ENERGY AND ENERGETICS PART ONE Keeping Track of Energy During a Chemical Reaction

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1 ENERGY AND ENERGETICS PART ONE Keeping Track of Energy During a Chemical Reaction ADEng. PROGRAMME Chemistry for Engineers Prepared by M. J. McNeil, MPhil. Department of Pure and Applied Sciences Portmore Community College Main Campus 1

2 REVISION (PART I) Distinguish between exothermic and endothermic reactions in terms of energy content of products and reactants. (Make use of H notation). Draw an energy profile diagram to illustrate endothermic and exothermic changes. Include the action of catalyst using energy profile diagrams. Calculate energy changes from experimental data - heat of solution and heat of neutralization. Identify the various forms of energy and trace their inter-conversions. Discuss the use of fossil fuels and any other two sources of energy. 2

3 LECTURE OBJECTIVES (PART II) 3

4 LECTURE OBJECTIVES PART II 4

5 DISCREPANT EVENT I Ammonium chloride + water Calcium chloride + water 5

6 DISCREPANT EVENT II Mix NH 4 Cl (s) + Ba(OH) 2(s) - The mixture becomes more viscous. The mixture becomes colder. If you add a small volume of water and let it sit on a board table, the reaction will freeze the water and the flask will stick to the board table over time. (HEAT IS ABSORBED FROM THE SURROUNDING). Mix KCl + C 12 H 22 O 11 (sucrose) onto a plate Then add 1 drop of H 2 SO 4. The reaction is explosive. A tremendous amount of heat is expelled when the acid is added. (HEAT IS GIVEN OFF TO THE SURROUNDING) 6

7 ANALYSIS OF DISCREPANT EVENTS Ammonium Chloride + water (2.5 o C) Calcium chloride + water (52.0 o C) What direction of heat flow would you expect for the above reaction? Temperature is a measure of the average kinetic energy of the particles in a system Heat is a transfer of thermal energy. Heat is not possessed by a system. Heat is energy flowing between systems. The study of energy changes that is associated with a chemical reaction is the main focus of this presentation. 7

8 PURCHASING ENERGY Purchasing energy in a chemical form: a litre of gasoline for your car. natural gas to heat your home. a small battery for your flashlight. Super question is - where does all that enormous energy specifically came from? 8

9 CHEMISTRY AND ENERGY Chemistry? It can be considered as the study of matter and its inter-conversions for e.g. physical / chemical / nuclear changes etc. Hydrogen can undergo three changes Physical change: H 2(l) H 2(g) (Hydrogen boils at -25 o C) Chemical change: 2H 2(l) + O 2 H 2(g) (H is burned in space shuttle s main engine.) Nuclear change: H + H He (sun, where H undergo nuclear fusion) What is Energy? What is energetics? Energetics is the study of the chemical changes associated with chemical reactions. This branch of chemistry is commonly referred to as thermochemistry. What is the Law of Conservation of Energy? Let us now consider some familiar forms of energy 9

10 FORMS OF ENERGY AND INTERCONVERSIONS Light (electromagnetic radiation) Electrical Sound Mechanical (kinetic and potential) Chemical energy Nuclear energy Heat (thermal energy) etc... 10

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12 ENERGY TRANSFORMATIONS Chemical Potential Energy (E p ) is energy stored in the bonds of a substance and relative intermolecular forces. Which direction will the heat flow if your hands were cold? Kinetic Energy (E k ) is related to the motion of an entity. Thermal Energy is the total kinetic energy of all of the particles of a system. Increases with temperature. emistry/animations/chang_7e_esp /enm1s3_4.swf (Heat flow video) Why is the study of energy and energy changes important? 12

13 INTERCONVERSION OF CHEMICAL ENERGY 13

14 ORIGIN OF CHEMICAL ENERGY All compounds have a certain amount of energy stored in them. This energy is stored mainly in the form of bonds (chemical bond). During a chemical reaction, chemical bonds are being broken and new chemical bonds are being formed. (Law of conservation of energy) The energy that is released from broken bonds in the reactants is required to form the new bonds in the products. On the basis of energy change, chemical reactions can be classified into two types - endothermic and exothermic processes. (chemical system versus the surrounding) There are several ways to distinguish between both processes: i. study whether the energy flows into a chemical reaction or viceversa. ii. graphing the processes using energy profile diagrams. 14

