UNIT 2 - ALL DEFINITIONS YOU NEED TO KNOW WITH EXAMPLES

Hello all, today I am coming to you with all the definitions you need to know for your Unit 2 Chemistry exam. I included a thorough explanation of every single keyword and I also incorporated some examples for you to visualise my points.

So, let's get started with them...

2.1

Exothermic reaction- during this reaction energy is released to the surroundings, ΔH= negative. If during a reaction the reaction mixture is warm, this means it is giving away energy (heat).

Endothermic reaction- during this reaction energy is taken in from the surroundings, ΔH= positive. This reaction works the other way, the energy (heat) is taken in by the reaction mixture for it to proceed.

Enthalpy, H- amount of heat (energy) in a system (at constant pressure).

Enthalpy change, ΔH- this is the heat added to a system. If heat is added to the system, the ΔH is positive- therefore endothermic reaction and if heat is released from the system, ΔH is negative- exothermic reaction.

The principle of conservation of energy- energy cannot be created or destroyed, it can only be changed from one form to other.

Hess’s law- the total enthalpy change of a reaction is the same no matter of the route that is taken.

Bond enthalpy- the enthalpy (heat) needed to break a covalent bond.

e.g. In methane, we have four C-H bonds (always draw out the molecules first to make sure you remember about all the bonds present). Each C-H bond has a bond enthalpy but here we have 4 the same bonds so we just multiply the number by 4.

Average bond enthalpy – average value of enthalpy required to break a given type of bond.

2.2

Rate of reaction – change in concentration per unit time.

Activation energy – this is the minimum energy that is needed to start a reaction.

Catalyst- it increases the rate of reaction by providing an alternative route of lower activation energy.

A homogenous catalyst- catalyst that is in the same phase as the reactants.

A heterogenous catalyst- catalyst that is in a different phase than the reactants.

2.3

Carbon neutrality- chemical process which does not lead to overall increase in the level of carbon dioxide.

2.4

Functional group- group of atoms or an atom that gives compound its characteristic properties.

e.g. COOH, Br, Cl, (halogen), OH, etc.

Homologous series- series of compounds with the same functional group.

Saturated compound – the one with no C to C multiple bond (no C=C double bond)

There is no double bond, so nothing else can be joined to the molecule.

e.g. ethane, propane, butane, etc.

Unsaturated compound- the one with C to C multiple bond (the one with C=C double bond).

There is a double bond which can open up and accept another atom or group of atoms.

e.g. methane, ethene, butane, etc.

Molecular formula- shows all atoms in a molecule and the actual number of each atom in a molecule.

e.g. C2H6 (ethane), C4H9Br (2-bromobutane).

Displayed formula- this is the molecules drawn out with all the atoms. It shows how the molecule is arranged and all bonds between atoms.

e.g. Ethane (on the left) and 2-bromobutane (on the right).

Shortened (structural) formula- shows the atoms carbon by carbon with the hydrogens and functional groups attached.

e.g. Ethane (on the left) and 2-bromobutane (on the right).

Skeletal formula- it shows the carbon skeletal with any functional groups.

e.g. Ethane (on the left) and 2-bromobutane (on the right).

Structural isomers- compounds with the same molecular formula but different structural formula (the same atoms present, they are just structured differently).

E-Z isomerism- they occur in alkenes due to the restricted rotation about the double bond.

We need at least two different functional groups on each carbon to form and E-Z isomer.

e.g. E isomer (on the left)- the functional groups are opposite each other.

e.g. Z-isomer (on the right)- the functional groups are on the same 'zame' side of the molecule.

2.5

Fossil fuel- derived from organisms that lived long ago.

Non-renewable resources- resources that cannot be reformed.

Renewable resources- resources that can be reformed.

Greenhouse gas- gas that causes an increase in the Earth’s temperature.

e.g. carbon dioxide, methane, ozone, nitrous oxide, water vapour.

Acid rain- rain with much lower pH (sulfuric and nitric acids are present in acid rain).

Complete combustion- combustion that occurs with excess oxygen (carbon dioxide, CO2 produced).

Incomplete combustion- combustion where not enough oxygen is provided (carbon monoxide, CO produced- toxic).

Halogenation- reaction with a halogen (initiation, propagation and termination).

Initiation- starts the process (Cl2--> 2Cl• forms a radical).

Radical- species with an unpaired electron (like Cl above- represented by a dot).

Propagation- chain reaction, reaction grows due to this stage (a radical reacts with a molecule continuously).

Termination- reactions that ends the process (two radicals react together to form one molecule).

Homolytic bond fission- bond is broken and each of the atoms gets one electron.

Substitution reaction- where one atom (or group) is replaced by another atom (or group). When hydrogen is replaced by for example a halogen or OH- group.

π bond- bond formed in alkene by the sideways overlap of p electrons.

Electrophile- electron ‘loving’ species, they want to accept a pair of electrons.

Heterolytic bond fission- bond is broken and one of the atoms get both electrons.

Addition reaction- when two reagents combine to give a product.

Carbocation- carbon which has a positively charged ion.

We form a carbocation in some reactions which is then attacked by some kind of a nucleophile, which wants to get rid of electrons.

Polymerization – joining many monomers together to make a polymer.

Monomer- small molecule that can be made into a polymer.

Repeat unit- section of a polymer that is repeated to make a structure.

2.6

Halogenoalkane- an alkane with a halogen group attached (instead of a hydrogen).

Nucleophile- electron ‘hating’ species, they want to donate a pair of electrons.

Reflux- a process of continuous evaporation and condensation.

Hydrolysis- a reaction with water which produces a new product.

Elimination reaction- there is a loss of a small molecule and a double bond is produced. For example, if you have an alkane and you remove 2 hydrogens, you'll form an alkene.

CFCs- halogenoalkane that contains chlorine and fluorine only- very toxic and contributes to ozone depletion.

Ozone layer- layer surrounding the earth (contains O3 molecules).

HFCs- halogenoalkane that contains fluorine only (no chlorine)- safer substitution to CFCs.

2.7

Alcohol- homologous series that contains OH functional group.

Fermentation- reactions that converts sugars to ethanol.

Biofuel- fuel produced biologically.

Primary alcohol- OH is bonded to a carbon which is bonded to one other carbon (or no other carbons).

Secondary alcohol- OH is bonded to a carbon which is bonded to 2 other carbons.

Tertiary alcohol- OH is bonded to a carbon which is bonded to three other carbons.

Carboxylic acid- homologous series that contains COOH functional group.

2.8

Molecular ion- positive ion that is formed in a mass spectrometer.

Fragmentation – splitting of a molecule in a mass spectrometer into smaller parts.

Wavenumber- energy absorbed in a IR spectra.

Chemical shift- energy absorbed is shown by a chemical shift in NMR spectrometer.

Environment- what atoms/ groups surround a molecule.

e.g. in this molecule, we have 3 carbon environments. To find them you look at each carbon separately and see what is connected to them. If the things that are connected to one carbon are also connected to another carbon, it means both of the carbons are in the same environment. If the things connected are different, that means the carbons are in two different environments.

PS. Please remember, I am only a student, and as anyone, I can make mistakes. If you think you can see one, don't hesitate and comment (either here on on my youtube channel) Thank you!