Energetics Flashcards
(18 cards)
Define Enthalpy change
The amount of heat energy transferred in a reaction under constant pressure
Give an example of an exothermic reaction
Combustion of fuel
Give an example of an endothermic reaction
Thermal decomposition
Define the Enthalpy change of formation
The Enthalpy change when one mole of a compound is made from its elements in their standard states under standard condition, 100KPa 298K
What is the Enthalpy of formation of an element
0
Define Enthalpy change of combustion
The Enthalpy change when one mole of a substance is combusted completely in oxygen under standard condition, with all reactant and products being in their standard states
How does incomplete combustion affect exothermic value
Reaction is less exothermic
If there is a temperature increases , what sign will Enthalpy change have
Negative sign
What mass is used in q=mct equation of both reactants are solutions
Total volume of the solutions
Define Hess’s law
The Enthalpy change for a reaction is in dependant of the route taken
How doo you find the Enthalpy change of a reaction
Enthalpy formation ( products) - Enthalpy formation ( reactants)
Define mean bond energy
The Enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules
Why are mean bond energy values positive
It requires energy to break a bond
Is energy taken in or out when a bond is made
Out
When does the definition of mean bond energy apply
When the substances start and end in the gaseous state
How do you calculate Enthalpy change when given bond data
Enthalpy change = bond energies BROKEN - bond energies MADE
Why are Enthalpy change values using bond energy data less accurate than using formation/combustion data
The mean bond energies are not exact
Explain how does Enthalpy of combustion change for successive members of a homologous series
There is a constant rise as the number of carbon atoms increases
Explain how experimental values of Enthalpy of combustion are different from theoretical values
They’re lower due to heat loss and incomplete combustion