15 CLASSIFICATION OF CHEMICAL REACTION (in terms of energy) Photosynthesis ENDOTHERMIC PROCESSES Reaction of steam and carbon. Electrolysis of water Respiration EXOTHERMIC PROCESSES Acid and base reactions (neutralization) reactions Dissolving NaOH in water Dissolution of ammonium nitrate in water 15

16 ENDOTHERMIC PROCESS Endothermic energy is taken in, but not necessarily given out (photosynthesis) energy + H 2 O (s) H 2 O (l) 16

17 ENDOTHERMIC PROCESSES 17

18 Ca + H 2 O Ca(OH) 2 + H 2 + heat EXOTHERMIC PROCESS More energy is given off than is taken in or absorbed (combustion, respiration) 18

19 EXOTHERMIC PROCESSES 19

20 COMPARING ENERGY PROFILE DIAGRAMS Compare both energy profile diagrams. State the differences you observe. An endothermic reaction Energy Profile diagram for: An exothermic reaction 20

21 COMMUNICATING ENTHALPY Exothermic processes release energy C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4H 2 O (g) kj Endothermic processes absorb energy C(s) + H 2 O (g) +113 kj CO(g) + H 2 (g) 21

22 ENERGY RELATED TO BOND BREAKING & BOND FORMATION ENDOTHERMIC REACTIONS In an endo-reaction, the energy required in breaking the bonds in the reactants is greater than the energy released in forming the bonds in the products (reactants contain stronger bonds). Products less stable than reactants (higher energy) ΔH = H products - H reactants EXOTHERMIC REACTIONS In an exo-reaction, the energy required in breaking the bonds in the reactants is less than the energy released in forming the bonds in the products (products contain stronger bonds). Products more stable than reactants (lower energy). Since the products have more energy than the reactants, the ΔH value is positive. ΔH = H products - H reactants Since the products have less energy than the reactants, the ΔH value is negative. 22

23 ENTHALPY Enthalpy is the energy contained in a chemical bond that can be converted into heat. A tree trunk has more enthalpy than ash in terms of its potential to produce energy. Enthalpy is given the symbol H. Enthalpy cannot be measured directly, but we can measure the enthalpy change in a reaction as H. 23

24 COMMUNICATING ENTHALPY EXAMPLE: Communicate the following enthalpies of reaction as a chemical potential energy diagram. The burning of magnesium to produce a very bright emergency flare. The decomposition of water by electrical energy from a solar cell. 24

25 DIFFERENCES BETWEEN ENERGY PROFILES ENDOTHERMIC REACTION Heat content of the products is greater than that of the reactants Heat (enthalpy) change, H is positive (+ H) H 2 O(s) H 2 O(l) ΔH = kj mol -1 ½N 2 + O 2 NO 2 ΔH = kj mol -1 EXOTHERMIC REACTION Heat content of the products is less than that of the reactants Heat (enthalpy) change, H is negative (- H) C 8 H ½O 2 8CO 2 + 9H 2 O ΔH = kj mol -1 H 2 + ½O 2 H 2 O ΔH = -286 kj mol -1 25

26 PARAMETERS EXOTHERMIC ENDOTHERMIC Definition process that releases energy into the surroundings process that absorbs energy from the surroundings Heat flow direction Heat flows out of the system Heat flows into the system ΔH value Heat change (ΔH) < 0 Heat change (ΔH) > 0 E.g. / chemical e.g.s Respiration / refer to demos Photosynthesis / refer to demos Stability of reactants and products Products more stable than reactants (lower energy) Products less stable than reactants (higher energy) Bonding Bond formation Bond breaking Change in temperature Increases Decreases 26

27 CHECKING CONCEPTS 1. Define EACH of the following terms: Exothermic reaction [1] Endothermic reaction: [1] Draw a fully labelled diagram to illustrate the conversion. [2] 2. The conversion of steam to water is an exothermic process. H 2 O(g) H 2 O(l) 3. Suggestion a reason for this. [2] 27

28 Enthalpy (H) is used to quantify the heat flow into or out of a system in a process that occurs at constant pressure. DH = H (products) - H (reactants) H = heat given off or absorbed during a reaction at constant pressure H products < H reactants DH < 0 H products > H reactants DH > 0 28

