1. Chapter 5 - Thermochemistry Being able to measure the amount of energy that flows either into or out of a system is an important part of chemistry. Chemists can measure the amount of energy released by burning fuel. Food also contains energy that can be measured in calories. How can one figure out how much potential energy a particular type of food contains, or chemical reaction may have ?

2. Thermochemistry Energy Is defined as the ability to do work. Work = (force Applied) x (distance object move) W = F. d

3. Thermochemistry Energy Energy can take on different forms; Heat Light Sound

4. Thermochemistry Energy Kinetic Energy (Latin: Kinesis = to move) The energy of motion. If an object, molecule, or atom moves as a result of a force being applied, we can say that it possesses kinetic energy.

5. Thermochemistry Energy Kinetic Energy (Latin: Kinesis = to move) In chemistry, molecules that move possess kinetic energy. Kinetic energy creates heat. Heat is a measure of kinetic energy.

6. Thermochemistry Energy Kinetic Energy KE = 1/2m.v 2 m = mass (kg) v = velocity (m/s)

7. Thermochemistry Energy Potential Energy A stick of dynamite has potential energy. When it explodes, the potential energy is converted into kinetic energy (heat). 

8. Thermochemistry Energy Potential Energy The amount of potential energy contained in food is a popular topic. Some foods contain a very large amount of kinetic energy.

9. Thermochemistry Energy Potential Energy The amount of potential energy contained in food is expressed in Calories. There are 813 Calories in a Cinnabon. There is enough energy in 1 Cinnabon to heat 813000 grams of water by 1°C. That’s 813 L (215 gallons!)

10. Thermochemistry Energy Potential Energy E el = kQ 1 Q 2 d The electrostatic potential energy is equal to the product of two electrical charges times a proportionality constant (k = 8.99 x 10 9 J.m/C 2 ) and inversely proportional to the distance separating them, d (meters).

11. Thermochemistry Energy Units for Measuring Energy The most common units for energy is the Joule (J), calorie (cal), and the Calorie (Cal). 1 calorie is defined as the amount of energy required to raise the temperature of 1.o gram of water by 1.0°C.

12. Thermochemistry Energy Units for Measuring Energy Therefore 1 Calorie is the amount of energy required to raise 1.0 kg of water by 1.0°C. 1 calorie = 4.18 Joules 1000 calories = 1 Calorie

13. Thermochemistry Energy Units for Measuring Energy Convert 2.0 x 10 3 calories into Joules. 4.5 x 10 4 J = ? Cal

14. Thermochemistry Energy Chemical Reactions and Energy A chemical reaction that releases energy is called an exothermic reaction. The prefix ‘exo’ means outside, or to leave.

15. Thermochemistry Energy Chemical Reactions and Energy An Endothermic reaction absorbs energy from its surroundings. The prefix ‘endo’ means to enter or absorb. An ice-pack absorbs energy from your body in the form of body heat.

16. Thermochemistry Energy Physical Changes and Energy Condensation and freezing are examples of an exothermic process. Energy has to be released when a substance changes from a gas to a liquid, or a liquid to a solid.

17. Thermochemistry Energy Physical Changes and Energy Vaporization, evaporation, and melting are examples of an endothermic phase change. Energy is absorbed when a substance changes from a liquid to a gas or a solid to a liquid.

18. Thermochemistry Energy Heat Capacity (Specific Heat) The amount of energy required to raise the temperature of a substance by 1°C. The Specific Heat Capacity (Specific Heat) of a substance is the amount of energy required to raise the temperature of 1.0 gram of the substance by 1.0 °C.

19. Thermochemistry Energy Heat Capacity (Specific Heat) The unit of the calorie is defined using the specific heat of water. 1 calorie is the defined as the amount of energy required to raise the temperature of 1.0 gram by 1.0°C.

20. Thermochemistry Energy Energy Calculations We can relate energy, mass, and temperature with the following equation; q = m x c x ΔT q = energy (J. cal, or Cal) m = mass (g or Kg) c = specific heat constant (J/g.°C) The units of specific heat may vary. (J/g.°C) (cal/g.°C) (cal/kg.°C) (Cal/g.°C) (Cal/kg.K)

21. Thermochemistry Energy Specific Heat Values

22. Thermochemistry Energy The sign of ‘q’ If ‘q’ has a positive value, energy is being absorbed by the substance. If ‘q’ has a negative value, energy is being released from the substance.

