Textbook Ch.8: Thermochemistry

the part of the universe in which attention is focused

exchange energy within the system, make up the rest of the universe

composition, temperature, and pressure

depend ONLY on the state of the system, not on the way the system reached the state

the variable for heat flow (magnitude of heat flow) • expressed in joules and kilojoules • q is positive when heat flows into the system from the surroundings • q is negative when heat flows out of the system into the surroundings

a reaction in which heat flows from the surroundings into the reaction system • q is positive (q>0) • Temperature of the surroundings DROPS • Ex: melting ice

a reaction in which heat flows from the reaction system into the surroundings • q is negative (q<0) • Temperature of the surroundings RISES • Ex: combustion of methane • Reaction evolves heat

the amount of heat required to raise the temperature of 1g of water 1°C • 1 cal = 4.194 J • 1 kcal = 4.184 kJ

q = C * ∆t q = magnitude of heat flow C = heat capacity ∆t = tfinal - tinitial

the amount of heat required to raise the temperature of the system 1°C and has the units J/°C

q = m * c * ∆t

the amount of heat required to raise 1g of the substance 1°C • It is like density or melting point because it is an intensive property that can be used to identify a substance or determine its purity

a device used to measure the heat flow in a reaction • If the reaction is exothermic, the heat flow in the calorimeter is positive because heat flows from the reaction to the calorimeter • If the reaction is endothermic, the heat flow in the calorimeter is negative because the calorimeter gives up heat to the reaction mixture reaction = - Ccalt

two foam cups partially filled with water and have a thermometer inserted Heat capacity of the coffee-cup calorimeter = heat capacity of the water CcalWater * water = water * 4.18 J/g°C q reaction = -water * 4.18 J/g°C * ∆t

more versatile than a coffee-cup calorimeter because it can be used with gases, and reactions in which the products reach high temperatures Ccal = 9.33 kJ/°C • All heat given off by the reaction is absorbed by the bomb calorimeter q reaction = -calorimeter reaction = -Ccal * ∆t

a type of chemical energy sometimes referred to as “heat content” • The heat flow for the reaction system = the difference in enthalpy (H) between the products and reactants • In exothermic reactions, the enthalpy is negative • In endothermic reactions, the enthalpy is positive • the enthalpy of a substance is a state property

a chemical equation that shows the enthalpy relation between products and reactions

• Sign of ∆H (enthalpy) indicates whether the reaction is endothermic (+∆H), or exothermic (-∆H) • The coefficients represent the number of moles • The phases of all species must be specified

• The magnitude of ∆H is directly proportional to the amount of reactant or product • ∆H for a reaction is equal in magnitude but opposite sign to ∆H for the reverse reaction (AKA heat evolved = heat absorbed) • The value of ∆H for a reaction is the same whether it occurs in one step of in a series of steps (Hess’s Law)

the heat absorbed when a solid melts (solid –> liquid)

the heat absorbed when a liquid vaporizes (liquid –> gas)

enthalpy change when one mole of the compound is formed at a constant pressure of 1 atm and a fixed temperature of 25°C

zero

the sum of the standard enthalpies of formation of the compounds minus the sum of the standard enthalpies of formation f the reactant compounds ΔH = ΣΔH(products) − ΣΔH(reactants)