Chem Chapter 5
(delta)H =
(Delta)(E+PV) = (Delta)E +P(Delta)V (P is constant) = q(p) + w - w = q(p) The p after q indicates that the process occurs at constant pressure. Thus, the change in enthalpy equals the heat q(p) gained or lost at constant pressure.
Pressure-volume work with constant pressure
(or P-V work) is work that is a result of a volume change against an external pressure. when gases expand, deltaV is +, but the system is doing work on the surroundings, so w is negative. w = -P * (delta)V Where w = work, P = pressure, and (delta) V = change in volume.
endothermic processes
+(delta)H: temp of surroundings will decrease
exothermic processes
-(delta)H: temp of surrounding will increase
1st law of thermodynamics (conservation of energy)
-energy is neither created nor destroyed -energy of the universe is constant -energy can be transferred between objects -energy can be transformed from one form to another (heat to light, kinetic energy to potential, or potential to kinetic)
atmospheric pressure
1 atm
1 J =
1 kg-m^2 / s^2 This is because KE = 1/2(mv^2) = 1/2 (2kg) (1m/s)^2 =1kg-m^2/s^2
guidelines for thermochemical equations and enthalpy
1. Enthalpy is an extensive property. The magnitude of (delta)H is proportional to the amount of reactant consumed in the process. 2. The enthalpy change for a reaction is equal in magnitude, but opposite in sign to (delta)H for the reverse reaction 3. The enthalpy change for a reaction depends on the state of the reactants and products
1. Radiant energy 2. Thermal energy 3. Chemical energy 4 Nuclear energy
1. comes from the sun and is earth's primary energy source (kinetic energy) 2. the energy associated with the random motion of atoms and molecules (kinetic energy) 3. the energy stored within the bonds of chemical substances (potential energy) 4. the energy stored within the collection of neutrons and protons in the atom (potential energy)
1 calorie =
4.184 J (exactly) (also the amount of energy needed to raise one gram of water by 1 degree Celsius)
state function
A function that depends only on the initial and final states of a system, not on the path in between. Internal energy (E), Pressure (P), Volume (V), and Enthalpy (H) are all state functions (along with delta of all of these).
endothermic
A process in which the system absorbs heat
closed system
A system that allows for the exchange of energy, but not matter, across its boundaries.
open system
An open system is a system that can freely exchange matter and energy with its surroundings.
q (heat) =
C X (delta)T C is the heat capacity ( J/degrees Celsius or J/Kelvin) delta T is the change in temperature of the system
kinetic energy
Energy of motion
potential energy
Energy stored due to an object's position or arrangement
work =
Force * Distance object moves
Work and is it extensive or intensive?
Force acting over a distance or force X distance. A force is any push or pull exerted on an object. Extensive
magnitude of kinetic energy (on eq sheet)
KE = 1/2 * (mv^2) KE = magnitude of kinetic energy m = mass v= velocity
molar heat capacity eq
Molar heat capacity = (quantity of heat transferred) / (number of moles X temperature change) C(m) = (q) / ((n) X (deltaT))
isolated system
No exchange of heat, work, or matter with surroundings
heat capacity
The amount of heat required to raise the temperature of a specified amount of a specified substance substance by 1°C or 1 K.
enthalpy of reaction or heat of reaction
The enthalpy change that accomponies a reaction, sometimes written (delta)H(rxn) where rxn is an abbreviation for reaction. By convention, we calculate enthalpy change for the number of moles of reactants in the reaction as written
Enthalpy
The heat content of a system at constant pressure H = E +PV Where H is enthalpy, E is internal energy, P is pressure, and V is volume
thermodynamics
The study of energy and its transformations
thermochemistry
The study of energy changes that occur during chemical reactions and changes in state
constant volume calorimetry with bomb calorimeter
When volume is constant, w=-p(delta)V v= 0 So w = 0 that means (delta)E = q + 0 This means, with a constant volume and insulated surroundings, we can directly measure (delta)E using q.
