GCSE Chemistry Revision Notes - everything!

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Chemistry

Syllabus

Atoms

Atomic structure

Relative molecular and formula masses

Chemical formulae and chemical equations

Ionic compounds

Covalent substances

Electrolysis

Metallic crystals

The Periodic Table

Group 1 elements

Group 2 elements

Group 7 elements

Oxygen and oxides

Sulphur and nitrogen

Hydrogen

The transition metals – iron and copper

Reactivity series

Preparing and analysing

Alkanes

Alkenes

Ethanol

States of matter

Acidic, alkalinity and neutralisation

Energetics

Rates of reaction

Dynamic Equilibria

Extraction and uses of metals

Natural oil and gas

Synthetic polymers

The manufacture of chemicals



Atoms

The simplest substances from which all other substances are made up are called elements. They are shown in the periodic table and can be represented by a chemical symbol. This consists of one or two letters. The first letter is always a capital letter.

Elements can be divided into groups into two ways:

  • Solid, liquid and gas
  • Metal and non-metal

Elements combine in fixed proportions to form compounds. The compound sodium chloride is composed of two elements: the metal, sodium, and the non-metal, chlorine. The compound calcium carbonate contains the metal, calcium, and two non-metals carbon and oxygen.

All elements are made up of atoms. An atom is the smallest part of an element that can exist.  Atoms are made up of protons, neutrons and electrons.  All elements are made up of these three basic particles and atoms of different elements contain different numbers of these three particles.

Atomic structure

The nucleus of the atom contains the protons and neutrons; they are tightly packed. The nucleus is positively charged. The electrons move around the nucleus in energy levels or shells. Because an atom has no overall charge (they arte electrically neutral) the number of protons in an atom is equal to the number of electrons.

Number of electrons= number of protons

The number of protons in an atom is called is atomic number or proton number. Each element has a different atomic number. So if an atom has 8 protons it must be oxygen.

Atomic number = number of protons.

The mass number is the total number of protons and neutron is the nucleus of the atom.

Mass number = number of protons + number of neutrons

12 6C The mass number of Carbon- 12 is 12 and its atomic number is 6.  Therefore a carbon–12 atom contains 6 protons (i.e. atomic number = 6), 6 electrons and 6 neutrons. The atomic number is written under the mass number).

It is possible for many elements to get more than one type of atom. For example, there are 3 types of oxygen atom:

Oxygen-16   8p, 8e, 8n

Oxygen-17   8p, 8e, 9n

Oxygen-18   8p, 8e, 10n

These different types of the same element are called isotopes. Isotopes are atoms of the same element containing the same number of protons and electrons but different numbers of neutrons.  Isotopes of the same element have the same chemical properties but slightly different physical properties.

The relative mass of chlorine is approximately 35.5 as 75% of chlorine is chlorine-35 and 25% is chlorine-27? The relative atomic mass of an element is the mass of an average atom.

Atoms are arranged in the Periodic Table in order of increasing atomic number. The atomic number is always the smallest number against an element; the other number will either be the mass number of the most common isotope of the relative atomic mass of the element.

Electrons move rapidly around the nucleus in distinct energy levels. They are found at considerable distances from the nucleus. Each energy level can only hold a certain number of electrons. Low energy levels are always filled before higher ones.

The 1st energy level can only hold 2 electrons.

The 2nd energy level can only hold 8 electrons.

The 3rd energy level can hold a maximum of 18 electrons, but it sometimes appears full with 8 electrons.

 

Elements with atoms having full outer electron energy levels are very un-reactive. Their electronic arrangement is said to be very stable. However elements whose atoms contain only one or two electrons in their outer shell are very reactive, these atoms tend to lose these electrons in order to achieve a stable electronic arrangement. Also elements whose atoms contain six or seven electrons are also very reactive. These atoms tend to gain one or more extra electrons so they achieve a stable electron arrangement. Elements with atoms contains three, four or five electrons in their outer energy levels are usually less reactive. The reactivity of elements is related to the electron arrangement in their atoms.

