Enthalpy Changes Flashcards
(17 cards)
Enthalpy change
The heat energy transferred in a reaction at constant pressure. The units of enthalpy change are kJmol-1
Symbol for enthalpy change
ΔH
Is enthalpy affected by standard conditions
Enthalpy changes under standard conditions
What is the symbol for the enthalpy change under standard conditions
ΔH°
Exothermic
Reactions give out energy, ΔH is negative, temperature increased
Oxidation (combustion) is it endo or exo
Exothermic
Endothermic
Reactions absorb energy, ΔH is positive, temperature decreases
Thermal decomposition is it endo or exo
Endothermic
How are mentally changes calculated
Using average bond enthalpies
=total energy absorbed to break bonds/total energy released in making bonds
Positive ΔH
More energy to break bonds than is released when bonds are made
Negative ΔH
Less energy to break bonds than is released when bonds are made
Are average bond enthalpies exact (H2O)
No
First bond in H2O: H-OH=+492
Second bond: H-O=+428
So average is +460
Average bond enthalpy
The energy needed to break one mole of bonds in the gas phase, average over many different compounds
What is bind enthalpy related to
Length of the bond
Bond length
The distance between two nuclei is the distance the attractive and repulsion forces balance each other in a covalent bond
Higher bond enthalpy
Stronger the attractions between atoms, the shorter the bond length
Lower bond enthalpy
Weaker attraction between molecules, longer the bond length