thermochemical equations
a balanced chemical equation that includes the physical states of all reactants and products and the energy change that accompanies the reaction
calorimeter
a measuring instrument that determines quantities of heat
exothermic
a process in which the system loses heat
algebraic expression of 1st law of thermodynamics (deltaE =)
change in internal energy of system = q + w w = work q= heat
any change is always...
directional. It is always final minus initial. For example, the change in energy is always final energy - initial energy
Is enthalphy of reaction an extensive or intensive property?
extensive, the more reactants you use the larger the magnitude of the enthalpy change
non state functions
heat and work
heat capacity equation
heat capacity = quantity of heat transferred / change in temperature C = q / (delta)T
Internal energy, E, of a system...
is the sum of all the kinetic and potential energies of the components of the system
change is enthalpy is always given in
kJ
calorimetry
measurement of heat flow
change in energy of system =
negative change in energy of surroundings because change in internal energy = final energy (energy of reactants) - initial energy (energy of products) the surrounding then either gets an equal amount of energy taken away from it (if final energy is greater than initial) or added to it (if final energy is less than initial) energy flow from the system to the surroundings is always negative (loss of energy from the system) energy flow from the surroundings to the system has a positive sign (gain of energy to the system)
calculating q given enthalpy of reaction
number of moles of given substance X enthalpy of reaction/coef of given substance from balanced equation
system in a reaction
reactants and products
formation reactions
reactions of elements in their standard state to form 1 mole of a pure compound -If you are not sure what the standard state of an element is, find the form in the appendix that has a standard change in enthalpy = 0 for that element. -Since the definition requires one mole of compound be made, the coefficients of the reactants may be fractions
fusion
solid to liquid phase change
calculating q given specific heat
specific heat = (quantity of heat transferred) / (grams of substance X temperature change) OR C(s) = (q) / ((m) X (delta)T) Therefore q= (m) X (Cs) X (delta)T
standard enthalpy change of reaction from tabulated standard enthalpy of formation values equation
standard enthalpy of reaction = the sum of all ( n * standard enthalpy of formation of products) - the sum of all (n * standard enthalpy of formation of reactants) where n is the number of moles of a certain product or reactant in the balanced equation.
standard state
state of material at a defined set of conditions pure gas at exactly 1 atm pure solid or liquid in its most stable form at exactly 1 atm pressure and temperature of interest (usually 25 degrees celcius)
difference between temperature and heat
temperature is a measure of average thermal energy of the atoms in a substance and is an Intensive property. Heat is the transfer of thermal energy between two bodies that are a different temperature and is an extensive property. For example, if you were to heat 2 containers of water, one much larger than the other, to 100 degrees Celsius, they would both have the same temperature, but the larger container would have much higher thermal energy. This is because the amount of energy that had to be transferred to the larger container to raise it to 100 degrees celcius is larger than the amount of energy that had to be transferred to the smaller container. another way to think of it is, (sum of the energies of each atom)/ number of atoms = temperature. So, for the larger container to be 100 as well, the sum of the energy of the atoms must have been grater in the larger container.
energy
the capacity to do work or transfer heat
Hess's law
the change in enthalpy for a stepwise process is the sum of the enthalpy changes of the steps
standard enthalpy change of formation
the enthalpy change for a reaction forming one mole of a pure compound from its constituent elements - the elements must be in their standard states -the change in elthalpy of formation for a pure element in its standard state is 0 kJ/mol
standard enthalpy change
the enthalpy change when all reactants and products are in their standard states
specific heat
the heat capacity of one g of a substance
molar heat capacity
the heat capacity of one mole of a substance
heat and is it extensive or intensive?
the transfer of thermal energy between two bodies that are at different temperatures. Extensive property
reactions done in aqueous solutions open to the atmosphere are
under constant pressure
units obtained by P-V work and conversion to joules
units of pressure (usually atm) multiplied by units of volume (usually liters). To express work in joules, the conversion factor 1 L-atm = 101.3 J is useful.
