The Ozone Story Flashcards
What is ppm? how does it work?
This is parts per million. It is the number of molecules of a gas per 1 million molecules.
What is 1% in ppm?
1% = 10,000ppm
What is the relationship with frequency and wavelength? What is the equation?
c= λν c = speed of light λ = wavelength ν = frequency
How can EM radiation transfer energy to molecules? (4 methods?
- Translation (molecule moving)
- Rotation (of the whole molecule)
- Vibration of the bonds
- Electron energy
Which transfer of EM radiation to molecules has the most energy? What is the order?
Highest 1. Electronic 2. Vibration 3. Rotation 4. Translation Lowest
How is energy quantised? (When talking about the transfer of EM radiation to molecules)
Energy comes in discrete values. So electronic, vibrational, rotational or translational energy is in discrete values
How can the method of transferring energy from EM radiation to bonds be determined?
The frequency of the EM radiation determines how energy is transferred to molecules. But the frequency is different for different molecules and different bonds§
What are the 3 different electronic outcomes of a molecule absorbing EM Radiation? e.g. Cl2
- Electrons may be excited to a higher energy state. This is why chlorine looks green because green light is not absorbed.
- The bonding of the molecule may break forming radicals. This is photodissociation. Cl ==> Cl• + Cl•
- the molecules may get so much energy that an electron leaves it and an ion is formed. Cl ==> Cl2+ + e-
Which type of electronic absorption is the least energy? What is the order?
Lowest 1. Exciting electrons 2. Dissociation 3. Ionisation Highest
What is a radical?
It is an atom that has a single unpaired electron in its outer shell
What is bond fission?
Bond breaking
What are the two forms of bond fission in covalent bonding and what is the difference?
Heterolytic - Both of the shared electrons go to one of the atoms in the bond
Homolytic - Each atoms gets one electron
How is the possibility of heterolytic bond fission increased?
If a molecule is polar it is more likely to do heterolytic bond fission
What does Br• mean? does it have a charge?
It means that there is a single unpaired electron on the bromine molecule. It does not have a charge because it is the same structure as found on the periodic table
Why are radicals reactive?
Because atoms are more stable with a filled outer shell
How do radicals react? (3 Methods)
- Initiation
- Propagation
- Termination
What is the process of initiation?
This is when a molecule absorbs energy and dissociates to form radicals
e.g. Cl2 + hν ==> Cl• + Cl•
What is the process of propagation?
This is when a radical takes the electron from a molecule because it is so reactive and this produces another radical.
e.g. Cl• + H2 ==> HCl + H•
What is the process of termination?
This is when two radicals collide and react to form no radicals
e.g. Cl• + Cl• ==>Cl2
How are curly arrows used?
They are used to indicate the movement of electrons.
The tail shows where the electrons start and the head shows where they finish.
What if a curly arrow has half a head?
It indicates the movement of a single electron
What if a curly arrow has a fill head?
It indicates the movement of an electron pair
Are alkanes reactive? How can they react with halogens? What are the 3 stages for this reaction?
They are not very reactive and only react with halogens in the presence of light. Initiation: Cl + hν ==> Cl• + Cl• Propagation: Cl• + CH4 ==> HCl + CH3• CH3• + Cl2 ==> CH3Cl + Cl• Termination: Cl• + Cl• ==> Cl2 CH3• + Cl• ==> CH3Cl CH3• + CH3• ==> C2H6
What is the equation for measuring the rate of a reaction?
Reaction Rate = Change in property / time taken
What is reaction kinetics?
This is the rate of a reaction
What factors affect the rate of a reaction?
Temperature Pressure Particle size Concentration Intensity of radiation Catalyst
How is the amount of gas evolved measured?
The reaction takes place in a conical flask with a bung and delivery tube into a tub of water. This then goes into an upturned cylinder. The reaction is started and the gas displaces the water in the beaker so the amount of gas evolved can be calculated
What are 5 methods of measuring the rate of a reaction?
- Gas evolved
- Mass change
- pH measurement
- Colorimetry
- Chemical analysis
How can mass change be used to measure reaction rate?
By graphing mass lost against time, the gradient is the rate of the reaction
How can colorimetry be used to measure reaction rate?
It measures the change in colour of a reaction.
How is chemical analysis used as a way of measuring reaction rate?
Samples are taken throughout the reaction and then the samples are analysed using chemical reactions.
What is collision theory for reactions?
This is that particles need to collide with enough kinetic energy to overcome the activation energy to form a bond. If this energy is exceeded a bond can form.
What is an enthalpy profile and what does it look like?
It is a diagram of enthalpy against reaction progress. It starts with reactants on the left and products on the right. Either above (Endothermic) or below (exothermic) the reactants level. Then a transition state is graphed which shows the activation enthalpy.
What is the activation enthalpy?
It is the minimum amount of energy for a bond to form between two reactants. It is the difference between the enthalpy of the reactants and the highest part of the transition state.
