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Ionic Bonding and Structures

Ionic bond: the electrostatic attraction between oppositely charged ions

Ionic bonding holds charged ions together in giant structures- the structure is regular because the ions pack together neatly.

Each ion in ionic compounds is held firmly in place because strong electrostatic forces of attraction act in all directions- each ion is surrounded by ions of the opposite charge

An example: Sodium chloride contains equal numbers of sodium and chloride ions which alternate to form a cubic lattice

Giant ionic structures:

  • Ionic compounds have giant structures because many electrostatic forces hold the ions together
  • This means they are solid at room temperature
  • A lot of energy is needed to overcome the strong ionic bonds so ionic structures have high melting points
  • Ionic compound can only conduct electricity when molten or dissolved in water when the particles can move freely and carry a charge
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Covalent Bonding

Covalent bonding joins two non-metals because they need to gain electrons to achieve stable electronic structures

  • They share electrons
  • Each shared pair of electrons strongly attracts the two atoms forming a covalent bond
  • Substances that have atoms held together by covalent bonds are called molecules

A covalent bond acts only between the two atoms it bonds together so many covalent substances consist of small molecules.

Some atoms, e.g. carbon, can form several bonds so join together to form giant covalent structures. These are known as macromolecules.

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Metals and Metallic Bonding

The atoms is a metallic element are all the same size

They form giant structures in which layers of atoms are arranged in regular patterns

Metallic Bonding:

When metal atoms pack together, the electrons in the highest energy level (outer shell) delocalise.

The delocalised electrons can move freely between atoms

This produces a lattice of positive electrons in a 'sea' of moving electrons

The delocalised electrons strongly attract the positive ions and hold the giant structure together

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Simple Molecules

The atoms within a molecule are held together by strong covalent bonds

--> these bonds act only between the atoms within the molecule so the molecules have little attraction for eachother

  • Intermolecular forces are weak so simple molecules have low melting and boiling points
  • They do not conduct electricity because molecules have no overall charge so cannot carry electrical charge

Intermolecular forces are weak- these forces are overcome when a molecular substance melts or boils

Substances made of small molecules have very weak intermolecular forces and are gases at room temperature (e.g. H2, Cl2, CH4)

Substances made or larger molecules have stronger attractions so may be liquids at room temperature (e.g. Br2, C6H14)

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Giant Covalent Structures

Atoms of some elements (e.g. carbon) can form several covalent bonds and can form giant covalent structures/macromolecules

Every atom in the structure is joined to several atoms by strong covalent bonds

-->It takes an enormous amount of energy to break the lattic so macromolecules have very high melting points


  • A form of carbon that has a regular 3D giant structure
  • Every carbon atom is covalently bonded to four others
  • This makes the diamond hard and transparent


  • A form of carbon that has a structure of atoms in giant flat 2D layers
  • Carbon atoms are covalently bonded to three others
  • There are no covalent bonds between the layers so they slide over eachother- making graphite slippery and grey
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Graphite and Fullerenes

Bonding in graphite:

Each carbon atom is covalently bonded to three others forming a flat sheet of hexagons

One electron from each carbon atom is delocalised which allow graphite to conduct heat and electricty


Large molecules formed from hexagonal rings of carbon atoms

The rings join together to form cage-like shapes with different numbers of carbon atoms- some are nano-sized

Uses: drug delivery into the body, lubricants, catalysts, reinforcing materials

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Giant Metallic Structures

Metal atoms are arranged in layers- when a force is applied, the layers of atoms can slide over eachother

They can move into a new position without breaking apart so the metal bends or stretches into a new shape - this makes metals useful for making wires, rods and sheet materials


  • Mixtures of metals or metals mixed with other elements
  • The different sized atoms in the mixture distort the layers in the metal structure and make it more difficult for them to slide over eachother
  • This makes alloys harder than pure metals

Shape memory alloys:

  • Can be bent or deformed into a different shape
  • When they are heated they return to their original shape
  • They can be used in many ways e.g. dental braces
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The properties of a polymer depend on:

  • The monomer used to make it
  • The conditions we use to carry out the reaction

Poly(propene) is made from propene and softens at a higher temperature than poly(ethene)

Low density poly(ethene) and high density poly(ethene) are made using different catalysts and different reaction conditions

HD poly(ethene) has a higher softening temperature and is stronger than LD poly(ethene)

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Bonding in Polymers

Thermosoftening polymers (e.g. poly(ethene)

  • Made up of individual polymer chains that are tangled together
  • When heated, it becomes soft and hardens again when cooled
  • This means it can be heated to mould into shape and can be remoulded by reheating
  • Forces between the polymer chains are weak- when we heat the polymer, the weak intermolecular forces are broken and it becomes soft
  • When the polymer cools down, the intermolecular forces bring the polymer molecules back together so the polymer hardens again

Thermosetting polymers

  • Do not melt or soften when heated
  • Set hard when they are first moulded because strong covalent bonds from corss-links between their polymer chains
  • The strongbonds hold the polymer chains in position
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When atoms are arranged into very small particles they behave differently to ordinary materials made of the same atom

