# ENERGETICS - MrsPage.com 5.1 ENERGETIC CALCULATIONS IB SL Chemistry Mrs. Page 2015-2016 Daily Objectives Understand the principle of experimental methods for determining enthalpy changes Work out enthalpy changes from experimental data Enthalpy Change of Combustion

Heat given off in combustion rxn is used to heat a substance of known specific heat capacity The mass and temp change of water must be measured cH2O = 4.18 J g-1K Note: 1C = 1K Calculating Enthalpy from Combustion Rxn The mass of a spirit lamp containing ethanol (C2H5OH) was determined accurately. The lamp was then lit and placed under a beaker containing 150. g of pure water. The water was continually stirred. After the temperature of the water had

increased by 12.0 C the flame was extinguished and the mass of the lamp containing the unburned ethanol was determined. Use the following data to determine the enthalpy change when 1 mol of ethanol is burnt completely. Mass of lamp + ethanol before burning = 42.597 g Mass of lamp + ethanol after burning = 42.200 g Mass of ethanol burned: 42.597-42.200 = 0.397 g Molar Mass of ethanol: 46.08 g mol-1 Calculating Enthalpy from Combustion Rxn 0.387 g C2H5OH

1 mol 46.08 g = 0.00840 mol C2H5OH How much heat energy required to raise the temp of 150. g of water by 12.0C q=mcT = 150. (4.18) (12.0) = 7524 J It take 7524 J to burn 0.00840 moles of ethanol so how much to burn 1 mol? 7524 J 0.00840 mol C2H5OH

= 895,714J/mol = 896,000J/ mol or 896 kJ/mol Combustion Rxns: Exothermic so H= - 896 kJmol-1 Enthalpy Change of Combustion Possible Sources of Error: Incomplete combustion (build up of soot, occurring in air, yellow flame) Loss of heat (using a beaker vs. insulated container) Heat loss to system (heating not only water but

also beaker, thermometer, stirring rod, ring stand, etc) Loss of alcohol to evaporation Errors will always be lower than actual value Enthalpy Change of Combustion Possible Improvements: Include the specific heat capacity of the container you are heating the water in as well Use Insulating can Use Draught Shield Use a Bomb Calorimeter

Enthalpy Change of Combustion T = 45.7 19.5 = 26.2C Mass of Ethanol = 121.67 120.62 = 1.05 g Mol Ethanol = 1.05 g (1mol/46.08 g) = 0.022786 mol q = mc T = 150.00(4.18)(26.2) = 16427.4 J = 164 kJ H = J/mol = 164.274kJ/0.022768 mol H= -721 kJmol-1 Lit Value = -1371kJmol-1 Enthalpy Changes in Solution (Hsol) In Lab: Measure known amounts of reagents

Record initial temp Mix reagents in styrofoam cup Record max/min temp Specific heat of final solution assumed to = specific heat of water Enthalpy Change of Solution(Hsol) Hsol when 1 mole of a solute is dissolved in excess solvent to form a solution of infinite dilution under STP May be endo or exothermic Example:

Example of Enthalpy Change of Solution When 50.0 cm3 of 1.00 mol dm-3 sodium hydroxide solution (NaOH(aq)) at 25.0 C was added with stirring to 50.0 cm3 of 1.00mol dm-3 hydrochloric acid solution HCl(aq), also at 25.0C, in a polystyrene coffee cup, the temperature of the resulting solution rose very quickly to 31.8 C. What is the change in enthalpy of this reaction? T = 31.8 25.0 = 6.8 C Total Volume = 100.0 cm3 Assume: resulting solution has same density & c of water

Note: system heated up so exothermic and H is negative q=mcT = 100.0(4.18)(6.8) = -2842 J Example of Enthalpy Change of Solution When 50.0 cm3 of 1.00 mol dm-3 sodium hydroxide solution (NaOH(aq)) at 25.0 C was added with stirring to 50.0 cm3 of 1.00 mol dm-3 hydrochloric acid solution (HCl(aq)), also at