29 HOW DO WE MEASURE (HEAT) ENTHALPY CHANGE, H? With a simple laboratory calorimeter, which consists of an insulated container (typically a coffee cup) made of two nested polystyrene cups, a measured quantity of water, and a thermometer. The chemical is placed in or dissolved in the water of the calorimeter. Energy transfers between the chemical system and the surrounding water is monitored by measuring changes in the water temperature. Calorimetry is the process of measuring energy changes of an isolated system called a calorimeter. Includes: Thermometer, stirring rod, stopper or inverted cup, two Styrofoam cups nested together containing reactants in solution 29

30 FACTORS AFFECTING HEAT QUANTITIES The amount of heat contained by an object depends primarily on three factors: The mass of material (m) The temperature (T) The type of material (c) and its ability to absorb or retain heat. i.e. Consider a bathtub and a teacup of water! All water has the same specific heat capacity which is 4.19 Jg -1 C -1. However, the bathtub would take considerably more energy to heat up! c is the specific heat capacity, the quantity of energy required to raise the temperature of one gram of a substance by one degree Celsius. 30

31 GENERAL EXPERIMENTAL STEPS TO DETEREMINE H Allowing a known mass or volume of reactants to reach the steady temperature of the surrounds. Record this temperature. Thoroughly mixing the reactants and recording the highest temperature (if its an exo-process) or lowest temperature (if its an endo-process) reached. Determining the temperature change, T = T f - T i for the reaction. Calculating the heat evolved or absorbed in the experiment. Calculating the enthalpy change for the reaction. Make use of the equation: Q = mc T The heat evolved or absorbed per mole of the substance under investigation can then be calculated as well. 31

32 CALCULATING HEAT (ENTHALPY) ENERGY Heat Energy Change q = m x c x ΔT q = heat (joules or calories) m = mass (g) c = specific heat (J g -1 o C -1 ) The amount of heat required to raise the temperature of 1 g of a substance 1 o C. Specific heat capacity of water = 4.2 Joules ΔT = change in temperature 32

33 A SIMPLE CALORIMETER FOR A NON-COMBUSTION REACTION A simple polystrene cup calorimeter of low heat capacity can be used for any non-combustion reaction that will happen spontaneously at r.t involving liquids or solid reacting with a liquid and it does not matter if the reaction is exo or endo. The reactants are weighed in if solid and a known volume of any liquid (usually water or aqueous solution). The mixture could be a 1. Salt and water (heat change on dissolving) 2. An acid and alkali (heat change od neutralization) 33

34 A SIMPLE CALORIMETER FOR COMBUSTION OF FUELS A simple calorimeter for combustion is specifically for determining the heat energy released for burning fuels. The burner is weighed before and after the combustion to get the mass of liquid fluid burned. The thermometer records the temperature rise of the known mass of water (1 g = 1 cm 3 ) since the density of water is 1.0 g cm -3. Sources of errors: Huge losses of heat e.g. radiation from the flame and calorimeter, conduction through the copper calorimeter, convection from the flame gases passing by the calorimeter. 34

35 SIMPLE APPARATUS USED TO DETERMINE THE ENTHALPY OF COMBUSTION OF ETHANOL The Philip Harris calorimeter used for determining the enthalpy change of combustion of a liquid fuel 35

36 ENTHALPY OF COMBUSTION OF FOOD 36

37 CALORIMETRY ASSUMPTIONS All the energy lost or gained by the chemical system is gained or lost (respectively) by the calorimeter; that is, the total system is isolated. All the material of the system is conserved; that is, the total system is isolated. The specific heat capacity of water (c) over the temperature range is 4.19 J g -1 C -1. The specific heat capacity (c) of dilute aqueous solutions is 4.19 J g -1 C -1. The thermal energy gained or lost by the rest of the calorimeter (other than water) is negligible; that is, the container, lid, thermometer, and stirrer do not gain or lose thermal energy. 37

38 ENERGY (HEAT TRANSFER) CALCULATIONS I Example: Determine the change in thermal energy when 115 ml of water is heated from 19.6 o C to 98.8 o C? The density of a dilute aqueous solution is the same as that of water; that is, 1.00g/mL or 1.00kg/L c water = 4.19J/g C 38

39 ENERGY (HEAT TRANSFER) CALCULATIONS II Calculate the heat gained in an aluminum cooking pan whose mass is 400 grams, from 20 o C to 200 o C. The specific heat of aluminum is J g -1 o C -1. Solution Q = mc T = (400 g) (0.902 J g -1 o C -1 )(200 o C - 20 o C) = 64,944 J 39

