Chemistry unit 2

Enthalpy change

Heat energy change measured at constant pressure

Standard conditions

Pressure - 100 kPa, Temperature - 298 K

Exothermic reaction

Reaction releasing energy to the surroundings

Endothermic reaction

Reaction absorbing energy from the surroundings

Sign of ΔH for exothermic and endothermic reactions

Exothermic: ΔH is negative, Endothermic: ΔH is positive

Standard enthalpy change of reaction, ΔrHθ

Enthalpy change when reactants react under standard conditions

Standard enthalpy change of formation, ΔfHθ

Enthalpy change when one mole of product is formed from it’s constituent elements under standard conditions, with all elements in their standard states

Standard enthalpy change of combustion, ΔcHθ

Enthalpy change when one mole of compound reacts with excess oxygen under standards conditions, with all elements in their standard states

Hess's law

Enthalpy change is independent of the reaction route

Mean bond enthalpy

Energy to break a specific covalent bond type averaged across compounds

Difference in calculation results involving enthalpies

Formation and combustion enthalpies are more accurate than mean bond enthalpies

Energy transfer equation

q = mcΔT, q = energy, m = mass, c = specific heat capacity, ΔT = temperature change

Insulated container purpose

Prevent heat energy loss in energy change reactions

Experiment for enthalpy change investigation

Procedure to study thermal decomposition of potassium hydrogencarbonate

Mass Spectrometry

Vaporized sample ionized into positive ions, accelerated, deflected, and detected.

Relative Atomic Mass Calculation

[(79.2 x 17) + (80.1 x 83)] ÷ 100 = 79.95 g mol-1

Molecular Ion Peak

Peak with highest m/z value in mass spectrum, representing molecular mass.

2+ Charge in Mass Spectrometer

True; less common than 1+ charge.

Chlorine Isotopes in Mass Spectrum

Peaks at m/z 70, 72, 74 in 9:6:1 ratio, with additional peaks at m/z 35 and 37.

Infrared Radiation Effect

Bonds absorb specific frequencies, causing vibration.

Functional Groups Identification

Identified by comparing peaks in infrared spectrum to known values.

Fingerprint Region

Region between 1500 cm-1 and 500 cm-1 with unique absorptions for each compound.

13C Nuclei in NMR

Align with or against magnetic field, energy change based on carbon environment.

Carbon Environment

Atoms or groups bonded to a carbon atom.

Carbon-13 NMR Spectrum Deduction

Number of peaks = number of different carbon environments.

Structure Determination from 13C NMR

2 carbons, peak at 200 ppm (C=O), 2 different carbon environments; compound is ethanal.

Number of Peaks Prediction in 13C NMR for 3-methylbutanal

4 peaks for 3-methylbutanal with 4 different carbon environments.

Protons in NMR

Align with or against magnetic field, resonance condition for alignment flip.

Proton Environment Identification

Compare chemical shift values to data book.

Ratio of Peak Areas in Proton NMR

Indicates relative number of protons in each environment.

Equivalent Protons

Protons in the same environment.

Splitting Pattern in Proton NMR

Shows number of protons on adjacent carbon atoms.

First 4 Splitting Patterns in Proton NMR

Singlet (1 peak), Doublet (2 peaks), Triplet (3 peaks), Quartet (4 peaks).

n+1 Rule

Number of peaks = number of adjacent protons + 1.

Exceptions to n+1 Rule

Alcohols, equivalent hydrogens, benzene.

Peak at 0 ppm in Proton NMR

TMS (tetramethylsilane) standard.

TMS Usage

Standard for chemical shift measurements in NMR spectroscopy.

TMS Standard Benefits

12 hydrogens in same environment, non-toxic, inert, produces single peak in both NMR types.

Functional group of alcohols

-OH

General formula for alcohols

CnH2n+1OH

Conditions for industrial ethanol production

Hydration of ethene with phosphoric acid catalyst at 300°C and high pressure

Chemical equation for hydration of ethene

CH2CH2 + H2O → CH3CH2OH

Production of ethanol from renewable source

Fermentation of sugars in crops to ethanol

Conditions for fermentation of glucose to ethanol

Yeast, 30-40°C, anaerobic conditions

Separation of ethanol after fermentation

Distillation

Formation from alcohols undergoing dehydration

Alkenes

Classification of alcohols

Primary (1°), Secondary (2°), Tertiary (3°) based on R group attachments

Oxidation of primary alcohols to aldehydes

Heating with acidified potassium dichromate(VI)

Oxidation of primary alcohols to carboxylic acids

Heating under reflux with acidified potassium dichromate(VI)

Oxidation of secondary alcohols to ketones

Heating under reflux with acidified potassium dichromate(VI)

