Energy Change

Question 1

What is energy often defined as?

Answer 1

The ability to do work.

Question 2

Name different forms of energy.

Answer 2

Potential (stored)

Question 3

Are the various forms of energy interconvertible?

Answer 3

Yes.

Question 4

Give an example of energy conversion in the human body.

Answer 4

Chemical energy in food is converted to heat energy and kinetic energy.

Question 5

What does an electric bulb convert electrical energy into?

Answer 5

Heat energy and light energy.

Question 6

What is the Law of Conservation of Energy?

Answer 6

Energy can neither be created nor destroyed

Question 7

What is an exothermic reaction?

Answer 7

A reaction that releases heat to the surroundings.

Question 8

What is the sign of ΔH for an exothermic reaction?

Answer 8

Negative (-).

Question 9

Give examples of exothermic reactions.

Answer 9

Combustion reactions

Question 10

What is an endothermic reaction?

Answer 10

A reaction that absorbs heat from the surroundings.

Question 11

What is the sign of ΔH for an endothermic reaction?

Answer 11

Positive (+).

Question 12

Give examples of endothermic reactions.

Answer 12

Melting of ice

Question 13

What is enthalpy (H)?

Answer 13

The heat content of a system at constant pressure.

Question 14

What is enthalpy change (ΔH)?

Answer 14

The difference between the enthalpy of the products and the enthalpy of the reactants.

Question 15

What occurs during chemical reactions?

Answer 15

Energy changes.

Question 16

Why do energy changes occur in chemical reactions?

Answer 16

Because reactants and products possess different amounts of chemical energy.

Question 17

Can we measure the amount of chemical energy in a substance?

Answer 17

No.

Question 18

What forms can energy changes in chemical reactions be observed as?

Answer 18

Heat

Question 19

Give an example of a reaction that produces heat.

Answer 19

Mixing a strong acid and a strong base.

Question 20

Give an example of a reaction that produces light and heat.

Answer 20

Magnesium burning in air.

Question 21

Give an example of a reaction that produces sound and heat.

Answer 21

Ignition of a hydrogen and oxygen mixture.

Question 22

Which energy change is easiest to measure?

Answer 22

Heat energy changes.

Question 23

What is standard enthalpy change (ΔH⁰)?

Answer 23

The enthalpy change when reactants are converted to products in their standard states under standard conditions.

Question 24

What are standard conditions?

Answer 24

100 kPa pressure and a specified temperature (usually 298 K or 25°C).

Question 25

What is the standard state of a substance?

Answer 25

The pure form of the substance at 100 kPa and the specified temperature.

Question 26

What is the standard enthalpy change of formation (ΔH⁰f)?

Answer 26

The enthalpy change when one mole of a substance in its standard state is formed from its constituent elements in their standard states.

Question 27

What is the standard enthalpy change of combustion (ΔH⁰c)?

Answer 27

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

Question 28

What type of reaction is neutralization?

Answer 28

Exothermic.

Question 29

What is the heat of neutralization?

Answer 29

The amount of heat evolved during a neutralization reaction in which one mole of water is formed.

Question 30

What is the standard heat of neutralization (ΔH⁰n)?

Answer 30

The amount of heat evolved when 1 mole of hydrogen ions (H+) from an acid reacts with 1 mole of hydroxide ions (OH-) from an alkali to form 1 mole of water under standard conditions.

Question 31

Approximately what value is the heat of neutralization for any strong acid and strong base?

Answer 31

-57.4 kJ/mol.

Question 32

Why is the heat of neutralization less when weak acids or weak bases are involved?

Answer 32

Because the reactants are only partially dissociated into ions in solution

Question 33

What is combustion?

Answer 33

A chemical process that is always accompanied by the evolution of heat.

Question 34

What is the heat of combustion?

Answer 34

The amount of heat evolved when one mole of a substance is burned completely in oxygen.

Question 35

What is the standard heat of combustion (ΔH⁰c)?

Answer 35

The heat evolved when one mole of the substance is burned completely in oxygen under standard conditions.

Question 36

Why is the heat of combustion an important quantity?

Answer 36

Because the combustion of fuels is the main source of energy for industrial and domestic purposes

Question 37

What is calorific value?

Answer 37

The amount of heat evolved per kilogram of the substance.

Question 38

What occurs in the first step when an ionic solid dissolves in water?

Answer 38

Water molecules split up the ions in the crystal into free ions.

Question 39

What is the energy needed for splitting up ions in a crystal called?

Answer 39

Lattice energy.

Question 40

What occurs in the second step when an ionic solid dissolves in water?

Answer 40

Free ions are hydrated by water molecules.

Question 41

What is the energy evolved when free ions are hydrated by water molecules called?

Answer 41

Hydration energy.

Question 42

What is the heat of solution?

Answer 42

The resultant heat change of the two steps (lattice energy and hydration energy) when an ionic solid dissolves in a solvent.

Question 43

Under what condition is dissolution exothermic?

Answer 43

If lattice energy < hydration energy.

Question 44

Under what condition is dissolution endothermic?

Answer 44

If lattice energy > hydration energy.

Question 45

Why is the dissolution of hydrated salts endothermic?

Answer 45

Because the ions are already extensively hydrated