  • Nanoparticles are a few nanometres (one billionth of a metre) in size
  • They contain a few hundred atoms arranged in a particular way 
  • Their very small sizes give them large surface areas and new properties that can make them useful

Nanotechnology uses nanoparticles as: highly selective sensors, very efficient catalysts, new coatings, new cosmetics (suncream, deodorant), to give construction materials special properties

  • Nanoparticles are used more and more so there is greater risk of them finding their way into our bodies 
  • This could have unpredictable consequences for our health and the environment
  • More research needs to be done to find out their effects
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Instrumental analysis

Seperating substances for analysis- Gas chromatography, Mass spectrometer

  • The mixture is carried by a gas through a long column lined with particles of a solid
  • The individual components travel at different speeds through the column and com eout at different times
  • The amount of substance leaving the column is recorded and shows the number of compunds in the mixture as well as their retention times
  • The retention times can be compared to the retention times of known compounds to help identify the compounds in the mixture
  • The output from a gas chromatography column can be linked to a mass spectrometer (GC-MS)
  • The mass spectrometer gives further data that a computer can use to identify the compounds

Measuring relative molecular masses:

A mass spectrometer can give the relative molecular mass of a compound- for an individual compound, the peak with the largest mass corresponds to an ion with only one electron removed.

The peak is called the molecular ion peak and is furthest to the right on a mass spectrum

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Rate of Reaction

The rate of reaction measures how fast it is - this can be found by:

  • Measuring the amount of a reactant used/the amount of a product made and the time taken
  • Measuring the time taken for a certain amount of reactant to be used or product to be formed

Rate of reaction = amount of reactant used / time taken


Rate of reaction = amount of product formed / time taken

The rate of reaction at a given time can be found from the gradient of the line on a graph of amount of reactant/product and time

A graph can be produced by:

  • Measuring the amount of gas released
  • The volume of gas produced at intervals of time
  • Measuring changes in colour
  • Measuring changes in concentration/pH
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Collision theory & Surface area

Collision theory states that reactions can only happen if particles collide with enough energy to change into new substances.

The minimum energy they need to react is the activation energy

Factors that increase the frequency of collisions (and the rate of reaction):

  • Temperature
  • Concentration of solutions
  • Pressure of gases
  • Surface area of solids

Surface area:

The larger the surface area, the faster the rate of reaction

--> there are more collisions at the same time

E.g. a powder reacts faster than lumps of a solid because it has a larger surface area

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The effect of temperature, concentration/pressure,

Increasing the temperature increases the speed of the particles in a reaction mixture

  • They collide more frequently
  • They collide with more energy

Increasing the concentration of a solution/ the pressure of a gas means there are are more particles in the same volume

  • The particles are closer together and they collide more frequently

Catalysts change the rate of chemical reactions 

  • Catalysts lower the activation energy so that more of the collisions can result in a reaction

Catalysts are not used up, catalysts that are solids are used in forms with a large surface area to make them as effective as possible, catalysts only work with one type of reaction

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Catalysts in action

Catalysts can be used in industrial processes because:

  • They do not need replacing very often (they can be reused)
  • They can reduce the time needed for the reaction
  • They reduce the amount of energy needed for the reaction so reduce cost and environmental impact

Many of the catalysts used in industry involve transition metals and their compounds

--> some of these are toxic and may cause harm if they get into the environment


Finding new/better catalysts is a major area of research

  • Nanoparticles offer exciting possibilities for producing highly efficient catalysts
  • Enzymes (biological catalysts) work at ordinary temperatures so could reduce cost even further
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Exothermic / Endothermic

When chemical reactions take place, energy is transferred as bonds are broken and made

Exothermic reactions: reactions that transfer energy to the surroundings 

  • The energy transferred often heats up the surroundings so the temperature increases

Examples of exothermic reactions:

  • Combustion
  • Oxidation reactions, e.g. respiration
  • Neutralisation reactions involving acids and bases

Endothermic reactions: reactions that take in energy from the surrondings 

Some cause a decrease in temperature and others require a supply of energy

  • When some solid compounds are mixed with water, the temperature decreases because endothermic changes happen as they dissolve
  • Thermal decomposition reactions need to be heated continuously to keep the reaction going
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Reversible reactions

Reversible reaction: If the products of a chemical reaction can react to produce the reactants, the reaction can go in both directions

An example of a reversible reaction:

ammonium chloride Equilibrium symbol ( + hydrogen chloride

When heated, ammonium chloride decomposes to produce ammonia and hydrogen chloride. When cooled, ammonia and hydrogen chloride react to produce ammonium chloride.

In reversible reactions, the energy transfers are equal and opposite.

A reversible reaction that is exothermic in one direction must be endothermic in the other direction. 

