TOPIC 2 Speed of Reactions

A chemical reaction can be represented by a Chemical Equation. A chemical reaction can be represented by a Chemical Equation. The starting chemicals are called Reactants and the new ones made the Products. The starting chemicals are called Reactants and the new ones made the Products. The reactants go on the left-hand side of the arrow and the products on the right- hand side. The reactants go on the left-hand side of the arrow and the products on the right- hand side. ie ie ReactantsProducts Mg +HClMg(Cl) 2 + H 2

Factors affecting the speed of reactions (Rate of reactions) There are four factors that affect the Rate of Reactions. There are four factors that affect the Rate of Reactions. They are: Concentration Particle Size Temperature Catalyst. They are: Concentration Particle Size Temperature Catalyst.

Let us consider each in turn For a reaction to occur, reacting molecules must collide and stick together. For a reaction to occur, reacting molecules must collide and stick together. For a successful collision they must be moving fast enough ie. They must have sufficient Kinetic or movement energy. The minimum energy they must possess is called the Activation Energy. For a successful collision they must be moving fast enough ie. They must have sufficient Kinetic or movement energy. The minimum energy they must possess is called the Activation Energy. Concentration : The Higher the concentration the greater the rate of reaction. This is because higher concentrations have greater numbers of reacting molecules and therefore a greater chance of successful collision Concentration : The Higher the concentration the greater the rate of reaction. This is because higher concentrations have greater numbers of reacting molecules and therefore a greater chance of successful collision

Factors affecting Reaction Rates continued Particle Size : The smaller the particle size the quicker the rate of reaction. This is a result of smaller particles allowing a greater surface area of reacting molecules to be in contact and therefore a greater chance of a successful collision. Particle Size : The smaller the particle size the quicker the rate of reaction. This is a result of smaller particles allowing a greater surface area of reacting molecules to be in contact and therefore a greater chance of a successful collision. Temperature : The higher the temperature the greater the rate of reaction. When we heat a substance we give the particles more energy. As a result they move faster thus increasing the chance of a successful collision(it can be said that more particles now have more than the Activation Energy). Temperature : The higher the temperature the greater the rate of reaction. When we heat a substance we give the particles more energy. As a result they move faster thus increasing the chance of a successful collision(it can be said that more particles now have more than the Activation Energy).

Factors affecting Reaction Rates(contd) Factors affecting Reaction Rates(contd) Catalysts : A catalyst speeds up the rate of reaction. It does so by providing a surface on which reacting molecules can meet and collide. Catalysts : A catalyst speeds up the rate of reaction. It does so by providing a surface on which reacting molecules can meet and collide. A catalyst will remain unchanged before and after a reaction. A catalyst will remain unchanged before and after a reaction. Catalysts in industrial reactions often have to be changed as they become Poisoned. Catalysts in industrial reactions often have to be changed as they become Poisoned. Catalysts are said to be specific ie. A catalyst that works for one reaction does not work for any other. Catalysts are said to be specific ie. A catalyst that works for one reaction does not work for any other. Enzymes: These are Catalysts in living things (Biological Catalysts). Enzymes only work when the reaction is at the correct temperature and pH. Enzymes: These are Catalysts in living things (Biological Catalysts). Enzymes only work when the reaction is at the correct temperature and pH.

Measuring The Speed of Reactions Marble chips are added to Hydrochloric Acid and the volume of gas given off was noted every 10secs. Marble chips are added to Acid and the mass of the flask noted ever 30secs as the CO 2 gas is given off.

Time(seconds) Volume of gas(cm 3 ) This line represents Mg metal + 2M HCl acid added together This line represents the final volume of gas produced.This will be the same if the mass of Mg and no. of moles of Acid remain constant This line represents a smaller Particle Size This line represents a higher Temperature This line represents a greater Concentration The steeper the slope the quicker the reaction The following is obtained by adding a metal to an acid and measuring the volume of H 2 gas given off every 10secs How do we measure the Rates of Reaction

Measuring Reactions rates We can measure the rate of reaction by following the change in mass throughout the course of the reaction. We can measure the rate of reaction by following the change in mass throughout the course of the reaction. Let us consider chalk and excess acid added together and the mass of the flask taken every 30 secs Let us consider chalk and excess acid added together and the mass of the flask taken every 30 secs Time (s) Mass (g) CaCO 3 + 2HClCaCl 2 + CO 2 + H 2 O The Mass of the flask gets lighter as the CO 2 is released The steeper the slope the faster the reaction.The line levels off when all the chalk has been used up When the line levels off the reaction has finished