[Year 1] OZ Flashcards
What is electronegativity?
The ability for an atom to attract electrons towards itself in a covalent bond.
What is the trend in electronegativity on the periodic table?
Further up and right you go the more electronegative the element is (Fluorine is the most electronegative).
What makes a bond more ionic?
The greater the difference in electronegativity the more ionic the compound will be.
What is the Pauling scale?
A scale that helps us quantify electronegativity.
What is the electronegativity difference of a purely covalent bond?
0
e.g. Cl-Cl
What make a bond more polar?
- A bigger difference in electronegativity
- which pulls the shared electrons in a covalent bond closer to the more electronegative atom.
How can we show polarity?
δ- and δ+
What is a polar molecule?
The uneven distribution of charge.
What are the three intermolecular forces/bonds in order of strength (Strongest first)?
Hydrogen Bonding.
Permanent dipole-permanent dipole.
Instantaneous dipole-induced dipole.
How are id-id forces made?
- Any molecule or atom with electrons moves near to another atom or molecule.
- The electrons in the molecule or atom can move from one end to another creating an instantaneous dipole.
- The δ- on one atom will repel electrons from neighbouring atoms inducing a δ+ near it.
- This will attract to the δ- on another and a force of attraction is created.
What happens when instantaneous dipoles move away from each other?
The id-id interaction is destroyed.
How does Iodine from crystal structures?
- Weak id-id forces hold iodine molecules together.
- Strong covalent bonds hold the 2 iodine atoms together.
- This creates a well-defined repeating structure.
Why does Iodine form crystals but Fluorine does not?
- The bigger the molecule or atom, the more id-id forces. As you have larger electron clouds.
What happens when we boil a liquid?
- We break the weak id-id forces NOT the covalent bonds.
- To do so we must have enough energy to overcome these forces.
Why does the boiling point of hydrocarbons increase as you get longer straight-chain hydrocarbons?
- Longer straight-chain hydrocarbons have more id-id forces.
- More energy is needed to overcome its force.
- Thus boiling point increases.
How does branching effect id-id forces?
- Hydrocarbons that are branched can’t pack as closely together.
- This weakens the id-id forces between the chains.
- Thus lowering there boiling point.
When do we get pd-pd interactions?
When we have molecules with a permanent polarity.
This creates a weak electrostatic force between δ- and δ+ atoms of different molecules. (In addition to id-id forces)
How can we test for polar molecules?
- Placing a charged rod near a steady stream of a polar liquid.
(the steady stream can be made by passing the liquid through burette)
- Polar liquids will bend towards the rod as the molecules align to face the opposite charges rod.
When does Hydrogen bonding occur?
When hydrogen on one molecule forms a bond with the lone pair on nitrogen, oxygen, or fluorine.
(alongside pd-pd and id-id forces)
What are the features of Hydrogen bonding?
- Hydrogen bonds in ice form a regular structure, there are further apart than in water, making ice less dense.
- Hydrogen bonds are the strongest intermolecular force hence hydrogen fluoride has a higher melting point than hydrogen iodide. As more energy is needed to overcome the electrostatic force.
How can we investigate the strength of an intermolecular bond?
- Place dry filter paper around the bulb of a thermometer and take the temperature (control)
- Dip the filter paper into 3 samples with different strengths of intermolecular bonds (Haxane, Ethanol, and Water) for about 5 minutes.
- Record temp change.
Hexane (id-id) highest temperature change.
Ethanol (1 hydrogen bond per molecule) lower temperature change.
Water (2 hydrogen bonds per molecule) lowest temperature change.
Define the rate of a reaction?
How can you calculate it?
The change of concetration/amount of a reactant or product per unit time.
Rate = (reactant used [OR] product made) / time
What is collision theory?
- For a reaction to occur the particles must collide in the right direction.
- And have the minimum amount of kinetic energy required.
What is the activation energy?
The minimum amount of energy required for a reaction to occur.
How might you show a change in energy in a reaction?
With an energy profile diagram.
- Enthalpy by Reaction progress.
What does the Maxwell-Boltzman distibution show?
The energy in gas particles.
