicals

what are free radicals?

free radicals are species containing an unpaired electron

what is the nature and reactivity of radicals?

they are unstable and highly reactive, they attack alkanes

what is formed when halogen radicals react with alkanes?

haloalkanes

what type of reactions do halogens react with alkanes?

photochemical reactions (started by light)

where is the ozone layer?

stratosphere

how are ozone molecules formed?

when UV radiation from the sun hits oxygen molecules

what does the ozone layer contain?

most of the atmosphere ozone molecules (O3)

what happens when UV radiation hits an oxygen molecule?

if the right amount of UV radiation is absorbed by an oxygen molecule, the oxygen molecule splits into separate atoms or free radicals

the free radicals then combine with other molecules to form ozone molecules (O3)

why is the ozone layer being constantly destroyed and replaced?

UV radiation can also reverse the formation of ozone

why is the ozone layer important?

it protects earth from UV radiation

what are the effects of high energy UV radiation?

damage DNA in cells

cause skin cancer

cause sunburn

how does the ozone layer protect us?

when ozone breaks down, it absorbs high energy UV radiation, so the ozone layer removes all the high UVC radiation and about 90% of the UVB

what do humans use UV radiation for?

produce vitamin D

why does ozone also occur in the troposphere

due to the effect of sunlight on mixtures of nitrogen dioxide and hydrocarbons, which are released from vehicle engines and power stations

how is photochemical smog formed?

when ozone mixes with solid particles of carbon and other substances

what are the dangers of photochemical smog?

respiratory problems

toxic to humans

what are chlorofluorocarbons? (CFCs)

haloalkanes that have all their hydrogen atoms replaced with chlorine and fluorine atoms

what do chlorofluorocarbons (CFCs) do?

break down ozone in the stratosphere

how are chlorofluorocarbons (CFCs) broken down in the stratosphere?

the C-Cl bond can be broken down by high energy UV radiation in the stratosphere to form chlorine radicals, which act as catalysts in the breakdown of the ozone

why are only a few chlorofluorocarbons (CFCs) broken down in the troposphere?

as most of the high frequency UV radiation has been absorbed by the ozone layer

how do chlorofluorocarbons (CFCs) break down ozone in the stratosphere?

chlorine free radicals are formed when CFCs are broken down by high energy UV radiation

these free radicals are catalysts

they react with ozone to form intermediate (ClO), and an oxygen molecule

the intermediate can then react with an oxygen atom to form an oxygen molecule and chlorine free radical

the reaction can only be terminated if two radicals react together

what are the initiation, propagation and termination reactions in this process?

initiation: CCl3F + hv --> CCl2F + Cl

propagation: Cl + O3 --> O2 + ClO

propagation ClO + O --> O2 + Cl

termination: Cl + Cl --> Cl2

how can you compare the reactivities of the haloalkanes?

1. put a chloroalkane, bromoalkane, iodoalkane in three different test tubes

2. add silver nitrate solution and ethanol to each of the test tubes

3. an insoluble silver halide compound will form a precipitation

4. the most reactive will form precipitate the fastest

give chloroalkane, bromoalkane and iodoalkane in order of reactivity

iodoalkane (most)

bromoalkane

chloroalkane (least)

why is chloroalkane the least reactive?

the C-Cl bond is the strongest and for a reaction to occur the carbon-halogen bonds need to break

why is iodoalkane the most reactive?

the C-I bond isn't as strong so breaks easily