6. General Chemistry 2 Flashcards
convection
movement of heat from hotter to colder portions
radiation
electromagnetic waves emitted from a hot body into the surrounding environment,
light colors radiate and absorb less, dark colors radiate and absorb more
black body radiator
theoretical perfect body that absorbs all energy incident upon it, and then emits 100% of this energy as electromagnetic radiation
conduction
molecular collisions carry heat along a conduit
heat capacity
about of energy (in joules or Calories) a system must absorb to give a unit change in temperature (J/K or cal/C)
C=q/delta T
constant volume
systems confined into rigid walls
constant pressure
open to atmosphere
What happens when heat enters a system? Does the temperature always increase? Is any increase in temperature always exactly proportional to the heat absorbed by that system? (Hint: Think of adding energy to a sealed steel container vs. adding energy to a balloon; remember that temperature is the average kinetic energy of the molecules, but an increase in temperature is NOT the only “place” where added energy can go.)
This is our first foray into pv work. PV work is the work necessary to produce an increase in volume. For example, when a sealed balloon is heated, the gases inside the balloon will expand and must do work on the rubber walls of the balloon and the air around it to accomplish this expansion. Because some of the heat energy added to the balloon was used for pv work, only the remaining portion of the heat will go toward increasing the average kinetic energy of the molecules (i.e., temperature). So, when heat enters a system, if the system is capable of volume change, heat can go to either pv work, increased temperature, or both. For this reason, the addition of a certain amount of heat will NOT necessarily be exactly proportional to the resultant increase in temperature. If the system is not capable of changing volume then no pv work can be done, so all of the added heat will go toward an increase in temperature.
For the same system, which heat capacity will be greater, the constant volume heat capacity or the constant pressure heat capacity?
If the volume is held constant, then 100% of the energy added will go toward an increase in temperature. If the pressure is held constant the volume can still change and therefore some of the added heat will go toward pv work. If we think of heat capacity as “the amount of energy we can add before the system increases by one temperature unit,” it is fairly easy to see that the system capable of pv work will be able to absorb more heat before increasing by one degree Celsius or Kelvin. It is much like asking how many gallons of water can be added to Tank A vs. Tank B? Tank A and Tank B are both 5-gallon tanks, but Tank B is connected via a hose to a reserve tank that holds 2 gallons. So, you can add 5 gallons to Tank A before it is “full” (analogous to a one unit increase in temperature). However, you can add 7 gallons to Tank B before it is full (the reserve tank being analogous to pv work). We would therefore say that Tank B has the higher “water capacity” in terms of our analogy. This indicates that the constant pressure heat capacity (allows for pv work; i.e., includes the 2-gallon reserve tank) will always be more than the constant volume heat capacity (does not allow for pv work; i.e., no reserve tank) for the same system.
specific heat capacity
Above we defined Heat Capacity as the energy absorbed/unit change in temperature for a system. This “system” could include a solution, the container holding the solution, and even a thermometer or stirring rod. Specific Heat Capacity, however, describes energy absorption for one individual substance only and is defined per unit mass. Little “c” is used instead of big “C”.
c=q/mdeltaT ==> q=mcdeltaT
specific heat of water
1.0 cal/gCelsius or 4.18 J/gCelsius
Beakers 1 and 2 contain 0.25 L of water and 0.5 L of water, respectively. How does the heat capacity of the water in Beaker 1 compare to that of the water in Beaker 2? How do the specific heat capacities compare?
The heat capacity of Beaker 2 will be greater than that of Beaker 1 because there is more water available to absorb heat in Beaker 2. However, the specific heat capacity of water in both beakers will be identical – specific heat capacity is an intensive property.
calorimeters
device used to calculate enthalpy change (deltaH), we assume q is equal to delta H
coffee cup calorimeter
q=mcdeltaT
bomb calorimeter
solve using q=CdeltaT, does NOT give enthalpy, but change in internal energy, deltaU or deltaE, ising heat capacity (C) instead of specific heat capacity (c)
Which type of calorimeter provides constant pressure and which type provides constant volume?
The bomb calorimeter is a sealed steel container, meaning volume cannot change. However, because volume is constant any gases produced or consumed during the reaction will change the pressure. The coffee cup calorimeter allows for an increase in volume of the solution inside the coffee cup, but remains at atmospheric pressure throughout. The bomb is constant volume, the coffee cup constant pressure.
Pressure-Volume (PV) work
work is energy transfer via force (physics) or via change in volume at constant pressure (chemistry)
PV Work = P delta V
requires constant pressure, any change in volume tells you there is pv work
pressure vs. volume graph
exponential graph, with area under the curve being work
Which of the calorimeters described above allows for pv work?
The coffee cup allows for pv work because it allows for a change in volume.
the First Law of Thermodynamics
energy cannot be created or destroyed (work done ON a system is POSITIVE, work done BY a system is NEGATIVE) (note:in physics we note that force and displacement is in the same direction then work is positive, if they are opposing than it is negative)
deltaE = q +w
energy change in a closed system is equal to the heat absorbed by that system plus any work done on that system by its surroundings
the Second Law of Thermodynamics
heat cannot be changed completely into work in a cylindrical process, entropy in an isolated system can never decrease.
the Third Law of Thermodynamics
Pure crystalline substances at absolute zero have an entropy of zero
the Zeroth Law of Thermodynamics
If object A is in thermal equilibrium with object B, and object C is also in thermal equilibrium with
object B, then objects A and C must be in thermal equilibrium with each other. (A = B and B = C, therefore A = C) (note: Everything tends to move toward thermal equilibrium with everything else. Objects with higher temperatures will always equilibrate over time with their surroundings, including other objects with which they are in contact. Finally, if two objects are in thermal equilibrium, by definition they have the same temperature.)
temperature
average kinetic energy of the molecules
kinetic energy
KE = 3/2(kb) T, kb= boltzmann’s constant, shows direct relationship between temperature and KE)
temperature conversions
F -> C = (5/9)(F-32)
C -> F = (C+(9/5))+32
C- > K = C+273.15
Enthalpy (∆H)
the energy contained within chemical bonds (Joules)
standard state
A set of specific conditions chosen as the reference point for measuring and reporting enthalpy, entropy, and Gibbs free energy. You may remember seeing these conditions specified at the top of a table of ∆H values. When values are given for standard state conditions a superscript, called “naught,” is added that resembles a degree symbol (∆H°). standard temperature and pressure, 25C or 298K
STP
standard temperature and pressure, 0C or 273K
Elements in their standard state have
Hformation° = zero. i.e., Gformation° = zero for elements, but the standard state entropy of elements is not zero.