40 ENTHALPY CHANGES In a simple calorimetry experiment involving a burning candle and a can of water, the temperature of 100 ml of water increases from 16.4 C to 25.2 C when the candle is burned for several minutes. What is the enthalpy change of this combustion reaction? Assuming: Δ c H = Q (The energy lost by the chemical system, (burning candle), is equal to the energy gained by the surroundings (calorimeter water), where c denotes combustion. Assuming: Q = mcδt then Δ c H = mcδt Is the value of Δ c H going to be positive or negative? If the surroundings gained energy (water), then the system (burning candle) lost it. So based on the evidence, the enthalpy change of combustion for this reaction is -3.69J. 40

41 ENTHALPY CHANGES When 50 ml of 1.0 mol/l hydrochloric acid is neutralized completely by 75 ml of 1.0 mol/l sodium hydroxide in a polystyrene cup calorimeter, the temperature of the total solution changes from 20.2 C to 25.6 C. Determine the enthalpy change that occurs in the chemical system. Is this an Endothermic or Exothermic reaction?? Based upon the evidence available, the enthalpy change for the neutralization of hydrochloric acid in this context is recorded as kj. 41

42 MOLAR ENTHALPY Molar enthalpy: the change in enthalpy expressed per mole of a substance undergoing a specified reaction (kj/mol) Can we measure the molar enthalpy of reaction using calorimetry? Yes, but indirectly. We can measure a change in temperature, we can then calculate the change in thermal energy (Q=mct). Then, using the law of conservation of energy we can infer the molar enthalpy. In doing so, we must assume that the change in enthalpy of the chemicals involved in a reaction is equal to the change in thermal energy of the surroundings. 42

43 SOME ENTHALPY TERMS AND THEIR CALCULATIONS MOLAR HEAT (ENTHALPY) OF SOLUTION The heat of solution is the amount of heat evolved or absorbed when 1 mole of solute is completely dissolved in water (so that further dilution produces no further heat change). MOLAR HEAT (ENTHALPY) OF COMBUSTION The heat of combustion is the heat evolved when 1 mole of a substance is completely burned in oxygen. MOLAR HEAT OF (ENTHALPY) NEUTRALISATION The heat of neutralization is the heat evolved when one mole of hydrogen ions (H + ), from an acid reacts with one mole of hydroxide ions (OH - ) 43

44 COMBUSTION & NEUTRALIZATON ARE EXOTHERMIC PROCESSES Combustion Exothermic reaction General Combustion Reaction Formula: Compound (usually hydrocarbon) + O 2 CO 2 + H 2 O + energy CH 4 + 2O 2 CO 2 + 2H 2 O + 890kJ H = -890kJ Neutralization Exothermic reaction Acid + Base Salt + Water + energy HCl + NaOH NaCl + H 2 O kj H = -57.3kJ 44

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46 HEAT OF SOLUTION PROBLEM I & SOLUTION 1. Add 3.00 g of NaOH to 100 cm 3 of water. 2. Note the initial temperature and the highest or lowest temperature reached. (If you are not sure, the change in temperature is given on the graph.) 3. Calculate the heat of solution of NaOH. (specific heat capacity of water is 4.2 J g -1 K -1 and the density of water is 1 g cm -3 ) 46

47 HEAT OF SOLUTION PROBLEM II 0.02 mol of anhydrous ammonium chloride was added to 45 g of water in a polystyrene cup to determine the enthalpy change of solution of anhydrous ammonium chloride. It is found that there was a temperature drop from 24.5 o C to 23.0 o C in the solution. Given that the specific heat capacity of water is 4200 J kg -1 K -1 and NH 4 Cl(s) + aq NH 4 Cl(aq) Calculate the enthalpy change of solution of anhydrous ammonium chloride. (Neglect the specific heat capacity of the polystyrene cup.) 47

48 HEAT OF SOLUTION II ANSWER Heat absorbed = m 1 c 1 DT ( c 2 0) = kg 4200 J kg -1 K -1 ( ) K = J (0.284 kj) g Heat absorbed per mole of ammonium chloride = 0.02 mol = 14.2 kj mol -1 The enthalpy change of solution of anhydrous ammonium chloride is kj mol -1. Enthalpy level diagram for the dissolution of NaCl 48