Qualitative test for primary and secondary alcohols

Add acidified potassium dichromate(VI) - color change from orange to green

Qualitative test for carboxylic acids

Add sodium hydrogencarbonate - effervescence with CO2 production

Products of carboxylic acids reacting with bases

Salt and water

Reaction of ethanoic acid with sodium hydroxide

CH3COOH + NaOH → CH3COO-Na+ + H2O

Products of carboxylic acids reacting with carbonates

Salt, CO2, and water

Reaction of methanoic acid with sodium carbonate

2HCOOH + Na2CO3 → 2HCOO-Na+ + CO2 + H2O

Products of carboxylic acids reacting with hydrogencarbonates

Salt, CO2, and water

Reaction of ethanoic acid with sodium hydrogencarbonate

CH3COOH + NaHCO3 → CH3COO-Na+ + CO2 + H2O

Alcohols reacting with carboxylic acids

Form esters via esterification with sulfuric acid catalyst

Reaction between methanol and ethanoic acid

CH3OH + CH3COOH → CH3COOCH3 + H2O (Product: Methyl ethanoate)

Separation of ethanol from water

Distillation based on different boiling points

Halogenoalkanes

Organic compounds with a halogen replacing at least one hydrogen on an alkane.

Nucleophile

Species donating a lone pair of electrons in a reaction.

Displayed Formula

Shows all atoms and bonds in a molecule.

Structural Formula

Shows how atoms are arranged in a molecule.

Skeletal Formula

A simplified organic formula, removing hydrogen atoms.

Primary Halogenoalkanes

Have one alkyl group attached to the carbon with the halogen.

Secondary Halogenoalkanes

Have two alkyl groups attached to the carbon with the halogen.

Tertiary Halogenoalkanes

Have three alkyl groups attached to the carbon with the halogen.

Alcohol

Product when a halogenoalkane reacts with an aqueous alkali.

Alkene

Product when a halogenoalkane reacts with ethanolic potassium hydroxide in an elimination reaction.

Amine

Product when a halogenoalkane reacts with warm ethanolic ammonia.

Nitrile

Product when a halogenoalkane reacts with alcoholic potassium cyanide.

Nucleophilic Substitution

Mechanism for the reaction between a halogenoalkane and a nucleophile.

Nucleophilic Substitution with Ammonia

Mechanism for the reaction between a halogenoalkane and ammonia.

Reactivity Trend of Halogenoalkanes

Tertiary > Secondary > Primary in reactivity.

Reactivity Trend of Halogenoalkanes by Halogen

Iodo- > Bromo- > Chloro- due to bond enthalpy.

Qualitative Identification of Halogenoalkanes

Add aqueous NaOH and and dilute nitric acid to neutralise and add aqueous silver nitrate to form a silver halide precipitate.

CFCs

Chlorofluorocarbons, organic compounds with carbon, chlorine, and fluorine.

Environmental Impact of CFCs

CFCs damage the ozone layer by releasing chlorine radicals.

Reactions with Chlorine Free Radical and Ozone

Cl• + O3 → O2 + ClO•; ClO• + O3 → 2O2 + Cl•; Overall: 2O3 → 3O2.

Impact of C-H, C-F, C-Cl Bonds on Atmosphere

C-Cl bond is weakest, breaking easily and impacting the environment.

Reflux Setup

Boiling setup with a vertical condenser allowing vapors to return once condensed.

Complete combustion products

Water (H2O) and carbon dioxide (CO2)

Balanced ethane combustion

2C2H6 + 7O2 → 4CO2 + 6H2O

Incomplete combustion products

Soot (C), CO2, CO, H2O

Incomplete methane combustion

CH4 + O2 → C + 2H2O

Fossil fuel burning drawbacks

Finite resources, CO2 emissions, toxic CO, acid rain

Fossil fuel burning benefits

High energy yield, current availability

Alkane bonds

C-C and C-H bonds are σ-bonds

Radical substitution example

Photochlorination of alkanes

Photochlorination

Replacement of alkane hydrogen with chlorine initiated by light

Alkenes reactivity vs. alkanes

Alkenes more reactive due to C=C electron density

Alkene C=C bond

Contains σ-bond and π-bond

Stereoisomerism definition

Isomers with different spatial arrangements due to limited rotation

E-Z isomer determination

E: Opposite priority groups, Z: Same priority groups

Z-2,5-dichloro-3-ethylpent-2-ene

Specific compound name

Alkenes electrophilic addition

Susceptibility to electrophiles due to C=C electron density

Bromine addition to ethene

Electron dense C=C induces bromine dipole

Alkene qualitative tests

Bromine water and acidified potassium dichromate tests