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Topic 5 – Chemical Reactions Revision Checklist . C2.24-25 Temperature Changes. Magnesium reacting with acid. Thermit reaction. Exothermic reactions. Exothermic reactions increase in temperature. Examples include: Burning reactions including the combustion of fuels.
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C2.24-25 Temperature Changes Magnesium reacting with acid Thermit reaction Exothermic reactions Exothermic reactions increase in temperature. • Examples include: • Burning reactions including the combustion of fuels. • Detonation of explosives. • Reaction of acids with metals.
C2.24-25 Temperature Changes 45° C 25° C magnesium hydrochloric acid Exothermic reaction: Mg + HCl • magnesium + hydrochloric acid Heat energy given out Gets hot
C2.24-25 Temperature Changes Exothermic reactions: source of energy • If heat is given out this energy must have come from chemical energy in the starting materials (reactants). 45° C 25o C Reactants convert chemical energy to heat energy. The temperature rises.
C2.24-25 Temperature Changes Exothermic reactions: energy changes • Almost immediately the hot reaction products start to lose heat to the surroundings and eventually they return to room temperature. 45o C 25° C Chemical energy becomes heat energy. The reaction mixture gets hotter. Eventually this heat is lost to the surroundings. It follows that reaction products have less chemical energy than the reactants had to start with.
Exothermic energy level diagram C2.24-25 Temperature Changes Reactants have more chemical energy. reactants Some of this is lost as heat which spreads out into the room. Energy / kJ) Products now have less chemical energy than reactants. products Progress of reaction (time)
Exothermic reactions and ΔH C2.24-25 Temperature Changes H (delta H) ishow much energy is given out reactants H is negative because the products have less energy than the reactants. H=negative Energy / kJ products Progress of reaction
Definition of an exothermic reaction C2.24-25 Temperature Changes Energy / kJ) Progress of reaction reactants products Exothermic reactions give out energy. There is a temperature rise and H is negative. His negative
C2.24-25 Temperature Changes Endothermic reactions Endothermic reactions cause a decrease in temperature. • Endothermic chemical reactions are relatively rare. • A few reactions that give off gases are highly endothermic - get very cold. • Dissolving salts in water is another process that is often endothermic.
Endothermic reaction: NH4NO3 + H2O C2.24-25 Temperature Changes Heat energy taken in as the mixture returns back to room temp. ammonium nitrate water Endothermic reactions cause a decrease in temperature. Cools Starts 25° C Cools to 5° C Returns to 25° C
C2.24-25 Temperature Changes Endothermic reactions: source of energy • Extra energy is needed in order for endothermic reactions to occur. • This comes from the thermal energy of the reaction mixture which consequently gets colder. 5° C 25o C Reactants convert heat energy into chemical energy as they change into products. The temperature drops.
C2.24-25 Temperature Changes Endothermic reactions: energy changes • The cold reaction products start to gain heat from the surroundings and eventually return to room temperature. The reactants gain energy. 25o C 5o C 25° C This comes from the substances used in the reaction and the reaction gets cold. Eventually heat is absorbed from the surroundings and the mixture returns to room temperature. Overall the chemicals have gained energy.
Endothermic reactions and ΔH C2.24-25 Temperature Changes This is how much energy is taken in products This is positive because the products have more energy than the reactants. H=+ Energy / kJ) reactants Progress of reaction
Definition of an endothermic reaction C2.24-25 Temperature Changes Energy / kJ Progress of reaction products reactants Endothermic reactions take in energy. There is a temperature drop and H is positive. H=+
C2.24-25 Temperature Changes reactants products H = - Energy / kJ) H = + Energy / kJ products reactants Progress of reaction Progress of reaction Energy diagrams Sketch the two energy diagrams and label exothermic and endothermic as appropriate.
