Enthalpy Flashcards
What is the symbol for delta, which represents change?
Triangle
What represents enthalpy?
H
What is kJmol^-1 the unit for?
How much enthalpy for each mole that reacts
Enthalpy profile diagram:-
Arrow along y axis labelled H. Each side of reaction on a line, product slightly across. Arrow up/down, touching lines between the two, labelled with the enthalpy change.
What is enthalpy change of reaction?
The enthalpy change when the number of moles shown in the equation react under standard conditions.
What indicates enthalpy change of reaction?
Delta r H
What is enthalpy change of combustion?
Enthalpy change when 1 mol of a substance burns completely in oxygen.
What are the products of combustion?
CO2 and H2O
What is enthalpy change of formation?
Enthalpy change when 1 mol of a compound is formed FROM ITS ELEMENTS.
What shows enthalpy change of formation?
Delta f H
What shows combustion?
Delta c H
What is enthalpy change of neutralisation?
Enthalpy change when 1 mol of H2O is formed in a neutralisation reaction (acid + alkali).
What shows enthalpy change of neutralisation?
Delta neut H
What are standard conditions?
1 atm pressure and 25oC (unless otherwise stated)
What is the equation for calculating enthalpy change from temp change?
q=MCdeltaT
Key:- q= J M= substance mass C= specific heat capacity deltaT = temp change (oC/K)
What is Hess’ law?
Enthalpy change in a reaction doesn’t depend on the pathway of the reaction, provided initial and final states are the same.
What does Hess’ law enable us to do?
Combine enthalpy change data to deduce enthalpy change that couldn’t be easily studied directly.
What is bond enthalpy?
Enthalpy change when 1 mole of bonds are broken in the gas phase
Why can’t method with XH2O simply as product (molecular) be used?
It doesn’t account for H2O’s intermolecular forces
What is the problem with bond enthalpy?
We use average bond enthalpies (measured in a variety of compounds and averaged) so it isn’t exactly correct for any specific compound so only gives an approx enthalpy change value.
How do you calculate total bond enthalpy?
Count number of bonds on each side and add enthalpies together.
How do you calculate reaction enthalpy change using bond enthalpies?
X - Y kJmol^-1
X = reactant total
Y= product total
Enthalpy change when exothermic/endothermic:-
Exo = negative Endo = positive
