Category: caterscam.github.io

S1.4.6 – Avogadro’s Law

📌 Moles in gaseous state

• Avogadro’s law relates the volume of gases to the number of moles
• The law states that equal volumes of any gas at equal temperature and equal pressure contain an equal number of particles
• One mole of any gas will occupy 22.7 dm³ of gaseous volume
• Using this, we can then calculate the number of moles in different volumes of a gas as well as the volume of gas that a given number of moles might occupy

S1.4.5 – Molar concentration

📌 Concentration

• Molar concentration is determined by the moles of solute and volume of solution
• In solutions, the number of moles is represented as a concentration

📌 Dilutions

• In chemistry, the term standard solutions refers to solutions prepared at a known concentration
• The exact mass of solute is measured and then carefully added to the solvent in a volumetric flask
• Standard solutions can be diluted to get different concentrations for specific experiments/reactions
• Since the number of moles must remain standard throughout the dilution the following formula is used

📌 Serial dilutions

• Serial dilution is where the concentration is reduced by a fixed amount at each step
• A series of solutions of known concentrations can be used under UV-vis spectroscopy which measures absorbance
• Doing so allows us to plot a calibration curve to then determine the concentration of a solution with knowing the absorbance

S1.3.6 & S1.3.7– Ionisation Energy

Ionisation energy
↑
|        |
|        |  ← large jump
|    |
|  |
| |
|________________________→ electron removed

S1.3.3, S1.3.4 & S1.3.5 – Electronic configuration

📌 Electronic configuration

• Electronic configuration shows how electrons are arranged in an atom
• Electrons occupy energy levels around the nucleus
• Energy levels are divided into sublevels and orbitals

📌 Energy levels and sublevels

• Energy levels are numbered 1, 2, 3, and 4
• Each energy level contains sublevels
• Sublevels are labelled s, p, d, and f

Sublevel	Number of orbitals	Maximum electrons
s	1	2
p	3	6
d	5	10

📌 Order of filling

1s → 2s → 2p → 3s → 3p → 4s → 3d

📌 Rules for filling orbitals

• Each orbital holds a maximum of two electrons
• Orbitals of lower energy fill first – Aufbau principle
• Electrons in the same orbital have opposite spins – Pauli’s exclusion principle
• Orbitals of equal energy fill singly before pairing – Hund’s rule

S1.2.3 – Mass spectra (HL)

📌 Relative atomic masses

• Mass spectrometers are used to produce mass spectra which help determine the abundance of isotopes of different atomic masses
• Mass spectrometers first vaporise the element and then ionise them into a cation. Following this, the positively charged ions are attracted to negatively charged plates and deflected by magnetic fields. The angle of deflection is used to calcualte the relative mass of the ion.
• Mass spectra are graphs that plot the % abundance against the mass/charge ratio of the ion. An example of this is given below

S1.2.2 – Isotopes

📌 What are isotopes?

• Isotopes are alternate forms of atoms that have varied number of neutrons in their nucleus
• The number of protons and electrons remain the same (thus the charge/proton number does not change) but the mass varies due to the varied number of neutrons
• The stability of isotopes depends on the number of protons and neutrons in the nucleus. If there are too many or too few neutrons, the isotope is unstable and can become radioactive. They behave the same way chemically as other isotopes.
• The average relative mass of isotopes can be calculated by multiplying the abundance of each isotope with it’s mass number and averaging this value

R2.1.1 Chemical Equations :

• Chemical equations show the ratio in which chemical species each with each other and the relationship to the amount of product formed
• Mass is conserved in all chemical reactions – total number of atoms on both sides must be same – balancing equations
• State symbols are added to chemical equations to provide information about the state of the reactants and products

R2.1.2 Using Mole Ratios in Equations :

• The mole ratio of an equation can be used to determine the masses of reactants and products
• Keep these formulas in mind
  • n = m/M
  • n = v//molar volume
  • c = n/v
• Most problems involve identifying mole ratio, converting to moles, and then using information from the question to solve for mass/concentration/etc
• Avogadro’s law : equal volumes of all gases measured under the same conditions of pressure and temperature contain equal number of moles
• Molar volume – volume occupied by 1 mole of gas (at STP) = 22.7 dm³ mol^-1
• Increase in temperature, increase in molar volume
• Increase in pressure, decrease in molar volume

🧠 Paper 2 Tip : STP refers to a temperature of 273K and a pressure of 100 kPa.

