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Chapter 6: ThermochemistryI. PRINCIPLES OF HEAT FLOWBasic Vocabulary Definitions-System:Surroundings:State of a System:State Property:Sign of Heat Flow-The symbol for heat flow is:q is + when:q is – when:Endothermic processSign of q:Heat flows from: ______________to____________.Effect on the temperature of the surroundings:Example of an endothermic process:Endothermic Process-Sign of q:Heat Flows from: _________to___________.Effect on the temperature of the surroundings:Example of an exothermic process:6. Heat capacity- A. Symbol: B. Definition:7. Specific Heat- A. Symbol: B. Definition: C. Mathmatical relationship between the magnitude of heat flow and specific heat: D. Units for specific heat:EXAMPLES:The specific heat of ethyl alcohol is 2.43 J/goC. If the Ethyl alcohol absorbs 17.85 J of heat when its temperature increases from 23.13oC, what is the mass of the ethyl alcohol present? Calculate the specific heat of an alloy if 17.98 J of heat must be absorbed to raise the temperature of a 6.00-g sample from 20.00oC to 28.35oC. What is the heat capacity of the alloy? II. CALORIMETRYExamples:1. When 1.507 g of NH4Cl are dissolved in 100.00g of water at 25.00oC, the temperature drops to 23.98oC. Calculate q for the water and q for the solution process.2. When 12.84 grams of MgSO4 are dissolved in 250.00g of H2O at 23.76oC, 9.745 kJ of heat are evolved: A. What is q for the solution process? B. What is q for the water? C. What is the final temperature of the solution?III. Enthalpy-A. Definition:B. Symbol:C. Relation to qreaction:D. ?H 1. Relationship of ?H to enthalpy of products and reactants: 2. Sign of ?H A. Exothermic reaction:B. Endothermic reaction:E. Enthalpy is a state property.Examples:1. When a piece of sodium metal is added to a beaker with water, the temperature in the beaker goes from 25oC to 42oC. Is the reaction exothermic or endothermic? What is the sign of ?H?2. When ammonium nitrate is dissolved in water, the temperature drops. A. Is heat evolved to the surroundings or absorbed from the surrounding? B. Write the chemical equation for the reaction involved.IV. Thermochemical EquationsA. Things to remember about a thermochemical equation:1. The sign of ?H indicates whether the reaction, when carried out at constant pressure, is exothermic or endothermic.2. The coefficients of the balanced equation represent the number of moles of the species in that particular equation.B. Laws of Thermochemistry1. The magnitude of ?H is directly proportional to the amount of reactant or product. A. The ?H written to the right of the equation indicates how many kilojoules of energy are either evolved or absorbed by the reaction. B. Once again, conversion factors can be obtained from the reaction. With a thermochemical equation, in addition to the mole and gram relationships you can write, you can also obtain a relationship involving moles, grams, and kilojoules for the substances in the reaction. C. Consider the reaction between sodium metal and liquid water. It produces solid sodium hydroxide, hydrogen gas, and 281.8kJ of heat energy. 1) Write the thermochemical equation for this reaction:2) Write all the relationships you can obtain from this reaction:3) Write the conversion factor for converting between grams of water and KJ of energy:Remember:1. The sign of ?H is included in every conversion factor that you write. Do NOT drop it.2. You will need to write balanced chemical equations.Examples:1. When four moles of iron metal are burned in oxygen, iron III oxide is formed and 1644.4 kJ of energy are evolved. A. Write a balanced thermochemical equation for this reaction: B. How much heat is evolved if 3.031 g of iron are burned?2. When a mole of solid calcium sulfate reacts with carbon dioxide, solid calcium carbonate and sulfur trioxide are formed and 223.8 kJ of heat are absorbed. A. Write a balanced thermochemical equation for the reaction: B. How many grams of CaCO3 can be produced if 4.843 kJ are absorbed? 