Theory

Question 1

(b) the bonding in organic compounds in terms of σ- and π-bonds

Answer 1

• C-C (single covalent) bonds have 1 σ-bond
  
  • due to overlap of two orbitals (bond = area of increased electron density)
  • usually very strong
• C=C bonds have 1 σ-bond and 1 π-bond
  
  • due to overlap of p-orbitals
  • π-bond = two areas of partial negative charge (above + below molecular axis)
  • π-bond = weaker than a σ-bond
    
    • so double bond (σ-bond and π-bond) = less than double single covalent bond

Question 2

(c) the relation of molecular shape to structural formulae and the use of solid and dashed wedges to represent 3-D shape

Answer 2

Question 3

(d) the terms: exothermic, endothermic, standard conditions, (standard) enthalpy change of reaction (∆_rH), (standard) enthalpy change of combustion (∆_cH), (standard) enthalpy change of formation (∆_fH), (standard) enthalpy change of neutralisation (∆_neutH)
Enthalpy change of neutralisation is per mole of water formed

Answer 3

• Exothermic: A reaction that gives out energy and heats the surroundings (negative enthalpy)
• Endothermic: A reaction that takes in energy and cools the surroundings (positive enthalpy)
• Standard conditions: Set conditions to allow us to compare enthalpy changes
  
  • temperature = 298K (25^oC)
  • pressure = 100kPa
  • concentration (for solutions) = 1 mol dm^-3
  • In these conditions substances are in their standard states
• standard enthalpy change of reaction (∆_rH): The enthalpy change when molar quantities of reactants react together under standard conditions
• standard enthalpy change of combustion (∆_cH): The enthalpy change that occurs when one of a substance is burnt completely in oxygen under standard conditions in standard states
• standard enthalpy change of formation (∆_fH): The enthalpy change when one mole of a compound is formed from its elements under standard conditions in standard states
• standard enthalpy change of neutralisation (∆_neutH): The enthalpy change when one mole of hydrogen ions react with one mole of hydroxide ions to form one mole of water under standard conditions and in solutions containing 1 mol dm^-3

Question 4

(e) the term average bond enthalpy and the relation of bond enthalpy to the length and strength of a bond; bond-breaking as an endothermic process and bond-making as exothermic; the relation of these processes to the overall enthalpy change for a reaction

Answer 4

• Average bond enthalpy: The quantity of energy needed to break one mole of bonds in the gas phase, averaged over many different compounds
• In covalent molecules:
  
  • Positively charged nuclei (protons) are attracted to shared (negatively charged) e^-
  • Repulsion between two positively charged nuclei where the attractive forces and repulsive forces balance
  • Stronger attraction between atoms –> nuclei pulled closer together = shorter bond length & higher bond enthalpy
• Making a bond = exothermic
  
  • Energy taken in
• Breaking a bond = endothermic
  
  • ​Energy released
• If a reaction is exothermic:
  
  • More energy is released making bonds than taken in breaking bonds (not necessarily more bonds are broken because bond enthalpies are different)
  • Temperature often increases
  • All combustion reactions are exothermic
  • ΔH = negative
• If a reaction is endothermic:
  
  • More energy taken in breaking bonds than released making bonds
  • Temperature often decreases
  • All thermal decomposition reactions are endothermic
  • ΔH = positive
• To work out ΔH:
  
  • Enthalpy change = Energy absorbed breaking bonds - energy released making bonds
  • if positive = endothermic
  • if negative = exothermic

Question 5

(h) the terms catalyst, catalysis, catalyst poison, heterogeneous

Answer 5

• Catalyst: Speeds up a reaction by providing an alternate pathway of lower activation enthalpy and is chemically unchanged (recovered) at the end of the reaction
• Catalysis: The process of speeding up a reaction using a catalyst
• Catalyst poison: A substance that stops a catalyst from functioning properly
  
  • e.g. For heterogeneous catalysts; poison adsorbed more strongly than reactants to the catalyst surface
• Heterogeneous: Catalyst and reactants are in different phases/physical states

Question 6

(i) a simple model to explain the function of a heterogeneous catalyst

Answer 6

Question 7

(k) the origin of atmospheric pollutants from a variety of sources: particulates, unburnt hydrocarbons, CO, CO₂, NO_x , SO_x ; the environmental implications and methods of reducing these pollutants

do i need to know how acid rain forms?

