Chemistry

• atoms are made of protons, neutrons and electrons.
• protons and electrons have equal and opposite electric charges.  Protons are positively charged, and electons are negatively charged.
• atoms are arranged in the periodic table in order of their atomic number
• neutrons have no electic charge.  They are neutral.

• the electrons in an atom are arranged in energy levels or shells
• atoms with the same number of electrons in their outer shell belong in the same group of the periodic table.
• the number of electrons in the outer shell of an atom determines the way that the atom behaves in chemical reactions.

• elements react to form compounds by gaining or losing electrons or by sharing electrons.
• the electrons in Group 1 react with the elements in Group 7 because Group 1 elements can lose an electron to gain a full outer shell.  This electrons can be given to an atom grom Group 7, which then also gains a full outer shell.

• ionic compounds are held together by strong forces between the oppositely charged ions.
• other elements that can form ionic compounds include those in Group 2 and 6.

• covalent bonds are formed when atoms share electrons.
• many substances containing covalent bonds consist of molecules, but some have giant covalent structures.

• the atoms (or ions) in metals are arranged in regular layers.
• the positive ions in metals are held together by electrons from the outer shell of each metal atom.  These delocalised electrons are free to move throughout the metal lattice.

• it takes a lot of eneergy to break the bonds which hold a giant ionic lattice together.  so ionic compounds have very high melting points- they are all solids at room temperature.
• ionic compounds will conduct electricity when we melt them or dissolve them in water because their ions can then move freely.

• substances made up of simple molecules have low melting points and boiling points.
• the forces between simple molecules are small.  These weak intermolecular forces explain their low melting points and boiling points.
• simple molecules have no overall charge, so they cannot carry electrical charge.  Therefore substances containing simple molecules do not conduct electricity.

• some covalently bonded substances contain giant structures.
• these substances have high melting points and boiling points.
• the giant structure of graphite contains layers of aoms that can slide over each other which make graphite slippery.  The atoms in diamond have a different structure and cannot slide like this- so diamond is a very hard substance.
• graphite can conduct electricity because of the delocalised electrons along its layers.

• we can bend and shape metals because the layers of atoms (or ions) in a metal can slide over each other.
• delocalised electrons in metals allow them to conduct heat and electricity well.

• the relative mass of protons and neutrons is 1.
• the mass number of an atom tells you the total number of protons and neutrons in its nucleus.
• isotopes are atoms of the same element with different numbers of neutrons.

• we compare the masses of atoms by measuring them relative to atoms of carbon-12.
• we work out the relataive formula mass of a compound from the relative masses of the elements in it.
• one mole of any substance always contains the same number of particles.

• the relative atomic masses of the elements in a compound can be used to work out its percentage composition.
• we can calculate emphirical formulae given the masses or percentage composition of elements present.

• chemical equations tell us the number of moles of substances in the chemical reaction.
• we can use chemical equations to calculate the masses of reactants and products in a chemical reaction from the masses of one mole of each of the substances involved in the reaction.

• the yield of a chemical reaction describes how much productis made.
• the percentage yield of a chemical reaction tells us how much product is made compared with the maximum amount that would be made (100%).
• factors affecting the yield of a chemical reaction include product being left behind in the apparatus and difficulty separating the products from the reaction mixture.
• it is important to maximise atom economy to conserve resources and reduce pollution.

• In a reverse reaction the products of the reaction can react to make the original reactants.
• in a closed system the rate of the forward and backwards (reverse) reactions are equal at equilibrium.
• changing the reaction conditions can change the amounts of products and reactants in a reaction mixture.

• ammonia is an important chemical for making ither chemicals, including fertilisers.
• ammonia is made from nitrogen and hydrogen in the Haber process.
• we carry out the Haber process under conditions which are chosen to give a seasonable yield of ammlonia as quickly as possible.

• knowing and controlling the rate of chemical reactions is important in living cels, in the laboratory and in industry.
• we can measure the rate of a chemical reaction by following the rate at which reactants are used up.  Alternatively, we can measure the rate at which products are made.

• the minimum amount of energy that particles must have in order to react is called the activation energy.
• the rate of a chemical reaction increases if the surface area of any solid reactant is increased.

• reactions happen more quickly as the temperature increases.
• a 10 degrees C increase in temperature at room temperature roughly doubles the rate of a reaction.
• the rate of a chemical reaction increases with temperature because the particles collide more often and they have more energy.

• increasing the concentration of reactants increases the frequency of collisions between particles, increasing the rate of reaction.
• increasing the pressure of reacting gases results in particles collide more often, increasing the rate of reaction.

• a catalyst speeds up the rate of a chemical reaction.
• a catalyst is not used up during a chemical reaction.
• catalysts are generally quite expensive because they are made of precious metals.
• catalysts lower the activation energy needed for a chemical reaction to occur.

• energy may be transferred to or from the reacting substances in a chemical reaction.
• a reaction where energy is transferred from the reacting substances into its surroundings is called an exothermic reaction.
• a reaction where energy is transferred to the reacting substances from its surroundings is called an endothermic reactions.

• in reverse reactions, one reaction is exothermic and the other is endothermic.
• in any reversible reaction, the amount of energy released when the reaction goes in one direction is exactly equal to the energy absorbed when the reaction goes in the opposite direction.
• we can change the amount of products formed at equilibrium by changing the temperature at which we carry out a reverse reaction.

• the Haber process uses a pressure of around 200 to 350 atmospheres to increase the amount of ammonia produced.
• although higher pressures would produce more ammonia, they would make the chemical plant too expensive to build.
• a temperature of about 450 degrees C is used for the reaction.  Although lower temperatures would increase the amount of ammonia at equilibrium, the ammonia would be produced too slowly.

• electrolysis involves splitting up a substance using electricity.
• ionic substances can be electrolysed when they are molten or in solution.
• in electrolysis positive ions move to the negative electrode (cathode) and negative ions move to the positive electrode (anode).

• in electrolysis, the ions move towards the oppositely charged electrodes.
• at the electrodes, negative ions are oxidised while positive ions are reduced.
• reactions where reduction and oxidation happen are called redox reactions.
• when electrolysis happens in water, the less reactive element is usually produced at an electrode.

• when we electrolyse brine we get three products- chlorine gas, hydrogen gas and sodium hydroxide solution.
• chlorine is used to kill microbes in drinking water and swimming pools, and to make hydrochloric acid, disinfectants, bleach and plastics.
• hydrogen is used to make margarine and hydrochloric acid.
• sodium hydroxide is used to mke bleach, paper and soap.

• copper extracted from its ore contains impurities such as gold and silver.
• copper is purified by electrolysis to remove these impurities.

• acids are substances which produce H+ ions when we add them to water.
• bases are substances that will neutralise acids.
• an alkali is a souble base.  Alkalis produce OH- ions when we add them to water.
• we use the pH scale to show how acidic or alkaline a solution is.

• when we react an acid with a base a neutralisation reaction occurs.
• the reaction between an acid and a base produces a salt and water.
• salts can also be made by reacting a metal with an acid.  This reaction prduces hydrogen gas as well as a salt.

• an indicator is needed when we produce a salt by reacting an alkali with an acid to make a soluble salt.
• insoluble salts can be made by reacting two solutions to produce a precipitate.
• precipitation is an important way of removing some substances from wastewater.

Mole: The relative formula mass of a substance in grams.

Precipitate: A solid material produced from a solution.