x Flashcards
enthalpy, H
measure of the heat energy in a chemical system
enthalpy change, ∆H
difference on enthalpy change
write the equation to find ∆H
∆H= H(products)- H(reactants)
law of conservation of energy
energy cannot be created or destroyed
where is the surrounding?
where the thermometer is
what is an exothermic change?
from the system to the surroundings
what is an endothermic change?
from the surroundings to the system
what is an activation energy?
minimum energy required for a reaction to take place
what is a standard pressure?
100kPa
what is the standard temperature?
25℃ and 298K
what is the standard concentration?
1 moldm⁻³
standard enthalpy change of formation definition
formation of 1 mol of a compound from its constituent elements, under standard conditions
standard enthalpy change of combustion definition
enthalpy change when 1 mole of substance is burnt in excess oxygen, under standard conditions
standard enthalpy change of neutralisation definition
formation of 1 mol of water from neutralisation, under standard conditions
how do you determine the temperature change of the surroundings?
∆T=T(final)- T(initial)
write the equation to calculate an energy change
q=mc∆T
why are combustion reactions less exothermic that expected?
- heat loss to the surroundings
- incomplete combustion of reactants
how do you find the enthalpy change by using the energy change?
∆H=q/n
what other factors could lead to an inaccurate ∆H result
non- standard conditions
average bond enthalpy definition
breaking of 1 mol of bonds in gaseous molecules
bond breaking
endothermic
bond making
endothermic
how to calculate the enthalpy change of reaction
∆H=Σ(bond enthalpies in reactions)- Σ(bond enthalpies in products)
what is the limitation of average bond enthalpies?
- actual energy involved in breaking and making individual bonds would ne slightly different
- bonds may be in different environments
how to work out the enthalpy change of formation
A=C-B
how to work out the enthalpy change of combustion
A=B-C
