# Quiz 2 - Thermochemistry | Thermochemistry

1

In an endothermic reaction, which of the following is true?

In an endothermic reaction, the system absorbs heat from the surroundings, resulting in an increase in the internal energy of the system.

2

Which of the following is true about bond enthalpies?

Bond enthalpies are positive values because they represent the energy required to break a bond in a molecule, which is an endothermic process.

3

Calculate the enthalpy change for the reaction C2H4 (g) + 3 O2 (g) → 2 CO2 (g) + 2 H2O (l), given the bond enthalpies: C=C (612 kJ/mol), C-H (412 kJ/mol), O=O (498 kJ/mol), C=O (799 kJ/mol), and O-H (463 kJ/mol).

Calculate the total bond enthalpies for reactants and products:

• Reactants:

C2H4: 1 x 612 + 4 x 412 = 612 + 1648 = 2260 kJ
3 O2: 3 x 498 = 1494 kJ
Total = 2260 + 1494 = 3754 kJ

• Products:

2 CO2: 4 x 799 = 3196 kJ
2 H2O:4 x 463 = 1852 kJ
Total = 3196 + 1852 = 5048 kJ

ΔH = Bonds broken − Bonds formed = 3754 − 5048 = −1294 kJ

4

How much heat is required to raise the temperature of 10 grams of water from 25°C to 75°C? (Specific heat capacity of water is 4.18 J/g°C)

The heat required is calculated using the formula q = smΔT = , where s = 4.18 J/g°C, m = 10 g, , and ΔT = 75°C − 25°C = 50°C.
q = 4.18 J/g°C x 10 g x 50°C = 2090 J.

5

Which of the following best describes the relationship between heat and work in the context of the first law of thermodynamics?

The first law of thermodynamics states that the change in internal energy (ΔU) of a system is equal to the heat added to the system (q) plus the work done on the system (w).

6

Calculate the standard enthalpy change (ΔHo) for the reaction: N2 (g) + 3 H2 (g) → 2 NH3 (g)
Given the standard enthalpies of formation: ΔHf(NH3 (g)) = −45.9 kJ/mol, ΔHfo (N2 (g)) = 0 kJ/mol, ΔHf(H2 (g)) = 0 kJ/mol.

Using the standard enthalpies of formation:
ΔHo =[2 x ΔHfo (NH3)] − [ΔHfo (N2) + 3 x ΔHfo​​​​​​​ (H2)]
ΔHo = [2 x (−45.9 kJ/mol)] − [0 + 3 x 0]
ΔHo = −91.8 kJ

7

Which of the following is an example of a state function?

A state function is a property that depends only on the current state of the system and not on the path taken to reach that state. Internal energy is a state function, whereas work and heat are path functions.

8

Hess's Law is based on which of the following principles?

Hess's Law states that the total enthalpy change for a reaction is the same regardless of the number of steps because enthalpy is a state function, depending only on the initial and final states of the system.

9

Which of the following equations correctly applies Hess's Law for a multi-step reaction?

Hess's Law states that the enthalpy change for a multi-step reaction is the sum of the enthalpy changes for each individual step.

10

Given the following reactions and their enthalpy changes:
C (s) + O2 (g) → CO2 (g)     ΔH = −393.5 kJ
CO (g) + $\frac{1}{2}$O2 (g) → CO2 (g)      ΔH = −283.0 kJ

What is the enthalpy change for the reaction: C (s) + $\frac{1}{2}$O2 (g) → CO (g)?

To find the enthalpy change for C (s) + $\frac{1}{2}$O2 (g) → CO (g), we can:

• Reverse the second reaction: CO2 (g) → CO (g) + $\frac{1}{2}$O2 (g)      ΔH = +283.0 kJ
• Add this to the first reaction: C (s) + O2 (g) → CO2 (g)     ΔH = −393.5 kJ
• Sum up the enthalpy changes: ΔH = -393.5 kJ + 283.0 kJ = -110.5 kJ