C2.24-25 Temperature Changes Energy in chemicals Energy needed Breaking chemical bonds • Most chemicals will decompose (break up) if we heat them strongly enough. • This indicates that breaking chemical bonds requires energy – is an endothermic process. Heat taken in Energy needed to overcome the bonding between the atoms
C2.24-25 Temperature Changes Energy in chemicals Energy given out Making chemical bonds • It is reasonable to assume that bond making will be the opposite of bond breaking • Energy will be given out in an exothermic process when bonds are formed. Heat given out Energy given out as bonds form between atoms
C2.24-25 Temperature Changes Energy given out as new bonds form Energy taken in as old bonds break Overall endothermic in this case Energy in chemicals +H products reactants Bonds and endothermic reactions • In most chemical reactions some existing bonds are broken (endothermic) • But new bonds are made (exothermic)
C2.24-25 Temperature Changes Energy taken in as old bonds break Overall exothermic – in this case Energy given out as new bonds form Energy in chemicals -H reactants products Bonds and exothermic reactions • Again some existing bonds are broken (endothermic) • And new bonds are formed (exothermic)
C2.24-25 Temperature Changes Exo Endo Bonds break Bond forming Bonds form Energy in chemicals Energy in chemicals Bonds break products -H +H reactants reactants products Summary of bond changes • Where the energy from bond forming exceeds that needed for bond breaking the reaction is exothermic. • Where the energy for bond breaking exceeds that from bond forming the reaction is endothermic.
C2.24-25 Temperature Changes C H H H H O O O O Bond Breaking Bond Forming H O Energy in chemicals O O C O O H H H H O H C O O H H Progress of reaction Burning methane • This is an exothermic reaction: -H
C2.26-28 Rates of Reaction and Collision Theory Reactions, particles and collisions Reactions take place when particles of reactants collide with a certain amount of energy. This energy is called activation energy, and is different for each reaction. The rate of a reaction depends on two things: • the frequency of collisions between particles; • the energy with which particles collide. If particles collide with less energy than the activation energy, they will not react. The particles will just bounce off each other.
C2.26-28 Rates of Reaction and Collision Theory Changing the rate of reactions Anything that increases the number of successful collisions between reactant particles will speed up a reaction. What factors speed up reactions? • Increased temperature; • increased concentration of dissolved reactants, and increased pressure of gaseous reactants; • increased surface area of solid reactants; • use of a catalyst.
C2.26-28 Rates of Reaction and Collision Theory hydrochloricacid magnesiumchloride + + magnesium hydrogen Measuring rates of reaction Measuring the rate of a reaction means measuring the rate of change over a period of time. This means measuring the change in the amount of a reactant or the amount of a product. What can you measure to calculate the rate of reaction between magnesium and hydrochloric acid? • The amount of magnesium used up (g/min). • The amount of hydrochloric acid used up (cm3/min). • The amount of magnesium chloride produced (g/min). • The amount of hydrogen produced (cm3/min).
C2.26-28 Rates of Reaction and Collision Theory Temperature The higher the temperature, the faster the rate of a reaction. In many reactions, a rise in temperature of 10°C causes the rate of reaction to approximately double. Why does increased temperature increase the rate of reaction? • At a higher temperature, particles have more energy. This means they move faster and are more likely to collide with other particles. • When the particles collide, they do so with more energy, and so the number of successful collisions increases.
C2.26-28 Rates of Reaction and Collision Theory sodiumchloride sulfurdioxide sodiumthiosulfate hydrochloricacid + + + + sulfur water SO2(g) H2O(l) Na2S2O3(aq) 2HCl(aq) 2NaCl(aq) S(s) + + + + Temperature and rate of reaction The reaction between sodium thiosulfate and hydrochloricacid produces sulfur. Sulfur is solid and so it turns the solution cloudy. The effect of increasing temperature on the rate of reaction can be measured by comparing how long it takes the solution to turn cloudy at different temperatures.
C2.26-28 Rates of Reaction and Collision Theory 5. Repeat the experiment at different temperatures using the same volume of reactants. Compare how long it takes the cross to disappear. Sodium thiosulfate and hydrochloric acid To run the experiment investigating the effect of temperature on the rate of reaction: 1. Mark a cross on a piece of paper. 2. Add a known amount of sodium thiosulfate to a beaker, and place it on the piece of paper. 3. Add a known amount of hydrochloric acid to the beaker and immediately start a stop-clock. The solution will begin to turn cloudy. 4. As soon as the cross can no longer be seen, stop the clock and note the time.
