1. Thermochemistry

2. Topic: Specific Heat and Calorimetry Objectives: Day 1 of 2 To understand specific heat and it’s units To understand calorimetry and and the equation q = (m)(C p )(ΔT) To understand the difference between endo and exothermic changes To understand the first law of thermodynamics Unit: Thermochemistry

3. Heat Transfer & Specific Heat If you recall, heat is the energy transferred between objects that have different temperatures Heat is always transferred from an object with a higher temperature to an object with a lower temperature Temperature is a measure of the average kinetic energy

4. What is Heat? The transfer of kinetic ______from a hotter object to a colder object When heat is abosrbed molcules move more _______ Answer Bank Gram Absorbs into Energy constant Heat system Colder 1 transfer joules energy rapidly

5. The temperature change of an object depends on: –The amount of heat that is transferred to or from the object –The mass of the object –The specific heat of the object Heat Transfer & Specific Heat

6. Specific Heat Specific heat is the amount of heat needed to raise the temperature of 1 gram of a substance by 1ºC We represent specific heat with symbol (C p ) It is a constant for a given substance at a certain pressure The units of specific heat are (J/gºC) –Where J is the SI unit for energy, the joule (J)

7. Specific Heat Think of specific heat as how much heat a substance can hold Substances with a high specific heat act like a “heat sponge” They can absorb a lot of heat with just a small change in temperature

8. What is specific heat? The amount of ______needed to raise the temperature of 1 _____ of a substance by ___ºC It is symbolized by the symbol C p It is a constant for a given substance at a certain pressure Units are in (J/gºC) Answer Bank Gram Absorbs into Energy constant Heat system Colder 1 transfer joules heat gram 1

9. Calorimetry Calorimetry is the measure of heat transfer Suppose a chemist want s to calculate the amount of heat transferred when the temperature of a sample changes by so many degrees The amount of heat absorbed or released by a sample is symbolized by the letter q qq q It is calculated using the following equation: q qq q = (m)(Cp)(ΔT)

10. What is calorimetry? A measure of heat _______ given by the equation: q = (m)(C p )(ΔT) Answer Bank Gram Absorbs into Energy constant Heat system Colder 1 transfer joules transfer

11. Calorimetry q = (m)(C p )(ΔT)Here is what each symbol means for the equation: q = (m)(C p )(ΔT) q = (m)(C p )(ΔT) q is the symbol for heat If q is positive heat is absorbed by the system & the reaction is endothermic Cp is the specific heat of the Substance it is a constant M is the mass of the sample in grams If q is negative heat is lost by the system & the reaction is exothermic ΔT is the change in temperature

12. Calorimetry If energy flows into the system (endothermic) q is positive(+) qOn the other hand if energy is flowing out of the system (exothermic) q is negative( - ) System Surroundings Energy System Surroundings Energy

13. What is the difference between endo- and exothermic changes? An endothermic change ______ heat (q+) and energy flows ____the system An exothermic change releases heat (q-) and energy flows out of the_____ Answer Bank Gram Absorbs into Energy constant Heat system Colder 1 transfer joules absorbs into system

14. Thermodynamics The internal energy, E, of a system can be defined most precisely as the sum of the kinetic and potential energies of all “particles” in the system The internal energy of a system can be changed by a flow of work, heat or both Mathematically we say: ΔE = q + w ΔE means a change in the function W represents work q represents heat

15. Thermodynamics The study of energy is called thermodynamics The law of conservation of energy is often called the first law of thermodynamics and is stated as follows: The energy in the universe is constant

16. What is the First Law of Thermodynamics? The First Law of Thermodynamics states that the energy in the universe is________ Answer Bank Gram Absorbs into Energy constant Heat system Colder 1 transfer joules constant

17. Thermodynamics The same is true for w If the system does work on the surroundings (energy flows out of the system) w is negative If the surrounding do work on the system (energy flows into the system) w is positive System Surroundings Energy System Surroundings Energy

18. Practice: What amount of heat is needed to increase the temperature of 2006 grams of Mercury (Hg) by 7.5ºC? The specific heat of mercury is 0.139 J/gºC. Use the equation q = (m)(C p )(ΔT) and Solve for q q = (m)(C p )(ΔT) q = (2006 g)(0.139 J/gºC)(7.5ºC) q = 2090 Joules

19. First Law of Thermodynamics The First Law of Thermodynamics states that the energy in the universe is constant This is another way of stating the law of conservation of energy

