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 mixtures separated are called fractions

• 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