Propose a method for measuring your results.

Transcription

1 System vs. Surroundings In thermodynamics, a system is defined as that part of the universe that is under consideration (the part of the universe that you are studying). A hypothetical boundary separates the system from the rest of the universe, which is referred to as the environment, surroundings, or reservoir. Problem: You are studying your own body's metabolism. You are trying to measure the amount of energy burned by your body during a 30min. cardio workout. 1. Propose a method for measuring your results. 2. Define the system, the boundary line, and the surroundings for this study. 3. Is it possible to completely isolate a portion of the universe to study, such that no energy or material crosses the boundary line? Explain. 1

2 In reality, a system can never be absolutely isolated from its environment!!! WHY? 2

3 Endothermic Reactions An ENDOTHERMIC ice pack: Ba(OH) 2 8H 2 O (s) + 2NH 4 SCN (s) + energy Ba(SCN) 2(aq) + 2NH 3(aq) + 10H 2 O (l) When you break the pack of Barium hydroxide octahydrate (which is located in the middle of the ice pack), it mixes and begins to react with the ammonium cyanide. Energy (in the form of heat) is absorbed from the surroundings making the surroundings feel cold!! Summary: H >0 (positive) ENDOTHERMIC System absorbs energy from surroundings (energy) What are some physical signs that can be observed during a reaction to help identify an endothermic reaction? 3

4 Enthalpy Diagram: An ENDOTHERMIC ice pack: Ba(OH) 2 8H 2 O (s) + 2NH 4 SCN (s) + energy Ba(SCN) 2(aq) + 2NH 3(aq) + 10H 2 O (l) Ba(OH) 2 8H 2 O (s) + 2NH 4 SCN (s) Endothermic H > 0 Low Energy Bonds High Energy Bonds Exothermic H < 0 Ba(SCN) 2(aq) + 2NH 3(aq) + 10H 2 O (l) The forward reaction: Ba(OH) 2 8H 2 O (s) + 2NH 4 SCN (s) + energy Ba(SCN) 2(aq) + 2NH 3(aq) + 10H 2 O (l) Low energy bonds are absorbing energy (which causes a rearrangement of bonds some of the existing bonds to break and some new bonds to form). New products are produced that have higher energy bonds relative to the original reactants. This is due to the fact that the additional energy that was absorbed is now stored in the bonds of the products. The amount of energy that is absorbed during the forward reaction is the same amount of energy that would be released should the reverse reaction be performed. 4

5 Exothermic Reactions BURNING SUGAR An EXOTHERMIC Reaction: C 12 H 22 O 11(s) + 3O 2(g) 9C (s) + 3CO 2(g) + energy When sugar is burned (broken down) in the presence of oxygen (like is done in your body), the product is solid carbon (the char left after a fire), gaseous carbon dioxide, and excess energy is released into the surroundings. Summary: H <0 (negative) EXOTHERMIC System releases energy into the surroundings (energy) What are some physical signs that can be observed during a reaction to help identify an exothermic reaction? 5

6 Enthalpy Diagram: Combustion (burning) of Sugar: C 12 H 22 O 11(s) + 3O 2(g) 9C (s) + 3CO 2(g) + energy C 12 H 22 O 11(s) + 3O 2(g) Exothermic H < 0 High Energy Bonds Low Energy Bonds Endothermic H > 0 9C (s) + 3CO 2(g) + energy The forward reaction: C 12 H 22 O 11(s) + 3O 2(g) energy + 9C (s) + 3CO 2(g) High energy bonds are breaking apart and release energy in the process. (The energy that was previously stored in the bonds is now released into the environment). New products are produced that have lower energy bonds relative to the original reactants. The amount of heat that is released during the forward reaction is the same amount of heat that would be absorbed should the reverse reaction be performed. 6

7 The CHANGE IN ENTHALPY ( H) is defined as the heat gained or lost by a system when a process occurs under constant pressure. H = H final H initial = H Products H reactants Energy is stored in the bonds of compounds: So if the reactants are made of high energy bonds and a reaction occurs that produces products with lower energy bonds, the reaction must have been EXOTHERMIC (heat releasing): H = H Products H reactants = H LOW H HIGH H <0 (negative) For example: For example: If 10kJ (a smaller number) 15kJ (a bigger number) A smaller number minus a bigger number will always produce a negative result!!! 2H 2(g) + O 2(g) 2H 2 O (l) + 484KJ H = 5kJ Then 2H 2(g) + O 2(g) 2H 2 O (l) H= 484KJ If the reactants are made of low energy bonds and a reaction occurs that produces products with higher energy bonds the reaction must have been ENDOTHERMIC (heat absorbing): H = H Products H reactants = H HIGH H LOW H >0 (positive) For example: For example: 15kJ (a bigger number) 10kJ (a smaller number) H = +5kJ A bigger number minus a smaller number will always produce a positive result!!! If 2H 2 O (l) + 968KJ 4H 2(g) + 2O 2(g) Then 2H 2 O (l) 4H 2(g) + 2O 2(g) H = +968kJ 7

8 Combustion of Ethanol C 2 H 5 OH (l) + 3O 2(g) 2CO 2(g) + 3H 2 O (l) Exothermic or Endothermic? Write this equation, including the heat, in two different ways. Calculate the ΔH rxn for the combusion of 1 mole of ethanol using your pink table. Does this calculation show that the combustion reaction is exothermic or endothermic? Explain. C 2 H 5 OH (l) + 3O 2(g) 2CO 2(g) + 3H 2 O (l) Draw an enthalpy diagram for this reaction and on the diagram show a dashed arrow for the reverse reaction Do the products contain high energy bonds or low energy bonds? Explain. Calculate the ΔH rxn for the combusion of 10mL of ethanol, recall that the density of ethanol is 0.789g/mL. 8

