Chemical Energy

Question 1

What is the enthalpy of combustion?

Answer 1

The energy released when 1 mole of a substance burns completely in oxygen

(measured in kJ mol^-1)

Question 2

What does Hess’s Law state?

Answer 2

That the enthalpy change of a chemical reaction does not depend on the route taken during the reaction

Question 3

According to Hess’s Law…

Answer 3

△H1 = △H2 + △H3

Question 4

What are formation reactions?

Answer 4

Chemical reactions that form one mole of a substance from its constituent elements in their standard states

For example, the formation of methane CH4 -
C(s) + 2H2(g) –> CH4(g)

Question 5

What do the letters in the formula Eh=mc△t stand for?

Answer 5

Eh = Heat energy released
m = Mass of water (in grams)
c = 4.18
△t = Change in temperature

Question 6

What are bond enthalpies?

Answer 6

The energies required to break 1 mole of a particular bond between a pair of atoms in gaseous state

Question 7

What is molar bond enthalpy?

Answer 7

The energy required to break one mole of bonds in a diatomic molecule

Question 8

What is a mean molar bond enthalpy?

Answer 8

The average energy required to break 1 mole of bonds, for a bond that occurs in a number of compounds

Question 9

What are bond enthalpies used for?

Answer 9

To estimate the enthalpy change occurring for a gas phase reaction

Question 10

What formula is used to calculate △H from bond enthalpies?

Answer 10

△H = E△H bonds broken + E△H bonds made