I. The Nature of Energy A. Energy

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1 I. The Nature of Energy A. Energy is the ability to do work or produce heat. It exists in 2 forms: 1. Potential energy is energy due to the composition or position of an object. 2. Kinetic energy is energy of motion.

2 The Nature of Energy Explain the potential and kinetic energy of this system.

3 The Nature of Energy B. Chemical systems contain both kinetic energy and potential energy. 1. Kinetic Energy is affected by temperature. 2. The potential energy depends upon its composition: a. the type of atoms in the substance b. the number and type of chemical bonds joining the atoms, c. the particular way the atoms are arranged.

4 C. Law of conservation of energy The law of conservation of energy states that in any chemical reaction or physical process, energy can be converted from one form to another, but it is neither created nor destroyed.

5 II. Chemical potential energy A. The energy stored in a substance because of its composition is called chemical potential energy. B. Heat, which is represented by the symbol q, is energy that is in the process of flowing from a warmer object to a cooler object.

6 C. Measuring heat 1. In the metric system of units, the amount of heat required to raise the temperature of one gram of pure water by one degree Celsius (1 C) is defined as a calorie (cal). 2. The SI unit of heat and energy is the joule (J). One joule is the equivalent of calories, or one calorie equals joules.

7 Relationships Among Energy Units

8 Specific Heat Capacities Table D. Specific Heat 1. The specific heat of any substance is the amount of heat required to raise the temperature of one gram of that substance by one degree Celsius. Substance J/kg/ o C cal/g/ o C Water (0 o C -100 o C) Methyl Alcohol Ice (-10 o C - 0 o C) Steam (100 o C) Soil (typical) Air (50 o C) Aluminum Glass (typical) Iron/Steel Copper Silver Mercury Gold Lead

9 E. Calculating heat evolved and absorbed 1. The heat absorbed or released by a substance during a change in temperature depends on: a. the specific heat of the substance b. the mass of the substance c. the amount of the temperature change. 2. You can express these relationships in an equation.

10 Calculating heat evolved and absorbed 3. In the equation, q = the heat absorbed or released, c = the specific heat of the substance, m = the mass of the sample in grams, and T is the change in temperature in C (T final T initial ).

11 F. Calculating Specific Heat 1. In the construction of bridges and skyscrapers, gaps must be left between adjoining steel beams to allow for the expansion and contraction of the metal due to heating and cooling. 2. The temperature of a sample of iron with a mass of 10.0 g changed from 50.4 C to 25.0 C with the release of 114 J heat. What is the specific heat of iron?

12 Calculating Specific Heat Solve the equation using the known values. - -

13 F. Measuring Heat 1. A calorimeter is an insulated device used for measuring the amount of heat absorbed or released during a chemical or physical process.

14 Determining specific heat 2. Suppose you put 125 g of water into a foam-cup calorimeter and find that its initial temperature is 25.6 C. A 50.0-g sample of an unknown metal is heated to a temperature of C and put into the water.

15 Determining specific heat The flow of heat stops only when the temperature of the metal and the water are equal. The final temperature is 29.3 C. Solve for the heat capacity of the unknown metal.

16 Determining specific heat 4. Assuming no heat is lost to the surroundings, heat gained (water)= the heat lost (metal). First, calculate the heat gained by the water.

17 Determining specific heat The heat gained by the water equals the heat lost by the metal, q metal, so you can write this equation. - - Now calculate for c metal. -

18 Determining specific heat Substitute the known values of m and T (50.0 g and 85.7 C) into the equation and solve. - -

19 Determining specific heat The unknown metal has a specific heat of 0.44 J/(g C).

20 Warm Up: Using Data from Calorimetry A piece of metal with a mass of 4.68 g absorbs 256 J of heat when its temperature increases by 182 C. What is the specific heat of the metal? Known mass of metal = 4.68 g metal quantity of heat absorbed, q = 256 J T = 182 C Unknown specific heat, c =? J/(g C)

21 Using Data from Calorimetry The calculated specific heat is the same as that of strontium.

22 III. Energy and the Change of State Some astonishing demos! A. During a change of state, energy is absorbed or released, but no temperature change is observed.

23 B. Calculating energy needed to change the state of matter. 1. Freezing energy needed to freeze (or melt) a substance Q = moles x H fusion H fusion = 6.01 kj/mol water 2. Boiling Energy needed to vaporize (or condense) a substance Q = moles x H vaporization H vaporization = 40.7 kj/mol water

24 C. Calculate the heat released when 10.0 grams of water freeze. moles water =10.0 g/18.0 g/mol = mol Q = 0.55mol x kj/mol = kj

25 ` D. Calculate the heat needed to melt.0 g of ice at 0.0 C and then warm it up to 25.0 C. First calculate heat needed to melt the ice: Q fusion = moles x H fusion Next calculate the heat needed to warm up the water. Q warming = mass x c x T Finally add the Q s together! 8.76 kj

26 IV. Chemical Energy and the Universe A. Thermochemistry is the study of heat changes that accompany chemical reactions and phase changes. B. Enthalpy is the heat content of a system (at constant P).

