Thermochemistry Quizzes: Master Energy Changes in Chemical Reactions

Question 1

What is the correct definition of enthalpy change of a reaction?

Accepted Answer

The enthalpy change of a reaction is the difference in enthalpy between the reactants and the products.

Answer

The enthalpy change of a reaction is always positive.

Answer

The enthalpy change of a reaction is always zero.

Question 2

At equilibrium, how is the equilibrium constant (Kc) related to the concentrations of the reactants and products?

Accepted Answer

Kc equals the ratio of product to reactant concentrations.

Answer

Kc is directly proportional to the product concentrations.

Answer

Kc is inversely proportional to the reactant concentrations.

Answer

Kc is independent of reactant and product concentrations.

Question 3

In a closed system at constant temperature and pressure, the change in entropy of a chemical reaction is directly proportional to:

Accepted Answer

Change in disorder or randomness

Answer

Change in temperature

Answer

Change in volume

Answer

Change in energy

Question 4

Enthalpy change refers to the energy change that occurs during a chemical reaction. Which of the following best describes this concept?

Accepted Answer

The heat energy released or absorbed as a reaction proceeds at constant temperature and pressure.

Answer

A decrease in the total energy of the products compared to the reactants.

Answer

The difference in activation energies between the reactants and the products.

Answer

The ratio of reactant moles to product moles.

Question 5

In a chemical reaction, the activation energy (Ea) is the minimum energy required for the reaction to occur. Which factor primarily determines the magnitude of the activation energy?

Accepted Answer

Nature of reactants and products

Answer

Concentration of reactants

Answer

Enthalpy change of the reaction (ΔH)

Answer

Temperature of the reaction

Question 6

A reaction has a negative enthalpy change (ΔH < 0) and a negative entropy change (ΔS < 0). Which of the following statements about the spontaneity of this reaction is correct?

Accepted Answer

The reaction is nonspontaneous at all temperatures.

Answer

The reaction is spontaneous at all temperatures.

Answer

The reaction is spontaneous at high temperatures and nonspontaneous at low temperatures.

Answer

The reaction is nonspontaneous at high temperatures and spontaneous at low temperatures.

Question 7

Which of the following is NOT a characteristic of a spontaneous process?

Accepted Answer

Positive enthalpy change and large positive entropy change

Answer

Negative Gibbs free energy change

Answer

Positive entropy change

Question 8

How can Hess's law be applied to calculate the enthalpy change of a reaction that cannot be directly measured?

Accepted Answer

By combining the enthalpy changes of multiple reactions that, when added together, equal the target reaction.

Answer

By subtracting the enthalpy change of the reverse reaction from the enthalpy change of the forward reaction.

Answer

By multiplying the enthalpy change of the reaction by the number of moles of reactants.

Answer

By dividing the enthalpy change of the reaction by the number of moles of products.

Question 9

Which is not a unit of enthalpy?

Accepted Answer

eV

Answer

kJ/mol

Answer

J/mol

Answer

cal/g

Question 10

Which statement about entropy is true?

Accepted Answer

It measures randomness or disorder.

Answer

It is always positive.

Answer

It is the same for all substances in the same physical state.

Answer

It is not affected by temperature.

Question 11

Which of the following definitions accurately describes an exothermic reaction?

Accepted Answer

A reaction that releases heat into its surroundings

Answer

A reaction that absorbs heat from its surroundings

Answer

A reaction that neither absorbs nor releases heat

Answer

A reaction that proceeds spontaneously

Question 12

The entropy of a system undergoes what change during the transition from solid to liquid?

Accepted Answer

Increases

Answer

Decreases

Answer

Remains the same

Answer

Cannot be determined

Question 13

The formula ΔG = ΔH - TΔS represents:

Accepted Answer

Gibbs free energy

Answer

Enthalpy

Answer

Entropy

Answer

Activation energy

Question 14

If a reaction has a negative change in entropy, what does this imply about the spontaneity of the reaction?

Accepted Answer

The reaction is non-spontaneous.

Answer

The reaction is spontaneous.

Answer

The entropy of the surroundings is increasing.

Answer

The reaction is at equilibrium.

Question 15

Out of the following, which is the correct definition of the standard enthalpy of formation?

