5th year Chem

exothermic reaction

a reaction in which heat is released e.g fuel burning

endothermic

a reaction in which heat is taken in e.g photosynthesis

heat of reaction

this is the heat change which occurs when a reaction takes place according to a given balanced chemical equation

when is ΔH positive

when heat is gained in the reaction (endothermic reaction)

when is ΔH (delta H) negative

when heat is lost in the reaction (exothermic reaction)

heat of combustion

when 1 mole of a substance is burned in an excess of oxygen

kilogram calorific value

the heat energy produced when 1kg of the fuel is completely burned in oxygen

bond energy

the energy required to break one mole of covalent bonds and separate the neutral atoms completely from each other

heat of neutralisation

this is the heat change when one mole of an OH+ ion from an acid reacts with one mole of an OH- ion from a base

heat of formation

this is the heat change which occurs when one mole of a compound is formed from its elements in their standard states

(standard state)

the elements normal form at 25 degrees and at one atmospheric pressure

Hess’s law

if a chemical reaction takes place in a number of stages, the sum of the heat changes in the separate stages is equal to the heat change if the reaction is carried out in one stage

law of conservation of matter

energy cannot be created or destroyed but can only be converted from one form to another

catalytic cracking

the breaking down of long chained hydrocarbon molecules by the action of heat and catalysts into short chained molecules.

fractional distillation

the heating of crude oil and separating the different mixtures based on their boiling points

characteristics of oil with small molecules

• low boiling point

• light in colour

• easy to light

• runny

characteristics of oils with large molecules

• high boiling point

• dark in colour

• hard to light

• thick

how does fractional distillation occur

• the oil enters the column partially vapourised

• the tower is 50cm

• the temp drops as it moves up the tower

• substances with large molecules come off as liquids at the bottom of the column (large hydrocarbons)

• substances with low boiling points come off as gases at the top of the column (small hydrocarbon)

Dalton’s atomic theory

1. all matter is made of very small particles called atoms

2. all atoms are indivisible and cannot be broken down into simpler particles

what are cathode rays

cathode rays are streams of electrons that travel in straight lines that are deflected by electric and magnetic fields which the sufficient energy to move a objects such as a paddle wheel

who discovered cathode rays

William Crookes

what colour is given off when lithium is present

crimson

what colour is given off when potassium is present

lilac

what colour is given off when barium is present

green

what colour is given off when strontium is present

red

what colour is given off when copper is present

blue-green

what colour is given off when sodium is present

yellow

j

what is the ground state of an atom

the state where the electrons occupy the lowest available energy levels

how do electrons jump from the ground state to the excited state

when an atom in the ground state is provided with energy (by heating or electricity) and a specific amount of energy is absorbed causing the electrons to jump from lower energy levels to higher energy levels

what is the excited state of an atom

the electrons occupy higher energy levels than those available in the ground state

what are the properties of electrons in the ground state

• they have fixed values

• they occupy the lowest available energy levels

what are the properties of electrons in the excited state

• the energy absorbed is equal to the difference in energy

• electrons in the excited state are unstable and they fall back down to lower energy levels after a short period of time

what is a compound

a substance made up of two or more different substances that are chemically combined

why are group 8 of the periodic table seen as inert

they are seen as inert as the are very unreactive and don’t form many compounds

what is the octet rule

when bonding occurs atom tend to reach an arrangement with 8 electrons in their outermost shell

what are the exceptions to the octet rule

• transition metals such as zinc and zirconium don’t usually obey the octet rule

• elements near helium in the periodic table (hydrogen, lithium beryllium) can achieve electronic arrangement with two electrons in their outermost shells

what is an ion

a charged atom or groups of atoms

when does an atom become an ion

an atom becomes an ion when it gains or loses electrons

what is a cation

a cation is a positively charged ion

what are anions

negatively charged ions

what is an ionic bond

the force of attraction between oppositely charged ions in a compound

how are ionic bonds formed

by the complete transfer of electrons from one atom to another

example of an ionic substance

sodium chloride

what is an ionic substance

a substance held together by ionic bonding

what is a molecule

a group of atoms joined together

what is an arrhenius acid

a substance that dissociates in water to produce H+ ions

what is a monobasic/monoprotic acid

molecules that dissociate to give one H+ ion in an aqueous solution

examples of monobasic acids

hydrochloric acid, nitric acid

what is a dibasic acid

molecule that can dissociate to give two H+ ions in an aqueous solution

example of dibasic acid

sulfuric acid

what is a tribasic acid

a molecule that dissociates in water to produce 3 H+ ions

what is a example of a tribasic acid

phosphoric acid (H3PO4)

what is an arrhenius strong acid

a substance that completely dissociates in water to give hydrogen ions

examples of strong acids

sulfuric acid (found in car batteries) hydrochloric acid (found in gastric juice in the stomach)

what is an arrhenius weak base

a substance that only slightly dissociates in water to give hydrogen ions

examples of weak bases

ethanoic acid (found in vinegar)

what is an arrhenius acid

a substance that dissociates in water to produce H+ ions

what is a solution of hydrogen chloride in water called

hydrochloric acid

what is the effect of acids on litmus

acids turn moist blue litmus paper red

what is the effect of bases on litmus

bases turn litmus indicator from red to blue

what is an arrhenius base

a substance that almost completely dissociates in water to give hrydroxide ions

what is the common term for OH- ion

hrydroxide ion

what are examples of bases

NaOH (caustic soda) found in oven cleaners, limewater used to detect carbon dioxide

what are the properties of a base

• soapy feel (convert the oil in the skin to soap)

• convert oil an grease into soluble soaps

how do cars start

• in the internal combustion engine a mixture of petrol and air is drawn into the cylinder

• the mixture is compressed and ignited by a spark

• the gases produces by the explosion drives the pistons down which rotates the crankshaft causing the wheels to turn

what is knocking

when in the running of an engine the explosion between petrol and air occurs too early

what is auto ignition

the early explosion of the petrol- air mixture before normal ignition by a spark takes place

what are the reasons for auto ignition

• straight chained molecules ignite more easily e.g heptane, octane

• branched chains like 2,2,4 trimethylpentane don’t auto ignite

what is the octane number

the measure of the tendency of a fuel to resist knocking

what is an example of a molecule with a low octane number

heptane which is prone to auto ignition and has an octane number of 0

what is an example of a molecule with a high octane number

2,2,4 trimethylpentane which has a low tendency to auto-ignite and has an octane rating of 100

what is said about octane numbers and chain length

the shorter the alkane chain the higher the octane number

how do you increase the octane number of a fuel

1. isomerisation

2. adding oxygenates

3. catalytic cracking

4. (Reforming) dehydrocyclization

what is isomerisation

changing straight chained molecules into their branched equivalents

how does isomerisation occur

• the alkanes are heated in the presence of a catalyst to break the chains apart and then they join together again resulting in a more branched product

what is an example of isomerisation

pentane - 2 Methylbutane