Thermochemistry Exam 2 and Problem Solutions

Thermochemistry Exam 2 and Problem Solutions

1. Which ones of the following statements must be known to find enthalpy of ;

CO2(g) + H2(g) → CO(g) + H2O(g)

I. Molar formation enthalpy of H2O(g)

II. Molar formation enthalpy of  CO(g) and CO2(g)

III. Molar combustion enthalpy of C(s) + O2(g) → CO2(g)


Enthalpy of given reaction is found by;

∆H=[∆HCO + ∆HH2O] - [∆HCO2 + ∆HH2]

Since enthalpy of H2 is zero, we must know molar formation enthalpies of CO2(g), CO(g) and H2O(g).

2. During reaction of formation Al2O3 from 5,4 g Al and enough amount of O2, temperature of 2 kg water increases 20 0C. Find formation enthalpy of Al2O3 ? (Al=27, cwater=1 cal/g.0C)


Amount of heat required for increasing temperature of 2 kg water 20 0C is;


Q=2000g.1 cal/g.0C. 20 0C

Q=40000 cal=40 kcal

2Al + 3/2O2 → Al2O3

Energy released from combustion ıf 2mol Al (54 g) gives formation enthalpy of Al2O3.

If 5,4 g Al gives 40 kcal heat

54 g Al gives ? kcal heat


?= 400 kcal

Since reaction is exothermic, formation enthalpy of Al2O3 is -400kcal.

3. Enthalpies of two reaction are given below.

I. A + B → C + 2D ∆H1=+X kcal/mol

II. C + E → A + F  ∆H2=-Y kcal/mol

Find enthalpy of A + 2B + E → C + 4D + F reaction in terms of X and Y.


To get this reaction A + 2B + E → C + 4D + F; we should multiply first reaction by 2 then sum it up with second reaction.

2A + 2B → 2C + 4D ∆H1=+2X kcal/mol

+  C + E → A + F  ∆H2=-Y kcal/mol


A + 2B + E → C + 4D + F ∆H3=2X-Y

4. C(s) reacts with O2(g) and after reaction, 8,96 L CO2 gas is formed and 37,6 kcal heat is released. According to this information, which one of the following statements are true? (C=12, O=16)

I. Reaction is exothermic

II. 94 kcal heat is required to decompose CO2(g) into its elements

III. 23,5 kcal heat is required to form 11g CO2(g)

IV. Sum of enthalpies of products is smaller than sum of enthalpies of reactants


I. Since heat is released, reaction is exothermic. I is true.

II. Number of moles of CO2(g);


During formation of 0,4mol CO2, -37,6 kcal heat is released

During formation of 1mol CO2,  ? kcal heat is released


?=-94kcal heat is released

Since -94kcal heat is releases during formation of CO2(g), in decomposition of CO2(g) into its elements 94 kcal heat is required. II is true.

III. Molar mass of CO2=12+2.(16)=44g

Mole of CO2(g);


For 1mol CO2 -94kcal heat is released

For 0,25mol CO2 ? kcal heat is released



As you can see, 23,5 kcal heat is released not required. III is false.

IV. Reaction is exothermic. So, this statement is true.

5. Which one of the given reaction-name couple is false?

I. MgSO4(s) → Mg+2(aq) + SO4-2(aq) : Decomposition

II. CO(g) + 1/2O2(g) → CO2(g) : Combustion

III. Al(s) + 3/2N2(g) + 9/2O2(g) → Al(NO3)3(s) : Formation


I. It is dissolution of 1mol MgSO4(s), I is false.

II. It is combustion of 1mol CO. II is true.

III. It is formation of 1mol Al(NO3)3(s). III is true.