Answer 7

• Particulates:
  
  • Solid particulates, of carbon, produced by burning fossil fuels in vehicles + power stations
  • Can cause lung cancer + heart attacks
• Unburnt hydrocarbons:
  
  • Not all fuel burnt (due to incomplete combustion)
  • Photochemical smog contributor
• CO:
  
  • Incomplete combustion of hydrocarbons in fossil fuels
  • CO = toxic gas (poisionous)
  • Photochemical smog contributor
• CO₂:
  
  • combustion of fossil fuels
  • greenhouse effect
• NO_x:
  
  • Produced because of high pressure and temperature in car engine causing nitrogen and oxygen in air to react together
  • Acid rain contributor
  • Photochemical smog contributor
• SO_x:
  
  • burning of fuels containing sulfur
  • toxic gas
  • Acid rain contributor

_​Methods of reducing:

Question 8

(l) the terms aliphatic, aromatic, arene, saturated, unsaturated, functional group and homologous series
Arenes defined here as compounds containing groups represented as either of (below):
Unsaturated compounds contain C=C or C≡C.

Answer 8

• Aliphatic: Compounds that do not contain any benzene rings
• Aromatic: Compounds that contain one or more benzene rings
• Arene: The homologous series containing aromatic compounds (containing benzene rings)
• Saturated: Hydrocarbons containing the maximum number of hydrogen atoms possible, no carbon-carbon double or triple bonds
• Unsaturated: Organic compound containing one or more double/triple bond
  
  • Alkenes (C_nH_2n) are unsaturated
• Functional group: Modifiers that are responsible for the characteristic chemical reactions of molecules
• Homologous series: A series of compounds in which all members have the same general molecular formula

Question 9

(m) the nomenclature, general formulae and structural formulae for alkanes, cycloalkanes, alkenes and alcohols (names up to ten carbon atoms)

Answer 9

• (Need to be able to name; alkanes, cycloalkanes, alkenes and alcohols)

Question 10

(r) structural formulae (full, shortened and skeletal)

Answer 10

Question 11

(j) the term cracking; the use of catalysts in cracking processes; techniques and procedures for cracking a hydrocarbon vapour over a heated catalyst
Specific examples of catalysts are not required
• cracking a hydrocarbon vapour over a heated catalyst and testing the product

Answer 11

• Cracking: Any reaction in which a larger molecule is made into smaller molecules
• Cracking reactions give shorter chains of hydrocarbons (more useful)
• Same molecule can crack differently to give different products
  
  • products separated by in a fractional distillation column
• Cracking process requires extremely high temperatures and pressure (expensive)
  
  • Process of cracking = passing hydrocarbon vapour over a heated solid catalyst (heterogeneous)
  • Catalyst used so that it can occur at a much lower temperature (450^oC) and pressure (saving money)
• Testing product
  
  • bromine water test for alkenes

Question 12

(n) balanced equations for the combustion and incomplete combustion (oxidation) of alkanes, cycloalkanes, alkenes and alcohols

Answer 12

• Complete combustion:
  
  • Fuel_(l/g) + O_2(g) –> CO_2(g) + H₂O_(g)
  • Produces blue flame
• Incomplete combustion:
  
  • May not produce CO₂
  • Will produce CO or C or both
  • Produces orange flame

Question 13

(t) (i) stereoisomerism in terms of lack of free rotation about C=C bonds when the groups on each carbon differ; description and naming as:
E/Z for compounds that have an H on each carbon of C=C

Question 14

(t) (ii) stereoisomerism in terms of lack of free rotation about C=C bonds when the groups on each carbon differ; description and naming as:
cis/trans for compounds in which one of the groups on each carbon of C=C is the same

Question 15

(a) the concept of amount of substance in performing calculations involving: volumes of gases (including the ideal gas equation pV = nRT), balanced chemical equations, enthalpy changes; the techniques and procedures used in experiments to measure volumes of gases
The molar gas volume at room temperature and pressure, RTP(24.0 dm³ mol^–1) and the gas constant R (8.314 J mol^–1 K^–1) are given on the Data Sheet.

Answer 15

• If conditions (temperature and pressure) are room temp and pressure (298K/25^oC and 100kPa), one mole of any gas occupies same volume - 24.0 dm³
  
  • this is because in a gas, molecules are far apart, so the size (M_r) of each molecule, so has a negligible effect on volume occupied
  • n = (volume in dm³) / 24.0
    
    • 1000 cm³ = 1 dm³
    • 1000 dm³ = 1 m³
• For reactions involving only gases –> moles of each gas reacting is its coefficient
• Ideal gas equation: pV = nRT
  
  • Used when conditions (temperature and pressure) are different from room temp and pressure
    
    • p - pressure (in Pa)
    • V - Volume (in m³)
    • n - number of moles
    • R - Gas constant (Data sheet: 8.314 J K^-1 mol^-1
    • T - temperature (K)
• Experiments to measure volumes of gas:
  
