1 Energy in Chemical Reactions

2 Potential Energy: Potential Energy is stored energy. It is energy something has because of its position or composition. What has more potential energy – a bowling ball on the ground, or a bowling ball three feet directly above your head? What has more potential energy – a barrel of water or a barrel of gasoline?

3 Chemical potential energy n That’s the type of energy that gasoline has. n Defined as the potential energy stored in the bonds between atoms of a substance.

4 Chemical potential energy n When you burn gasoline, or wood, or anything flammable, much of the chemical potential energy is released as heat.

5 Chemical potential energy n Burning isn’t the only chemical reaction that releases heat. n Virtually every chemical reaction either releases heat (exothermic) or absorbs heat (endothermic). Exothermic: heat is exiting.

Exothermic process is any process that gives off heat – transfers thermal energy from the system to the surroundings. Endothermic process is any process in which heat has to be supplied to the system from the surroundings. System? Surroundings? The “system” is generally the container where the reaction is taking place.

Exothermic process is any process that gives off heat – transfers thermal energy from the system to the surroundings. If energy is listed as a product, the reaction is exothermic: Endothermic process is any process in which heat has to be supplied to the system from the surroundings. If energy is listed as a reactant, the process is endothermic: 2H 2 ( g ) + O 2 ( g ) 2H 2 O ( l ) + energy energy + 2HgO ( s ) 2Hg ( l ) + O 2 ( g ) 6.2

Endothermic or Exothermic? (how to decide) Fires give off heat, and feel hot to your hands. They are exothermic. Cold packs feel cold to your hands, so they must be the opposite: endothermic. They feel cold because they’re absorbing heat from your hands.

Endothermic reactions feel cool because they absorb heat from the surroundings.

10 Chemical energy Is this reaction exothermic or endothermic?

11 Potential energy Heat Is this reaction exothermic or endothermic?

12 Chemical energy What about this one?

13 Potential energy Heat What about this one?

14 Practice Questions Is this reaction endothermic or exothermic? What is the change in energy for the reaction?

15 Practice Questions Does the graph represent an endothermic or exothermic reaction? What is the change in energy for the reaction?

16 Energy of chemical reactions: Every energy measurement has three parts: 1. A unit ( Joules or calories). 2. A number (how many). 3. and a sign to tell direction: negative = exothermic positive = endothermic

17 Enthalpy n The heat lost or gained by a system during a physical or chemical change is called the enthalpy change (ΔH) or the heat of reaction. Remember that a negative enthalpy change means that heat is lost from the system to the surroundings, making the process exothermic. n A positive enthalpy change tells us that heat is gained by the system and the process is endothermic.

18 System Surroundings Energy  <0

19 System Surroundings Energy  >0

20 Thermochemical Equations H 2 O ( s ) H 2 O ( l )  H = 6.01 kJ Is  H negative or positive? System absorbs heat; the system will feel cold. Endothermic  H > kJ are absorbed for every 1 mole of ice that melts at 0 0 C and 1 atm. 6.4

21 Thermochemical Equations CH 4 ( g ) + 2O 2 ( g) CO 2 ( g) + 2H 2 O ( l )  H = kJ Is  H negative or positive? System gives off heat Exothermic  H < kJ are released for every 1 mole of methane that is combusted at 25 0 C and 1 atm. 6.4

22 H 2 O ( s ) H 2 O ( l )  H = 6.01 kJ The coefficients always refer to the number of moles of a substance Thermochemical Equations If you reverse a reaction, the sign of  H changes H 2 O ( l ) H 2 O ( s )  H = kJ If you multiply both sides of the equation by a factor n, then  H must change by the same factor n. 2H 2 O ( s ) 2H 2 O ( l )  H = 2 x 6.01 = 12.0 kJ 6.3

23 Practice Question Which statement correctly describes an endothermic chemical reaction? (1) The products have higher potential energy than the reactants, and the ΔH is negative. (2) The products have higher potential energy than the reactants, and the ΔH is positive. (3) The products have lower potential energy than the reactants, and the ΔH is negative. (4) The products have lower potential energy than the reactants, and the ΔH is positive.

