Chemistry Revision Unit 4

7/31/2019 Chemistry Revision Unit 4

1/6

Chemistry revision

Kinetics

- Rate of Change is the change in concentration with unit time.- The order of reaction with respect to one of the species is the power of the concentration.The overall order is the sum of the orders

For example 2A + B ---> 3C + D has a rate equation rate = k[A]2

[B] order respect to A = 2 ,

respect to B =1 and the overall = 3

- The order can also be calculated using a graph y=n is 0 order , y=x is 1st order and y=x2is 2ndorder

- It also can be calculate using an initial rate in a table form- In the rate equation the constant k can be affected by temperatures, the hotter it is the

faster the rate so higher K is

- The increase in temperature increase the speed of particles so there is more collisions withhigher energies hence the rate is increase.

- *The rate determining step is the slowest step in a long reaction. Otherwise it depends onthe reactants in the rate equation. The reactant with the highest order has the greater affect

Equilibrium

- An equilibrium mixture will always form the same ratio of products and reactants so long asthe conditions are maintained. Hence there is an equilibrium constant Kc which is calculate

by aA + bB xX + yY = [X]x[Y]

y

[B]b[A]

a

- For an exothermic reaction (-H) increasing the temperature decrease the equilibriumconstant.

- For an endothermic reaction (H) increasing the temperature increase the equilibriumconstant

- IfKc is greater than 1 there is more products than reactants so the equilibrium over to theright.

- Catalyst and change in concentration have no effect on Kc. Catalyst affects the forward andbackward reaction equally hence only the rate is increased.

Optical isomerism

- They occur when four different substituents are attached to one carbon atom.- They are mirror images of each other- They are said to be chiral and the two isomers are called enantiomers- The carbon is the chiral centre- Polarised light can be passed through the enantiomers rotate the light in opposite

directions. This occurs in a polarimeter.- The + isomer produces a clockwise rotation, the - isomer produces a anticlockwise rotation.- Many reactions which involve optical isomers produce a racemic mixture which is 50% of

each enantiomers. Hence there is no optical activity it cancels out


2/6

- Racemic mixtures are produced because a nucleophile can attack from above or below thedouble bond carbons.

- Optical isomers in the drug industry means one enantiomers might not work or bedangerous. To sort this out they can be separated which is difficult and expensive, keep it in

which is wasteful or find a new synthetic root.

Carbonyl Compounds (C=O)

- The polar C=O means it has a higher boiling point than alkanes, but no hydrogen bondsoccur.

- Shorter chains can dissolve in water.- They Undergo Nucleophile addition.- Aldehydes can be oxidised to carboxylic acid using acidified (dilute H2SO4) potassium

dichromate K2Cr2O7/H+. In ketones a C-C bond has to be broken for this to occur and so it

does not.

- Fehlings test contains Cu2+ which is coloured blue, when warmed with an aldehyde a brickred precipitate (copper oxide is formed), Ketones have no reaction.

- Tollens silver mirror test contains a reagent [Ag (NH3)2]+, when added to an aldehyde, thealdehyde is oxidised to a carboxylic acid. The Ag

+is reduced to metallic silver, Ketones do not

react.

- A reduction of an aldehyde produces a primary alcohol and a ketone a secondary alcohol.The reaction requires H

+form a solvent (NaBH4)

Carboxylic acid + esters

- Carboxylic acid forms hydrogen bonds with water and in there solid state meaning they havea higher boiling point than there alkanes.

- To identify carboxylic acid you can measure there melting point- Esters name by the alcohol first then the carboxylic acid, for example

Ethanoic acid + Propanol --- > Propylethanoate

- The carboxylic acid can dissociate to form a carboxylate ion. The negative charge is shared,the delocalisation makes the ion more stable.

- Esters are formed by adding carboxylic acid and an alcohol with a strong acid catalyst. Tofrom an ester and water. The reaction is reversible and the ester can be hydrolysed. This

can happen at room temperature.

- By adding NaOH instead of a strong acid to catalyse the reaction the salt of the acid isproduced and water

- Uses of esters , involves glycerol and 3 fatty acids to produce an ester adding this to NaoHproduces soap (salt). These dissociate and hence have two distinct ends. The hydrocarbon is

non-polar and mixes with grease whilst the COO- does not allowing grease and water to mix.

- Propane 1,2,3-triol is used in pharmaceutical and cosmetic preparation, medicines andtoothpaste, a solvent in food. And is used to plastics various materials. Biodiesel is also an

ester.