23. Thermochemistry Energy Problems How many Joules of energy is required to heat 1.0 L of water from 23.0°C to 100.0 °C? (c H2O = 4.18 J/g.°C)

24. Thermochemistry Energy Problems How many calories of energy is lost when a 400.0 gram piece of iron cools from 200.0 °C to 50.0°C? (c Fe = 0.11 cal/g.°C)

25. Thermochemistry Calorimetry A calorimeter is an insulated container that can measure the amount of energy that either flows into or out of an object or chemical reaction. Water is usually used to capture, or release the energy.

26. Thermochemistry Calorimetry When using a calorimeter, it is assumed that the energy lost or gained by the object or reaction, is equal to the energy lost or gained by the water in the calorimeter. q (lost by object) = q (gained by calorimeter) or q (gained by object) = q (lost by calorimeter)

27. Thermochemistry Calorimetry For an exothermic reaction, we can assume that; -q (reaction or object) = +q (calorimeter) -mcΔT (reaction or object) = +mcΔT (calorimeter)

28. Thermochemistry Calorimetry A 200.0°C piece of lead, with a mass of 12.0 g, is placed into a calorimeter that holds 300.0 grams of water. The temperature of the water rose by 18.0°C. Calculate the experimental value for the specific heat constant for lead. (specific heat of water = 4.18 J/g. °C)

29. Thermochemistry Calorimetry 10.0 mL of a 1.0 M aqueous solution of HNO 3 is added to 10.0 mL of a 1.0 M aqeous solution of NaOH. Each solution is recorded as having a temperature of 23.0°C before they are mixed. After the two solutions are mixed, the temperature increases to 30.0°C. Assuming that each solution is mostly water, calculate the enthalpy change (ΔH) of this chemical reaction.

30. Thermochemistry Energy and Phase Changes The graph shows the continuous heating of water versus time. Describe what is happening.

31. Thermochemistry Energy and Phase Changes The temperature of a substance remains constant when it is going through a phase change. We can calculate the amount of energy absorbed in the sloped regions by using q=mcΔT.

32. Thermochemistry Energy and Phase Changes We can calculate the amount of energy absorbed during the phase changes by using the heat of fusion, ΔH fus, or the heat of vaporization, ΔH vap.

33. Thermochemistry Energy and Phase Changes Calculate the amount of energy absorbed when 5.00 grams of ice melts and changes into water. ΔH fus H 2 O = 334 J/g

34. Thermochemistry Energy and Phase Changes Calculate the amount of energy released when 18.00 grams of steam condensed and changes into water. ΔH vap H 2 O = 2260 J/g

35. Thermochemistry Energy and Phase Changes Calculate the amount of energy that is required to heat water from -20.0°C to 80.0°C. ΔH fus H 2 O = 334 J/g

36. Thermochemistry Energy and Phase Changes Cooling Curves

37. Thermochemistry Thermochemical Equations The change in the amount of energy of a chemical equation is expressed as an Enthalpy Change (ΔH). Endothermic Reaction = +ΔH Exothermic Reaction = -ΔH

38. Thermochemistry Thermochemical Equations We can calculate the theoretical ΔH of a chemical reaction if we know the Heats of Formation (ΔH f ) of the reactants and products. N 2(g) + 2H 2(g)  2NH 3(g) ΔH f N 2 = 0 kJ/mol ΔH f H 2 = 0 kJ/mol ΔH f NH 3 = -46 kJ/mol

39. Thermochemistry Thermochemical Equations Hess’s Law – The Heat of a chemical reaction (ΔH reaction ) can be calculated by taking the difference of the heat of formation of the product(s) (ΔH products ) and heat of formation of the reactant(s) (ΔH reactants ). ΣΔH reaction = ΣΔH products - ΣΔH reactants

40. Thermochemistry Thermochemical Equations

41. Thermochemistry Thermochemical Equations Hess’ Law Calculate ΔH reaction for the combustion of methane, CH 4(g) using Hess’ Law.

42. Thermochemistry Thermochemical Equations Hess’ Law Calculate ΔH reaction for the neutralization reaction between HCl and NaOH.