27 Enthalpy and enthalpy changes C. H rxn is the difference between the enthalpy of the products of the reaction and the enthalpy of the reactants. 1. H = q p

28 Enthalpy and enthalpy changes 2. Energy Diagrams reactants

29 Enthalpy and enthalpy changes 3. Example, the highly exothermic combustion of glucose (C 6 H 12 O 6 ) occurs in the body as food is metabolized to produce energy for activities. 4. The enthalpy (heat) of combustion ( Hcomb) of a substance is the enthalpy change for the complete burning of one mole of the substance.

30 Enthalpy and enthalpy changes 5. Standard enthalpy changes have the symbol H Measured at standard conditions (1 atm pressure and 298 K)

31 V. Calculating Enthalpy of Reaction A. The H comb for methanol (CH 3 OH) is 726 kj/mol. How much heat is released when 82.1 g of methanol is burned?

32 Calculating Enthalpy of Reaction First, calculate the number of moles of methanol that is burned. Now find the enthalpy of reaction for the combustion of 82.1 g (2.56 mol) of methanol.

33 Basic Assessment Questions Question 1 A 15.6-g sample of ethanol absorbs 868 J as it is heated. If the initial temperature of the ethanol was 21.5 C, what is the final temperature of the ethanol?

34 Answer Basic Assessment Questions 44.3 C

35 Basic Assessment Questions Question 2 If 335 g water at 65.5 C loses 9750 J of heat, what is the final temperature of the water?

36 Answer Basic Assessment Questions 58.5 C

37 Basic Assessment Questions Question 3 How much heat is evolved when 24.9 g of propanol (C 3 H 7 OH) is burned? H comb = 10 kj/mol

38 Answer Basic Assessment Questions 833 kj

39 Calculating Enthalpy Change B. Hess s law: If two or more thermochemical equations can be added to produce a final equation for a reaction, then the enthalpy change for the final reaction equals the sum of the enthalpy changes for the individual reactions.

40 Applying Hess s Law Example: Use thermochemical equations a and b to determine H for the oxidation of ethanol (C 2 H 5 OH) to form acetaldehyde (C 2 H 4 O) and water. a. b.

41 Applying Hess s Law Add these two equations, and cancel any terms common to both sides of the combined equation.

42 Hess s Law: Heat of Combustion of Mg Lab Overall Equation: Mg + ½ O 2 MgO Known Equations: MgO + 2HCl MgCl 2 + H 2 O Mg + 2HCl MgCl 2 + H 2 H 2 + ½ O 2 H 2 O

43 Standard enthalpy (heat) of formation C. Standard enthalpies of formation may be used with Hess s law to calculate enthalpies of reaction under standard conditions. Note: the standard heat of formation of an element in its standard state is zero.

44 Example: Use Heats of formation to calculate the heat of reaction for ethanol reacting with oxygen to form acetaldehyde and water. Substance H f C 2 H 5 OH(l) C 2 H 4 O(g) kj/mol -166 kj/mol H 2 O(l) kj/mol [2(-166 kj/mol) + 2( kj/mol)] [2( kj/mol)] = -348 kj

45 Energy and Chemical Change: Additional Concepts VI. Reaction Spontaneity A. Entropy (S) is a measure of the disorder or randomness of the particles that make up a system. 1. Spontaneous processes always result in an increase in the entropy of the universe. 2. The change in the entropy of a system is given by the following equation.

46 Energy and Chemical Change: Additional Concepts Reaction Spontaneity B. Factors that affect entropy: 1. Changes of state increase from solid to liquid to gas; decrease in opposite direction. 2. Dissolving of a gas in a solvent decrease in entropy 3. Change in the number of gaseous particles increased particles = increase in entropy 4. Dissolving of a solid or liquid to form a solution - Entropy increases. 5. Change in temperature - A temperature increase results in increased entropy.

47 Energy and Chemical Change: Additional Concepts C. Free energy 1. Def. For a reaction or process occurring at constant temperature and pressure, the energy available to do work is the free energy (G).

48 Energy and Chemical Change: Additional Concepts Free energy 3. If G system is negative, the reaction or process is spontaneous What conditions of H and S support this? - H and + S 4. If G system is positive, the reaction or process is nonspontaneous.

49 Energy and Chemical Change: Additional Concepts Demo: Thermite Reaction! 2Al (s) + Fe 2 O 3 (s) 2Fe(s) + Al 2 O 3 (s) + E Substance H f (kj/mol) S (J/mol-K) Fe 2 O Al Fe Al 2 O Calculate H rxn Calculate S rxn Calculate G rxn

50 Energy and Chemical Change: Additional Concepts Determining Reaction Spontaneity Because G system is negative, the reaction is spontaneous.

51 Additional Assessment Questions Question 1 Predict the sign of S system for

52 Additional Assessment Questions Answer negative

53 Additional Assessment Questions Question 2 Predict the sign of S system for

54 Additional Assessment Questions Answer positive

55 Additional Assessment Questions Question 3 Predict the sign of S system for

56 Additional Assessment Questions Answer positive

57 Additional Assessment Questions Question 4 Calculate G system for each of the following processes, and state if the process is spontaneous or nonspontaneous.

58 Additional Assessment Questions Question 4a H system = 147 kj, T = 422 K, S system = 67 J/K Answer 4a 175 kj; nonspontaneous

59 Additional Assessment Questions Question 4b H system = 43 kj, T = 21 C, S system = 118 J/K Answer 4b 8 kj; spontaneous

60 Additional Assessment Questions Question 4c H system = 227 kj, T = 574 K, S system = 349 J/K Answer 4c 27 kj; nonspontaneous