49 HEAT OF NEUTRALISATION PROBLEM I The heat absorbed or evolved when one mole of water is produced when an acid reacts with a base. Problem: When 50 cm 3 of 2 mol dm -3 hydrochloric acid at 25 o C is reacted with 50 cm 3 of 2 mol dm -3 sodium hydroxide at 25 o C, the temperature increased to 36 o C. Determine the heat of neutralisation. Assume that the density and the specific heat capacity of the solution is the same as that for water. 49

50 HEAT OF NEUTRALISATION I SOLUTION Initial Temperature = 25 o C (average of both solutions) Final Temperature = 36 O C Density of solution = mass/volume Mass = volume x density = 100 cm 3 x 1 g cm -3 = 100 g 50

51 HEAT OF NEUTRALISATION I SOLUTION Calculation of the number of moles of reactants: Number of moles of HCl (and NaOH): 1000 cm 3 contain 2 moles 50 cm 3 contain x x = 50 x = 0.1 moles NaOH + HCl NaCl + H 2 O 1 mol NaOH 1 mol HCl 1 mol H 2 O 0.1 mol NaOH 0.1 mol HCl 0.1 mol H 2 O 51

52 HEAT OF NEUTRALISATION I SOLUTION Calculation of the heat of neutralisation: Formation of 0.1 mole of water produces 4620 J Hence formation of 1 mole of water would produce 4620 J 0.1 mol = J H(neut) = J mol -1 = kj mol -1 52

53 HEAT OF NEUTRALIZATION PROBLEM II A student tried to determine the enthalpy change of neutralization by putting 25.0 cm 3 of 1.0 M HNO 3 in a polystyrene cup and adding 25.0 cm 3 of 1.0 M NH 3 into it. The temperature rise recorded was 3.11 o C. Given that the mass of the polystyrene cup is 250 g, the specific heat capacities of water and the polystyrene cup are 4200 J kg -1 K -1 and 800 J kg -1 K -1 respectively. Determine the enthalpy change of neutralization of nitric acid and aqueous ammonia. (Density of water = 1 g cm -3 ) 53

54 HEAT OF NEUTRALIZATION II SOLUTION (a) No. of moles of NH 3 used = 1.0 M dm 3 = mol No. of moles of H 2 O formed = mol Heat evolved per mole of H 2 O formed J = mol = kj mol -1 The enthalpy change of neutralization of nitric acid and aqueous ammonia is kj mol

55 HEAT OF COMBUSTION PROBLEM I Determine the enthalpy change of combustion of ethanol using the following data: Mass of spirit lamp before experiment = g Mass of spirit lamp after experiment = g Mass of water in copper calorimeter = 50 g Mass of copper calorimeter without water = 380 g Initial temperature of water = 18.5 o C (291.5 K) Final temperature of water = 39.4 o C (312.4 K) The specific heat capacities of water and copper calorimeter are 4200 J kg -1 K -1 and 2100 J kg -1 K -1 respectively. 55

56 HEAT OF COMBUSTION I SOLUTION Heat evolved by the combustion of ethanol = Heat absorbed by the copper calorimeter = (m 1 c 1 + m 2 c 2 ) DT = (0.05 kg 4200 J kg -1 K kg 2100 J kg -1 K -1 ) ( )K = J C 2 H 5 OH(l) + 3O 2 (g) 2CO 2 (g) + 3H 2 O(l) Mass of ethanol burnt = ( ) g = 0.78 g Number of moles of ethanol burnt = 0.78 g = mol g mol 56

57 HEAT OF COMBUSTION Heat given out per mole of ethanol = J mol II SOLUTION = J mol -1 = 1239 kj mol -1 The enthalpy change of combustion of ethanol is kj mol -1. There was heat loss by the system to the surroundings, and incomplete combustion of ethanol might occur. Also, the experiment was not carried out under standard conditions. Therefore, the experimentally determined value (-1239 kj mol -1 ) is less than the theoretical value of the standard enthalpy change of combustion of ethanol (-1371 kj mol -1 ). 57

58 HEAT OF COMBUSTION PROBLEM II A student used a calorimeter to determine the enthalpy change of combustion of methanol. In the experiment, 1.60 g of methanol was used and 50 g of water was heated up, raising the temperature by 33.2 o C. Given that the specific heat capacities of water and copper calorimeter are 4200 J kg -1 K -1 and 2100 J kg -1 K -1 respectively and the mass of the calorimeter is 400 g, calculate the enthalpy change of combustion of methanol. 58

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