How is the enthalpy of an endothermic reaction shown on an enthalpy diagram?
The level of the products will be higher than the level of reactants. So more energy it needed to reach the activation enthalpy than is released between the activation enthalpy being reached and the formation of the products.
How is the enthalpy of an exothermic reaction shown on an enthalpy diagram?
The level of the products will be lower than the level of reactants. So less energy it needed to reach the activation enthalpy than is released between the activation enthalpy being reached and the formation of the products.
What is the transition state?
This is the moment when old bonds stretch and new bonds start to form. It only happens for a very short time
How do catalyst increase reaction rate?
They provide an alternative reaction path with a lower activation enthalpy
What is a Maxwell-Boltzmann distribution?
This is a graph of kinetic energy on the x-axis and a graph of the fraction of particles with that kinetic energy. The kinetic energy of molecules in a reaction are distributed along the diagram
What happens to a Maxwell-Boltzmann distribution when temperature is increased?
More molecules get kinetic energy so the distribution moves towards the right.
What else can be plotted on a Maxwell-Boltzmann distribution? and how does this help explain reaction rate?
Activation enthalpy can be plotted. All molecules with a kinetic energy higher than the activation enthalpy can react. This shows how by increasing the temperature more molecules have enough energy to react
What happens to the activation enthalpy on a Maxwell-Boltzmann distribution when a catalyst is added?
The activation enthalpy moves to the left because a lower activation enthalpy is needed
How does a catalyst work?
It does this by creating an alternative reaction pathway with a lower activation enthalpy to make a product from the reactants. This then allows more atoms which now surpass the activation enthalpy threshold to react.
Do catalysts affect the position of equilibrium?
No, they have no effect
Do catalysts affect the rate at which equilibrium is obtained?
Yes they speed up the time it takes for reactants and products to reach equilibrium
What is a homogeneous catalyst?
The catalyst is in the same physical state as to what it is catalysing. So it needs to be removed / filtered from the products formed.
What is the state that reactants are in when reacting with a catalyst before they from products?
The transition state
What are the reactions between a chlorine radical and ozone? What type of reactions are these?
Cl• + O3 ==> ClO• + O2
ClO• + O ==> Cl + O2
These are homogeneous catalyst reactions
What is the overall reaction for the removal of ozone from the atmosphere?
O + O3 ==> 2O2
What is the general equation for the removal of ozone with radicals?
X• + O3 ==> XO• + O2
XO• + O ==> X• + O2
What are 3 examples of radicals that could react with ozone?
Cl•
Br•
OH•
How does nitrogen monoxide react with ozone?
NO + O3 ==> NO2 + O2
What are NO and NO2 both? but why are they a bit different?
Radicals but they are quite stable
What is a haloalkane?
It is an alkane with one or more halogens bonded to the carbons.
How are haloalkanes named?
- The halogen name is put infront of the alkane name.
- Di, tri or poly is put if necessary before the halogen name.
- The carbon number that the halogen is attached to is put at the front.
- If there is more than one halogen then steps 1 to 3 are done twice with the halogen with a name starting closer to the letter A first.
e. g. 3-Bromo-1-chlorobutane
What is it about the carbon-halogen bond that causes it to have certain physical properties?
The bond is polar
How are atoms in covalent molecules held together?
Each atom attracts a pair of electrons
What affects the attraction of electrons in a covalent bond?
The number of protons and the number of shells. The more protons the stronger the attraction. The more shells the further away the electrons are held so the weaker the attraction of the protons in the nucleus
What is a polar bond?
When atoms attract the share electrons unevenly, each atom gets a partial charge. The atom furthest away from the shared electrons get a partially positive charge and the atom closest to the shared electrons gets a partially negative charge.
What is the symbol of a partial charge?
δ+ or δ-
What is electronegativity?
It is an atoms attraction to the shared electrons in a covalent bond
How does electronegativity trend in the periodic table?
The more to the right, the more electronegative. The further up the table the more electronegative
What is the most electronegative molecule?
Florine
How can the polarity of a bond be calculated?
Each element has a value for electronegativity. This can be compared to see the polarity of a bond
What is an intermolecular bond? and how are they often broken?
This is the attraction of different molecules to one another. The intermolecular bonds in a solid are most present and are stronger in a solid. They are often broken with heat. This causes solids to melt and liquids to turn to gasses
What is the trend in the boiling points of alkanes?
The longer the alkane the higher the boiling point
What is the trend for non-strightchain alkanes?
The more it is ‘broken up’ the lower the boiling point. Pentane has a higher boiling point than 2,3-dimethylbutane
What is a dipole?
It is a molecule (or part of a molecule) with different charges on either end of the molecule
What does polarised mean?
It means that each end of the molecule (or part of the molecule) has a different charge.
What are the 3 types of dipoles?
Permanent dipoles
Instantaneous dipole
Induced dipole
What is a permanent dipole?