9 Barium Hydroxide Octahydrate with Ammonium Thiocyanate Ba(OH) 2. 8H 2 O(s ) + 2 NH 4 SCN(s ) Ba(SCN) 2 (s ) + 10 H 2 O(l ) + 2 NH 3 (g ) Write a word equation for the is reaction. What happens when this reaction takes place? Why does this happen? Recall that bases turn red litmus paper blue and acids turn blue litmus paper red. Which one happens and why? If the ΔHrxn is +8541kJ for the reaction involving 2.5mol of Barium hydroxide octahydrate, calculate the amount of heat that is required to react 1.8g of Ammonium thiocyanate. Ba(OH) 2. 8H 2 O(s ) + 2 NH 4 SCN(s ) Ba(SCN) 2 (s ) + 10 H 2 O(l ) + 2 NH 3 (g ) Do the products contain high energy bonds or low energy bonds? Explain. 9

10 Exothermic and Endothermic Reactions Calculations HANDOUT Example #1: C 3 H O 2 3 CO H 2 O H = 635kJ If 9.5g of H 2 O (l) is produced in the reaction, calculate the amount of heat produced/released in calories. C 3 H O 2 3 CO H 2 O Draw an enthalpy diagram for this reaction. 10

11 Exothermic and Endothermic Reactions Calculations HANDOUT Example #2: C 3 H O 2 3 CO H 2 O H = 635kJ If 7.31x10 23 molecules of CO 2 are produced in the reaction, calculate the amount of heat produced/released in kcal. Is it necessary to draw an enthalpy diagram for this reaction? Explain. 11

12 USING THE THERMODYNAMICS TABLE GIVEN: 3C (s, graphite) + 2Fe 2 O 3(s) 4Fe (s) + 3CO 2(g) 1. Calculate the ΔH for this reaction. Is this reaction endothermic or exothermic? ΔH = Hproducts Hreactants 2. Re-Write the equation for the reaction if only 1 mole of Graphite is reacted. How much heat is released or produced in this case? GIVEN: 3C (s, graphite) + 2Fe 2 O 3(s) 4Fe (s) + 3CO 2(g) 3. Do the reactants consist of high energy or low energy bonds, relative to the products? Explain. 12

13 Problem: Determine the amount of heat produced/required to completely react 7.5g of NH 3(g). 4NH 3(g) + 5O 2(g) 4NO (g) + 6H 2 O (g) Write the chemical equation with the energy as: (a) part of the chemical equation (b) as a ΔH separate from the chemical equation 4NH 3(g) + 5O 2(g) 4NO (g) + 6H 2 O (g) 4NH 3(g) + 5O 2(g) 4NO (g) + 6H 2 O (g) 13

14 STATE CHANGES A.K.A. Phase Changes 1. Write the equation for the phase change of Carbon Tetrachloride gas to liquid. Calculate the molar heat for this state change. 2. Write the equation for the phase change of Water from liquid to gas. Calculate the ΔH for this state change in units of kj/gram. 14

15 1. Quiz: For the following reaction: 1CH 3 OH (l) + 1O 2(g) 1/3 CO (g) + 2/3 CO 2(g) + 2H 2 O (g) + 1/3C (s, graphite). If your reaction produces 5.6g of CO (g), how many molecules of CO 2(g) would be produced? 2. Consider a capped bottle of ice water left in an area of room temperature. Let the bottle be the boundary around the system. How would the system behave different if the system was closed vs. if the system was open. 3. 6CH 3 OH (l) + 6O 2(g) 2CO (g) + 2CO 2(g) + 12H 2 O (g) + 2C (s, graphite). Rebalance this equation for 3 mol CH 3 OH (l). 4. Use your thermodynamics table to calculate the Hrxn for: 6CH 3 OH (l) + 6O 2(g) 2CO (g) + 2CO 2(g) + 12H 2 O (g) + 2C (s, graphite). Use this result to calculate the amount of energy that would be released/required when 5.9g of O 2(g) reacts completely. Draw an enthalpy diagram for the reaction that occurs when 5.9g of O 2(g). 5. How are endothermic and exothermic reactions similar? Different? 15

16 How was the H f table created? Why do some compounds have a H f value of 0KJ, some have a negative value and others have a positive value? The table was set up in the following way: The H f value is the amount of energy gained/released during the formation of the compound from it s elements in standard form (the way the element is normally found in nature). A 0kJ for the H f value of a compound identifies this as the NATURAL form of the compound. RECALL: Energy transfers occur during EVERY process on earth!!! HOWEVER: In creating this table, scientists identified the natural form of each element. The amount of energy necessary to create the element in standard form was set to zero, and the amount of energy requires/released in forming ALL OTHER COMPOUNDS from their elements in standard form was measured relative to zero and recorded. 16

17 Example: Calculate the amount of heat released or absorbed during the formation of each compound below from their elements in standard form. (a) 1 moles of C 3 H 8(g) (b) 0.25 moles of C 3 H 8(g) (c) 10.5g of BeO (s) 17

18 Question Set #2 (review) 18