24 Practice Question What is  for this reaction? What is  for this reaction? Number and sign)

25 LecturePLUS Timberlake25 Chapter 8 Energy in Chemical Reactions, Calorimetry, Enthalpy and Thermochemistry Heating and Cooling Curves

26 LecturePLUS Timberlake26 Heating Curve for Water 120 °C steam 100 °C water  steam 50°C liquid water 0 °C ice  liquid -10 °C ice Heat added 

27 LecturePLUS Timberlake27 Learning Check CS1 A. The flat lines on a heating curve represent 1) a temperature change 2) a constant temperature 3) a change of state B. The sloped lines on a heating curve represent 1) a temperature change 2) a constant temperature 3) a change of state

28 LecturePLUS Timberlake28 Solution CS1 A. The flat lines on a heating curve represent 2) a constant temperature 3) a change of state B. The sloped lines on a heating curve represent 1) a temperature change

29 LecturePLUS Timberlake29 Temperature Changes T(1) beginning temp.1  T(1) T(f) final temp of both T°C  T(2) T(2) final temp. 2

30 LecturePLUS Timberlake30 Cooling Curve Using the heating curve of water as a guide, draw a cooling curve for water beginning with steam at 110°C and ending at -20°C.

31 LecturePLUS Timberlake31 Learning Check CS2 A. Water condenses at a temperature of 1)0°C2) 50°C3) 100°C B. At a temperature of 0°C, water 1) freezes2) melts3) changes to a gas C. When a gas condenses, heat is 1) released2) absorbed D. Freezing is 1) endothermic2) exothermic

32 LecturePLUS Timberlake32 Solution CS2 A. Water condenses at a temperature of 3) 100°C B. At a temperature of 0°C, water 1) freezes2) melts C. When a gas condenses, heat is 1) released D. Freezing is 2) exothermic

33 LecturePLUS Timberlake33 Learning Check CS3 Is energy absorbed (1) or released (2) in each of the following: ____A. Ice to liquid water ____B. Water vapor to rain ____C. Water to ice When it rains, the air becomes 1) warmer2) cooler3) does not change

34 LecturePLUS Timberlake34 Solution CS3 Is energy absorbed (1) or released (2) in each of the following: _1__A. Ice to liquid water _2__B. Water vapor to rain _2__C. Water to ice When it rains, the air becomes 1) warmer

35 LecturePLUS Timberlake35 Learning Check CS4 Complete using the terms gains or loses In the cooling coils of a refrigerator, liquid Freon ___________ heat from the food and changes to a gas Food ___________heat and becomes colder In the back of the refrigerator, Freon _________ heat and condenses back to a liquid

36 LecturePLUS Timberlake36 Solution CS4 Complete using the terms gains or loses In the cooling coils of a refrigerator, liquid Freon absorbs heat from the food and changes to a gas Food loses heat and becomes colder In the back of the refrigerator, Freon loses heat and condenses back to a liquid

37 LecturePLUS Timberlake37 Combining Heat Calculations To reduce a fever, an infant is packed in 1250 g of ice. If the ice at 0°C melts and warms to body temperature (37.0°C) how much heat in joules is absorbed?

38 LecturePLUS Timberlake38 Combining Heat Calculations Step 1: Diagram the change of state 37°C  T = 37.0°C - 0°C = 37.0°C 0°C S L

39 LecturePLUS Timberlake39 Step 2: Calculate the heat to melt ice (fusion) = 1250 g ice x 334 J 1 g ice = 418,000 J Step 3: Calculate the heat to warm the water from 0°C to 37°C = 1250 g x 37.0°C x 4.18 J g °C = 193,000 J

40 LecturePLUS Timberlake40 Total: Step 2 + Step 3 = 418,000 J + 193,000 J = 611,000 J

41 LecturePLUS Timberlake41 Learning Check CS8 A. Why do drops of liquid water form on a glass of iced tea ? B. When it snows, the air temperature seems warmer. How can that be? C. How much heat is needed to change 1.00 g of water at 0° to steam at 100°C? 1) 540 cal2) 640 cal 3) 720 cal

42 LecturePLUS Timberlake42 Solution CS8 A. Why do drops of liquid water form on a glass of iced tea? Condensation of water in the air that cools B. When it snows, the air temperature seems warmer. How can that be? Condensation is exothermic; heat is released. C. How much heat is needed to change 10.0 g of water at 0° to steam at 100°C? 2) 640 cal