20. The First Law of Thermodynamics Suppose you want to measure the amount of heat transferred from a hot piece of metal (system) to a sample of cooler water (surroundings) If you know the specific heat of water, the water’s mass and the water’s temperature change, you can calculate the amount of heat absorbed by the water This equals the amount of heat released by the metal System Surroundings System

21. Practice: A 118 gram piece of tin at 85ºC is dropped into a 100 grams of water at 35ºC. The final temperature of the mixture is 38ºC The specific heat of water is 4.18 J/gºC 1)What amount of heat is absorbed by the water? 2)What amount of heat is released by the tin? 3)What is the specific heat of tin? Solving for the specific heat of water we get: q = (100g)(4.18 J/gºC)(3.0ºC) = 1254 J The value is positive because heat is absorbed This same amount of heat is released by tin, and when heat is released the q value is negative q water = -q tin 1254 J = -1254 J

22. Visualize: What is hotter, the water or piece of tin? In which direction will heat move? System Surroundings System 118 gram piece of tin at 85 o Celsius 100 grams of water at 35 o Celsius Heat moves from the metalTo the water (system  surroundings) q water Is positive (+) heat is gained q metal Is negative (-) heat is lost

23. Practice: Because the same amount of heat that is absorbed by water is also released by tin, we can use 1254 J for to solve for the amount of heat released by tin However when heat is released, it’s value is negative and we get -1254 J, So, the amount of heat released by tin is -1254 J Substituting -1254 in for q, we can solve for specific heat (C p ) q tin = -1254 J = (118g)(C p )(-47 º C) C p = __ -1254 ___ (118g)(-47ºC) C p = 0.23 J/gºC

24. Summarize: In the equation, q = (m)(C p )(ΔT), q represents the heat ____or_____, m is the ____ in grams, C p is the _____heat, and ΔT is the change in _____ An endothermic change ______ heat (q+) and An exothermic change ______heat (q-) In the last practice problem the amount heat lost by the ____ is equal to the amount of heat ______by the water (Or q = -q) Answer Bank gram Mass gained Absorbs releases temperature specific metal system lost

25. Topic: Enthalpy and Reaction Rates Objectives: To understand enthalpy and how it applies to a reaction To understand how reactions proceed and the factors that effect them

26. Enthalpy So far we have seen that some reactions are exothermic and other reactions are endothermic Chemists also like to know exactly how much energy or heat is produced or absorbed by a given reaction We will use the term enthalpy to describe the change in heat that occurs during a reaction ΔH = Heat

27. Enthalpy Enthalpy (H) is the amount of heat a sample has at a certain temperature and pressure It is the sample’s heat content Enthalpy cannot be measured, only changes in enthalpy can be measured As a reaction takes place and atoms are rearranged, heat is either released or absorbed

28. What is Enthalpy? Enthalpy (H) is the amount of ______a sample has at a certain temperature and pressure Think of it as a sample’s heat____ Answer Bank enzyme released Content activation Absorbed minimum products Heat chemical

29. Enthalpy The change in heat that takes place during a reaction is called the heat of reaction (ΔH ) It is the amount of heat released or absorbed in a chemical reaction, given 1 mol of a reactant or product ΔH rxn is the difference between enthalpy of the reactants and products In other words, ΔH rxn = (H of products) – ( H of reactants)

30. What is the heat of reaction? The amount of heat _______or _______in a chemical reaction, given 1 mol of a reactant or product represented by ΔH Where ΔH rxn = (H of______ ) – ( H of reactants) Answer Bank enzyme released Content activation Absorbed minimum products Heat chemical

31. Practice: Classify the reactions below as wither exo- or endothermic: CaCO 3  CO 2 + CaO ΔHrxn = +178.4 kJ/mol CaCO 3 CH 4 + 2O 2  CO 2 + 2H 2 O ΔHrxn = -890.2 kJ/mol CH 4 ΔHrxn is positive so the reaction is endothermic ΔHrxn is negative so the reaction is exothermic

32. Practice: When 1 mol of methane is burned at constant pressure, -890 kJ of energy is released as heat. Calculate ΔH for a process in which a 5.8 gram sample of methane is burned at constant pressure q = ΔH = -890 kJ/mol CH 4 The negative sign indicates an exothermic process kJ/mol is our conversion factor First convert grams to moles using the molar mass of methane 16 g/mol, then use our conversion factor of -890 kJ/mol 1 mol CH 4 -890kJ 5.8 g CH 4 16.0 g CH 4 1 mol CH 4 = -320 kJ CH 4