Elements, Compounds and Mixtures

An element is a substance made up of only one type of atom.

A compound is two or more different type of atom, chemically combined (bonded).

A mixture is two or more substances that are not chemically combined. Their chemical properties are unchanged and they can be separated by physical methods.

There are many different ways of separating mixtures. Some are listed here:

  • Filtering
  • Sublimation
  • Chromatography
  • Dissolving
  • Evaporation
  • Condensing/ Distillation
  • Fractional Distillation
  • Crystallisation

Bonding

The joining of atoms together is called bonding. An arrangement of particles together is called a structure. Three methods of bonding atoms together are ionic, covalent, and metallic bonding. 

Ions are atoms that have lost or gained electrons. They have a charge. Ionic bonding involves a complete transfer of electrons from one atom to another.  A sodium atom has an electron arrangement of 2, 8, 1 (i.e. one more electron than the stable arrangement of 2, 8). A chlorine atom has an electron arrangement of 2, 8, 7 (i.e. one electron less than the electron arrangement of 2, 8, 8). So each sodium atom loses one electron to form a sodium ion Na+ and each chlorine atom gains one electron and forms a chloride ion Cl-. Both ions formed have stable electron arrangements. Strong electrostatic forces hold the ions together.  Although sodium is a dangerously reactive metal and chlorine is a very poisonous, reactive gas, sodium chloride is safe to eat.

A positive ion is called a cation and a negative ion is called an anion. Cations are positive because they have lost one electron, so it has the same number of positively charged protons but one less electron to balance them. Anions are negatively charged because they have gained an extra negative electron.

A lot of energy is needed to overcome the strong forces of attraction between oppositely charged ions. Ionic substances have high melting and boiling points. When ionic solids are molten or dissolved in water, the ions are free to move. Therefore they conduct electricity. Ionic compounds have regular crystalline structure.

Covalent bonding involves the sharing of electrons, rather than the complete transfer.  In a covalent bond, a pair of electrons is shared between two atoms. Each of the positively charged nuclei is attracted to the same negatively charged pair of electrons. A molecule is a small group of covalently bonded atoms.  They can either be elements, Os or compound CH4. 2 shared electrons denotes a single covalent bond. 4 shared electrons means a double bond and 6 shared electrons means a triple bond.

Metallic bonding is found only in metals. A metal consists of a close-packed regular arrangement of positive ions, which are surrounded by a ‘sea’ of electrons that bind the ions together.

The sea of electrons can move throughout the structure, this explains the high electrical conductivity of solid metals. Metals are crystalline due to the regular arrangement of particles in the structure.

Structure

You can divide substance into two quite different types, those belonging to giant structures and those that have molecular structures.

Ionic compounds such as NaCl and MgO have high melting points and high boiling points because of strong electrostatic forces between ions. However, MgO has a much high melting and boiling point than NaCL because of the increased charge on its ions. They have double the charges on those in the NaCl compound, which leads to a stronger attraction between the oppositely charged ions.

Ionic compounds tend to be crystalline has this reflects the regular arrangement of the ions in the lattice.

Ionic crystals tend to brittle as any distortion of a crystal will bring ions of the same charge close together. Since like charges repel, the crystal splits itself apart.

Ionic compounds tend to be soluble in water but insoluble in organic solvents.

Most metals are hard and have high melting points suggesting that the intermolecular forces that hold the particles together in a metal are very strong. A metal consists of a close-packed regular arrangement of positive ions, which are surrounded by a ‘sea’ of delocalised electrons which bind the ions together.

The outer electron of each atom becomes free to move throughout the whole structure. Once an atom loses an electron, it becomes an ion. The attraction of each positive ion to the delocalised electrons holds the structure together.

