Energy, Enthalpy, and Thermochemistry Flashcards
Energy
Capacity to do work/ produce heat
Law of conservation of energy
Energy can be converted from one form to another, but it can neither be created or destroyed
Thermochemistry
the study and measurement of heat evolved or absorbed during chemical reactions
2 types of energy
Kinetic: energy due to the motion of an object
Potential: energy due to position or composition
Temperature
Reflects the random motion of particles
Heat
energy transferred due to a temperature difference
Work:
Force acting over a distance
State function
a property of a system that depends only on its present state
I.e it is path independent
State function include
- internal energy
- temperature
- pressure
- Volume
- Enthalphy
Path-dependent properties include
- Work
- Heat
Exothermic reaction
Heat is released, heat in the product
Endothermic reaction
Heat is absorbed, Heat in the reactant
Internal energy
Sum of the kinetic and potential energy of all particles
Internal energy formula
change in E = q + w
q>0 E system increase (Endothermic)
q<0 E system decrease (exothermic)
Internal energy formula (work)
W>0 E system increase -> work is done to the system
W<0 E system decrease -> system performs work on the surrounding
Units of energy
J=Kgm^2/s^2
Heat = energy = J
Work = Energy = J
Common type of work in gases
w= -P x change in V
Expansion: (w < 0) work done by the gas
Compression: (w>0) work done on the gas
Expansion
(w < 0) work done by the gas
change in V > 0 -> w < 0
Compression
change in V < 0 -> w > 0
(w>0) work done on the gas
Enthalpy
Measure of heat
H= E+PV
A change in enthalpy is amount of heat (q) given off or absorbed at a constant pressure
Molar heat capacity
Energy required to raise the temp of 1 mol of that substance by 1K
3/2R = valid for monoatomic gas at constant volume
Constant pressur molar heat capacity (Cp)
Cp = (3/2)R + R = Cv + R = (5/2)R
3 ways of molecule can accommodate heat energy
- Translational
- Vibrational
- Rotational
Calorimetry
Science of measuring heat
Based on observations of temperature changes when a body absorbed or discharges energy as heat
Calorimeter
Device used to determine the heat associated with chemical reactions
Heat capacity
Represent the amount of energy necessary to increase the temp by 1 C or 1 K
= Heat absorbed/ Increase in temp
Two types of calorimeters ( constant pressure )
Measure change in enthalpy
Two types of calorimeters (Constant Volume)
Measure changes in internal energy
bomb calorimeter
Hess’s Law of constant Heat Summation
Indirect determination of change in H
H is a state function: path independent
Standard states
For a compound
- gas = 1atm
- Liquid/solid= pure substance
- solution = 1M
For element
- 1atm
- usually 25 C