3. When 1 mol of hydrogen gas reacts with 1 mol of iodine gas to produce hydrogen iodide gas, 51.8 kJ of energy are absorbed. A. Write the balanced thermochemical equation for this reaction: B. How much heat is involved in the decomposition of 8.164 g of HI into H2 and I2? C. Is the heat in the decomposition absorbed or evolved?V. HESS’S LAWA. The value of ?H for a reaction is the same whether it occurs directly or in a series of steps. If equation C is the sum of equations A and B, then ?H for equation C is the sum of the ?H’s for equations A and B. This is known as Hess’s Law. 1. You may need to reverse a reaction to get the substances on the correct side of the arrow. If you do you should change the SIGN of the ?H value also. 2. You may need to multiple or divide the coefficients by a factor. If you do you should do the same to the ?H value. 3. Cancel all substances that appear on opposite sides of the arrow and add the ?H’s. DON’T drop the signs.Examples:1. Add the following thermochemical equations:2Na2O2(s) + 2H2O(l) 4NaOH(s) + O2(g) ?H = -126.4kJ (1)NaOH(s) + HCl(g) NaCl(s) + H2O (l) ?H = - 179.2 kJ (2) To produce the overall reaction:2Na2O2(s) + 4HCl(g) 4NaCl(g) 4NaCl(s) + O2(g) + 2H2O(l)2. Use the equations:2C2H2(g) + 5O2(g) 4CO2(g) + 2H2O(g) ?H = -2512 kJN2(g) + ? O2(g) N2O (g) ?H = 104 kJ To obtain ?H for the reaction:C2H2(g) + 5N2O(g) 2CO2(g) + H2O(g) + 5N2(g)B. Knowing the amount of heat evolved or absorbed by a reaction gives a conversion factor that you can use to calculate ΔH for the equation. Consider calorimetry example #1. Here you were given data to calculate ΔH for the solution of 1.507 g of NH4Cl. You calculated ΔH to be 427J. Your conversion factor is therefore: 1.507g or 427J 427J 1.507g The reaction is:NH4Cl(s) NH4+(aq) + Cl-(aq) You therefore want ΔH for the dissolving of one mole of NH4Cl. You thus want to convert moles of NH4Cl to kilojoules. Since you do not have a conversion factor involving moles and kilojoules directly, you will have to convert 1 mole of NH4Cl to grams of NH4Cl using the conversion factor 53.49 g of NH4Cl = 1 mole. Now you can convert 53.49 g NH4Cl to kilojoules.53.49g NH4Cl x 427 J x 1 kJ 1.507 g 1000J = 15.2 kJ The thermochemical equation is therefore:NH4Cl(s) NH4+(aq) + Cl-(aq) ΔH = 15.2 kJVI. ENTHALPIES OF FORMATIONA. Definition:B. Formula for determining ΔHo:C. Heat of formation of elements in their stable state is:D. Formula for determining ΔHo for ions in water solution:E. Heat of formation of H+:Examples:1. The reaction between one mole of sodium oxide and water yields an aqueous solution of sodium ions and hydroxide ions. The reaction evolves 237.5 kJ of heat. A. Write the balanced thermochemical equation using the smallest single whole number coefficients. B. Calculate the heat of formation of sodium oxide using your thermodynamic quantities table.2. The balanced thermochemical equation for the reaction between sulfur dioxide gas and liquid bromine is:Br2(l) + SO2(g) + 2H2O(l) 4H+ + 2Br-(aq) + SO42-(aq) ΔHo = -280.9kJ Calculate the heat of formation of the sulfur dioxide gas.VII. FIRST LAW OF THERMODYNAMICSA. Types of energy in thermodynamics? 1. Heat a. Symbol: b. Positive when: c: Negative when: 2. Work a. Symbol: b. Positive when: c. Negative when: B. First law of thermodynamics 1. Statement: 2. Mathematical statement:Examples:1. Calculate ΔE for a gas involved in a process for which the gas evoves 25 J of heat and does 48J of work expanding.2. A gas has ΔE of -115J. The gas absorbs 38 J of heat. How much work is involved in the process? Is it done by the system or on the system? ................
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