Accepted Answer

The enthalpy change accompanying the formation of 1 mole of a compound from its constituent elements in their standard states

Answer

The enthalpy change during the combustion of 1 mole of a compound

Answer

The enthalpy change when 1 mole of a compound dissolves in water

Answer

The enthalpy change upon the reaction of 1 mole of a compound with oxygen

Question 16

Which field of study primarily employs thermochemical principles to examine energy processes and chemical reactions within living organisms?

Accepted Answer

Biochemistry

Answer

Chemical engineering

Answer

Environmental science

Answer

Astrophysics

Question 17

In exothermic reactions, which way does heat flow: from the system to the surroundings or vice versa? And in endothermic reactions, which way does heat flow?

Accepted Answer

Exothermic: system to surroundings, Endothermic: surroundings to system

Answer

Exothermic: surroundings to system, Endothermic: system to surroundings

Answer

Both exothermic and endothermic: system to surroundings

Answer

Both exothermic and endothermic: surroundings to system

Question 18

Which field applies thermochemistry to study energy transformations in living organisms?

Accepted Answer

Biochemistry

Answer

Physics

Answer

Geology

Question 19

Which reaction represents the standard enthalpy of formation of carbon dioxide gas?

Accepted Answer

C(graphite) + O₂(g) → CO₂(g)

Answer

C(s) + O₂(g) → CO₂(g)

Answer

CO(g) + ½ O₂(g) → CO₂(g)

Answer

2CO(g) + O₂(g) → 2CO₂(g)

Question 20

In the context of thermochemistry, what is enthalpy change?

Accepted Answer

Heat absorbed or released by a reaction at constant pressure

Answer

Net energy change of a reaction

Answer

Maximum work done by a reaction

Answer

Entropy change of a reaction

Question 21

What does the Gibbs free energy change of a reaction represent?

Accepted Answer

Maximum non-expansion work obtainable from a reaction

Answer

Enthalpy change of a reaction

Answer

Entropy change of a reaction

Answer

Net energy change of a reaction

Question 22

How is equilibrium related to free energy in chemical reactions?

Accepted Answer

Equilibrium is reached when the free energy change is zero

Answer

Equilibrium is reached when the free energy change is negative

Answer

Equilibrium is reached when the Gibbs free energy change is positive

Answer

Equilibrium is not related to free energy

Question 23

Which of the following is NOT a valid unit of enthalpy change?

Accepted Answer

eV

Answer

kJ mol<sup>-1</sup>

Answer

J g<sup>-1</sup>

Answer

kcal mol^-1

Question 24

The standard enthalpy change of formation of a compound is defined as the enthalpy change when:

Accepted Answer

1 mole of the compound is formed from its elements in their standard states at 298 K.

Answer

1 gram of the compound is formed from its elements in their standard states at 298 K.

Answer

1 mole of the compound decomposes into its elements in their standard states at 298 K.

Answer

1 gram of the compound decomposes into its elements in their standard states at 298 K.

Question 25

The equilibrium constant of a reaction is defined as the:

Accepted Answer

Ratio of the concentrations of the products to the concentrations of the reactants at equilibrium.

Answer

Ratio of the concentrations of the reactants to the concentrations of the products at equilibrium.

Answer

Difference in concentrations between the products and reactants at equilibrium.

Answer

Sum of the concentrations of the products and reactants at equilibrium.

Question 26

Which of the following is a characteristic of a fuel?

Accepted Answer

It reacts with oxygen to release heat.

Answer

It is a solid at room temperature.

Answer

It is a good conductor of electricity.

Answer

It is soluble in water.

Question 27

What is the formula for Gibbs free energy?

Accepted Answer

G = H - TS

Answer

G = H + TS

Answer

G = T - HS

Answer

G = TS - H

Question 28

Which reaction is considered spontaneous at all temperatures?

Accepted Answer

A + B → C + D (ΔG < 0)

Answer

A + B → C + D (ΔG > 0)

Answer

A + B → C + D (ΔG = 0)

Answer

A + B ↔ C + D (ΔG < 0)

Question 29

Define the standard enthalpy change of formation of a compound:

Accepted Answer

The enthalpy change when 1 mole of the compound is formed from its constituent elements

Answer

The enthalpy change when 1 mole of the compound is decomposed into its constituent elements

Answer

The enthalpy change when 1 gram of the compound is formed from its constituent elements

Answer

The enthalpy change when 1 mole of the compound is dissolved in water

Question 30

Which unit is used to measure entropy?