What is the area under a Maxwell-Boltzman distribution curve?
Total number of molecules.
What does the peak of a Maxwell-Boltzman distribution curve show?
The most likely energy of a particle in a sample.
Describe why a Maxwell-Bolzman distribution curve changes if the temperature of a reaction increases.
- Particel have, on average, more kinetic energy when they are heated.
- A larger proportion of the molecules will have energy greater than the activation energy.
- So there will be a larger area under the curve beyond the activation energy.
State what happens to a Maxwell-Bolzman distribution curve if the temperature of a reaction increases.
- The curve shifts RIGHT
- The peak is LOWER.
- The area under the curve is the SAME.
- The area under the curve beyond the activation energy INCREASES.
Why do we see a faster rate of reaction when temperature increases?
- Particles move around more at higher temperatures.
- So they collide more often.
- These collisions are more energetic (seen in Maxwell-Boltzman distribution curve).
- So a small increase in the temperature leads to a large increase in rate.
How does pressure effect the rate of reaction?
- Inreace in pressure will increas the rate of reaction.
- As particles are closer together.
- So the collide more often.
- More frequent collisions results in a higher chance of a reaction.
How does concentration effect the rate of reaction?
- Increase in concentration will increase the rate of reaction.
- As particles are closer together.
- So the collide more often.
- More frequent collisions results in a higher chance of a reaction.
What is a catalyst?
- A substance that increases the rate of reaction.
- By providing an alternative pathway that has a lower activation energy.
- The catalyst is chemically unchanged at the end of the reaction.
How does a catalyst effect a Maxwell-Boltzman distribution curve?
- Shifts the activation energy further to the left.
- This means the area under the graph beyond the activation enthergy is increased.
- So more molecules are able to react successfully.
How does a catalyst effect a energy profile diagram?
- It provides an alternative pathway.
- And thus lowertes the activation energy curve.
- So less energy is required for the reaction to progress.
What is a homogeneous catalyst?
- A catalyst that is in the same phase as the reaction.
- For example aqueous in aqueous reactants.
How do homogeneous catalysts behave differently to heterogeneous ones?
- They for intermediate species.
- By reactants combining with the catalyst wich then react to form the products.
- Being reformed again.
Decribe what a energy profile graph looks like with a homogeneous catalyst as oppose to a heterogeneous one?
- They have two activation energies.
- Like a camel hump
What are methods of measuring rate?
How long it takes for a precipitate to form:
- Place a cross and time how long it takes for the cross to disappear (precipitate to form).
- Use the same person to observe to reduce error.
Amount of mass lost:
- For reactions that produce a gas, place on a balance and measure the mass loss as gas periodically.
- Use a fume cupboard if gas is harmful or toxic.
Volume of gas produced:
- If gas is produced, measure the amount of gas produced using a gas syringe.
- Measure this over a specified time.
Measure change in temperature.
- Usign thermometer and timing periodic change.
Measure the change in pH.
- Usign pH probe and timing periodic change.
Measure via titration:
- by taking samples of the reaction at regular intervals.
How can rate be calulated from a graph (linear)?
- From the gradient.
use a bigger section of a graph to get a better accuracy
How can rate be calulated from a graph (curved)?
- draw a tangent (at specified time).
- From the gradient.
(use a bigger section of a graph to get a better accuracy)
What is a halo alkane?
Alkanes with one or more halogen attached to it.
How are haloalkanes named?
- Find longest carbon chain (the last part of the name)
- The names and positions of the halogens on the molecule comes fist.
- Flouro-
- Chloro-
- Bromo-
- Iodo-
IN ALPHABETICAL ORFER IF THERE IS MORE THAN ON HALOGEN. - If there are more than one of the same halogen use the prefix ‘di’, ‘tri’, or ‘tetra’.
Draw 1-bromo-1,1-dichloro-2-iodoethane.
[google image if not clear]
- 2 carbons.
- 2 ‘H’ and 1 ‘I’ on one carbon.
- 2 ‘Cl’ and 1 ‘Br’ on the other.
What is the trend in boiling point of halogenoalkanes?
And why?
- Increase as they go down the group.