The sea of electrons can move throughout the whole structure which is why they able to carry current, and explains the electrical conductivity of solid metals. Metals are also crystalline; this is because of the regular arrangement of particles in the structure.  Metals are also malleable as when a large force is applies, the particles can slide over each other and stay in their new positions.

Allotropes

Allotropy is the existence of two or more forms of an element in the same physical state.  These different forms are called allotropes. Allotropy is caused by the possibility of more than one arrangement of atoms. Carbon can exist in allotropy forms including diamond and graphite. They have very different properties- diamond is the hardest natural substance, whereas graphite is one of the softest.

Solid lead and molten lead are not allotropes as they are not in the same physical state. One is solid, the other is liquid.

Diamond is a form of pure carbon. In its structure, each carbon atom is strongly bonded (covalent bonding) to four other carbon atoms tetrahedrally. A large giant structure is built up, continuing on and on in three dimensions. It is a not a molecule as the number of atoms joined up in a diamond depends on the size of the diamond. Molecules always contain fixed number of atoms joined by covalent bonds.

Diamond is very hard because of the strong carbon-carbon bonds which extend though the whole crystal. Because diamond is so hard, it is often used in drills and to cut stones.

Diamond doesn’t conduct electricity as all the electrons in the outer levels of the atoms are held together tightly in covalent bonds, and are therefore not free to move around. This is also why diamond doesn’t dissolve in water or any other solvent, as the powerful covalent bonds would have to be broken for the diamond to dissolve.

Graphite is also a form of pure carbon, but it has a layer structure. In each layer the carbon atoms are also bound covalently, which means that the bonds within the layers are very strong, but the bonds between the layers of graphite are very weak, which enables layers to slide over one another.

Graphite is a soft material with a slimy feel so it is often used as a lubricant.

Charges

‘ide’ means two elements only. When 2 elements combine chemically to form a compound, the non-metal changes its name to end in ‘-ide’.

‘ate’ endings includes oxygen. When the non-metal part also contains oxygen, the ending is ‘-ate’.

‘ite’ endings also include oxygen, but you will not come across them often.

Relative formula masses and molar volumes

Atoms are amazingly small, so it would be silly to measure the masses of atoms in conventional mass units like grams. Instead their masses are compared with the mass if an atom of the carbon-12 isotope, taken as standard. The relative atomic mass of an atom is the number of times an atom is heavier than one-twelfth of a carbon-12 atom (approximately the mass of the commonest hydrogen atom). Flourine1-9 has a relative atomic mass of 19 because one of its atoms has a mass of 10 times that basic unit.

The symbol for relative atomic mass is Ar and it is defined as the weighted average mass of the isotopes of the element. It is measure on a scale on which a carbon-12 atom has a mass of exactly 12 units.

You can measure the masses of compounds on the same carbon-12 scale by adding the relative atomic masses of all the elements involved.  This will give you the relative formula mass (Mr).  The relative formula mass of water is 18. (2x1)+ 16 = 18

As an alternative to trying to consider masses of individual atoms, it is possible to consider a large number of atoms. The mass of magnesium atoms will always be twice the mass of carbon-12 atoms, provided equal numbers of atoms are compared. In Chemistry, the mole is a measure of amount of substance. To work out the mass of 1 mole of a substance, work out of the relative formula mass and attach the unit grams. So the mass of one mole of carbon atoms is thus 12g. A mole is that amount of matter than contains 6 x 1023 particles. This number is called Avogadro’s constant.

One mole of any gas occupies 24000 cm3 (or 24 dm3) at room temperature and atmospheric pressure.  One mole of any gas occupies 22.4 dm3 at standard temperature and pressure.

States of Matter

There are 3 states of matter: solid, liquid and gas. To go from solid to liquid, the substance is melting, and from liquid to gas it is evaporating, from gas to liquid – it is called condensation and from liquid to solid, it’s freezing. In the usual case of going straight from solid to gas and vice versa, the process is called sublimation.

