REVISION BOOKLET

C2A

SADE TAIWO

Atomic Structure

The nucleus of an atom contains protons and neutrons. Round the nucleus are electrons

All atoms of an element have the same number of protons and electrons. An atom has no overall charge.

During a chemical reaction, the elements remain the same. Only electrons are involved. They are transferred or shared between atoms. A small amount of energy is either taken or given out.

In nuclear reactions, nuclei of atoms either split or join together. In both cases, huge amounts of energy are released: In nuclear fission, the nucleus of one atom of a large element

splits to give two nuclei of smaller elements. In nuclear fusion, the nuclei of two small atoms join together

to form a new element

Electronic Structure

Electrons are arranged in shells. The innermost shell holds up to two electrons, the second up to eight. Each shell represents a different energy level. The inner most shell has the lowest energy level. Electrons occupy the lowest available energy levels.

Atoms with a full outer electron shell are stable and unreactive. When an atom reacts, it gains, loses or shares one or more electrons to achieve a full outer electron shell

Groups in the periodic table contain elements with the same number of outer electrons, so elements in a group undergo similar reactions; in group 2 elements react by losing 2 outer electrons.

Mass Number and Isotopes

Elements, groups and relative atomic mass

A group in the periodic table contains elements with the same number of electrons in their outermost shell, so elements in a group have similar chemical properties.

The mass number of an element is the sum of protons and neutrons in one of its atoms,

Atoms of an element may have different numbers of neutrons. Such atoms are isotopes of the element and have a different mass number.

Ionic Bonding

To form ionic compounds, the atoms of reacting elements either lose electrons to become positively charges ions, or gain electrons to become negatively charged ions. Ions achieve a full outermost electron shell, with the electronic structure of a noble gas.

Ionic bonds are strong forces of attraction between oppositely charged ions. Ionic compounds are solids at room temperature.

Metal atoms lose electrons to form positive ions.Non-metal atoms gain electrons to form negative ions.

Ionic CompoundsIonic Compound – when positive metal ions and negative non-metal ions are arranged in a regular lattice to form a giant ionic structure.

Ionic compounds are solids with high melting points and boiling points. Don’t conduct electricity unless they are in a molten or aqueous solution (because the ions are free to move) E.g. Solid copper sulphate does not conduct electricity because the ions cannot move. But in a watery (aqueous) solution the ions are free to move (positive copper ions and negative sulphur ions)

The diagram shows a paper clip as the negative electrode. When the power pack is switched on. Positive copper ions from the copper strip move to the negative electrode, where they receive electrons and become copper atoms which are deposited onto the paper clip.

Covalent Bonding

A covalent bond is formed when two atoms share electrons so that each achieves a stable outermost shell of electrons. (2 in the first, 8 in the second)

Covalent bonds are strong. Holding atoms together…like: - Hydrogen- Chlorine- Oxygen- Hydrogen Chlorine

- Water- Ammonia

Simple Molecules

Strong covalent bonds join the atoms in simple molecules. Simple molecular substances are gases, liquids or solids with

relatively low melting and boiling points. They don’t conduct electricity.

Simple Molecules –

Elements

- Hydrogen (H2), H-H- Oxygen (O2), O=O- Chlorine (Cl2), Cl-Cl

Compounds

- Water (H20), H-O-H- Hydrogen Chloride (HCl), H-Cl

- Methane (CH4) H - - H

- Ammonia (HN3), H - - H

Giant Covalent Structures

In giant covalent structures, very strong covalent bonds join many atoms arranged in a 3d lattice forming macromolecules.

They have high melting points. Many are very hard. They do not conduct electricity or dissolve in water.

Diamond and graphite (forms of carbon) are giant covalent structures.

Diamond, covalent bonds join each carbon to four others, making diamond the hardest known mineral.

In Graphite, each carbon atoms bonds to three others, forming layers, making it slippery and soft. Each atom is delocalised allowing graphite to conduct electricity.

HCH

N H

Compound silicon dioxide (silica=sand) has strong silicon-oxygen covalent bonds in a giant structure, so it’s very hard and has a high melting point.

Metals

Metals are reactive and so in ores they occur in combination with other elements. In chemical reactions, metals lose the electrons in the outermost shell and form positive ions.

The layers of atoms in metals can slide over each other, allowing metals to be bent and shaped. Heavier metals have high densities and strength, and high melting and boiling points.

Metals consist of giant lattice structures of regularly arranged atoms. Electrons in the outermost shell are delocalised – free to move. The structure is held together by strong electrostatic forces – attractions between oppositely charged ions.

Metals conduct heat well. Heating increase the kinetic energy of the free electrons (delocalised) they move faster and transfer thermal energy to the positive ions.

The free electrons also make metals good conductors of electricity.

Metal alloys contain atoms different sizes. This prevents atoms from regularly arranged and the delocalised electrons cannot move so freely.

Nichrome, (containing: nickel, iron and chromium) Is such an alloy and is used as the heating element in toasters.

Alkali Metals

(Group 1)

The elements in group 1, in column 1 of the periodic table are the alkali metals.

Alkali metals are very reactive, with reactivity increasing down the group. In reactions the form ions with a 1+ charge by losing the single electron in their outermost shell and leaving them with the full outer shell of a noble gas.

All alkali metals react vigorously with water. With non-metals they form salts which are ionic compounds.

