Chem lec 1st year study
Chemistry 86: Chemistry for Engineers
Thermochemistry
Branch of thermodynamics that deals with heat flow in chemical reactions.
Thermodynamics
Scientific study of energy transformations.
Nature of Energy
Chemistry studies matter but is affected by energy.
Energy: Capacity to do work.
Work = Force x Distance
Energy can be exchanged through contact (collisions).
Classification of Energy
Kinetic Energy: Energy of motion; includes thermal energy as it's associated with molecular motion.
Potential Energy: Stored energy; associated with the position or composition of an object.
Example: Energy stored in a compound's structure.
Forms of Energy
Electrical Energy: Kinetic energy from the flow of charge.
Heat/Thermal Energy: Kinetic energy associated with molecular motion.
Light/Radiant Energy: Kinetic energy from atomic transitions.
Nuclear Energy: Potential energy in atomic nuclei.
Chemical Energy: Potential energy due to atom arrangements.
Units of Energy
Kinetic Energy (KE): KE = ½mv²
Example: 1 Joule (J) moves a 1 kg mass at 1 m/s.
Energy Units:
Joules (J): 1 J = 1 N·m = 1 kg·m²/s²
Calorie (cal): Energy to raise 1g of water by 1°C; 1 kcal = 1,000 cal.
Energy conversion: 1 cal = 4.184 J, 1 kcal = 4,184 J.
Energy Use Units
Joule (J) for heating water: 4.18 J to raise temp of 1g water by 1°C.
Average Energy Use:
Light 100 W bulb for 1 hr: 3.60 x 10^5 J
Average U.S. citizen uses 9.0 x 10^8 J/day.
Systems in Thermochemistry
System: The focus of study in energy changes.
Types of systems: Open, closed, and isolated.
Open systems exchange mass & energy; closed systems exchange energy but not mass.
Isolated systems exchange neither.
Conservation of Energy
First Law of Thermodynamics: Energy cannot be created or destroyed.
Energy can only be transformed or transferred.
Symbolically: ΔEnergy_universe = 0 = ΔEnergy_system + ΔEnergy_surroundings.
Internal Energy
Total energy (kinetic + potential) within a system.
Change in internal energy depends only on initial and final states (state function).
ΔE = E_final - E_initial.
ΔE_reaction = E_products - E_reactants.
Energy Flow Diagrams
Show direction of energy change during processes.
Internal energy increases with energy addition (+ΔE) and decreases with energy removal (−ΔE).
Energy exchange with surroundings via heat (q) or work (w).
Heat and Work in Energy Exchange
Heat (q): Exchange of thermal energy.
Work (w): Energy spent doing task; can be calculated as Work = External Pressure x Change in Volume (w = −PΔV).
Heat Capacity
Heat capacity (C): Energy needed to raise temperature of an object.
q = C x ΔT.
Depends on mass and material type.
Specific Heat Capacity
Specific heat capacity (Cs): Heat needed to raise 1g of substance by 1°C. Units: J/(g·°C).
Calculating Heat Energy Absorbed
Use q = (mass) x (specific heat capacity) x (temp. change).
Example Calculations
Heat Absorption: Calculate the heat absorbed by different materials based on specific heat and temperature changes.
Pressure-Volume Work
PV work occurs when volume changes against external pressure in gases.
Use the constant external pressure for calculations.
Breathing Work Example
Demonstrates pressure-volume work involved in human breathing.
Calorimetry at Constant Volume
Bomb calorimeter: Used to measure ΔE at constant volume (w = 0), thus ΔE = q.
Enthalpy, ΔH
Enthalpy = internal energy + pressure x volume (H = E + PV).
Change in enthalpy (ΔH): Heat released/absorbed at constant pressure.
Exothermic and Endothermic Reactions
Exothermic: ΔH < 0, heat released.
Endothermic: ΔH > 0, heat absorbed.
Summary of Key Concepts
Energy conversion, internal energy changes, heat exchange mechanisms, and thermodynamic principles are critical in understanding thermochemistry.