Thermochemistry

Thermochemistry: Study of energy changes that occur during chemical reactions and changes in state Chemical Potential Energy: Energy stored in chemical bonds of a substance

What is heat, how does it flow? Heat (q) : energy transferred from one object to another due to temperature difference Heat always flows from warmer object to cooler object! Why? Hint – Energy…

Energy Law of conservation of energy: Energy is not created or destroyed Endothermic Process: absorbs heat from the surroundings Reactants + Heat  Products Exothermic Process: releases heat to surroundings Reactants  Products + Heat

1 Calorie = 1 kilocalorie = 1000 calories Units for Heat calorie: heat needed to raise 1.0 gram of water 1.0ᵒC 1 Calorie = 1 kilocalorie = 1000 calories Ex. Kudos Bar 130 Calories = 130 kcal = 130,000 calories Joule – SI unit for energy 1 J = 0.2390 cal 4.184 J = 1 cal How many joules are in a Kudos bar?

Specific Heat Specific Heat: Amount of heat needed to increase the temperature of 1g of a substance exactly 1ᵒC. specific heat (C) = Heat (Joules or calories) Mass (grams) x change in temperature (C)   C = q . ( m ) (T)

The temperature of a 95. 4g piece of copper increases from 25 The temperature of a 95.4g piece of copper increases from 25.0ᵒC to 48.0ᵒC when the copper absorbs 849J of heat. What is the specific heat of copper?

T = change in the temperature Calorimetry Calorimetry: measure of heat flow into or out of a system Enthalpy (H): the internal energy of the molecules of a material Heat(q) = H = m (C) T H = change in the heat m = mass C = specific heat T = change in the temperature

Thermochemical Equations Enthalpy change (ΔH) can be written as a product or a reactant. Endothermic 2NaHCO3(s) +129kJ  Na2CO3(s) + H2O(g) + CO2 (g) 2NaHCO3(s)  Na2CO3(s) + H2O(g) + CO2 (g) ΔH=129kJ Exothermic CaO(s) + H2O(l)  Ca(OH)2(s) + 65.2 kJ CaO(s) + H2O(l)  Ca(OH)2(s) ΔH=-65.2kJ

2NaHCO3(s) + 129kJ  Na2CO3(s) + H2O(g) + CO2(g) ΔH=129kJ How much heat (in kJ) is required to decompose 2.24mol of NaHCO3? ΔH = 2.24mol NaHCO3 x 129kJ 2mol NaHCO3 = 144kJ

C2H5OH(l) + 3O2(g)  2CO2(g) + 3H2O(l) ΔH=-1368kJ How much heat (in kJ) is released when 12.5g of ethanol burns?

Fusion and Solidification Molar Heat of Fusion (ΔHfus): heat absorbed by 1 mol of a substances as it melts (solid  liquid) ENDOTHERMIC solid + heat  liquid Molar Heat of Solidification (ΔHsolid): heat lost when 1 mol of a liquid solidifies (liquid  solid) EXOTHERMIC liquid  solid + heat ΔHfus = -ΔHsolid

Vaporization and Condensation Molar Heat of Vaporization (ΔHvap): amount of heat needed to convert 1 mol of liquid to gas (liquid  gas) ENDOTHERMIC liquid + heat  gas Molar Heat of Condensation: (ΔHcond): amount of heat released when 1 mol of vapor condenses (gas  liquid) EXOTHERMIC gas  liquid + heat ΔHvap = -ΔHcond

H2O(s)  H2O(l) ΔHfus = 6.01kJ/mol How many grams of ice at 0ᵒC will melt if 2.25kJ of heat are added?