Chapter 6: Thermochemistry Flashcards
What is enthalpy (H)
A state function that is defined as:
H = E + PV
E: Internal energy of the system
P: Pressure of the system
V: Volume of the system
What is true about the change in enthalpy at constant pressure
ΔH = q p
What is the formula to calculate the enthalpy change in a chemical reaction?
/\H = Hproducts-Hreactants
What happends when:
1. /\H = Hproducts is greater than Hreactants
2. /\H = Hproducts is smaller than Hreactants
the sign, and what happends
- /\H is positive (Heat is absorbed by the system - reation is endothermic)
- /\H is negatve (overall decreased in enthalpy is achieved by the generation of heat - reation is exothermic)
What is calorimetry (the science of measuing heat) based on?
Based on observations of temperature change when a body absorbs or discharges energy in the form of heat
what is a calorimiter
Device used to determine the heat associated with a chemical reaction
What is the formula for heat capacity
C = heat absorbed / increase in temperature
What is specific heat capacity
Energy required to raise the temperature of one gram of a substance by one degree Celsius
What is molar heat capacity
Energy required to raise the temperature of one mole of a substance by one degree Celsius
What is notable about the conditions for constant pressure calometry (ie. a coffe cup calorimeter)
Atmospheric pressure remains constant during the process (*ΔH = q p )
in constant pressure calorimetry, when two reactants at the same temperature are mixed, what happends to the solution (exothermic vs endothermic)
- An exothermic reaction warms the solution
- An endothermic reaction cools the solution
What is the calculation of heat for a neutralization reaction
s x m x Δt
what does it mean to say that heat of a reation is an extensive product
it depends entirely on the amount of substance
When do you use constant-volume calorimetry
Used in conditions when experiments are to be performed under constant volume
is work done in constant volume calorimetry? Why or why not
No work is done since V must change for PV work to be performed
How does a bomb calorimeter function, and what kind of calorimeter is is?
(Constant volume calorimetry)
- Weighed reactants are placed within a rigid steel container and ignited
- Change in energy is determined by the increase in temperature of the water and other parts
What is the formula for constant-volume calorimetry process
(ΔV = 0)
therefore, w = -PΔV = 0
ΔE = ΔT × heat capacity of the calorimeter
Concerning Hess’s law, what do you need to keep in mind?
(2)
- If a reaction is reversed, the sign of ΔH is also reversed
- If the coefficients in a balanced reaction are multiplied by an integer, the value of ΔH is multiplied by the same integer
What is the Standard Enthalpy of Formation (ΔHf)
Change in enthalpy that accompanies the formation of one mole of a compound from its elements with all substances in their standard states
What is the standard state?
Precisely defined reference state
What is the standard state for a gaseous substance?
Standard state for a gaseous substance is a pressure of exactly 1 atm
What is the standard state for a pure substance?
the standard state is the pure liquid or solid
What is the standard state for a substance in solution?
For a substance in solution, the standard state is a concentration of exactly 1M
What is the Conventional Definitions of Standard States for an Element
Standard state of an element is the form in which that element exists under conditions of 1 atm and 25 C
ex. standard state of oxygen: O2(g)
What is the ΔHf for an element in its standard state
zero
How many pairs of electrons are shared in each type of bond
1. Single bond
2. Double bond
3. Tripple bond
- Single bond : One pair of electrons is shared
- Double bond : Two pairs of electrons are shared
- Triple bond : Three pairs of electrons are shared
What happends to the bond length with the increse in number of shared electrons?
it shortens
