Patterns of Behaviour How we can predict what will happen in a chemical reaction
Contents • The Periodic Table • Chemical Reactions • Rates of Reactions • Reactions involving Enzymes • Reversible Reactions • Energy Transfer in Reactions
The Periodic Table The periodic table list elements according to their atomic number, if they were arranged According to atomic mass potassium and argon would be the wrong way round. Elements having the same number of electrons in their outermost shell are placed in vertical columns called groups. They have similar chemical properties From left to right, across each horizontal row (period) of the periodic table, a particular energy level is gradually filled up with electrons; in the next period, the next energy level is filled with electrons.
Chemical Reactions • Group 1 Metals • Transition Metals • Group 7 –The Halogens • Electrolysis of Brine • Acids and Alkalis
Group 1 metals • Physical Properties • Easy to cut, shiny when freshly cut, low density. • Chemical Properties • Metals are very reactive with chemical reactivity increasing down the group. • All the metals react • with oxygen to form oxides e.g. Li2O • With water to form hydroxides and hydrogen e.g. KOH • With halogens to form halides e.g. NaOH • Alkali Metal Compounds • All alkali metal compounds are stable, this is because the alkali metals are so reactive. Alkali metals have to be extracted from their ores by electrolysis • Alkali metal compounds are usually colourless • The hydroxides are strong alkalis
Chemical Properties All halogens form diatomic molecules, that is they go around in pairs e.g. I2 The halogens are reactive elements, with reactivity decreasing down the group. The larger atoms are less reactive as it is harder for them to capture an electron. Halogen compounds are called halides. Chlorine forms chlorides, bromine forms iodide etc. Group 7 – The Halogens
Acids and Alkalis • Acidity is measured on the pH scale,1 is a strong acid,7 is neutral and 14 is a strong alkali. • acid + alkali salt + water • naming salts • the alkali gives the first half of the name e.g. sodium hydroxide forms sodium salts. • the acid gives the second part of the name • hydrochloric acid gives chlorides • nitric acid gives nitrates • sulphuric acid gives sulphates • An acid is something which produces hydrogen ions in solution • Bases are the oxides and hydroxides of metals • Alkalis are soluble bases the produce hydroxide ions in solution. • Neutralisation occurs when hydrogen ions and hydroxide ions combine to form water: • H+(aq) + OH-(aq) H2O(l)
Rates of Reaction • Reactions can only occur when reacting particles collide with enough energy. This energy is called the activation energy • Increasing the temperature of the reaction increases the speed at which particles move. There are more collision and the collisions have a higher energy. This increases the number of effective collisions. Therefore the rate of reaction is increased. • Increasing the concentration of reactants increase the number of particles available to react. More collisions occur. Therefore the rate of reaction is increased. • Decreasing the size of particles increases the surface area. If there is a larger surface available for particles to collide with there will be more collisions. Therefore the rate of reaction is increased. • Catalyst provide a different route for the reaction to happen. This route has a lower activation energy. This means that more of the collisions will be effective. The rate of reaction is increased.
Reactions involving Enzymes • Enzymes are biological catalysts. • They are natural proteins. • An enzyme works because it is the right shape to hold the reactant particles together. • If an enzyme gets too hot this shape is destroyed. We say they have been denatured. he no longer work as a catalyst. • Enzymes are used to brew beer, make cheese and in biological washing powders
Reversible Reactions • Reversible reaction are one which occur in both directions at the same time. • We represent them using a double arrow: • reactantsÂ products • If a reversible reaction is exothermic (gives out energy) in one direction it is endothermic (takes in energy) in the other direction. • When a reversible reaction takes place in a closed system an equilibrium will be reached. This means that the forward and the backward reactions occur at the same rates. • In a closed system if the conditions are changed then the position of the equilibrium will move to minimise the effect of the change in conditions. • N2(g) + 3H2(g)Â 2NH3(g) this reaction is exothermic: • increasing the temperature will move the equilibrium in favour of the reactants, in the endothermic direction. The system ‘tries’ to take in the extra energy. • These factors, together with reaction rates, are important when determining the optimum conditions in industrial processes
Energy Transfer in Reactions • Chemical reactions involve the making and breaking of bonds. • Energy must be taken in so that bonds can be broken. • Energy is given out when new bond are made. • EA is the activation energy this is the energy required to break the bond of the reactants. • DH is the energy (enthalpy) change of the reaction if it is negative energy is given out if it is positive energy is taken in. • In an exothermic reaction more energy is released making new bonds than is taken in breaking bonds. • In an endothermic reaction less energy is released making new bonds than is taken in breaking bonds. • The energy change of a reaction can be calculated using the following formula • DH = Energy used in breaking bonds- Energy given out in making bonds • Remember for a particular bond the energy for breaking and making it is the same.
Summary • The Periodic Table • Elements are arranged according to their atomic number, each group of elements has similar chemical properties. • Chemical Reactions • The soluble oxides and hydroxides of group 1 metals are called alkalis, insoluble oxides and hydroxides of transition metals are called bases, both are neutralised by acids to form salts • Electrolysis of brine produces hydrogen, chlorine and sodium hydroxide solution. • Rates of Reactions • The rate of reaction can be increased by the use of a catalyst or enzyme, by increasing the temperature, concentration or surface area of reactants. • Reactions involving Enzymes • Enzymes are biological catalysts they can be denatured by over heating. • Reversible Reactions • Some reactions can proceed in both directions, in a closed system these reaction will reach an equilibrium in which both reactions proceed at the same rate. • Energy Transfer in Reactions • In all chemical reactions bonds are made and broken. These processes involve the transfer of energy.