Noadswood Science, 2012

 To be able to measure the rate of reaction Tuesday, September 08, 2015

Rate  Different reactions can happen at different rates: -  Reactions that happen slowly have a low rate of reaction  Reactions that happen quickly have a high rate of reaction  For example, the chemical weathering of rocks is a very slow reaction – it has a low rate of reaction, whilst explosions are very fast reactions – they have a high rate of reaction

Rate  Chemical reactions occur when particles of reactant collide with enough energy to react…

Measuring  How can you measure the rate of a reaction?  Measure the rate at which a reactant is used up  Measure the rate at which a product is formed  The method chosen depends on the reaction being studied – sometimes it is easier to measure the change in the amount of a reactant that has been used up; sometimes it is easier to measure the change in the amount of product that has been produced…  What can be measured?

Measuring  The measurement itself depends on the nature of the reactant or product…  Mass of a substance (solid, liquid or gas) is measured with a balance  The volume of a gas is usually measured with a gas syringe, or sometimes an upside down measuring cylinder or burette

Equation  The rate of reaction can be worked out simply, recording either the amount of reactant used / product formed and the time taken… Rate of reaction = amount of reactant used or amount of product formed time taken  E.g. 24cm 3 hydrogen gas is produced in 2 minutes – what is the rate of reaction? Rate of reaction = 24 ÷ 2 Rate of reaction = 12cm 3 hydrogen gas produced per minute

Experiment  Complete the rate of reaction worksheet… CaCO 3 + 2HCl  CaCl 2 H 2 O + CO 2

Rates Of Reaction  What factors effect the rate of reaction?  Temperature  Concentration of a dissolved reactant  Pressure of a reacting gas  Surface area of reactants  Catalysts  Chemical reactions occur when particles of reactant collide with enough energy to react – anything that increases the chance of effective collision increases the rate of reaction (e.g. surface area, temperature, pressure, using a catalyst etc…)

Surface Area  Look at the sugar cube and granulated sugar dissolving in the water – which will dissolve first?  The granulated sugar dissolves much quicker than the sugar cube – this is due to the difference in surface area…

Surface Area  The reactions of solids can only take place at the surface of the solid – if the solid is broken into smaller pieces we get more area and a faster reaction… Molecules collide with surface of the substance Extra surface for molecule to collide with

Surface Area  If we grind up a solid to a powder we massively increase the surface area, massively increasing the rate of any reaction… Slow Fast

Temperature  How do the particles differ in cold water and boiling water?  In boiling water the particles collide more often and with more force – they are moving quicker and with more energy…

Temperature  If the temperature is increased:  The reactant particles move more quickly  More particles have the activation energy or greater  The particles collide more often, and more of the collisions result in a reaction  The rate of reaction increases

Concentration  Concentration…

Pressure  Reactions involving gases are affected by the pressure of the gases present  Increasing the pressure squeezes the gas molecules closer together (making them more concentrated) – pressure speeds up reactions Compress

Catalysts  For chemical reactions to occur: -  Existing bonds have to begin breaking so that new ones can be formed  The molecules have to collide in such a way that the reacting parts of the molecules are brought together  Catalysts can help with either or both of these processes

Catalysts  Catalysts increase the rate of reaction without being used up – they do this by lowering the activation energy needed  With a catalyst, more collisions result in a reaction, so the rate of reaction increases – different reactions need different catalysts  Catalysts are important in industry because they reduce costs, e.g.  Biological soap powder uses biological catalysts (enzymes)  Enzymes in pineapple help cooked ham to be more tender  Manufacture of fertiliser via the Haber Process involves use of an iron catalyst  Catalytic converters in cars – the catalyst encourages decomposition of nitrogen oxide (poisonous) back into nitrogen and oxygen