Particles in a solid are closely packed and vibrate around a fixed point. When we warm a solid, the particles gain kinetic energy. The particles vibrate more strongly and eventually are able to slide over each other. They are still close. The temperature at which this happens is called the melting point.

If a liquid is heated eventually particles will have enough energy to overcome attractive forces. The temperature at which a liquid becomes a gas is called the boiling point.

There are forces of attraction between particles. In a solid the forces are strong, in liquids, they are weaker, allowing the particles to slide over each other and in gases- the forces are very weak.

We need to supply energy to make solids melt. A pure solid always melts at the same temperature, the melting point. The melting point of ice is 0°C. More energy is needed to make liquids boil at their boil points. Pure water boils at 100°C at normal air pressure. No two substances have the same melting and boiling point, so this information may be used to identify an unknown substance. Impurities prevent the melting and boiling points from being as sharp as in the pure substance. An impurity lowers the melting point and raises the boiling point.  

Solids have definite shape and volume. Liquids have definite volume but no definite shape. Gases have neither definite shape nor volume.

Solids tend to have high densities; liquids have lower whilst gases always have low densities.

Only fluids flow, solids do not,

Only gases can be compressed.

Diffusion is the movement of different particles amongst each other so they become evenly mixed. Some examples of this include:

  • The particles of potassium manganate (VII) spreading out in the surrounding water.
  • Bromine vapour diffusing from one gas jar to another.
  • Ammonia and hydrogen chloride gas diffusing towards each other in horizontal tube to give a white smoke ring where they meet and react with each other.

Separating mixtures

  • Distillation. Distillation separates mixtures of liquids. Since different liquids boil at different temperatures we can distil liquids that boil at different temperatures, as the liquid with the lower boiling point will boil off first leaving the other liquid. You can use a condenser to ensure that only one liquid evaporates.
  • Fractional distillation.  Some mixtures contain lots of liquids; this is harder to separate, as each liquid needs to be separated at a different temperature as each liquid has its own boiling point. A fractionating column containing glass beads is used as the beads in the column let the hot vapours condense and evaporate many times. This is better than simple distillation.
  • Filtration. If a solid is in suspension in a liquid, filtering will remove the solid from the liquid as (with the right strength of filter paper), the solid particles will be too big to squeeze through the holes in the filter paper.
  • Crystallisation.  Impure solids can sometimes be separated by crystallisation. It is possible when the pure substance and the impurity have different solubilities in a solvent. The impure mixture is first dissolved in a warm solvent. As the temperature falls, the material will become less and less soluble and the more abundant material then crystallises out. The impurity will remain in solution.
  • Paper chromatography. Place a piece of filter paper on top of a n evaporating basin. Put a colour sport in the centre of the circle and allow it to dry, Add drops of water to the centre of the coloured sport, very slowly, one by one. Watch the colours separate into bands.

Acidity, alkalinity and neutralisation

The pH scale

The pH scale ranges from 0 to 14 and tells you how acidic or how alkaline a solution is. pH 7 is neutral and 6 downwards is increasingly acidic, whereas 8 upwards is increasingly alkaline.

pH can be measured by using universal indicator. Universal indicator is a mixture of dyes which change colour gradually over a range of pHs. It can be used a solution but it also available in paper form. Universal indicator is not very accurate. Using a pH meter is more accurate, but before you us the meter, you have to adjust it to make sure it is reading accurately. This is done but putting it into a solution whose pH is known and adjusting the reading accordingly.

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There are also many simple indicators which are substances which have more than one colour form, depending on the pH. These will tell you if the solution is alkaline or acidic, but not the strength of the solution.

An acid can be defined as a proton donor, as acids produce H+ ions. The H+ ion is a proton. In water this proton is hydrated and is represented as H+ (aq).

A base can be defined as proton acceptor.  Alkalis, in aqueous solution, produce OH- (aq) ions.  So the process of neutralisation is the reaction between H+ (aq) and the OH- (aq) ions to give water.

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