3/6

Acylation

- Acyl groups is R-C=O- Acid derivatives all have acyl groups (these are derived from carboxylic acids and from a

groups called ethanoyl) This is attacked from nucleophiles which hence are acylated. This

occurs more readily depending on the magnitude of the charge on the carbonyl carbon,

which depends on the electron releasing or attacking of the power of the side group. How

easily the side group is lost and how good the nucleophile is. It follows the mechanism

nucleophile addition-elimination

- Nucleophiles consist of either ammonia (amide), primary amine (N-substituted amide) ,alcohol ( ester) and water ( carboxylic acid), this is order of the reactivity. Using an acid

chloride HCl is a by-product using an anhydride produce ethanoic acid.

- The anhydride is cheaper, less corrosive, does not react with water readily, it has a safer by-product.

Benzene

- Arenes are hydrocarbons based on benzene C6H6- It forms addition compounds- The electrons are delocalised. They form a region of density above and below the ring. It

produces an aromatic stability

- This can be proven as more energy is released by 3 double bonds than the benzene when itshydrogenated.

- Benzene is a colourless liquid boils at 353K and freezers at 279K. Because its flat they packcloser together and so are harder to separate.

- The ring is an area ofhigh electron density and so attacked by electrophiles.- The ring is very stable and so does not break- They react by electrophilic substation- Arenes can undergo combustion and their flames are very smoky- Nitration uses the substitution of NO2+ it is made from the following reaction

H2SO4 + HNO3 --- > H2NO3+

+ HSO4-

(The sulphuric acid is the stronger acid so the nitric acid

acts as a Bronstead-Lowry base)

- The H2NO3+ then loses a molecule ofwater to produce the NO2+ ( Nitronium ion) Molecule- Nitration is important to make explosives such as TNT- Friedal-crafts acylation has a catalyst of aluminium chloride- The acyl chloride is made by RCOCl + AlCl3 --- > RCO+ + AlCl4-- The aluminium atom accepts another chloride as the chloride gives a lone pair.- The aluminium chloride is a catalyst AlCl4- + H+ --- > AlCl3 + HCl

Amines

- A primary amine has one R group connected to the nitrogen , secondary two R groups andso on.

- Ammonia has a bond angle of 107 as t is a pyramidal shape ( The lone pair makes it act like atetrahedral)

- Amines are polar and can accept hydrogen bonds but they are not as strong as water asnitrogen has a lower electronegativity than oxygen


4/6

- Shorter chains are gases and slightly longer are volatile liquids which smell like rotting fish- Amines with a chain length of C4are soluble in both water and ethanol.- They undergo Nucleophlic substation and can act as a base- They act as Bronstead-Lowry Base and will form a salt which will crystallise after the

evaporation of water.

- Phenylamine will dissolve in excess HCl because of the soluble ion formed.- The strength of the base is based on how readily it accepts a proton. Amines with one or

more R groups are the strongest as the R groups provide an inductive affect. This means

they release electrons and increase the electron density meaning the proton is more

attracted to it. Tertiary amines are not as strong as secondary amines because they do not

dissolve in water. The nitrogen electron pair in Phenlyamine overlaps with the benzenes high

density which means the proton is attracted to it more (The nitrogen is a weaker electron

pair donor), meaning its a weaker base. Ammonia is the middle of the two.

- Without excess ammonia nucleophilic substation productions ammonium salts and withexcess haloalkane can produce secondary, tertiary and quaternary ammonium salts.

- Amines can be better made using nitriles by the following reactionIn aqueous ethanol RBr + CN

----> RCN (Triple bond) + Br

-

By hydrogenation with a nickel catalyst (Ni/H2) RCN + 2H2 ---> RCH2NH2

- To make Phenlyamine you first undergo nitration then by the following reaction using a Tinand HCl catalyst the Nitrobenzene is reduced.

C6H6NO2 + 6[H] ---- > C6H6NH2 + 2H2O

- The reaction is carried out with HCl hence NaOH is used to gain the phenylamine as its salt isinitially produced

C6H5NH3+

Cl-+ NaOH --- > C6H6NH2 + 2H2O + NaCl

- Amines are used in hair conditions.Amino Acids

- They exist of zwitterions in water and as a solid as they have positive and negative chargesthey are neutral overall

- In acids the amino acid gains a proton and has been protonated- In an alkaline solution they loses a proton and hence have been deprotonated- Amino acids can form an amide linkage (peptide bond) which releases H2O and the product

formed is a dipeptide.

