Energetics and Rates

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1 Energetics and Rates Specification points Year 0 Energetics Energy transfer during exothermic and endothermic reactions - know that an exothermic reaction transfers energy to the surroundings so the surrounding temperature increases eg: combustion, many oxidation reactions and neutralisation. Some everyday uses of exothermic reactions eg: self heating cans and hand warmers. that an endothermic reaction takes in energy from the surroundings so the temperature of the surroundings decreases eg: thermal decompositions and the reaction of citric acid and sodium hydrogencarbonate. Some sports injury packs are based on endothermic reactions. be able to distinguish between exothermic and endothermic reactions on the basis of the temperature change of the surroundings and to be able to evaluate uses and applications of exothermic and endothermic reactions given appropriate information. Reaction profiles -know that reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction. be able to draw energy level diagrams for exothermic and endothermic The energy change of reactions know that during a chemical reaction energy must be supplied to break bonds in the reactants and energy is released when bonds in the products are formed. be able to calculate the overall energy transferred during a reaction using bond energies supplied. in an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds. in an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds. Cells and batteries - know cells contain chemicals which react to produce electricity and that the voltage produced is dependent upon a number of factors including the type of electrode and electrolyte. how to make a simple cell and the a battery in non-rechargeable cells and batteries the chemical reactions stop when one of the reactants has been used up. Alkaline batteries are non-rechargeable. Fuel cells - know Fuel cells are supplied by an external source of fuel (eg hydrogen) and oxygen or air. The fuel is oxidised electrochemically within the fuel cell to produce a potential difference. in a hydrogen fuel cell involves the oxidation of hydrogen to produce water. Hydrogen fuel cells offer a potential alternative to rechargeable cells and batteries. how to evaluate the use of hydrogen fuel cells in comparison with rechargeable cells/batteries and to be able to write the half equations for the electrode reactions in the hydrogen fuel cell. Year 0 - Rates of Reaction

2 Calculating rates of reactions The rate of a chemical reaction can be found by measuring the quantity of a reactant used or the quantity of product formed over time: mean rate of reaction = quantity of reactant used time taken mean rate of reaction = quantity of product formed time taken The quantity of reactant or product can be measured by the mass in grams or by a volume in cm 3. The units of rate of reaction may be given as g/s or cm 3 /s or mol/dm 3. Students should be able to: calculate the mean rate of a reaction from given information about the quantity of a reactant used or the quantity of a product formed and the time taken draw, and interpret, graphs showing the quantity of product formed or quantity of reactant used up against time draw tangents to the curves and use gradient of the tangent to calculate rate. Factors which affect the rates of chemical reactions Factors which affect the rates of chemical reactions include: the concentrations of reactants in solution, the pressure of reacting gases, the surface area of solid reactants, the temperature and the presence of catalysts. Know how to investigate factors affecting rates by measuring the loss in mass of reactants, the volume of gas produced or the time for a solution to become opaque or coloured. Students should be able to explain the effects on rates of reaction of changes in the size of pieces of a reacting solid in terms of surface area to volume ratio. Collision theory and activation energy Be able to use Collision theory to explain how various factors affect rates of reactions. Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy. The minimum amount of energy that particles must have to react is called the activation energy. Students should be able to explain the effects on rates of reaction of: changes in the size of pieces of a reacting solid in terms of surface area to volume ratio changes in temperature, concentration and pressure in terms of collision theory. Factors that increase the rate of reaction Increasing the concentration of reactants in solution, the pressure of reacting gases, and the surface area of solid reactants increases the frequency of collisions and rate. Increasing the temperature increases the frequency of collisions and makes the collisions more energetic, and so increases the rate of reaction. Students should be able to predict and explain the effects of changing conditions on the rate of a reaction use simple ideas about proportionality when using collision theory to explain the effect of a factor on the rate of a reaction Catalysts Catalysts change the rate of chemical reactions but are not used up during the reaction. Different reactions need different catalysts. Enzymes act as catalysts in biological systems. Catalysts increase the rate of reaction by providing a different pathway for the reaction that has a lower activation energy be able to show this on a reaction profile diagram. Students should be able to identify catalysts in reactions from their effect on the rate of reaction and because they are not included in the chemical equation for the reaction. explain catalytic action in terms of activation energy. give the names of catalysts other than those specified in the subject content.

