Heating Curves and Enthalpy Main Concept: Chemical systems undergo three main processes that change their energy: heating/cooling, phase transitions, and chemical reactions.

Heating Curves and Enthalpy q = mcΔT q = nΔH Q = mL Energy Transferred Into System vs Energy Transferred Out of System Enthalpy: Exothermic vs Endothermic

- Heating a system increases energy of system; cooling a system decreases energy - A liter of water at 50°C has more energy than a liter of water at 25°C. - specific heat capacity (c)= amount of energy needed to heat one gram of a substance by 1°C (for water = 4.184 J/g°C)

- Energy must be transferred to a system to cause it to melt (or boil) ex: energy of a system increases as the system undergoes a solid-liquid (or liquid-gas) phase transition - a system gives off energy when it freezes (or condenses) ex: energy of the system decreases as the system undergoes a liquid-solid (or gas-liquid) phase transition.

- molar enthalpy of vaporization (ΔHvap, kJ/mol ) = amount of energy needed to vaporize one mole of a pure substance; energy released in condensation has an equal magnitude - molar enthalpy of fusion (ΔHfus, kJ/mol) = energy absorbed when one mole of a pure solid melts or changes from solid  liquid; energy released when liquid solidifies has an equal magnitude q = ΔHvap × n

Question: 2.055 liters of steam at 100°C was collected at 1 atm and stored in a cooler container. What was the amount of heat involved in this reaction? The ΔHvap of water = 44.0 kJ/mol. Answer: -2.95 kJ

- latent heat of vaporization (Lv)= amount of energy needed to vaporize one mole of a pure substance; energy released in condensation has an equal magnitude (2260 J/g for H2O) - latent heat of fusion (Lf) = energy absorbed when one mole of a pure solid melts or changes from the solid to liquid state; energy released when the liquid solidifies has an equal magnitude (334 J/g for H2O)

- In chemical reactions, energy of system decreases (exothermic reaction), increases (endothermic reaction), or remains the same - exothermic reactions  energy lost by reacting molecules (system) is gained by surroundings; energy is transferred to surroundings by either heat or work - endothermic reactions  system gains energy from surroundings by heat transfer or work done on system

- enthalpy change of reaction (ΔH°) = amount of energy released (negative values, –) or absorbed (positive values, +) by a chemical reaction at constant pressure

Calorimetry Main Concept: Calorimetry is an experimental technique that is used to determine the heat exchanged/transferred in a chemical system.

Calorimetry Setup for Calorimetry What we can learn from Calorimetry

- experimental setup for calorimetry: 1. A chemical system is put in thermal contact with a heat bath (a substance, such as water, whose heat capacity has been well established by previous experiments) 2. A process is initiated in chemical system (heating/cooling, phase transition, or chemical reaction), and change in temperature of heat bath is determined

- Because the heat capacity of the heat bath is known, the observed change in temperature can be used to determine amount of energy exchanged between system and heat bath

- energy exchanged between system and heat bath is equal in magnitude to change in energy of system Ex: If heat bath increased in temperature, its energy increased, and energy of system decreased by same amount If the heat bath decreased in temperature (therefore energy), energy of the system increased by same amount

- Because calorimetry measures change in energy of a system, it can be used to determine heat associated with heat capacities, enthalpies of vaporization, enthalpies of fusion, and enthalpies of reactions. ***Only constant pressure calorimetry is required in this course.