Various Enthalpy Change Definitions
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1C h e m g u i d e – q u e s t i o n s
VARIOUS ENTHALPY CHANGE DEFINITIONS
1. This question is about the word “standard” as in standard conditions and standard states.
a) What do you understand by standard conditions as applied to:
(i) temperature;
(ii) pressure;
(iii) concentrations of solutions?
b) What are the standard states for the following substances?
(i) oxygen
(ii) hydrogen
(iii) H
O
2
(iv) CO
2
(v) carbon
(vi) NaCl
(vii) sodium
(viii) CH
4
(ix) bromine
(x) ammonia
2. a) Define the term enthalpy change.
b) Define the term standard enthalpy change of reaction, using the following equation as an
illustration:
-1
2C
H
+ 7O
4CO
+ 6H
O
ΔH = -3120 kJ mol
2
6(g)
2(g)
2(g)
2
(l)
c) This reaction shows the combustion of ethane, but the standard enthalpy change of combustion of
ethane is not -3120 kJ mol
-1
. Define the term standard enthalpy change of combustion, and explain
why the standard enthalpy change of combustion of ethane isn't -3120 kJ mol
.
-1
3. a) Define the term standard enthalpy change of formation.
b) Write equations for the reactions for the enthalpy changes of formation for the following
compounds. State symbols are essential.
(i) H
O
2
(l)
(ii) C
H
3
8(g)
(iii) NaCl
(s)
(iv) C
H
OH
2
5
(l)
(v) NH
Cl
4
(s)
(vi) Na
SO
2
4(s)
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