lecture 2 - enthalpy & energy
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42 Terms
what is the equation for internal energy?
change in internal energy = heat + work
what is the equation for pressure-volume work?
work = -(pressure) (change in volume)
what does work represent in physics terms?
a force acting through a distance
how is work applied to gases in chemistry?
work occurs when a gas expands or contracts against an external pressure
what is pressure-volume work?
work associated with changes in volume under constant external pressure
what happens to work when a gas expands?
work is negative because the system does work on the surroundings
what happens to work when a gas contracts?
work is positive because work is done on the system
what is the sign of ∆V during expansion?
positive
what is the sign of ∆V during compression?
negative
what unit conversion is commonly used for work?
1 liter-atmosphere = ~101.3 J
what is heat capacity?
the amount of heat required to raise the temperature of a substance by one degree Celsius
what is specific heat capacity?
the heat capacity of one gram of a substance
what is molar heat capacity?
the heat capacity of one mole of a substance
what equation relates heat, mass, and temperature change?
q (heat) = (mass) (specific heat) (change in temperature)
what equation uses moles instead of mass for heat?
heat = (moles) (molar heat capacity) (change in temperature)
what are the units of specific heat capacity?
joules per gram per degree Celsius
what determines how much temperature changes for a given heat input?
the specific heat capacity of the substance
if two substances receive the same heat, which one changes temperature more?
the one with the lower specific heat capacity
how does heat capacity relate to intermolecular forces?
stronger intermolecular forces generally increase heat capacity
what is calorimetry?
the measurement of heat changes during chemical or physical processes
what is a coffee up calorimeter?
a constant pressure calorimeter used to measure heat flow in reactions in a solution
under constant pressure, what does heat equal?
heat equals the change in enthalpy
what is enthalpy? (∆H)
a state function defined as internal energy + pressure-volume work
what does ∆H represent at constant pressure?
the heat transferred at constant pressure
what is an exothermic reaction?
a reaction that releases heat to the surroundings
what is an endothermic reaction?
a reaction that absorbs heat from the surroundings
in an exothermic reaction, what is the sign of ∆H?
negative
in an endothermic reaction, what is the sign of ∆H?
positive
what is a bomb calorimeter?
a constant volume calorimeter used to measure heat released in reactions
what condition defines a bomb calorimeter?
volume is constant, so no pressure-volume work is done
what happens to work in a bomb calorimeter?
work is zero because there is no change in volume
under constant volume, what does heat equal?
heat equals the change in internal energy
what is the relationship between system and surroundings in calorimetry?
heat lost by the system equals heat gained by the surroundings
what contributes to heat absorbed by the surroundings in a bomb calorimeter?
both the water and the calorimeter itself
what equation is used for heat in a bomb calorimeter?
heat = (mass) (specific heat) (temperature change + calorimeter constant) (temperature change)
why is water commonly used in calorimetry?
because it has a relatively high specific heat capacity
why must water be stirred in a bomb calorimeter?
to ensure uniform temperature throughout the system
are calorimeters perfectly insulated?
no, there is always some heat loss, so measurements are approximations
what is the first law of thermodynamics in terms of system and surroundings?
the change in energy of the universe is zero, so energy lost by the system is gained by the surroundings
what happens in a closed system at constant volume?
no pressure-volume work occurs, so heat equals change in internal energy
what happens in an open system at constant pressure?
heat = ∆H
what is the relationship between enthalpy and internal energy?
∆H = ∆U + pressure-volume work