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)

Solution:

Enthalpy of given reaction is found by;

∆H=[∆H_CO + ∆H_H2O] - [∆H_CO2 + ∆H_H2]

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, c_water=1 cal/g.⁰C)

Solution:

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

Q=m.c.∆t

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.

Solution:

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

Solution:

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

II. Number of moles of CO₂(g);

n_CO2=8,96/22,4=0,4mol

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);

n_CO2=11/44=0,25mol

For 1mol CO₂ -94kcal heat is released

For 0,25mol CO₂ ? kcal heat is released

————————————————————————

?=-23,5kcal

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

Solution:

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.