- As you go down the group (7) the number of electrons in the halogen increases.
- This means there are stronger instantaneous diploe-induced dipole forces between the molecules.
- So more energy is needed to overcome these forces.
How does bond polarity and nucleophiles relate to haloalkanes?
- Haloalkanes have a polar bond and so are attacked by nucleophiles.
What is a nulceophile?
A substance that is an electron pair donor.
What are examples of nucleophiles?
- Ammonia (NH₃): one lone pair.
- Water (H₂O): two lone pairs.
- Hydroxide ions (OH⁻): two lone pairs.
How do haloalkanes react with hydoxide ions?
What are the conditions?
Via a nucleophilic substitition.
- Warm aqeous sodium hydroxide,
- Under reflux.
Decribe the mechanism for the reaction between Sodium hydroxide and chloroethane.
- The lone pair of electrons on the hydroxide ion (the nucleophile) will attack the 𝛿+ Carbon (with the chlorine bonded to it).
(the hydroxide ion was bonded to the sodium but this is ignored as it is the spectator ion.)
- This heterolyiclalt breaks the C-Cl bond, moving both electrons from the bond onto the 𝛿- chlorine.
- The halogen is kicked of leaving ethanol and a chlroine ion.
(the chorine can then ionically bond with sodium.)
What does heterolytic mean?
When both electrons from a bond move.
Shown by a double headed arrow.
How do haloalkanes react with water?
What are the conditions?
Via a nucleophilic substitition.
- Heat with water (steam).
Decribe the mechanism for the reaction between water and chloroethane.
- A lone pair of electrons on the water (the nucleophile) will attack the 𝛿+ Carbon (with the chlorine bonded to it).
- This heterolyiclalt breaks the C-Cl bond, moving both electrons from the bond onto the 𝛿- chlorine.
- The halogen is kicked of and an intermediate is foremed, as the whole molecule of water is bonded.
- This intermediate is not stable (the O is positively charged).
- electrons from a O-H bond breaks heterolytically, with its electrons moving to the O.
- Ethanol is formed leaving a H⁺ ion.
- The H⁺ ion can then react with the Cl⁻ ion to form HCl.
How do haloalkanes react with ammonia?
What are the conditions?
Via a nucleophilic substitition.
- Heat with ethanolic ammonia.
- Must have excess ammonia.
Decribe the mechanism for the reaction between ammonia and chloroethane.
- A lone pair of electrons on the ammonia (the nucleophile) will attack the 𝛿+ Carbon (with the chlorine bonded to it).
- This heterolyiclalt breaks the C-Cl bond, moving both electrons from the bond onto the 𝛿- chlorine.
- The halogen is kicked of and an intermediate is foremed, as the whole molecule of ammonia is bonded.
- This intermediate is not stable (the N is positively charged).
- The excess ammonia now act as a base and reacts with the H in an N-H bond
- electrons from a N-H bond breaks heterolytically, with its electrons moving to the N, stabilising it.
- An amine is formed leaving an NH₄⁺ ion (amonium).
- The NH₄⁺ ion can then react with the Cl⁻ ion to form ammonium chloride.
What happends to the reactivity of haloakalnes as you go down the group?
Why?
They become more reactive.
- because as you go down the group the bond enthaplies of C-X (X = halogen) becomes smaller.
- C-I has the lowest bond enthalpy and is easiest to break, therefore it is more reactive.
What experimanetal evidence can you give to test the reactivity of haloalkanes?
React with aqueous silver nitrate.
- place chloroalkane, bromoalkane, and iodoalkane into 3 test tubes.
- Add silver nitrate solution and ethanol (solvent) into each tube.
- Yellow precipitate forms first (silver iodide)
- Then cream precipitate (silver bromide)
- And then white precipitate (silver chloride)
What is bond fission?
- The breaking of a covalent bond.
- The electron pair in the bond can be distributed in 2 ways. (homolytic and heterolytic)
What are the types of bond fissions?
Homolytic:
- Electrons distributed equally to form 2 uncharged radicals.
(single-headed arrows showing a single electron moving)
Heterolytic:
- Electrons distributed unequally to form 2 different ions.
