Chapter 6 Quiz Flashcards
Thermochemistry
The study of the relationship between chemistry & energy
Work
The result of a force acting through a distance
Heat
The flow of energy caused by temp. difference
Kinetic energy
The energy associated with the motion of an object
Thermal energy
The energy associated with the temperature of an object
Potential energy
The energy associated with the position or composition of an object
Chemical energy
The energy associated with the relative positions of electrons & nuclei in atoms & molecules (form of potential energy)
Law of conservation of energy
States that energy can neither be created nor destroyed
System
A set of substances & energy that are being studied
Surroundings
Everything with which the system can exchange energy
KE =
1/2mv^2
1/2(mass)(velocity^2)
1kg m^2/s^2
1 joule (J)
calorie (cal)
4.184 J
Calorie (Cal)
4184 J
Joules
SI unit of energy
First law of thermodynamics
The total energy of the universe is constant / does not change
Internal energy
The sum of kinetic & potential energies of all of the particles that compose the system. It is a state function
State function
The value depends only on the state of the system
ΔE =
E(final) - E(initial)
E(products) - E(reactants)
q + w OR heat + work
When is ΔE positive?
When energy is absorbed (endothermic reaction)
When is ΔE negative?
When energy is released
Thermal equilibrium
There is no additional net transfer of heat
Heat capacity (C)
A measure of the system’s ability to absorb thermal energy without undergoing a large change in temp OR the amount of heat required to change its temperature by 1 degree C.
Specific heat capacity (Cs)
The amount of heat required to raise the temp of 1 gram of substance by 1 degree C
Molar heat capacity
The amount of heat required to raise the temp of 1 mol of a substance by 1 degree C
q =
m times Cs times ΔT
Heat (J) = mass (g) times Specific heat capacity (J/gC) times Temp change (degrees C)
Pressure-volume work
Occurs when force is caused by a volume change against an external pressure
w =
f times d OR force times distance
p times a times d
p times a times Δh
p times ΔV
ΔE(rxn) =
qv+w
Calorimetry
measuring the thermal energy exchanged between the reaction (or system) and the surroundings by observing the change in temp of the surroundings
Bomb calorimeter
Piece of equipment designed to measure ΔE for combustion reactions
q(cal) =
C(cal) times ΔT
C(cal) = heat capacity of the entire calorimeter assembly
Enthalpy
The sum of a system’s internal energy & the product of its pressure & volume
ΔH =
ΔE + PΔV
the heat at constant pressure
Endothermic reaction
A chemical reaction that absorbs heat from its surroundings
Exothermic reaction
A chemical reaction that releases heat to its surroundings
When is ΔH positive?
When it’s endothermic
When is ΔH negative?
When it’s exothermic
Enthalpy of reaction / Heat of reaction
Extensive property - depends on the amount of material undergoing the reaction. The enthalpy change for a chemical reaction
Coffee-cup calorimeter
Consists of 2 styrofoam coffee cups, one inserted into the other, to provide insulation from the laboratory equipment. Equipped with a thermometer & stirrer
Compare & contrast bomb calorimeters & coffee-cup calorimeters
Bomb calorimeters & coffee-cup calorimeters both measure changes in temp. However, bomb calorimeters measure ΔE & have constant volume while coffee-cup calorimeters measure ΔH(rxn) & have constant pressure
Relationships involving ΔH(rxn)
- If a chemical equation is multiplied by the same factor, then ΔH(rxn) is also multiplied by the same factor
- If a chemical equation is reversed, then ΔH(rxn) changes signs
- If a chemical equation can be expressed as the sum of a series of steps, then ΔH(rxn) for the overall equation is the sum of the heats of reactions for each step
Hess’s Law
The change in enthalpy for a stepwise process is the sum of the enthalpy changes of the steps
Standard state
Pure gas at a pressure of exactly 1atm
Pure liquid or solid in its most stable form at a pressure of 1atm & at the temp of interest (usually 25 degrees C)
Substance in solution at a concentration of exactly 1M
Standard enthalpy change (ΔH w/ degree sign)
The change in enthalpy for a process when all reactants & products are in their standard states. The degree sign indicates standard states
Standard enthalpy (heat) of formation (ΔH(f) w/degree sign)
ΔH(f) w/ degree sign = 0 for a pure element in its pure state.
For a pure compound: The change in enthalpy when 1 mol of the compound forms its constituent elements in their standard states
ΔH (w/ degree sign) (rxn)
Sum of n(p) times ΔH (w/ degree sign) (products) - Sum of n(r) times ΔH (w/ degree sign) (reactants)
List 3 connections between energy use & the environment
- Ozone production
- CO2
- Solar panels