• A standard solution is a solution of known concentration
• Titration is a technique of volumetric analysis used to find the unknown concentration of a solution by reacting it with a solution of known concentration and volume
• C₁V₁/ N₁ = C₂V₂/N₂
  • C = concentration
  • V = volume
  • N = coefficient of substance in balance eq

🧠 Paper 2 Tips (for Data Based Responses) : Steps of titration

• A pipette is used to measure known volume of one solution into a conical flask
• The other solution is poured using a funnel into a burette
• The point at which the two solutions have reacted fully is called the equivalence point
• The equivalence point is determined using an indicator, which changes color at the end point

• Back titration – unknown excess of A added to X
• Excess A titrated with B – to find how much of A reacted with X – find n(X)
• Works when X = solid

R2.1.3 The limiting reactant and theoretical yield :

• Reaction finishes when limiting reactant runs out
• Limiting reactant determines quantity of product
• Theoretical yield = maximum amount of product produced assuming 100% of reactants are converted to products
• For calculations of actual and theoretical yield – ensure to used limiting reactant in the mole ratios

⭐️ Limiting reactant is reactant with smallest n.

R2.1.4 Percentage Yield :

• % yield = experimental yield/theoretical yield * 100
• If experimental yield < theoretical yield
  • Losses during transfer of substances
  • Side reactions
  • Product decomposing
  • Incomplete reactions
• If experimental yield > theoretical yield
  • Residual solvent
  • Impurities

R2.1.5 Atom Economy :

• Atom economy is a measure of how efficient a reaction is in converting as much of reactants into useful product
  • Molar mass of product/ Total molar mass of all reactants * 100
• Principles of green chemistry
  • Reduce waste
  • Using readily available and safe materials
  • Using little solvent
  • Using little energy
  • Safe disposal

S3.2.1 Structural Representations of Organic Compounds :

📌 Definition Table :

🧠 Examiner’s Tip : Remember, solid wedges go out (of the paper) and dashed wedges go in( to the paper).

S3.2.2 Functional Groups and Classes of Compounds :

📌 Definition Table :

• Organic chemistry is the study of carbon compounds
• Catenation property of carbon allows for carbon atoms to join together to form chains/rings
• Functional groups give characteristic physical and chemical properties to a compound
• Saturated compounds only contain single bonds
• Unsaturated compounds contain double/triple bonds
• Compounds can be classified based on their saturation
• Functional groups on different compounds can react in a specific reaction to form new compounds
  • Eg : Two amino acids can link together in a condensation reaction to form a product called a dipeptide with an amide link
  • As the dipeptide will still have functional groups at both ends of the molecule, it can react with more amino acids to form a polypeptide (also known as a protein)

🧠 Paper 2 Tip : You do not require knowledge of arenes as a class of compounds, but you will be expected to recognize a phenyl group if it is part of a structure.

* 🧠 Examiner’s Tip : The difference between an aldehyde and a ketone is that in an aldehyde one of the R chains is an H.

S3.2.3 and S3.2.4 Homologous Series :

• A homologous series is a family of compounds in which successive members differ by a common structural unit
• Members of a homologus series show graduation in physical properties
• As the length of the carbon chain increases, boiling point of these compounds also increases – as the strength of the london dispersion forces are increasing
• Alkanes – increase is not linear as effect is proportionally greater for smaller compounds (steep in the beginning)
• For other homologous series – the increase is linear because stronger molecular forces than LDF are present – owing to their functional groups
• As carbon chain length increases, difference in boiling points between members of different homologous series decreases as the functional group is only a small part of a large molecule (most of the molecule is same)
• Alkanes are insoluble in water
• Solubility of alcohols decreases as length of carbon chain increases – functional group has proportionally smaller effect as molecule size increases

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S3.2.5 IUPAC Nomenclature :

⭐️ IUPAC Nomenclature refers to a set of rules used by the International Union of Pure and Applied Chemistry to apply systematic names to organic and inorganic compounds.

📌 IUPAC Rules :

1. Identify the longest straight chain of carbon atoms. This gives the stem of the name.

🧠 Examiner’s Tip : Note that straight chain means continuous/unbranched chains of carbon, don’t get confused by the 3d appearance of some molecules.

2. Identify the functional group. Ensure to number the carbons so that the functional group occupies the lowest position. This gives the suffix of the name (see S3.2.2).

3. Identify substituent groups. Where two different substituents would have the same number when numbering from different directions, the one that comes first in the alphabet is assigned the lower number. Use a prefix eg – di/tri/tetra to show the number of each substituent present. Arrange in alphabetical order.

