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
What can be inferred if a change in E (+): E final > E initial
The energy is absorbed by the system from the surroundings because the final energy of the system is greater than the initial energy.
What are the two main mechanisms of changing the IE of a system
Energy transfers of Heat (q) and Work (w)
Energy transfers of heat (q) if +q and -q
(+)q: System absorbs heat from surroundings
(-)q: System releases heat into surroundings
Energy transfers of work (w) if +w and -w
(+)w: Work is done ON the system BY the surroundings
(-)w: Work is done BY the system ON the surroundings
Mathematical representation of the first law of thermal dynamics
Change in E = P + W
How can the thermal energy of a system be changed?
By transferring energy as heat
Adding or removing heat will lead to a change in temperature
What does the magnitude of the change in T also depend on?
The amount of matter within the system and the intrinsic the capability of the matter to absorb heat
Thermal equilibrium
When the surroundings and system reach the same temperature and there is no additional net transfer of heat
What is the relationship between q (amount of heat transferred) and delta T (change in temperature)
They are proportional to one another (as one increases the other increases)
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
Molar heat capacity
The heat required to raise the temperature of 1 mole of a substance by 1 C° .
What substance has the highest specific heat capacity
Water with a specific heat capacity
Equation that quantifies the relationship between the amount of heat added to a given amount of the substance and the corresponding temperature increase.
Work formula
Pressure x Change in Volume
Heat capacity (C) formula
C = q/delta t
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
What is calorimetry
A method of measuring the thermal energy of the reaction (defined as the system)
and the surroundings exchange by observing the change in temperature of the
surroundings.
Constant-Volume Calorimetry
Conducted in a sealed, rigid container (bomb calorimeter).
Measures the internal energy change of reactions, commonly for combustion reactions.
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 does qcal mean?
quantifies the heat absorbed or released by the calorimeter components, including the water and the calorimeter itself.
What is qrxn and its relationship with qcal
qrxn is the heat released or absorbed as a result of the system releasing or absorbing.
ex. If the system absorbs, its qcal is + and the surroundings releasing is -qrxn
If not heat escapes from the container
qcal = -qrxn
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
Endothermic reaction (+delta H)
absorbs heat from its surroundings
ex. Cool compress absorbing heat
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?
What are the 3 relationships involving Change in Enthalpy (Delta Hrxn)
- If a chemical equation is multiplied by some factor, then 횫Hrxn is also multiplied by the same
factor. - If a chemical equation is reversed, then 횫Hrxn changes sign.
- If a chemical equation can be expressed as the sum of a series of steps, then ∆Hrxn for the overall
equation is the sum of the heats of reactions for each step. (Hess’s law)
Hess’s law
The change in enthalpy for a stepwise process is the sum of the enthalpy changes of the steps.
You find delta Hrxn (change in enthalpy) using known change of enthalpy values
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
How do we know if a reaction is exothermic based on its bond behavior?
A reaction is exothermic when weak bonds break and strong bonds form
How do we know if a reaction is endothermic based on its bond behavior
When strong bonds break and weak bonds form
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
What is heat of formation?
The heat of formation (ΔHf∘ΔH f∘) of a compound is defined as the enthalpy change for the reaction that forms 1 mole of the compound from its elements in their standard states under standard conditions
