Transcript Unit 10
Unit 12 Thermochemistry 1. Energy is… The capacity for doing work or supplying heat. a) Energy has neither volume _______ mass nor _________ b) List the types of energy--- Thermal/heat potential chemical kinetic electrical Solar/light/electromagnetic 2. Thermochemistry is Study of energy changes during chemical reaction and change in state. FYI Temperature is The average kinetic energy. Units: K (kelvin), oC (celsius), 3. Heat is … Energy that transfers from one object to another. Heat flows: from a Warmer object a colder object Conversion factors FYI 1 Calorie = 1000 calorie 4.184 J = 1 cal Practice 1.262 cal 301.1 cal 4. The law of conservation of energy In any chemical or physical process, energy is neither created or destroyed. 5. Endothermic process: In an endothermic process, the system absorbs heat as the surroundings cool down. 6. Exothermic process: In an exothermic process, the system releases heat as the surrounding warm up. q surr =mC∆T q sys 7. Enthalpy a) Enthalpy (H) is The heat content of a system b) The change in enthalpy (∆H)= The heat released or absorbed by a reaction c) ∆H = heat FYI: Energy Diagram Exothermic Reaction Endothermic Reaction 8. Endo or Exo during state change a. Solidliquidgas (absorb heat) endo b. Gasliquidsolid (release heat) exo Gas Endothermic reaction Melting and vaporization Liquid Exothermic reaction Freezing and Solid Condensation 16. Heating curve of water 9. Thermochemical equation a. Heat of reaction: the enthalpy change for the chemical equation. b. Heats of reaction reported as ∆H c. ∆H is negative Exothermic reaction, heat as a product d. ∆H is positive Endothermic reaction, heat as a reactant 9. Thermochemical equation e. Ex: CaO (s) + H2O (l) Ca(OH)2 (s) ∆H= -65.2kJ (endo or exo?) Exo, heat as a product f. Example of heat of combustion: CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) ∆H= -890kJ (endo or exo?) exo g. Thermochemical equation is same as stoichiometry Practice Consider the following chemical equation 2NaHCO3 (s) + 129kJ Na2CO3 (s) + H2O (g) + CO2 (g) i. Is this an exothermic or endothermic reaction? Explain. Endo, heat as a reactant. Practice mole to heat conv., use stoich 65 kJ 65 kJ of heat are absorbed Practice mole to heat conv., use stoich Consider the following chemical equation 2NaHCO3 (s) + 129kJ Na2CO3 (s) + H2O (g) + CO2 (g) iii. How many kJ of heat are absorbed when 25.0 gram of sodium hydrogen carbonate is decomposed? 19.2 kJ 19.2 kJ of heat are absorbed 9. Specific heat capacity a) The heat capacity is the amount of heat needed to increase the temperature of an object exactly 1oC b) The heat capacity depends on both its mass and its chemical composition. c) The specific heat capacity is the amount of heat it takes to raise the temperature of 1g of the substance 1oC. 9. Specific heat capacity d. Low specific heat capacity easy to raise temperature e. High specific heat capacity need more heat to raise temperature Substance Specific heat capacity (J/g oC) H2O (l) 4.18 H2O (s) 2.03 Al (s) 0.89 Fe (s) 0.45 Hg (l) 0.14 C (s) 0.71 10.Calorimetry a. Calorimetry is the measurement of the heat flow into or out of a system for chemical and physical process. b. Heat released by the system = Heat absorbed by its surrounding c. Heat absorbed by the system = Heat released by its surrounding q surr =mC∆T q sys 10.Calorimetry d. Calorimeter is the insulated devise used to measure the absorption or release of heat in chemical or physical process. i. qsurr = mC∆T (We can measure this) ii. qsys = - qsurr = - mC∆T (We can’t measure this, but put opposite sign) Important formula q = mCΔT q = heat absorbed/heat released, enthalpy change(J) m = mass (g) C = The specific heat capacity (specific heat) (J/g･oC) ΔT = final temperature - initial temperature (oC) IF you have specific heat, use this formula Practice e. How many joules of heat are needed to raise the temperature of 238 grams of water from 20.0oC to 50.0oC? (Cwater= 4.18 J/g oC) 29800J Practice Convert from