(double-headed arrows showing a pair of electrons moving)
What are the stages of a free radical chain reaction?
- Initiation.
- Propagation.
- Termination.
What happens in the initiation stage of a radical chain reaction?
- Radicals are produced using visable or UV light - called a photochemical reaction.
- The bond breaks homolytically producing 2 radicals.
What happens in the propagation stage of a radical chain reaction?
- Radicals react with non radical molecules.
- New eadicals are created which go onto react with other non radicals.
- Hence called a chain reaction.
What happens in the termination stage of a radical chain reaction?
- Two radicals react, forming a non-radical molecule.
- This ends the chain reaction.
What are CFCs?
Chlorofluorocarbons.
What happends to CFCs in the stratosphere?
- Their C-Cl bonds are broken homolytically by UV radiation.
- Radicals are formed which catalyse the break down of ozone.
Are more CFCs broken down in the stratoshere or troposphere?
More in stratosphere.
- Troposhpere is below the stratosphere so most of the high frequency UV has already been absorbed by the ozone layer further up.
- Hence little CFCs are broken down here.
What determines the ability for a photochemical reaction to take place?
The frequency of radiation (usually UV).
Why are C-Cl bonds in CFCs more likely to be broken than C-F bonds by UV?
C-Cl bonds have the lowest bond enthalpy in CFCs.
Describe how CFCs break down ozone.
Initiation:
- Sunlight (UV) breaks the C-Cl bonds in a CFC molecule and produces two radicals.
- There reacti with ozone.
Propigation:
- Cl• reacts with O₃ to from the ClO• intermediate and O₂
- ClO• reacts with more O₃ to make 2O₂ and Cl•
- As Cl• is reformed it acts as a homogeneous catalyst.
Termination:
- 2 radicals react (e.g. 2Cl• → Cl₂)
What is the overall equation for CFCs reaction with ozone?
2O₃ → 3O₂
Cl• is a catalyst
Other than CFCs what can also break down ozone?
Nitrogen oxides
How can we quntify electronegativity?
The pauling scale.
Is CO₂ a polar molecule?
No.
- although it contains polar bonds, the overall distribution of electrons within the molecule is symmetricall.
- and therefore evenly distributed.
How does size of molecule relate to the number of Instatanous dipole - induced dipole attractions?
- The bigger the molecule or atom, the more instantaneous dipole -inmduced dipole forced.
- As you have larger electron clouds.
What bonds are broken when water is boiled?
No bonds are broken.
- The weak instatanous dipole - induced dipole attractions are.
Where is ozone found?
The stratosphere.
How is ozone formed?
When UV radiation hits an oxygen molecule.
O₂ + hν → O• + O•
O₂ + O• → O₃
How is ozone broken down? (naturally)
When UV radiation hits it.
O₃ + hν → O₂ + O•
What happens when ozone is formed in the troposphere?
- Photochemical smog.
- when solid carbon particulate and ozone mix.
- this harms the respiratory system in animals and damages plants.
- Ozone is also toxic to humans.
How is ozone formed in the troposphere?
When sunlight, hydrocarbons and nitrogen dioxide react together.
(a lot of the hydrocarbons and nitrogen dioxide comes from cars and factories.)
Why do we use parts per million?
Because when measuring air, there is a tiny percentage of many gasses.
How is radiation from the sun remitted by the earth?
- Nuclear fusion occurs in the sun and emits IR, Visible, and UV light.
- The atmosphere absorbs some of this radiation.(ozone layer in the stratosphere)
- The surface of the earth absorbs radiation and remits it as infrared (a lower frequency)
Describe what must happen for oxygen free radicals to form.
- Electrons sit in discrete energy levels.
- If they absorb the right amount of energy they can move to another energy level as they are quantised.
- If UV or visible light hits a gas molecule the electrons absorb the energy but can only absorb specific frequencies.
- If the right energy is absorbed bonds break forming free radicals.
O=O → O• + O•
How is energy related to wavelength?
E=hν (h= planks constant, ν= frequnecy) ν = c/λ (c= speed of light, λ= wavelngth) ∴ E=hc/λ
