Chemistry Unit 1

Unit 1Topic 1- Rates of ReactionThe rate of a chemical reaction is a measure of how fast the reactants are being used up and how fast the products are being made. Chemical reactions are sped up by:Concentration: More particles in the same space means more collisions, which means more effective collisions.Temperature: Particles move faster when they get hotter. When they move faster they collide more often, which means more effective collisions.Particle size: Using smaller particles increases surface area and allows more collisions at surface, which means more effective collisions.A catalyst is a substance which speeds up a chemical reaction, but is not used up and can be recovered unchanged at the end of the reaction. Many are either transition metals or compounds containing transition metals e.g. nickel (used in the manufacture of margarine from vegetable oil) and platinum (used in car exhausts). When the reaction is complete, the mass of the catalyst will be the same as it was at the beginning. Average rate= change in quantity measured change in timeTopic 2- Atomic StructureEvery element is made up of very small particles called atoms. Elements are the simplest substances. Each element is made up of just 1 particular type of atom which is different to the atoms in any other element. There are about 100 or more different elements.Protons are located in the nucleus of an atom. They have a mass of 1 amu and a charge of +1.Neutrons are located in the nucleus of an atom. They have a mass of 1 amu and a charge of 0.Electrons are located in the orbitals surrounding the nucleus of an atom. They have a mass of almost 0 amu and a charge of -1. The atoms of an element contain equal numbers of protons and electrons and so have no overall charge. Atoms can be represented by nuclide notation.The atoms of a particular element always contain the same number of protons. The number of protons in an atom is known as the atomic number. If the number of protons changes, then the atom becomes a different element. Changes in the number of particles in the nucleus is very rare. It only takes place in nuclear processes such as radioactive decay, nuclear bombs and nuclear reactors. The mass of an atom results almost entirely from the number of protons and neutrons in the nucleus. The mass number is the sum of the protons and neutrons.Electrons are arranged in energy levels in an atom. The first energy level can hold only 2 electrons. The second energy level can hold up to 8 electrons (4 orbitals). The third energy level can hold up to 8 electrons (4 orbitals). The elements of the periodic table are arranged in terms of their atomic number and chemical properties. Elements with the same number of outer electrons have similar chemical properties.When there is an imbalance in the number of positive protons and negative electrons, the particle is known as an ion. Although atoms of the same element always have the same number of protons, they may have different numbers of neutrons. Atoms that differ in this way are called isotopes. The isotopes of an element are virtually identical in their chemical reactions. The uncharged neutrons make no difference to chemical properties, but do affect physical properties such as melting and boiling point, and density.The relative atomic mass is the average mass of the isotopes present taking into account their relative proportions. It is the average mass of an atom in a sample of the element. To calculate the RAM of a mixture of isotopes, multiply the percentage of each isotope by its atomic mass (mass number) and add them together. Topic 3- Bonding and PropertiesThe most stable atoms (and therefore the most unreactive) are in group 8/0 on the periodic table: the Noble Gases. Atoms form bonds to achieve chemical stability. This is achieved by bonding atoms becoming like the Noble Gases, which have full or complete outer shells of electrons. The octet rule states that elements gain or lose electrons to attain an electron arrangement of the nearest noble gas. There are different types of ones that can hold particles together:CovalentIonicMetallicAtoms can bond with each other by sharing outer electrons or by losing or gaining electrons to form ions. When atoms share electrons a covalent bond forms. When atoms from ions, ionic bonds result.Metallic bonds occur in metals (see Unit 3 Topic 1).Hydrogen and the elements in groups 4,5,6 and 7 are covalently bonded. When non-metal atoms have similar electron attracting powers the half-filled orbitals with unpaired electrons will overlap. The electrons are then shared and the outer orbitals of both atoms are filled by this process. A covalent bond is formed. A covalent bond involves a balance between attraction and repulsion. It occurs when the positive nuclei from two atoms are held together by their common attraction for the shared pair of electrons held between them. Covalent bonds are strong bonds- a lot of energy is required to break them.There are 7 diatomic elements in the periodic table. These elements do not exist as single atoms. They are molecules made up of two atoms. The formulae of these elements are: H2,N2,O2,F2,Cl2,Br2,I2There are two types of covalent structure: Covalent molecularCovalent networkThese covalent molecules can exist in all three physical states. Examples of Covalent Molecular elements: Sulphur- S8 - Solid Bromine- Br2- Liquid Hydrogen- H2-GasExamples of Covalent Molecular compounds:Glucose-C6H12O6-SolidWater- H2O-LiquidMethane- CH4-Gas Individual (discrete) molecules with strong covalent bonds between atoms exist. They have relatively low melting and boiling points due to the much weaker attractions between the molecules. It is these weaker intermolecular attractions that are broken on melting and boiling. The molecules can then move apart. The stronger the intermolecular attractions, the more energy and therefore the higher temperature required for melting and boiling. The strong covalent bonds are not broken during these changes of state. Chemical formulae indicate the elements present in a compound. When the structure is covalent molecular it also indicates the number of atoms present in each molecule.All molecules are 3D. The shape of a molecule is determined by the orbitals and the electrons that are used in forming the covalent bonds. The orbitals with non-bonding electron pairs must also be taken into consideration.