Halogens

(Group 7)

The elements in group 7 of the periodic table are the halogens. All exist as molecules containing two covalently bonded atoms.

The halogens are:- Fluorine: a pale yellow gas, the halogen with the smallest

atoms and most reactive.- Chlorine: a green gas- Bromine: a dark red liquid- Iodine: a dark grey crystalline solid that sublimes on

heating to give a purple vapour- Astatine: a highly radioactive solid

The halogens have similar chemical properties. Reactivity decreases down the group. With alkali metals they form salts – ionic compounds; the halide ion has a single negative charge.

Sodium metal burnt in chlorine gas forms the ionic compound sodium chloride:

- Sodium + chlorine sodium chloride Reactivity decreases down group 7, so the most reactive

halogen is fluorine. A more reactive halogen can displace a less reactive halogen from a compound.

Nanoparticles

A nanoparticle is a tiny manufactured structure of a few hundred atoms, from 1 to about 100 nanometres long.

Nanoparticles are made from carbon, metals and metal compounds.

The properties of nanoparticles are different from the properties of the materials in bulk.

The atoms in nanoparticles are regularly arranged in hallow structures such as tubes and spheres are one atom thick. (nanoparticles have high surface area to volume ratio)

There is a concern that nanoparticles can pass through the body undetected.

Smart Materials

A smart material has a particle property. It responds to environmental change.

- Photochromic: materials used on glasses. Darkens on exposure to strong light.

- Thermochromic: materials that respond to change in temperature, by changing colour. E.g. paper thermometers.

- Electroluminescent: materials that emit light of different colours when an alternating current is passed through.

Compounds

A mixture contains different substances that are not chemically combined. The substance can be separated.

A compound contains atoms of more than one element, chemically combined and joined by chemical bonds.

Writing Balanced Equations…

- The formula equation that represents a chemical reaction must be balanced. The number of atoms must be the same. E.g. word equation: Hydrogen + oxygen water Formula equation: 2H2+O22H2

Percentage Composition

The formula used to calculate the percentage mass of an element in a compound

=% mass of an element in a compound

Relative atomic mass of element

Number of atoms of element in the formula

Relative formula mass of compound

100=

Compound: Potassium Nitrate KNO3

Relative formula masses: K = 39, N = 14, 0 = 16 Relative formula mass of KNO3 = 39 + 14 + (3 x 6) = 101 % of mass of potassium in potassium nitrate = 39 x 1 X 100 =

38.6%

Moles

A formula of the compound water: H2ORelative formula mass of H2O = (2x1) + (1x16) = 18So mass of 1 mole of water molecules is 18g

Water is formed when hydrogen is burnt in air (oxygen)Balanced equation for the reaction is: 2H2+O22H2O

Yield of Product In a chemical reaction, no atoms are gained or lost. Yet the

reactants do not always give as much product as expected from the formula equation.

101

The reaction is reversible and does not go to completion

Why the actual amount of product may be less than the theoretical amount.

Some of the product may be lost when it is separated

Some of the reactants may react in an unexpected way

The maximum amount of product that could form in a reaction is the theoretical yield. The amount that actually forms is the actual yield.

We say a reaction has a high atom economy if the actual yield is high.

It is important for sustainable development and for economic reasons to use reactions with a high atom economy.

Percentage yield, ratios and empirical formulae

The cost of making a chemical depends on the percentage yield.

= 100

A reaction has an actual yield of 3.2g and the theoretical yield is 4.0g Percentage yield = 3.2 x 100 = 80% 4.0

Reversible Reactions

Examples of reversible reactions:

In reversible reactions the products react to produce the original reactants

Heating the white solid ammonium chloride gives the gases ammonia and hydrogen chloride. Ammonium chloride reforms when the gases cool down

White anhydrous copper sulphate reacts with water to give blue hydrated copper sulphate, this reforms when heated

In the reaction used to test whether water is present, blue anhydrous cobalt chloride reacts with water to form pink hydrated cobalt chloride. This reforms when heated

Equilibrium 1

Percentage Yield

Actual Yield

Theoretical Yield

General equation for a reversible reaction is: A + B ↔ C + D

A reversible reaction does not go to completion (not all of A and B changes to C and D) when the proportions of A, B, C, and D become steady, it has reached equilibrium

If the conditions do not change we say the reaction is happening is a closed system

If the conditions change for A, B, C, or D

- If A and B are gases and C and D are liquids, applying pressure will speed up the forward reaction rate (the equilibrium moves right)

- If the forward reaction gives out heat (its exothermic), raising the temperature still further will speed up the reverse reaction rate (the equilibrium moves left)

- If C and D are removed as soon as they are formed the forward reaction rate will speed up

Haber Process

Ammonia NH3 is used to make fertiliser. It is produced in the haber process. Nitrogen (from air) combines with hydrogen (from natural gas)

N2(g) + 3H2(g) ↔ 2NH3(g)The reverse reaction also takes place

To produce a reasonable yield of ammonia quickly, the conditions used in the Haber process are:

- A high pressure of approximately 200 atmospheres to drive the forward reaction.

- A compromise temperature of about 450oC to reach equilibrium quickly. The reaction is exothermic but at a cool temperature the reaction would be far too slow.

- A iron catalyst to speed up the reaction rate- Cooling the reaction mixture to liquefy and remove the

ammonia- Recycling the unreacted nitrogen and hydrogen