Accepted Answer

J/K

Answer

kJ/mol

Answer

cal/K

Answer

cal/g

Question 31

Define the activation energy of a reaction:

Accepted Answer

The minimum energy required for a reaction to proceed

Answer

The maximum amount of energy released by a reaction

Answer

The difference in energy between reactants and products

Answer

The enthalpy change of the reaction

Question 32

Which of the following factors does NOT affect the equilibrium constant of a reaction at a constant temperature?

Accepted Answer

Volume of the container

Answer

Concentration of reactants and products

Answer

Pressure (for gaseous reactions)

Answer

Nature of the reactants and products

Question 33

What is the relationship between standard enthalpy change and standard entropy change?

Accepted Answer

They can be either directly or inversely proportional

Answer

They are always inversely proportional

Answer

They are always directly proportional

Answer

There is no relationship between them

Question 34

How does increasing temperature generally affect the rate of a chemical reaction?

Accepted Answer

Increases the rate

Answer

Decreases the rate

Answer

Does not affect the rate

Answer

Cannot be determined from the information given

Question 35

What is a catalyst?

Accepted Answer

A substance that increases the rate of a reaction without being consumed

Answer

A substance that decreases the rate of a reaction without being consumed

Answer

A substance that increases the yield of a reaction

Answer

A substance that decreases the yield of a reaction

Question 36

Define the standard enthalpy change of a reaction.

Accepted Answer

Enthalpy change when 1 mole of a substance is formed from its elements under standard state conditions

Answer

Enthalpy change when 1 mole of a substance reacts with excess oxygen

Answer

Enthalpy change when 1 mole of a substance is dissolved in water

Answer

Enthalpy change when 1 mole of a substance is heated by 1 degree Celsius

Question 37

Which factor does NOT influence the enthalpy change of a reaction?

Accepted Answer

Concentration of reactants

Answer

Temperature

Answer

Pressure

Answer

Phase of the reactants and products

Question 38

What is the enthalpy change sign for an exothermic reaction?

Accepted Answer

Negative

Answer

Positive

Answer

Zero

Answer

Variable

Question 39

State the unit of entropy.

Accepted Answer

Joules per Kelvin

Answer

Joules per mole

Answer

Kelvin per Joule

Answer

Mole per Kelvin

Question 40

Classify the following process as entropy-increasing or entropy-decreasing: Formation of a crystal

Accepted Answer

Entropy-decreasing

Answer

Entropy-increasing

Answer

Entropy-neutral

Answer

Cannot be determined

Question 41

Which condition is NOT a standard state condition?

Accepted Answer

pH of 14

Answer

Temperature of 298 K

Answer

Pressure of 100 kPa

Answer

Concentration of 1 mol dm^-3

Question 42

At what point does a chemical reaction reach equilibrium in relation to the change in Gibbs free energy?

Accepted Answer

When the change in Gibbs free energy is zero.

Answer

When the change in Gibbs free energy is positive.

Answer

When the change in Gibbs free energy is negative.

Answer

The change in Gibbs free energy does not affect reaction equilibrium.

Question 43

Calculate the enthalpy change for the neutralization reaction between sodium hydroxide and hydrochloric acid using standard enthalpies of formation. What is the sign of the enthalpy change?

Accepted Answer

Negative, indicating an exothermic reaction

Answer

Positive, indicating an endothermic reaction

Answer

Zero, indicating no energy is released or absorbed during the reaction

Question 44

Identify the strong electrolyte among the following:

Accepted Answer

NaCl

Answer

NH3

Answer

C6H12O6

Answer

CH3COOH

Question 45

The free energy change for a reaction is negative. What does this indicate about the spontaneity of the reaction?

Accepted Answer

The reaction is spontaneous

Answer

The reaction is at equilibrium (incorrect)

Answer

The reaction is non-spontaneous (incorrect)

Answer

The reaction requires a catalyst (incorrect)

Question 46

The entropy of a closed system always:

Accepted Answer

Increases

Answer

Decreases (incorrect)

Answer

Fluctuates randomly (incorrect)

Answer

Remains constant (incorrect)

Question 47

Which of the following is the correct expression for the Gibbs free energy change?