cal to joule f. A 20.0 gram sample of iron at 25.0oC was given 50.0 calories of heat. What is its final temperature? Ciron=0.45 J/g oC) Tf -25.0oC= 23.2 Tf = 48.2oC 12. Application A student performed a calorimetry lab to determine the specific heat of an unknown metal. Consider the following data and answer the questions from a) to e). Assume Cwater = 4.18 J/goC Mass of unknown metal 12.50g Mass of water 75.00g Initial temperature of water in calorimeter 15.8oC Final temperature of water + unknown metal in calorimeter 25.1 oC Initial temperature of unknown metal in calorimeter 88.3OC a) What is the temperature difference (∆T) of water? 25.1 oC– 15.8 oC = 9.3 oC b) Calculate the heat absorbed by the water. Q = (75.00g)(4.18J/goC)(9.3)=2920J c) What is the heat released by unknown metal? Q = -2920J Change the sign!! d) What is the temperature difference (∆T) of unknown metal? 25.1 oC- 88.3 oC = -63.2 oC e) Calculate the specific heat capacity of this metal. (12.50g)(C)(-63.2 oC)=-2920J C = 3.70J/goC 11. Molar heats of solution: (∆Hsoln) a. b. c. d. e. f. The heat is either released or absorbed during formation of a solution. Example of thermochemical equation: + (aq) + OH- (aq) 2O ( l ) NaOH (s) H Na ∆Hsoln = - 445.1 kJ/mol (- 445.1 kJ per 1 mol) Is this endo or exo? Is the temperature of the solution increasing or decreasing? Because system release heat, so surrounding absorbs heat. Practice A student dissolved 2.500 mol NaOH (s) in water a. Write the thermochemical equation (l ) NaOH (s) HO Na+ (aq) + OH- (aq) + 445.1kJ 2 (∆Hsoln = -445.1 kJ/mol) b. Is it an endo or exo? exo Practice A student dissolved 2.500 mol NaOH (s) in water (l ) Na+ (aq) + OH- (aq) + 445.1kJ NaOH (s) HO 2 (∆Hsoln = -445.1 kJ/mol) c. How much heat (in kJ) is released when 2.500 mol NaOH (s) is dissolved in water? 2.500mol NaOH x -445.1 kJ = -1113kJ 1mol NaOH Lab report Data table II Mass of water (g) Use density formula: d = m/v D(water)=1.0 g/mL Lab report 2. Use qsys = - qsurr Post lab question/Analysis 3. Calculate the Calories per gram of marshmallow burned. Energy released by the marshmallow divided by the mass of marshmallow burned Post lab question/Analysis 4. Find the average Calculate the Calories per gram of marshmallow burned. Energy released by the marshmallow divided by the mass of marshmallow burned Marshmallow lab Introduction Heat unit in science joule (J) kilojoule (kJ) calorie (cal) kilocalorie (kcal) Heat unit in Food Calorie (Cal) Marshmallow lab Procedure 1. Measure and record the mass of 1. Marshmallow 2. Paper clip 3. Watch glass together Marshmallow lab Procedure 2. Make your paper clip into a holder for marshmallow Marshmallow lab Procedure 3. Measure 25.0 mL of cold water with a graduated cylinder. Record the volume of water used precisely. Pour it into the aluminum can. Marshmallow lab Procedure 4. Place chopstick through the tab so you can hang it from the ring support of the ring stand. Marshmallow lab Procedure 5. Measure and record the initial temperature of the water. Measurements Initial mass marshmallow, paperclip, and watch glass (g) Volume water (mL) Initial temperature water (°C) Final temperature water (°C) Trial 1 Trial 2 Marshmallow lab 6. Light the marshmallow. Keep the thermometer above the bottom of the can. Marshmallow lab 7. Allow the marshmallow to stop burning on its own, It does not have to completely burn to black. Keep an eye on the temperature to figure out the highest temperature reached. Record the temp. Marshmallow lab 8. Measure and record the mass of the burned marshmallow, paper clip, and watch glass together. To find the mass of burned marshmallow, subtract final mass from initial mass. Marshmallow lab Perform trial 2 Burned marshmallow should be thrown away use the same paper clip but washed use a new can Measure all over again when finished trial 2, return 2 cans, paper clip, chop stick Marshmallow lab Assumption Heat released by marshmallow is same as heat absorbed by water. q sys = q surr Pre lab quiz q sys q surr