3 Independent Study suggestions. Look at the specification points above use the textbook pages first edition and second edition, 4-22 to make a few notes/spider diagram/revision cards 2. Watch the Fuse School short 3-4 minute explanation videos on any area where you need more help. Energetics Calorimetry Bond breaking and making calculations: Collision Theory Catalysts Hydrogen peroxide breakdown: Rates of reaction I Rates of reaction II Ionic Equations: Electrochemical series: Fuel cells and energy 3. Have a go at the redox quizzes 4. Have a go at the rates of reaction quizzes: 5. Now have a go at the past paper questions below which you can then mark

If metal is more reactive than metal 2 then the voltage measured is negative.

11 Q.A student investigated simple cells using the apparatus shown in the figure below. If metal 2 is more reactive than metal then the voltage measured is positive. If metal is more reactive than metal 2 then the voltage measured is negative. The bigger the difference in reactivity of the two metals, the larger the voltage produced. The student s results are shown in the table below. (a) The ionic equation for the reaction occuring at the zinc electrode in the simple cell made using copper and zinc electrodes is: Zn Zn e Zinc is oxidised in this reaction. Give a reason why this is oxidation. ()

12 (b) Look at the table above. Which one of the metals used was the least reactive? Give a reason for your answer. Metal... Reason... (2) (c) Predict the voltage that would be obtained for a simple cell that has iron as metal and copper as metal 2. Explain your answer. (3) (d) Hydrogen fuel cells have been developed for cars. Write a word equation for the overall reaction that takes place in a hydrogen fuel cell. () (e) Write the two half equations for the reactions that occur at the electrodes in a hydrogen fuel cell. (2)

() (b) Owners of cars powered by fuel cells buy hydrogen from hydrogen filling stations.

13 (Total 9 marks) Q2.Some cars are powered by hydrogen fuel cells. Figure Robert Couse-Baker (CC BY-SA 2.0) via Flickr (a) What type of energy is released by hydrogen fuel cells? () (b) Owners of cars powered by fuel cells buy hydrogen from hydrogen filling stations. Figure 2 shows how the number of hydrogen filling stations in the UK is expected to increase up to the year Figure 2 Year Use the information in Figure 2 and your own knowledge to answer this question. Suggest two reasons why the UK government might encourage the building of more hydrogen filling stations. (2)

14 (c) The equation for the reaction of hydrogen with oxygen is: 2 H 2 + O 2 2 H 2 O During the reaction, energy is used to break the bonds of the reactants. Energy is released when new bonds are made to form the product. Bond energies for the reaction are given in the table below. Bond Bond energy in kj The structures of the reactants and product are shown in Figure 3. Figure 3 hydrogen oxygen water (i) Calculate the energy change for the reaction: 2 H 2 + O 2 2 H 2 O Energy change =... kj (3)

15 (ii) The reaction of hydrogen with oxygen is exothermic. Complete the energy level diagram for this reaction on Figure 4. Clearly label the activation energy. Figure 4 (3) (Total 9 marks)

16 Answers:

18 M.(a) (zinc has) lost electron(s) accept loss of electrons (b) copper is the least reactive because it gave the most negative voltage when it was metal 2 or it gave the biggest voltage with chromium or it gave the most positive voltage when it was metal (c) 0.7 V

19 The voltage with chromium and copper is.2 accept use of other cell pairings such as tin with copper and tin with iron The voltage with chromium and iron is 0.5 and copper is less reactive (than iron) (d) hydrogen + oxygen = water (e) H 2 2H + + 2e O 2 + 4H + + 4e 2H 2 O [9] M2.(a) (b) electrical using hydrogen saves petrol / diesel / crude oil allow crude oil is non-renewable ignore hydrogen is renewable using hydrogen (in fuel cells) does not cause pollution accept no carbon dioxide produced allow less carbon dioxide produced allow hydrogen produces only water (c) (i) ( )486 correct answer with or without working gains 3 marks if answer is incorrect: (2 436) or 370 gains mark or 856 gains mark correct subtraction of ecf gains mark 3 (ii) products lower than reactants reaction curve correctly drawn activation energy labelled

Rates. Specification points. Year 10 - Rates of Reaction

Rates. Specification points. Year 10 - Rates of Reaction Rates Specification points Year 10 - Rates of Reaction Calculating rates of reactions The rate of a chemical reaction can be found by measuring the quantity of a reactant used or the quantity of product