This is when the atoms in a bond have significantly different electronegativities. So each atom has a permanent partial charge
What happens to the dipoles of a carbon with 4 chlorine atoms attached?
The dipoles cancel out because of the symmetry of the molecule
What is an instantaneous dipole?
This is temporary dipole that is formed on atoms that have the same electronegativities. It happens when electrons in the cloud around the atoms happens to tend towards one side. This causes a partial charge to form for a small amount of time
What is an induced dipole?
This is when an unpolarised molecule comes near to a polarised one. A partial charge is induced on the originally unpolarised molecule because like charges repel. It can be induced from permanent and instantaneous dipoles
What are the different types of dipoles that make a bond? (4 types)
Instantaneous dipole - induced dipole
Permanent dipole - Permanet dipole
Permanent dipole - induced dipole
Hydrogen bond
What is the order of strongest type of intermolecular bond to weakest?
Strongest Hydrogen bond Permanent dipole - Permanent dipole Permanent dipole - Induced Dipole Instantaneous dipole - Induced dipole Weakest
What is a diatomic molecule?
A molecule made from two atoms of the same element
What bonds form between diatomic molecules?
instantaneous dipole - induced dipole because they are non polar
The larger the atom in a diatomic molecule…
… the larger the attraction
Why does the boiling point of halogens increase down the group?
Because the atoms get larger so there is a larger chance for an instantaneous dipole - induced dipole to occur.
How strong is a hydrogen bond?
It is the strongest type of intermolecular force
What is a hydrogen bond?
It is a special type of permanent dipole - permanent dipole bond. Formed between a hydrogen and another electronegative atom bonded together
What are the 3 criteria for a hydrogen bond to form?
- A large dipole between a hydrogen and a highly electronegative atom.
- A small hydrogen atom
- A lone pair on the electronegative atom.
How does a hydrogen bond work?
The hydrogen becomes partially positively charged and this is then attracted to the partially negative charge on the electronegative atom on another molecule
How do hydrogen halides intermolecularely bond?
They hydrogen bond
Does water or hydrogen halides form better hydrogen bonds?
Water because it has two hydrogens so can form many more bonds. There are two lone pairs on the oxygen and two hydrogens so it is a 1 to 1 ratio.
With Hydrogen halides its a 1 to 3 ratio and with ammonia its a 3 to 1 ratio. Water is special
How does hydrogen bonding affect the physical properties?
Hydrogen bonds cause compounds to have higher boiling points than ones without.
Hydrogen bonds increase the viscosity of liquids (makes them flow more easily)
Hydrogen bonds make compounds soluble in water more easily
What do reactions involving haloalkane often involve?
Breaking the carbon halogen bond
What is homolytic bond fission?
This is when a bond breaks from uv light and each molecule/atom produced has one unpaired electron in its outer shell
What is heterolytic bond fission?
This is when a bond breaks from uv light and one molecule/atom gets both electrons from the bond and the other atom/molecule gets no electrons and has two unpaired electrons
What is a substitution reaction?
This is when a negatively charged molecule reacts with a partially polarised carbon atom
What is a nucleouphile?
This is a molecule or atom that is attracted to positive charge. They are often negatively charged
What is the mechanism for 1-bromo-butane when reacted with OH-? What type of mechanism is this?
- The OH- attacks the carbon next to the bromine (because it is δ+ because bromine is electronegative)
- The electrons in the carbon bromine bond move to the bromine and the lone pair of electrons on the OH- bond with the carbon
This forms butan-1-ol and Br-
It is a heterophilic nucleophile substitution reaction.
What type of reaction is the reaction between 1-bromo-butane and OH-?
It is a nucleouphilic substitution reaction
What is the general symbol for a nucleouphile?
Nu-
What is the reaction with water as a nuclouphile?
nucleophile substitution
The water substitutes the halide and the Oxygen becomes O+ so a hydrogen donates its electrons to the oxygen and becomes H+ this then reacts the halide.
R-X + H2O ==> R-OH2 + X- ==> R-OH + H+ + X-
What is the reaction with ammonia as a nucleouphile?
nucleophile substitution
The ammonia substitutes the halide and the nitrogen becomes NH3+ . A hydrogen donates its electrons to the nitrogen and becomes H+ and this reacts with the halide.
R-X + NH3 ==> R-NH3+ + X- ==> R-NH2 + HX
What type of a reaction is nucleophilic substitution with water?
Hydrolysis
What is the mechanism for the nucleophilic substitution of alcohols with h+?
It is the reverse of other nucleophilic substitution reactions.
R-OH + H+ ==> R-OH2+
R-OH2+ + X- ==> R-X + H20
What are the factors that affect nucleophilic substitution with halogens?
Bond enthalpy and bond polarity.
Bond enthalpy says that C-I is the weakest bond so is most easy to break
Bond polarity says that C-F is the most polar
