Thermochemistry Flashcards
Thermochemistry
Study of the relationship between chemistry and energy
Energy
The capacity to do work and the sum of both the work (w) done and the heat (q) generated or lost.
E = w + q
Work
Transfer of energy resulting from a force acting over a distance
Heat
Transfer of energy resulting from a temperature difference
SI unit: Joules
Law of Conservation of Energy
Energy can be neither created nor destroyed; it can be converted from one form to another
Kinetic energy
energy associated with motion
Thermal energy
energy associated with the temperature of an object
Potential energy
stored energy; energy associated with an object’s position or composition
Chemical energy
Chemical energy is a form of potential energy stored within the bonds of atoms and molecules.
It’s the energy that holds particles together in chemical compounds and is released or absorbed during chemical reactions.
When bonds between atoms are broken or formed, chemical energy is either consumed or released.
What does bond breaking require?
absorbing energy (endothermic)
What does bond forming require?
releasing energy (exothermic)
SI unit for heat and energy
Joules (J)
KJ to Joules conversion
1 KJ = 1000 J
Calorie (cal)
defined as the amount of energy needed to raise the temperature of 1 gram of water by 1 °C.
cal to joules conversion
kcal to joules conversion
1 cal = 4.184 J
1 kcal = 4184 J
Between what two frames of reference do energy transfers always occur?
System and Surroundings
System
The object under investigation
ex. In combustion reactions the system is considered the reactants and products
Surroundings
Everything else that can interact with and transfer energy with the system
ex. In combustion reactions the surrounding air, metal, torch
What is considered a key step in many thermochemical problems?
Specifically defining or identifying the system
Thermodynamics
The study of energy and its interconversions (converting from one type of energy to another)
First Law of Thermodynamics
Total energy of the universe is constant
Internal energy of a system (E)
The total energy of a system
Sum of PE and KE of all particles in a system
State function
Internal energy (E) is an example of a state function whose value is dependent on its current state.
The current state of the function is specified by parameters such as temperature, pressure, concentration, and physical state
Internal Energy Change Formula
E final - E initial
In a chemical equation
E products - E reactants
Are combustion reactions endothermic or exothermic?
Combustion reactions with substances (hydrocarbons usually) reacting with oxygen to form CO2 and H20, releasing energy in the form of heat and light thus it is exothermic
Energy is released when forming new bonds
Path function
DOES depend on the path that the system goes to achieve a certain state
What can be inferred if a change of E (-): E final < E initial
The energy is released by the system into the surroundings, making the final energy of it less than the initial energy
Specific heat capacity (Cs)
a measure of how much heat a system must absorb to undergo a specified change in temperature
quantity of heat required to change its temperature by 1 C
Units: J/g x C
Equation that quantifies the relationship between the amount of heat added to a given amount of the substance and the corresponding temperature increase.
What happens in Thermal Energy Transfer
When two substances of different temperatures are combined, thermal energy flows as heat from the hotter substance to the cooler one.
Assume the two substances are thermally isolated from everything else, then the heat lost by one substance exactly equals the heat gained by the other (according to the law of energy conservation).
Relationship is quantified as
The formula for measuring the heat absorbed by
the entire calorimeter assembly (qcal)?hea
Ccal is the heat capacity of the entire calorimeter assembly (which is usually determined in a separate measurement involving the burning of a substance that gives off a known amount of heat).
What is delta Ernx? How is change of total energy of specific reaction burnt (delta Erxn) found using the qrxn?
Delta Erxn is a measure
of the total energy change (both heat and work) that occurs during a reaction
To determine change in Erxn per mole, divide by the number of moles of the substance burned
What is enthalpy (H)? What is the formula?
The sum of internal energy and the product of pressure and volume of a system
Relationship between change in Enthalpy and heat at constant pressure
They are both equal to one another
Exothermic reaction (-delta H)
gives off heat to its surroundings
ex. Ethanol burning
What is Enthalpy change of a chemical reaction (Delta Hrxn) and how can it be found?
It is defined as the total heat content of a system at constant pressure.
an extensive property, one that depends on the amount of heat generated or absorbed when a chemical reaction occurs when a specific amount of reactants react.
The formula for measuring constant-pressure calorimetry
qsol is the heat absorbed by or lost from the solution
qrxn = -qsoln
as the insulated calorimeter prevents heat from escaping
What are the key differences between bomb calorimetry and coffee-cup calorimetry?
How do you find change of Entropy using bond energy
- Identify bonds broken and formed
- The change of entropy is the sum of the sums of the bond energies broken and bond energies formed
Standard state for
- gas
- liquid or solid
- substance in solution
Standard Enthalpy Change
Standard Enthalpy of Formation (or standard heat of formation) for
- A pure compound
- Purse element in its standard state
Definition: The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
