WorksheetsHess's Law
Total questions: 10
Worksheet time: 11mins
Heat of Reaction
The heat released after burning 1 mole of a substance.
The amount of heat needed to melt 1 mole of a solid.
The heat released by 1 mole of a substance as it changes from a liquid to a solid.
The heat released or absorbed during a chemical reaction.
Enthalpy (H)
The heat content of a system at constant pressure.
A chemical equation that includes the amount of heat released or absorbed during the reaction.
The heat released or absorbed during a chemical reaction.
The heat released after burning 1 mole of a substance.
Endothermic reactions feel
warm
cold
Exothermic reactions feel
warm
cold
Chemical reactions that release energy. The ΔH reaction is negative.
Specific Heat
Endothermic Reaction
Exothermic Reaction
Enthalpy
The enthalpy change for the reaction
C(s, graphite) + 1⁄2O2(g) --> CO(g) cannot be measured directly since some carbon dioxide is always formed in the reaction.
It can be calculated using Hess’s Law and the enthalpy changes of combustion of graphite and of carbon monoxide.
C(s, graphite) + O2(g) --> CO2 ΔH=-394 kJmol–1
CO(g) + 1⁄2O2(g) --> CO2 ΔH=-283 kJmol–1
The enthalpy change for the reaction of graphite with oxygen to give carbon monoxide is
-677 kJmol–1
-111 kJmol–1
+111 kJmol–1
+677 kJmol–1
The enthalpy change for the reaction, ∆Hr , is equal to
∆H1 + ∆H2
-∆H1 - ∆H2
∆H1 - ∆H2
-∆H1 + ∆H2
What is the relationship between enthalpies p, q, r and s?
S(s) + H2(g) -> H2S(g) ∆H=p
H2(g) + ½O2(g) -> H2O(l) ∆H=q
S(s) + O2(g) -> SO2(g) ∆H=r
H2S(g) + 1½O2(g) -> H2O(l) + SO2(g) ∆H=s
p = q + r – s
p = q + r – s
p = s – q – r
p = s – q – r
The enthalpies of combustion of C(s), H2(g) and C4H9OH(l) (in kJmol-1) are as follows
C(s) + O2(g) -> CO2(g) ∆H=a
H2(g) + ½O2(g) -> H2O(l) ∆H=b
C4H9OH(l) + 6O2(g) -> 4CO2(g) + 5H2O(l) ∆H=c
What is the enthalpy change for the reaction shown below?
4C(g) + 5H2(l) + ½O2(g) -> C4H9OH(l)
c – 4a – 5b
4a + 5b - c
2a + 10b - c
2a + 5b + c
The standard enthalpy changes of combustion of carbon, hydrogen and methane are shown in the table.
Which one of the following expressions gives the correct value for the standard enthalpy change of formation of methane in kJ mol–1?
C(s) + 2H2(g) → CH4(g)
394 + (2 × 286) – 891
–394 – (2 × 286) + 891
394 + 286 – 891
–394 – 286 + 891
