Heat and Chemical Change Thermochemistry Heat and Chemical Change

Thermochemistry: the study of heat changes that occur in a chemical reaction Heat (q) is energy that transfers from one object to another because of temperature difference

Energy the capacity to do work or supply heat Chemical potential energy: energy stored within the structural units of chemical substances

System: The part of the universe you are studying Everything else in the universe  region in the immediate vicinity to the system

Exothermic Reactions The system loses heat and the surroundings heat up Ex. Fire, sodium metal + water, acid/base neutralization The D H < 0

Endothermic Reactions The system gains heat as the surroundings cool down Ex. Melting Ice, Boiling Water The DH is >0

Calories 1 calorie is the amount of energy required to raise 1 g H2O 1oC 1000cal= 1kcal= 1Cal 4.184 J = 1 cal

Specific Heat Capacity (C) Specific Heat: the amount of energy required to raise 1 g of a specific substance 1oC

Calorimetry

Calorimetry The accurate and precise measurement of heat change for chemical and physical processes q = m X C X DT Heat Mass Specific Change Heat in Temperature DT = Tf - Ti

Enthalpy (DH) Enthalpy = heat at constant pressure q = DH so… q = m X C X DT = DH Remember: +DH Endothermic reaction -DH Exothermic reaction

Example 1 How much energy (J) will it take to heat 100.0 mL of H2O, from room temperature 25oC to boiling? Hint: The density of H2O = 1g/mL

Ex. 2 A student mixed 50.0 mL of water containing 0.50 mol. of HCl at 22.5oC with 50.0 mL of water containing 0.50 mol NaOH at 22.5oC in a foam calorimeter. The temperature of the resulting solution increased to 26.0oC. How much heat in kJ was released by this reaction? Assume the density of solution to be 1g/mL

Ex. 3 When 435 J of heat is added to 3.4g of olive oil at 21.0 oC, the temperature increased to 85oC. What is the specific heat of olive oil?

Thermochemical Equations An equation that includes the heat of change CaO(s) + H2O (l)  Ca(OH)2 (s) + 65kJ

Heat of Reaction Heat change for the equation exactly as written Standard Temperature 25oC = 278K Standard Pressure = 1 atm= 101.3 kPa Physical states MUST be given

The amount of heat releases depends on the number of moles involved. Decomposition of 2 mol NaHCO3 = 129 kJ Decomposition of 4 mol NaHCO3 = 258 kJ

Example 1 2NaHCO3 (s) + 129 kJ = Na2CO3 + H2O (g) + CO2 (g) If 2.24 mol NaHCO3 decomposes = ?kJ

Example 2 C(s) + 2S (s)  CS2 (l) If DH = 89.3 kJ 5.66 g CS2 = ? kJ

The amount of heat released also depends on the physical state H2O (l)  H2 (g) + ½ O2 (g) DH= 258.8 kJ H2O (g)  H2 (g) + ½ O2 (g) DH= 241.8 kJ

Heat of Combustion (DHcom) The heat of reaction for the complete burning of 1 mole of a substance

Molar Heat of Fusion (DHfus) The heat absorbed by 1 mole of a substance in melting from a solid to liquid at constant temperature

Molar Heat of Solidification (DHsolid) The heat lost when 1 mole of a liquid solidifies at constant temperature The quantity of heat absorbed by a melting solid is exactly the same as the heat lost when the liquid solidifies. Therefore - DHsolid = DHfus

Example 1 How many grams of ice at 0oC and 101.3 kPa could be melted by the addition of 0.400 kJ of heat? DHfus= 6.01 kJ/mol

Heat of Vaporization (DHvap) The amount of heat necessary to vaporize 1 mole of a given liquid

Molar heat of Condensation (DHcond) The amount of heat released when 1 mole of vapor condenses Therefore DHvap = -DHcond

Molar Heat of Solution (DHsoln) The heat change caused by dissolution of 1 mole of a substance

Experiment Design an experiment to determine the DHsoln for 4 compounds NaHCO3 CaCl2 NaCl NH4Cl