Name: General Chemistry Chapter 11 Thermochemistry- Heat and Chemical Change

Transcription

1 Name: General Chemistry Chapter 11 Thermochemistry- Heat and Chemical Change Notepack 1

2 Section 11.1: The Flow of Energy Heat (Pages ) 1. Define the following terms: a. Thermochemistry b. Energy c. Chemical Potential Energy d. Heat (q) e. System f. Surroundings g. Universe h. Law of conservation of energy i. Endothermic process j. Exothermic process k. Calorie l. Joule m. Heat Capacity n. Specific heat capacity (specific heat) i. What is the specific heat of water in units of Calories/g-C? ii. What is the specific heat of water in units of Joules/g-c? 2. Calculating heat (q) a. We use this equation: 2

3 Example problems: 1. The temperature of a piece of copper with a mass of 95.4 grams increases from 25 C to 48 C when the metal absorbs 849 Joules of heat. What is the specific heat of copper? 2. When 435 J of heat is added to 3.4 grams of olive oil at 21 C, the temperature increases to 85 C. What is the specific heat of olive oil? 3. A piece of stainless steel weighing 1.55 grams absorbs 141 Joules of heat when its temperature increases by 178 C. What is the specific heat of the stainless steel? 4. How much heat is required to raise the temperature of 250 grams of mercury 52 C? The specific heat for mercury is 0.14 J/g-C. 5. Will the specific heat of 50 grams of a substance be the same as, or greater than, the specific heat of 10 grams of the same substance? Explain. 6. On a sunny day, why does the concrete deck around an outdoor swimming pool become hot, while the water stays cool? 7. Using calories, calculate how much heat 32.0 grams of water absorbs when it is heated from 25 C to 80 C. How many Joules is this? 8. A chunk of silver has a heat capacity of 42.8 J/C. If the silver has a mass of 181 grams, calculate the specific heat of silver. 9. How many kilojoules of heat are absorbed when 1.0 Liter of water is heated from 18 C to 85 C? 3

4 11.2 Calorimetry: Measuring and Expressing Heat Changes CONSTANT PRESSURE CALORIMETER (coffee cup calorimeter) 1. What is calorimetry? 2. What is Enthalpy a. What is the sign of H for exothermic reactions? b. What is the sign of H for endothermic reactions? Example Problems for Constant Pressure Calorimetry 1. To study the amount of heat released during a neutralization reaction, 25.0 ml of moles of HCl is added to 25.0 ml of moles of NaOH in a foam cup calorimeter. At the start, the solutions and the calorimeter are all at 25 C. During the reaction, the highest temperature observed is 32 C. Calculate the heat (in kj) released during this reaction. Assume the densities of the solutions are 1.00 g/ml. 4

5 2. A student mixed 50 ml of water containing 0.5 mol HCl at 22.5 C with 50 ml of water containing 0.50 mol MaOH at 22.5 C in a foam cup calorimeter. The temperature of the resulting solution increased to 26 C. How much heat in kilojoules (kj) was released by this reaction? 3. A small pebble is heated and placed in a foam cup calorimeter containing 25.0 ml of water at 25 C. The water reaches a maximum temperature of 26.4 C. How many joules of heat were released by the pebble? Thermochemical Equations Show the thermochemical equation for calcium oxide reacting with water and releasing 65.2 kj of heat energy. o You can treat heat change in a chemical reaction like any other reactant or product in a chemical equation. Draw the Enthalpy diagram for this process: o Write the thermochemical equation for sodium hydrogen carbonate decomposing into sodium carbonate, water and carbon dioxide. It absorbs 129 kj of heat energy. o Draw the enthalpy diagram for this process. 5

6 To solve problems involving thermochemical equations, treat the enthalpy value as any other stoichiometric value. Examples: 1. Using the equation from above, calculate the kilojoules of heat required to decompose 2.24 mole of sodium hydrogen carbonate. 2. When carbon disulfide is formed from its elements, 89.3 kj of heat is absorbed. Calculate the amount of heat (in kj) absorbed when 5.66 grams of carbon disulfide is formed. 3. The production of iron and carbon dioxide from iron III oxide and carbon monoxide is an exothermic reaction, releasing 26.3 kj of heat energy. How many kilojoules of heat are produced when 100 grams of iron III oxide reacts with excess of CO? 4. Combusting one mole of methane, CH 4, releases 890 kj of heat energy. How many grams of methane are needed to release 1250 kj of heat energy? 11.3 Heat in Changes of State Phase diagrams display the state of a substance at various pressures and temperatures and the places where equilibria exist between phases. 6