The amount of energy released by one direction equals the amount of energy taken in by the reverse reaction

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Using energy transfers from reactions

Exothermic reactions can be used to heat things

--> e.g. hand warmers use the oxidation of iron or the reaction of calcium oxide with water

  • some hand warmers use reversible reactions (such as the crystallisation of a salt) so the hand warmer can be reused. The warmer is heated to re-dissolve the salt and allow the reaction to happen again

Endothermic changes can be used to cool things

--> e.g. chemical cool packs contain ammonium nitrate and water- they are kept seperated but when mixed together, the ammonium nitrate dissolves and takes in energy from the surroundings

  • This type of pack can only be used once even though the reaction is reversible
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Acids and Alkalis

Acids are substances that produce hydrogen ions, H+(aq) when they are added to water

When a substance is dissolved in water it is called an aqueous solution (aq)

Bases react with acids and neutralise them

Alkalis are bases that dissolve in water to make alkaline solutions, alkalis produce hydroxide ions, OH-(aq) in the solution

Indicators have different colours in acidic and alkaline solutions

--> Universal indicator (UI) and full-range indicators have different colours at different pH values

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Making salts from metals/bases

Acids react with metals that are higher than hydrogen in the reativity series

When metals react with acids they produce a salt and hydrogen

acid + metal ---> salt + hydrogen

Metal oxides and metal hydroxides are bases

When an acid reacts with a base, a neutralisation reaction takes place and a salt and water are produced

acid + base ---> salt + water

  • These reactions can be used to make salts
  • A metal or base that is insoluble in water is added a little bit at a time to the acid until all of the acid has reacted
  • The mixture is then filtered leaving a solution of the salt
  • The solid salt is made when water is evaporated from the solution so it crystallises
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Making salts from solutions

We can make soluble salts by reacting an acid with an alkali:

acid + alkali ---> salt + water

We represent the neutralisation reaction between an acid and alkali with the equation:

H+(aq) + OH-(aq) ---> H20(l)

There is no visible change when acids react with alkalis so we use indicator or a pH meter

  • The solid salt can be obtained from the solution by crystallisation

Example: Ammonia solution is alkaline, it reacts with acid to produce ammonium salts such as ammonium nitrate. Ammonium salts can be used as fertilisers

We make insoluble salts by mixing solutions of soluble salts that contain the ions needed

  • E.g. we can make lead iodide by mixing solutions of lead nitrate and potassium iodide

Some pollutants, such as metal ions, can be removed from water by precipitation- the water is treated by adding substances that react with the pollutant metal ions to form insoluble salts

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Electrolysis: the process that uses electricity to break down ionic compounds into elements

  • The substance that is broken down is called the electrolyte
  • The electrolyte is an ionic compound in molten or solution form

The electrodes are made of inert substances that do not react with the products

Positively charged ions go to the negative electrode where they gain electrons to become neutral atoms- the gain of electrons is called reduction

Negaitvely charged ions go to the positive electrode where they lose electrons to become neutral atoms- this is called oxidation

  • Water contains hydrogen and hydroxide ions
  • When soutions of ions in water are electrolysed, hydrogen may be produced at the negative electrode
  • This happens if the other positive ions are those of a metal more reactive than H+

At the positive electrode, oxygen is usually produced from aqueous solutions, HOWEVER, if the solution contains a halide ion then a halogen will be produced

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The Extraction of Aluminium

Aluminium is more reactive than carbon so it must be extracted from its ore by electrolysis

  • The aluminium oxide in the ore (bauxite) is purified 
  • It is mixed with cryolite (another ionic compound) so that its melting point is reduced from 2000 to 850 degrees

aluminium oxide --> aluminium + oxygen

  • The cryolite remains in the cell and fresh aluminium oxide is added as the reaction happens
  • At the negative electrode: aluminium ions are reduced to aluminium atoms

Al3+(l) +3e- --> Al(l)

  • At the positive electrode: oxide ions are oxidised to form oxygen atoms. The atoms then form O2 molecules 

2O2-(l) --> 2O2(g) +4e-

  • The positive electrodes used in the cell are made of carbon 
  • At the high temperature of the cell, the oxygen reacts with the carbon to form carbon dioxide- this means that the carbon electrodes gradually  burn away and have to replaced
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Electrolysis of Brine

Brine is a solution of sodium chloride in water

The solution contains:

  • Sodium ions Na+(aq)
  • Chloride ions Cl-(aq)
  • Hydrogen ions H+(aq)
  • Hydroxide ions OH-

At the negative electrode: hydrogen is produced from the hydrogen ions because sodium is more reactive than hydrogen

At the positive electrode: chlorine is produced from the chloride ions

This leaves a solution of sodium and hydroxide ions (sodium hydroxide- NaOH(aq)) 

  • Sodium hydroxide is a strong alkali and is used in making soap, paper, bleach, controlling pH
  • Chlorine is used to kill bacteria in drinking water and swimming pools, to make bleach, disinfectants and plastics
  • Hydrogen is used to make margarine and hydrochloric acid
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Electoplating uses electrolysis to put a thin coat of metal onto an object to:

  • Make the object more attractive
  • Protect a metal object from corroding
  • Increase the hardness of the surface 
  • Reduce costs by using a thin layer of the metal instead of the pure metal

The electrolyte is a solution containing ions of the plating metal

At the negative electrode (the object to be plated): metal ions from the solution gain electrons to form metal atoms which are deposited on the object to be plated

At the positive electrode (made from the the plating metal): atoms of the plating metal lose electrons to form metal ions that go into the solution

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