  • Using gas syringe or inverted burette
  • system needs to be gas tight
  • when using an inverted burette –> volume of gas collected = initial volume minus final volume of gas

Question 16

(f) techniques and procedures for measuring the energy transferred when reactions occur in solution (or solids reacting with solutions) or when flammable liquids burn; the calculation of enthalpy changes from experimental results
Using the formula: q = mc∆T

Answer 16

• q = mc∆T
  
  • q - energy transferred (in joules)
  • m - mass of water in calorimeter, or solution in the insulated container (in grams)
  • c - specific heat capacity of water (given on Data sheet: 4.18 J g^-1 K^-1)
  • ∆T - change in temperature of water or solution (in K/^oC)
• Using a calorimeter to measure energy transferred when burning fuel
  
  • Use a measuring cylinder to pour a known volume of water into a copper calorimeter
  • Record initial temperature
  • Weigh a spirit burner (keeping cap on - reduces loss of fuel by evaporation)
  • draught excluder reduces energy losses to surroundings
  • Record highest temperature reached
  • Weigh spirit burner again

Question 17

(o) (i) the addition reactions of alkenes with the following, showing the greater reactivity of the C=C bond compared with C–C:
bromine to give a dibromo compound, including techniques and procedures for testing compounds for unsaturation using bromine water

Answer 17

• Test for double (C=C) bond = Electrophilic addition
  
  • Shake sample with bromine water
  • If double bond present colour change from orange to colourless occurs
  • Bromine is being added across the double bond to form a (colourless) dibromoalkane
  • Tests for unsaturation
    
    • Saturated compounds like alkanes dont react

Question 18

(o) (ii) the addition reactions of alkenes with the following, showing the greater reactivity of the C=C bond compared with C–C:
hydrogen bromide to give a bromo compound

Answer 18

• Alkenes and hydrogen halides (hydrogen bromide)
  
  • addition reaction to form haloalkanes (bromoalkanes)
  • If the hydrogen halide (HBr) adds onto an unsymmetrical alkene (e.g. propene)
    
    • two products are possible

Question 19

(o) (iii) the addition reactions of alkenes with the following, showing the greater reactivity of the C=C bond compared with C–C:
hydrogen in the presence of a catalyst to give an alkane (Ni with heat and pressure or Pt at room temperature and pressure)

Answer 19

• Alkenes to Alkanes
• H₂ and Nickel (Ni) catalyst at a temperature of 150^oC and high pressure

or

• H₂ and Platinum catalyst at room temperature and pressure

Question 20

(o) (iv) the addition reactions of alkenes with the following, showing the greater reactivity of the C=C bond compared with C–C:
water in the presence of a catalyst to give an alcohol (concentrated H₂SO₄, then add water; or steam/H₃PO₄/ heat and pressure)

Answer 20

• Adding water and alkenes in the presence of an acid catalyst makes alcohols
  
  • electrophilic addition​
• cold concentrated sulfuric acid
• add cold water
• warm product

or

• hydrated by steam at 300^oC and pressure of 60 atm
• solid phosphoric acid(V) acid catalyst
• reversible reaction with a low yield

Question 21

(s) structural isomerism and structural isomers

Answer 21

• Isomers: Two molecules that have the same molecular formula but differ in the way their atoms are arranged (different physical and chemical properties)
• Structural isomers: Atoms bonded together in a different order, have different structural formulae
• 3 types of structural isomers
  
  • Chain isomerism: Branching/Straight chain
  • Position isomerism: Functional group on a different carbon
  • Functional group isomerism: Same molecular formula but have different functional groups (e.g. Ether and Alcohol)

Question 22

(q) the terms addition, electrophile, carbocation; the mechanism of electrophilic addition to alkenes using ‘curly arrows’; how the products obtained when other anions are present can be used to confirm the model of the mechanism
Either a carbocation or a bromonium ion may be shown for bromination.

Answer 22

• Addition: A reaction where two or more molecules react together to form a single larger molecule
• Electrophile: A positive ion or a molecule with a partial positive charge, that is attracted to a negatively charged region and react by accepting a lone pair of electrons (electron pair acceptor) to form a covalent bond
  
  • e.g. positively charged ions, H⁺, NO₂⁺
  • e.g. molecules than can become polarised; Br₂
• Carbocation: An ion containing a positively charged carbon atom
  
  • Carbocations react rapidly
• Electrophilic addition reaction mechanism:
  
  • two step mechanism
  • for addition of HBr (H is partially positive and added first, Br partially negative and added last)