33. Practice: 2SO 2 +O 2  2SO 3 ΔHrxn = -98.9 kJ/mol SO 3 Consider the above equation: If 8.9 mol of SO 2 reacts with excess oxygen, how much heat is released? ΔHrxn = -98.9 kJ/mol SO 3 The negative sign indicates an exothermic process kJ/mol is our conversion factor We are given the ΔH for SO 3 So, we need to use our mol ratio to go from mol of SO 2 to mol SO 3 2 mol SO 3 -98.9kJ 8.9 mol SO 2 2 mol SO 2 1 mol SO 3 = -880.2 kJ SO 3

34. Enthalpy and Reaction Rates (Reaction Coordinates) Chemists often use diagrams to show how energy is transferred during reactions Remember, a change in energy always accompanies a reaction whether positive or negative

35. Enthalpy and Reaction Rates The diagram to the right illustrates how energy changes from reactants to products The activation energy is the minimum energy needed for the reaction to take place Once the activation energy is met, the reaction can proceed

36. Enthalpy and Reaction Rates A common analogy is pushing a boulder over a hill. Actually over a "pass". The reactants are on one side like the boulder. The energy needed to push the boulder to the crest of the hill is like the activation energy. The products are like the condition when the boulder is at the bottom of the far side of the "pass".

37. What is Activation Energy? The _______energy needed for the reaction to take place Answer Bank enzyme released Content activation Absorbed minimum products Heat chemical

38. Energy Reaction Rate (time) Reactants Products E A = Activation energy minimum amount of energy needed for a chemical reaction to take place ΔHΔH ΔH = The amount of heat released or absorbed in a chemical reaction (heat change) EAEA

39. Energy Reaction Rate (time) Reactants H 2 O + CO 2 Products C 6 H 12 O 6 + O 2 E A = Activation energy minimum amount of energy needed for a chemical reaction to take place ΔH = 1274.5 endothermic (+) EAEA

40. Energy Reaction Rate (time) Reactants CH 4 + O 2 Products CO 2 + H 2 O E A = Activation energy minimum amount of energy needed for a chemical reaction to take place ΔH = -890 exothermic (-)

41. Catalyst In order to lower the activation energy, some reactions utilize what’s know as a catalyst By lowering the activation energy, a catalyst (substance) accelerates a chemical reaction without itself being affected

42. Catalyst A catalyst is a substance that lowers the activation energy and accelerates a chemical reaction without itself being affected A biological catalyst is called an enzyme Enzymes help orientate molecules or atoms in specific manner so they are more likely to react Your body has millions of enzymes!!

43. What is a catalyst? A substance that lowers the _____energy and ______ a reaction without itself being affected A biological catalyst is called an _____ Answer Bank enzyme released Content activation Absorbed minimum products Heat Chemical accelerates

44. Review: Is the reaction endothermic or exothermic? What number represent the activation energy? What number represents the products?

45. Summarize: Review: ______ is a measure of the average kinetic energy of a molecular motion As heat is added, molecules move ______ The amount of heat released or absorbed in a chemical reaction is the _____ of reaction A substance that lowers the activation energy is called a _____ Draw & Label the diagrams below: Energy Reaction Rate (time) Reactants CH 4 + O 2 Products CO 2 + H 2 O E A = Activation energy ΔH = - 890 exothermic (-) Energy Reaction Rate (time) Reactants H 2 O + C O 2 Products C 6 H 12 O 6 + O 2 ΔH = + 1274.5 endothermic (+) EAEA Answer Bank catalyst faster activation products Heat temperature

47. Temperature rises as we add heat At a certain temperature (0 0 C) the ice melts and becomes water, The temperature stabilizes. This is the Molar Heat of Fusion H fusion The energy required to melt 1 mole of ice into water Now that the ice has melted, the temperature of the water continues to rise If we continue to add heat, the Temperature Of the steam will rise The energy required to change one mol of a liquid into vapor At a certain temperature (100 0 C) Water becomes steam and once again the temperature stabilizes. This is the Molar Heat of Vaporization H vaporization

49. Empirical Formula Empirical Formula - A formula that gives the simplest whole-number ratio of atoms in a compound. For example: If carbon and hydrogen are present in a compound in a ratio of 1:2, the empirical formula for the compound is CH 2.

50. Energy Reaction Rate (time) Reactants CH 4 + O 2 Products CO 2 + H 2 O E A = Activation energy ΔH = -890 exothermic (-) Energy Reaction Rate (time) Reactants H 2 O + C O 2 Products C 6 H 12 O 6 + O 2 E A = Activation energy minimum amount of energy needed for a chemical reaction to take place ΔH = 1274.5 endothermic (+) EAEA