3.1.4 Energetics Flashcards
What is an exothermic reaction? (3)
As the reaction progresses, the temperature of the system increases
In order to return the system to its original state, temperature decreases
This means energy is lost to the surroundings so enthalpy change is negative
What is an endothermic reaction? (3)
As the reaction progresses, the temperature of the system decreases
In order to return the system to its original state, temperature must increase
This means energy is gained from the surroundings so enthalpy change is positive
What is enthalpy change of formation? (2)
The energy change when one mole of a compound is formed from its constituent elements in their standard states
The equation must be balanced so only one mole of the desired product forms
What is enthalpy change of combustion? (2)
The energy change when one mole of a substance is burned completely in oxygen under standard conditions
The equation must be balanced so only one mole of substance is burned
What are standard conditions? (2)
100 kPa (about 1 atm) pressure A stated temperature (usually 298K)
What is the equation for specific heat capacity?
q = mcΔT
q is heat energy (J)
m is mass (g)
c is specific heat capacity
ΔT is temperature change
How is change in enthalpy for one mole calculated?
ΔH = q / n
ΔH is change in enthalpy for one mole (J mol^-1)
q is heat energy (J)
n is number of moles
State Hess’s Law
The energy change of a reaction is independent of the route taken
How is temperature change calculated by extrapolation? (4)
Measure the temperature of one of the reactants for three minutes
At the fourth minute add the other reactant (do not measure temperature)
Continue to record temperature at one minute intervals for 10 minutes
Plot a graph and extend both lines to find a temperature change at four minutes
What is the change in enthalpy of formation for elements?
0
What is mean bond enthalpy?
The energy required to break one mole of a covalent bond in the gaseous state
What sort of process is bond breaking?
Endothermic
What sort of process is bond formation?
Exothermic
Why do values from mean bond enthalpy vary to Hess’s law calculations?
Bond enthalpy uses mean data that is an average across a range of compounds containing that bond