C2.26-28 Rates of Reaction and Collision Theory increasing time Sodium thiosulfate and hydrochloric acid When looking down into the beaker, the cross will become fainter over time: The time taken for the cross to disappear can be used as the time of the reaction.
C2.26-28 Rates of Reaction and Collision Theory low concentration high concentration Concentration The higher the concentration of a dissolved reactant, the faster the rate of a reaction. Why does increased concentration increase the rate of reaction? At a higher concentration, there are more particles in the same amount of space. This means that particles are more likely to collide with other particles.
C2.26-28 Rates of Reaction and Collision Theory magnesiumchloride hydrochloricacid + + magnesium hydrogen + + MgCl2 (aq) Mg(s) 2HCl (aq) H2 (g) Reaction between acid and metal Reactive metals such as magnesium react with acid to produce hydrogen gas. The effect of increasing concentration on the rate of reaction can be measured by comparing how quickly hydrogen is produced using different concentrations of hydrochloric acid.
C2.26-28 Rates of Reaction and Collision Theory glasstube rubber connector gas syringe conicalflask rubber bung hydrochloric acid magnesium ribbon Mg + HCl: experiment set-up What equipment do you need for the experiment investigating the effect of concentration on the rate of reaction?
C2.26-28 Rates of Reaction and Collision Theory Magnesium and hydrochloric acid To run the experiment investigating the effect of concentration on the rate of reaction: 1. Measure out a fixed volume of hydrochloric acid into the conical flask. 2. Add a known mass of magnesium to the flask, immediately attach the gas syringe and start a stop-clock. 3. Measure the volume of hydrogen collected in the syringe at regular intervals until no more gas is produced. 4. Repeat the experiment using a different concentration of hydrochloric acid but using the same volume of acid and the same mass of magnesium. Compare the rate at which hydrogen is produced.
C2.26-28 Rates of Reaction and Collision Theory Surface area Any reaction involving a solid can only take place at the surface of the solid. If the solid is split into several pieces, the surface area increases. slow rate This means that there is an increased area for the non-solid reactant particles to collide with. The smaller the pieces, the larger the surface area. This means more collisions and a faster rate of reaction. fast rate
C2.26-28 Rates of Reaction and Collision Theory calciumchloride carbondioxide calciumcarbonate hydrochloricacid + + + water CaCO3(aq) 2HCl (aq) CaCl2(aq) H2O(aq) CO2(g) + + + Reaction between a carbonate and acid Marble chips are made of calcium carbonate. They react with hydrochloric acid to produce carbon dioxide. The effect of increasing surface area on the rate of reaction can be measured by comparing how quickly the mass of the reactants decreases using marble chips of different sizes.
C2.26-28 Rates of Reaction and Collision Theory cotton wool ‘plug’ conicalflask hydrochloricacid calciumcarbonatechips weighing scales CaCO3 + HCl: experiment set-up What equipment do you need for the experiment investigating the effect of surface area on the rate of reaction?
C2.26-28 Rates of Reaction and Collision Theory Calcium carbonate and hydrochloric acid To run the experiment investigating the effect of surface area on the rate of reaction: 1. Measure out a fixed volume of hydrochloric acid into a conical flask and place the flask on weighing scales. 2. Add a fixed mass of calcium carbonate chips to the flask, and place a cotton wool plug in the neck. This stops the liquid from spitting while allowing the CO2 to escape. 3. Begin taking mass readings straight away, and continue until there is no further change in mass. 4. Repeat the experiment using the same mass of calcium carbonate but of a smaller chip size, and the same volume of hydrochloric acid. Compare the rate at which the mass of reactants decreases.
What are catalysts? C2.29 Catalysts Ea withoutcatalyst energy (kJ) Ea withcatalyst reaction (time) • Change the rate of reaction without being used up. • Don’t produce more product – just produce the same amount of product more quickly! Most catalysts work by lowering the reaction’s activation energy (Ea).
Catalysts in industry C2.29 Catalysts • Products can be made more quickly, saving time and money. • Catalysts reduce the need for high temperatures, saving fuel and reducing pollution.