• When naming molecules – use commas between numbers and dashes between numbers and letters
• Every substituent must have a number to indicate its position
• Alkenes
  • x-alkene or alk-x-ene
  • Eg. but-2-ene or 2-butene
• Alcohols
  • alkan-x-ol or x-alkanol
  • Lowest number to OH rather than an alkyl substituent (functional group takes precedence)
  • If there is more than one OH group – the ‘e’ is retained
    • Eg. butanediol
• Aldehydes/Ketones
  • Aldehydes : alkanal
  • C of CHO is numbered 1 in aldehydes
  • Ketones : alkan-x-one or x-alkanone
• Carboxylic Acid – alkanoic acid
• Esters
  • R-COOH + R’-OH –> R-COO-R’ + H₂O
  • Alkyl group of alcohol gives the prefix
  • Stem comes from parent acid
  • Eg. methanoic acid and butanol gives butyl methanoate
• Ethers
  • Longer chain gives stem – retains alkane name
  • Shorter chain – alkoxy

🌍 Real World Perspective : Ethers are used in the medical field, especially in anaesthetics. Diethyl ether is a general anaesthetic used to induce unconsciousness during surgery.

S3.2.6 Structural Isomers :

• More branched an isomer, the lower its boiling point – due to reduced strength of LDF
• Positional isomers can exist in alcohols, halogenalkanes and amines and can be classified as primary/secondary or tertiary

• Naming primary/secondary/tertiary halogenalkanes or alcohols
  • Primary – One C bonded to the C bonded to the halogeno/hydroxyl group
  • Secondary – Two C bonded to the C bonded to the halogeno/hydroxyl group
  • Tertiary – Three C bonded to the C bonded to the halogeno/hydroxyl group
• Naming primary/secondary/tertiary amines
  • If N is attached to 1C – primary
  • If N attached to 2C – secondary
  • If N attached to 3C – tertiary

🧠 Examiner’s Tip : Make note of the difference between naming primary/secondary/tertiary alcohols, halogenalkanes and amines. Don’t let it slip you up!

📌 Structural isomers in disubstituted Benzene [HL] :

• Substituted benzene is formed when a hydrogen atom is replaced by a halogeno atom or a functional group like amino or nitro groups
• No structural isomers are possible for monosubstituted benzenes because all 6 positions in the carbon ring are identical
• In disubstituted benzenes – 3 structural isomers are possible
  • More would be possible if benzene has a straight chain structure – offering more evidence supporting the Kekulé structure of benzene

S3.2.7 Stereoisomers [HL] :

• Cis/trans isomerism
  • Double bonded molecules – π component restricts rotation
  • Position of molecules with respect to plane
  • Cis isomer – same group on same side of plane
  • Trans isomer – same group on different sides of plane
  • Is seen when a molecule contains 2 or more different groups attached to the double bond
  • Also occurs in cyclic molecules (disubstituted benzene)
• Optical isomers
  • Molecules are chiral if their mirror images are non superimposable
  • The non superimposable mirror images of a chiral molecules are called enantiomers
  • Enantiomers have opposite configurations at each chiral centre
  • When a molecule has opposite configurations at more than one but not all chiral centers it is called a diastereomer

🧠 Paper 2 Tip : Enantiomers are mirror images of each other. Diastereomers are not mirror images of one another.

📌 Properties of Enantiomers :

• Optical activity
  • Two enantiomers of chiral compounds rotate plane polarised light in opposite directions
  • Non polarised light runs through a polarising filter to create a plane polarised light
  • If light is passed through samples containing equal moles – it rotates in opposite directions by equal amounts
  • d – clockwise direction, l – counterclockwise
  • Polarimeter can be fixed to 90 where no light passes, then sample can pass through and amount of rotation can be recorded
• Racemic mixture – equimolar mixture of two enantiomers of a chiral compound
• Racemic mixtures have no effect on plane polarised light (optically inactive)
• Physical properties
  • Enantiomers have identical physical properties except for optical activity
• Chemical properties
  • Enantiomers have identical chemical properties
  • Process of resolution – method of separating enantiomers from a racemic mixture
  • Racemic mixture is reacted with single enantiomer to produce different products that have distinctive chemical and physical properties

🌍 Real World Connect : The reactivity of a pair of enantiomers with other chiral molecules is especially significant in medical fields, because our bodies have chiral environments. One example of this occured in the 1960s when the drug thaliomide was used to treat morning sickness in women. Whereas one enantiomer is therapeutic, the other causes defects in the fetus. This event pioneered research into chiral compounds and optical isomers.