Accepted Answer

ΔG = ΔH - TΔS

Answer

ΔG = ΔS - TΔH (incorrect)

Answer

ΔG = ΔH + TΔS (incorrect)

Answer

ΔG = TΔH - ΔS (incorrect)

Question 48

What is the sign of ΔS for a reaction that results in the formation of a solid precipitate?

Accepted Answer

Negative

Answer

Positive (incorrect)

Answer

Cannot be determined (incorrect)

Answer

Zero (incorrect)

Question 49

The standard enthalpy change of combustion for methane is -890 kJ/mol. What is the enthalpy change when 2.0 moles of methane are completely combusted?

Accepted Answer

-1780 kJ

Answer

-890 kJ

Answer

+1780 kJ

Answer

-445 kJ

Question 50

A reaction has a negative Gibbs free energy change (ΔG < 0). What does this indicate about the reaction under standard conditions?

Accepted Answer

It is spontaneous

Answer

It is exothermic

Answer

It is non-spontaneous

Answer

It is endothermic

Question 51

Which of the following is the standard unit of entropy?

Accepted Answer

J/K mol

Answer

kJ/mol

Answer

mol/L

Answer

J/mol

Question 52

The enthalpy of formation of water is -286 kJ/mol. What is the enthalpy change when 10 g of water is formed from its elements?

Accepted Answer

-159 kJ

Answer

-286 kJ

Answer

159 kJ

Answer

-2.86 kJ

Question 53

The equilibrium constant (K) for a reaction is 10. What is the relationship between the concentrations of products and reactants at equilibrium?

Accepted Answer

The concentration of products is 10 times greater than the concentration of reactants.

Answer

The relationship cannot be determined from the equilibrium constant.

Answer

The concentration of products is 0.1 times the concentration of reactants.

Answer

The concentration of products is equal to the concentration of reactants.

Question 54

Entropy is a thermodynamic property that measures the:

Accepted Answer

Degree of disorder or randomness in a system

Answer

Total energy of a system

Answer

Volume of a system

Answer

Degree of order in a system

Question 55

The Gibbs free energy change (ΔG) of a reaction indicates its:

Accepted Answer

Spontaneity under specific conditions

Answer

Equilibrium position

Answer

Change in enthalpy

Answer

Change in entropy

Question 56

The standard enthalpy of formation of a compound refers to the:

Accepted Answer

Enthalpy change when one mole of the compound is formed from its elements in their standard states

Answer

Enthalpy of the compound in its standard state

Answer

Enthalpy change when one mole of the compound is decomposed into its elements

Answer

Enthalpy change when one gram of the compound is formed from its elements

Question 57

According to the Second Law of Thermodynamics, the entropy of the universe:

Accepted Answer

Always increases for spontaneous processes

Answer

Always decreases for spontaneous processes

Answer

Can increase or decrease depending on the process

Answer

Remains constant

Question 58

The equilibrium constant (K) for a reaction is calculated as the:

Accepted Answer

Ratio of the concentrations of products to reactants at equilibrium, each raised to the power of their stoichiometric coefficients

Answer

Sum of the concentrations of products and reactants at equilibrium

Answer

Difference between the concentrations of products and reactants at equilibrium

Answer

Product of the concentrations of products and reactants at equilibrium

Question 59

What is the term for the enthalpy change that occurs during a process carried out at constant pressure?

Accepted Answer

Enthalpy change

Answer

Free energy change

Answer

Heat change

Answer

Entropy change

Question 60

What is the sign of the enthalpy change for an exothermic reaction?

Accepted Answer

Negative

Answer

Positive

Answer

Zero

Question 61

Which of the following statements accurately describes the relationship between Gibbs free energy change and spontaneity?

Accepted Answer

A negative Gibbs free energy change indicates a spontaneous reaction.

Answer

A positive Gibbs free energy change indicates a spontaneous reaction.

Answer

Gibbs free energy change is unrelated to spontaneity.

Question 62

Which of the following processes is always exothermic?

Accepted Answer

Combustion

Answer

Evaporation

Answer

Melting

Answer

Sublimation

Question 63

What is the standard enthalpy change of formation of an element in its standard state?

Accepted Answer

Zero

Answer

Variable

Answer

Negative

Answer

Positive

Question 64

Which of the following reactions has the greatest positive entropy change?

Accepted Answer

Dissolution of a solid in water

Answer

Precipitation of a salt from solution

Answer

Combustion of methane