25.0C, in a polystyrene coffee cup, the temperature of the resulting solution rose very quickly to 31.8 C. What is the change in enthalpy of this reaction? Amt of HCl = amt NaOH (balance equation 50.0 cm3 1 dm3 1.00 mol = 0.0500 mol NaOH 1000 cm3 1 dm3 q = mcT= 50.0(4.18)(6.8) = 1421.2 J Positive or negative? Why? H = -1421.2 / 0.050 = -28424 Jmol-1 = - 28.4 kJmol-1

Enthalpy Change of Neutralization (Hn) Hn when 1 mole of water molecules are formed when an acid (H+) reacts with an alkali (OH-) under STP Neutralizations are ALWAYS exothermic (-H) + ( )+ ( ) 2 ( )

You Try A 25.0 g sample of an alloy was heated to 100.0C and placed in a beaker containing 90.0g of water at 25.32C . The temperature of the water rose to a final value of 27.18C. Neglecting heat losses to the room and the heat capacity of the beaker itself, Calculate the specific heat of the alloy. You Try Assuming no heat loss to the surroundings or to the container, calculate the final temperature

when 100g of silver at 40.0 C is immersed in 60.0 g of water at 10.0 C. The specific heat capacity of silver is 0.236 JgC You Try When sulfuric acid dissolves in water, a great deal of heat is given off. The enthalpy change for this process is called the enthalpy of solution. To measure it, 175 g of water was placed in a coffee-cup calorimeter and chilled to 10oC. Then 49.0 g of pure sulfuric acid, also at 10.0oC was added, and the mixture was quickly stirred with a thermometer. The temperature rose rapidly to

14.9oC. Assume that the value of the specific heat of solution is 4.184 J/goC. You may assume that the specific heat of the resulting sulfuric acid solution will also be 4.184 J/goC. Calculate q for the formation of this solution, and calculate the enthalpy of solution in kilojoules per mole of H2SO4. Molar Enthalpy Change for a Reaction 1. 2. 3. 4.

5. 6. 7. 8. 9. Write equation for reaction Determine the limiting reagent Measure mass of aqueous solution Use calorimeter and measure initial temperature (probe should measure temp. every 30 seconds until a constant temperature is reached) Add other reactant and continue measuring temperature for

about 5 minutes after maximum temperature is reached Produce a temperature vs time graph (Excel) Look at cooling section of curve to extrapolate back to the point of introduction of reactant. Calculate the heat evolved in the experiment. Find the enthalpy change for the reaction Molar Enthalpy Change for a Reaction 50.0 cm3 of 0.200 mol dm-3 copper (II) sulfate solution was placed in a polystyrene cup. After 2 minutes 1.20 g of powdered zinc was added. The temperature was taken every 30 seconds and the following graph was obtained. Calculate the enthalpy change for the reaction taking place.

Equation: CuSO4(aq) + Zn(s) ZnSO4(aq) + Cu(s) Determine Limiting Reagent: 50.0 cm3 1 dm3 0.200 mol 1000 cm3 1 dm3 = 0.0100 mol CuSO4 1.20 g Zn 1 mol 65.37 g LIMITING = 0.0184 mol Zn

Molar Enthalpy Change for a Reaction 50.0 cm3 of 0.200 mol dm-3 copper (II) sulfate solution was placed in a polystyrene cup. After 2 minutes 1.30 g of powdered zinc was added. The temperature was taken every 30 seconds and the following graph was obtained. Calculate the enthalpy change for the reaction taking place. Extrapolate from Graph: T = 27.5 17 = 10.5 C Calculate Heat: q = mc T = 50.0(4.18)(10.5) = 2194.5 J = 2.1945 kJ Calculate Change in Enthalpy:

H = kJ/mol = 2.1945 kJ/ 0.0100 mol = 2.19 kJ/mol Endo or Exo? ANSWER -2.19 kJ/mol STANDARD ENTHALPY CHANGE OF REACTION H = at standard conditions Temperature 25C/ 298K Pressure 100 kPa All species in standard states Standard enthalpy change of formation = Hf is the amount of energy change to form 1 mole of a compound from elements in standard states at 298 K and 100 kPa Standard enthalpy change of combustion = Hc is amount of

enthalpy change when 1 mole of a substance is completely combusted in oxygen under standard conditions. Data booklet Section 12 Hf for common compounds. Section 13 - Hc HOMEWORK Quick Questions pp 145 & 146