- This can be reversed by hydrolysis with a H+ catalyst to return to the two amino acids.

- These means amino acids can form long chains and contain hydrogen bonds producing alinear chain to a helix, pleating or spiral.

- Enzymes are proteins (complex formations of amino acids)- Wool is also a protein which can stretch due to the reformation braking of the hydrogen

bonds within it.

Polymerisation

- Addition polymers are made from alkenes. The Biodegradability of these polymers is nonebecause the linkage is between a c-c bond which is non-polar, so is unreactive.

- Condensation polymers come in two flavours.Polyesters have an ester linkage, and a esters are used. The linkage produces H2O


5/6

- Polyamides are formed by an amide linkage , a diaminioalkane and a dicarboxylic acid areused, Nylon and Kevlar are examples of polyamides

- The amide linkage occurs between amino acids to form proteins- They can be hydrolysed back to the original substances

Spectrometry

- In mass spectrometry the molecular ion is on the left.- Fragmentation occurs in the ionisation chamber and means there is a lot more peeks

involved

- Infrared spectrometry works as a range of frequencies of waves hit the particle and the acertain vibration matches the correct frequency meaning that there is absorptions which are

detected

- NMR spectroscopy ( For how it works read the bloody book )- Carbon 13 can have an nmr but it is hard to detect , furthermore the shielding of electrons

the smaller the magnetic field felt by the nucleus and the lower the frequency.- Chemical shift is the units that all nmr are compared to TMS. The chemical shift is relative to

the environment the C13 or H is

- In proton nmr there is an integration trace, it is the area of each peak and is related to thenumber of hydrogen atoms produced by it

- TMC , Tertamethlylsilane has a chemical shift value of 0 as all the hydrogen are in the sameenvironment and all other environments are worked out from this.

- Spin-Spin Coupling produce splitting patterns. It happens because of the applied magneticfield of neighbouring hydrogen atoms.

- N hydrogens on adjacent carbon atom will split into n+1 smaller peaks- Chromatography

The mobile phase carries soluble components. The more soluble the longer the phase

The stationary phase will hold back the components in the mixture that are attracted to it.

The more affinity a component has the slower it moves the solvent.

- Column chromatography uses resin such as aluminium oxide- Gas- liquid chromatography , the mobile phase the sample is carried by nitrogen and

separates down the line each at a different degree . This means they leave the tube at

different times meaning they have different retention times

Acids, Base and Buffers

- An acid is a substance that can donate a proton and a base is a substance that can accept aproton.

- H3O+is a oxonium ion and is produced when HCl dissociates in water- The proton due to its small size can never be found on its own and hence when any acid

dissociates an oxonium atom is produced

- The ionic product of water at 298k is 1x10-14(Kw) Kw= [H+][OH-]- Ph is defined aslog10[H+(aq)]- PH of water varies depending on the temperature as its dissociation is endothermic. The

hotter it is the more H+

ions are formed meaning the PH decreases, however the same

amount of OH-

is formed meaning the two cancel out and water is still neutral- In strong acids all the of the acid is dissociated so [HA] = [H+]


6/6

- In weak acids Hardly any of the solution dissociates so it is said to have no dissociation.- Ka is the constant (acid dissociation constant) and is in the form Ka= [H+][A-]/ [HA]- A strong acid is an acid which is fully dissociated- A concentrated acid is an acid which has many [H+] in a small volume- PkA = -log10Ka

Titrations

- The equivalence point is the point at which sufficient base has been added just to neutralisethe acid. Normally this is around 7 depending on the acids and bases used

- Indicators used- The end point is the volume of alkali or acid added when the indicator changes colour- Half neutralisation is between 0 and the equivalence point. At this point pKa = PH

Buffers

Acidic buffers are made from weak acids. If HA dissociates into is constituents equal

amounts of both ion are made. If you add a alkali to this it reacts with the acid to produce

water and the negative ion. When adding an acid the equilibrium shifts to the left toproduce more of the undissociated acid. Once the negative ion runs out the acidity starts to

increase. So we add extra, by adding the salt which dissociates.

- An acidic buffer is made from a mixture of a weak acid and soluble salt of that acid. I willmaintain a Ph below 7.

- Buffers do not ensure no change they reduce the change.- By neutralising halfof the weak acid with a alkali, we end up with a buffer which ph is equal

to kpa.

- Basic buffers are buffers which resist change but maintain a pH above 7. They are madeform a weak base and the salt of that base

Documents

Chemistry Revision Unit 4