Chapter 7: Enthalpy Change

Question 1

What is energy?

Answer 1

The ability to do work or move an object against an opposing force.

Question 2

What are some forms of energy?

Answer 2

Heat, light, sound, electricity, and chemical energy.

Question 3

What is heat in terms of energy transfer?

Answer 3

Heat is a mode of energy transfer due to a temperature difference, increasing disorder.

Question 4

How does heat affect molecular behavior?

Answer 4

It increases the average kinetic energy of molecules in a disordered fashion.

Question 5

What is enthalpy (H)?

Answer 5

A measure of the heat energy contained in a substance, stored as potential energy.

Question 6

What is enthalpy change?

Answer 6

The energy exchange between a chemical reaction and its surroundings at constant pressure.

Question 7

Can enthalpy be directly measured?

Answer 7

No, only enthalpy change can be measured.

Question 8

What happens to the surroundings during an exothermic reaction?

Answer 8

The temperature of the surroundings increases.

Question 9

What happens to the surroundings during an endothermic reaction?

Answer 9

The temperature of the surroundings decreases.

Question 10

What is the sign of ∆H for exothermic reactions?

Answer 10

Negative (∆H < 0).

Question 11

What is the sign of ∆H for endothermic reactions?

Answer 11

Positive (∆H > 0).

Question 12

Give examples of exothermic reactions.

Answer 12

Combustion, respiration, reaction of water with calcium oxide.

Question 13

Give examples of endothermic reactions.

Answer 13

Decomposition of limestone, photosynthesis, dissolving certain ammonium salts.

Question 14

What is the standard enthalpy change?

Answer 14

The enthalpy change measured under standard conditions.

Question 15

What is Hess’s Law?

Answer 15

The total enthalpy change of a reaction is independent of the pathway taken.

Question 16

How can Hess’s Law be used in calculations?

Answer 16

By constructing an enthalpy cycle.

Question 17

What is bond energy?

Answer 17

The energy required to break a specific covalent bond.

Question 18

Is bond energy always positive or negative?

Answer 18

Positive, as it involves breaking bonds.

Question 19

What is the relationship between bond breaking and energy?

Answer 19

Bond breaking is endothermic.

Question 20

What is the relationship between bond making and energy?

Answer 20

Bond forming is exothermic.

Question 21

How do you determine if a reaction is exothermic using bond energies?

Answer 21

If energy released during bond formation is greater than energy absorbed during bond breaking.

Question 22

How do you determine if a reaction is endothermic using bond energies?

Answer 22

If energy absorbed during bond breaking is greater than energy released during bond formation.

Question 23

What is the formula for enthalpy change using bond energies?

Answer 23

Enthalpy change = Energy to break bonds - Energy to form bonds.

Question 24

Why are average bond energies used?

Answer 24

Bond energies vary between compounds, so averages are used for calculations.

Question 25

What is the enthalpy change of combustion?

Answer 25

The enthalpy change when 1 mole of a substance is burned in oxygen under standard conditions.

Question 26

What is the enthalpy change of formation?

Answer 26

The enthalpy change when 1 mole of a compound is formed from its elements under standard conditions.

Question 27

What is the enthalpy change of atomization?

Answer 27

The enthalpy change required to produce 1 mole of gaseous atoms from an element in its standard state.

Question 28

Why is Hess's Law important?

Answer 28

It allows the calculation of enthalpy changes that cannot be measured directly.

Question 29

What does an enthalpy cycle illustrate?

Answer 29

The conservation of energy in a chemical reaction.

Question 30

What does ∆H depend on in chemical reactions?

Answer 30

The strength and number of bonds broken and formed.

Question 31

What is the relationship between bond energy and bond strength?

Answer 31

Higher bond energy indicates stronger bonds.

Question 32

Why is the combustion of fuels exothermic?

Answer 32

It releases more energy when forming bonds than it absorbs to break bonds.

Question 33

Why is photosynthesis endothermic?

Answer 33

It absorbs more energy to break bonds than it releases to form bonds.

Question 34

How is bond energy calculated for diatomic molecules?

Answer 34

Directly from experimental data on bond dissociation.

Question 35

What does the decomposition of limestone produce?

Answer 35

Calcium oxide and carbon dioxide.

Question 36

Why is dissolving ammonium salts in water endothermic?

Answer 36

The energy absorbed to break ionic bonds is greater than the energy released during hydration.

Question 37

How is ∆H calculated in the Haber process?

Answer 37

Using bond energies of reactants and products.

Question 38

Why are bond enthalpies averaged?

Answer 38

The same type of bond can have slightly different energies in different molecules.

Question 39

What is an exothermic enthalpy profile diagram?

Answer 39

A graph showing products lower in energy than reactants.

Question 40

What is an endothermic enthalpy profile diagram?

Answer 40

A graph showing products higher in energy than reactants.

Question 41

What does a negative ∆H signify in combustion?

Answer 41

Heat is released.

Question 42

What does a positive ∆H signify in decomposition reactions?

Answer 42

Heat is absorbed.

Question 43

How is energy transfer visualized in enthalpy cycles?

Answer 43

With arrows showing energy flow between reactants, products, and intermediates.

Question 44

What is a standard state in enthalpy measurements?

Answer 44

A reference state of a substance at 1 atm pressure and 298 K.

Question 45

What is the role of oxygen in combustion?

Answer 45

To act as a reactant and release energy.

Question 46

Why is the oxidation of carbohydrates in respiration exothermic?

Answer 46

It releases stored chemical energy.

Question 47

Why is calcium oxide an important product in industrial processes?

Answer 47

It is used in cement and neutralizing acidic soils.

Question 48

What is the significance of Hess's Law in thermochemistry?

Answer 48

It provides a practical way to calculate enthalpy changes indirectly.

Question 49

How does temperature affect the enthalpy of a reaction?

Answer 49

Higher temperatures increase kinetic energy but do not change ∆H.

Question 50

What is the main concept behind Hess’s Law?

Answer 50

Conservation of energy.