I. Energy A. Terms and Definitions B. Energy Transfer as Heat C. Energy Transfer as Work D. Internal Energy

Transcription

1 Chapter 7 1 Thermochemistry is HOT! I. Energy A. Terms and Definitions B. Energy Transfer as Heat C. Energy Transfer as Work D. Internal Energy II. Chemistry and Energy A. Enthalpy and Enthalpies of Reaction B. Hess s Law C. Enthalpies of Formation III. Energy in Foods and Fuels

2 I. Energy A. Terms and Definitions 2 Energy- the ability to do work or transfer heat 1. Types of Energy a. Kinetic Energy energy of motion EK 1 mv 2 2 Thermal Energy kinetic energy of random molecular motion b. Potential Energy energy of position or composition e.g. - gravitational E p = mgh Q Q - electrostatic E el 1 d d - energy stored in chemical bonds

3 2. Units of Energy SI Unit = Joule (J) 1 J = 1 kg-m 2 /s 2 1 cal = J (exact) 1 Cal = 1000 cal 3 3. Keeping Track of Energy Changes 1 st Law of Thermodynamics Universe 1. System 2. Surroundings Chemistry System = Surroundings = NaCl(aq) + AgNO 3 (aq) NaNO 3 (aq) + AgCl(s)

4 4. Transferring Energy 4 Energy is the ability to do work or transfer heat 2HCl(aq) + Mg(s) MgCl 2 (aq) + H 2 (g) Two Types Energy Transfer 1. Temp. Change - Energy transferred as heat from hotter objects to colder objects 2. work - moving something against a force Physics - lift a book against gravity, push against a force Chemistry -

5 B. Energy Transfer as Heat 5 Heat Transfer (q) T = 1000 K T = 274 K T = 637 K Isothermal Heat Transfer

6 1. How much Energy is required for a temperature change? q 6 2. Terms (Energy storage) Heat Capacity (C) - energy required to raise temp of system or object 1K (or 1 C) Molar heat capacity - energy required to raise temp of 1 mole of a substance 1K (or 1 C) Specific heat (c or s) - energy required to raise temp of 1 g of a substance 1K (or 1 C) 9DM2ZfKxg5hL8wErdulSX3qE

7 3. How Do Substances Store Energy? 7 a.) elements - b.) Molecules - In General (Molar Heat Capacity) Elements vs. Molecules Molecules vs. Molecules Elements vs. Elements Physical States (g) (l) (s)

8 4. Calculations q = heat transferred to or from system (+ or -) 8 Calculating Heat Transfer (object) q = (heat capacity) * (ΔT) = ( J ) ( ) = J Calculating Heat Transfer (substance - moles) q = (moles)*(molar heat capacity)* (ΔT) = (moles) ( ) ( ) = J moles J Calculating Heat Transfer (substance - mass) q = (mass)*(specific heat)* (ΔT) = (g) ( J ) ( ) = J g Units and Info From T

9 Do the units of T matter? ( C or K) 9 T i = 0.00 C = K Tf = C = K T = C-0.00 C T= K K = C = K

10 Example 1. Calculate the quantity of heat required to raise the temperature of 9.25 L of water from 22.0 to 29.4 C. (specific heat of water = 4.18 J/g C throughout the temp range) 10

11 Where does the Energy for T come from? 11 Law of Conservation of Energy (1 st law of thermo) Total energy of universe remains constant Energy transfer just as heat

12 5. Calorimetry 12 The measurement of heat flow during a chemical reaction in an isolated environment How do we measure heat flow Calorimeters insulated devices used to contain and isolate chemical reactions (T i and T f ) Two Types of Calorimeters 1. Bomb Calorimeter (object) - constant volume - measure energy change (heat) as a result of combustion 2. Coffee-Cup Calorimeter - constant pressure (open to atm) - measure energy change (heat) associated with reactions in aqueous solutions

13 How do we use Calorimeters? - treat calorimeters as micro-universes Bomb Calorimeter micro-univ = reaction + calorimeter 2. Coffee-Cup Calorimeter micro-univ = reaction + solution Assumptions - Calorimeter is a perfect insulator - solution absorbs all the energy (neglect products and calorimeter) Procedure measure the temp. change of solution and relate to reaction

14 Example 2. When 50.0 ml of M HCl and 50.0 ml of M NaOH are mixed in a coffeecup calorimeter, the temperature of the resultant solution increases from 21.0 C to 24.3 C. Both solutions were originally at a temperature of 21.0 C. a.) What is the balanced chemical equation for this reaction? b.) Is this reaction endothermic or exothermic? c.) Calculate the heat of the reaction. Assume that all of the heat of the reaction is absorbed by the solution, that the total volume of the solution is ml, that density of the solution is 1.00 g/ml, and that its specific heat is J/g-K. d.) What is the amount of energy (heat) transferred per mole of H 2 O that forms? 14