7 The AB line is the interface. It starts at the (A), the point at which all three states are in equilibrium. It ends at the (B); above this critical temperature and critical pressure the liquid and vapor are indistinguishable from each other. Each point along this line is the of the substance at that pressure. The AD line is the interface between. The at each pressure can be found along this line. Below A the substance cannot exist in the. Along the AC line the phases are in equilibrium; the point at each pressure is along this line. 7

8 Energy Changes Associated with Changes of State Phase Changes Intermolecular Forces : Energy required to change a solid at its melting point to a liquid. Hfus : Energy required to change a liquid at its boiling point to a gas. Hvap Notice that the heat of vaporization is always larger than its heat of fusion. The heat of fusion, or enthalpy of fusion, for ice is 6.01 kj/mol. The heat of vaporization, or enthalpy of vaporization, for water is 40.7 kj/mol. The heat of sublimation is the sum of heats of vaporization and fusion. For water = approx 47 kj/mol 8

9 The heat added to the system at the melting and boiling points goes into pulling the molecules farther apart from each other. The temperature of the substance does not rise during the phase change SAMPLE PROBLEM #1: Calculate the enthalpy change upon converting 1 mol of ice at -25 o C to water vapor (steam) at 125 o C under a constant pressure of 1 atm. The specific heats of ice, water, and steam are 2.09 J/g-K, 4.18 J/g-K, and 1.84 J/g-K respectively. For H 2 O, Hfus = 6.01 kj/mol and Hvap = kj/mol. SAMPLE PROBLEM #2: What is the enthalpy change during the process in which 100 g of water at 50.0 o C is cooled to ice at -30 o C? 9

10 Heat of Solution 1. Heat changes can also occur when a solute dissolves in a solvent. The heat change caused by dissolution of one mole of substance is the. ( Hsoln) 2. When one mole of sodium hydroxide dissolves in water, sodium ions and hydroxide ions separate and interact with the water. The temperature of the solution increases, releasing kj of heat. Write the thermochemical equation. 3. The heat of solution for the dissolving process of calcium chloride is kj/mol. Write this thermochemical equation. 4. The heat of solution for the dissolving process of ammonium nitrate is kj/mol. Write this thermochemical equation. Sample Problems: 1. How much heat (in kj) is released when 2.5 mol of NaOH is dissolved in water? (Use the thermochemical equation from above) 2. How much heat (in kj) is released when 100 grams of sodium hydroxide are dissolved in water? 3. How many grams of ammonium nitrate must be dissolved in water so that 88 kj of heat is absorbed? 10

11 Section 11.4: Hess s Law Enthalpy is a state function. It is independent of the path. We can add equations to come up with the desired final product, and add the H Two rules o o Sample Problem #1: If the reaction is reversed the sign of H is changed If the reaction is multiplied or divided, so is H The following information is known: o C(s) + O 2 (g) CO 2 (g) H 1 = kj o CO(g) + ½ O 2 (g) CO 2 (g) H 2 = kj Using these data, calculate the enthalpy for: C(s) + ½ O 2 (g) CO(g) Sample Problem #2 Calculate H for the reaction 2C(s) + H 2 (g) C 2 H 2 (g) Given the following chemical equations and their respective H. C 2 H 2 (g) + 5/2O 2 2CO 2 (g) + H 2 C(s) + O 2 (g) CO 2 (g) H 2 (g) + ½ O 2 (g) H 2 O(l) H = kj H = kj H = kj 11

12 Sample Problem #3 Calculate H for the reaction NO(g) + O(g) NO 2 (g) Given the following information: NO(g) + O 3 (g) NO 2 (g) + O 2 (g) O 3 (g) 3/2 O 2 (g) O 2 (g) 2 O (g) H = kj H = kj H = kj 12