S3.2.8 Mass Spectroscopy :

• Mass Spectroscopy
  • The sample is vaporised and bombarded with high energy electrons – resultant ions are separated by m/z ratio
  • The fragmentation pattern can provide useful information about what groups and ions are present in the compound
  • The molecular ion/parent ion is the ion that passes through the spectrometer without breaking. It corresponds to the relative molecular mass of the compound.
  • The molecular ion has the highest m/z ratio, therefore it corresponds to the farthest peak
  • Other ions that break are also detected – these peaks can be analysed in terms of the groups that were lost or the groups that remained
    • Eg. If you know that the molecular mass of a compound is 46 and there is another peak at 15 – you know that this could correspond to the loss of a CH₃ group (1+1+1+12). From here, you could subtract 15 from 46 and continue to analyse what groups could produce the leftover (31) eg a CH₂OH group – which means your compound could be CH₃CH₂OH or ethanol.

🧠 Paper 2 Tip : Do not forget to include a positive charge when identifying the fragmented ions. The data booklet will help with some characteristic groups and their corresponding relative atomic masses.

S3.2.9 Infrared Spectroscopy :

• Infrared Spectroscopy
  • Used to identify bonds and functional groups
  • Molecules absorb infrared radiation at characteristic vibrational frequencies
  • $\lambda$ ∝ E (wavelength is proportional to energy)
  • Wavelength is inversely proportional to frequency
  • c = f$\lambda$
  • The region above 1500 on the IR spectrometer – used to identify
  • Region below 1500 (fingerprint region) used to confirm presence of bonds
• Absorbing IR radiation excites the bonds in a molecule
• The ability of a molecule to absorb IR radiation depends on the change in dipole moment that occurs when in vibrates
• A bond in a diatomic molecule will only interfere with IR if it is polar
• The change in vibrational energy causes a corresponding change in the dipole moment of these molecules
• In polyatomic molecules – the absorption causes bending and stretching of the molecules
• Homonuclear diatomic molecules are IR inactive
• Linear molecules symmetric stretch is also IR inactive as it produces no change in dipole moment
• Greenhouse effect
  • Shortwave radiation from the Sun to the Earth is reflected and passed (absorbed at surface)
  • Surface radiates long wave IR which is absorbed by greenhouse gasses – causes increase in kinetic energy and temperature
  • Global warming potential of a greenhouse gas compares amount of IR 1 tonne of the gas would absorb as compared to 1 tonne of CO₂
• Matching bonds with wavelengths
  • Characteristic infrared absorption bands are shown in the data table
  • Some bonds can be identified by the distinctive shape of their signals

S3.2.10 and S3.2.11 Nuclear Magnetic Resonance Spectroscopy :

• Hydrogen atoms have the property of nuclear spin – ability to act like a bar magnet
  • They align with lower energy and against higher energy
• HNMR provides information that can be analysed
  • Number of signals in the spectrum
  • Chemical shift of each signal
  • Size/area under each signal
  • Splitting pattern for each signal
• Signals correspond to groups of protons in different chemical environments
• Area under the signal corresponds to the number of protons in each chemical environment
  • Integration trace gives ratio of protons in each chemical environment
• Chemical shift – horizontal scale measured relative to TMS (tetramethylsilane)
  • TMS
    • 12 protons in the same environment (serves as a baseline)
    • Lower chemical shift than all organic molecules (does not interfere)
    • Non-toxic, inert, volatile and easy to remove
• Protons in the same chemical environment are said to be equivalent as they produce the same trace
• H attached to N/O also produce signals
• The closer the hydrogen atom is to an electronegative atom – the higher the chemical shift (measured in parts per million)
  • Chemical shift data is also available in the data booklet – hydrogen nuclei in particular environments have characteristic chemical shifts
• Splitting occurs due to the H atoms on the adjacent C
• Signal splits into n+1 where n is H atoms on adjacent C
• Singlet means no H on adjacent atom
• This multiplicity is also characteristic of some groups
  • Eg CH₃CH₂ group – signal due to 2H splits into quartet, signal due to 3H splits into a triplet
• Complex multiplet – when non equivalent protons on each side contribute to splitting

Overall structural analysis depends on combination of analytical techniques and cross referencing using multiple data points.