2B5 Energy Concepts and Calculations

Question 1

Define:

cell potential

Answer 1

The difference in electrical potential between two half-cells in an electrochemical cell.

Cell potential drives the flow of electrons from the anode to the cathode.
It’s affected by temperature, concentration, and pressure, and can be measured using a voltmeter.

Question 2

List the two types of electrochemical cells.

Answer 2

1. Galvanic (voltaic) cells
2. Electrolytic cells

Galvanic cells generate energy while electrolytic cells require energy input.

Question 3

What is a galvanic cell?

Answer 3

An electrochemical cell that converts chemical energy into electrical energy spontaneously.

It consists of two half-cells connected by a wire and a salt bridge.

Electrons travel through the wire.

The salt bridge keeps the two reactions separate while allowing ions to flow, thus completing the circuit.

Question 4

What is a redox reaction?

Answer 4

The process of the movement of electrons between reactants, involving reduction and oxidation half-reactions.

During reduction, electrons are gained, and during oxidation, electrons are lost.

Reduction occurs at the cathode while oxidation occurs at the anode.

Question 5

What is the standard cell potential?

Answer 5

The cell potential measured under standard conditions (1atm, 25°C, 1M).

Standard conditions ensure consistent measurement.

Question 6

Define:

standard reduction potential

Answer 6

The potential of a half-cell relative to a standard hydrogen electrode.

Standard hydrogen electrode has a potential of 0.00 V.

It serves as a reference for all reduction potentials.

Question 7

State the formula for calculating cell potential.

Answer 7

E°cell = E°cathode - E°anode

This formula calculates the standard cell potential.

The standard cell potential chart is used to find the values for the half-reactions.

Question 8

What does a positive standard potential value indicate?

Answer 8

The reaction is spontaneous and will go in the forward direction.

Reactions in galvanic cells are spontaneous.

If the standard potential value is negative, the reaction is non-spontaneous and may go in the opposite direction.

Non-spontaneous reactions require external energy.

Question 9

Define:

oxidation potential

Answer 9

The tendency of a species to lose electrons.

Reduction potential is the tendency of a species to gain electrons.

A positive standard reduction potential indicates a strong tendency for reduction.

Question 10

Which equation relates cell potential to temperature and concentration?

Answer 10

Nernst equation: E = E° - (RT/nF) lnQ

Where R is the gas constant, T is the temperature, n is the number of electron moles transferred in the reaction, F is Faraday’s constant, and Q is the reaction quotient.

Cell potential increases with an increase in concentration and temperature.

Question 11

True or False:

Cell potential depends on the path of the reaction.

Answer 11

False

Cell potential is a state function; it depends only on the initial and final states.

Question 12

Define:

latent heat

Answer 12

The heat energy absorbed or released during a phase change at a constant temperature.

Latent heat governs energy changes during phase transitions.

Phase changes involving latent heat include melting, freezing, vaporization, and condensation.

Question 13

Define:

heat of fusion

Answer 13

It is the energy required to change a solid to a liquid or a liquid to solid at a constant temperature.

It is also known as the enthalpy of fusion.

Phase change from solid to liquid (melting) is an endothermic process while phase change from liquid to solid (freezing) is an exothermic process.

Question 14

State the formula for heat of fusion.

Answer 14

Lf = Q/m

Where Lf is the heat of fusion, Q is the heat absorbed or released, and m is the mass of the substance.

Question 15

List three units commonly used for heat of fusion.

Answer 15

1. Joules per gram (J/g)
2. Kilojoules per kilogram (kJ/kg)
3. Calories per gram (cal/g)

Question 16

True or False:

Heat of fusion depends on the path of the reaction.

Answer 16

False

Heat of fusion is a state function, depending only on the initial and final states.

Question 17

List two substances with high heat of fusion.

Answer 17

1. Water
2. Metals like aluminum

Substances with strong intermolecular forces have higher heat of fusion.

The heat of fusion of water is 334 J/g while that of aluminum is 397 J/g at 0°C.

Question 18

What is the heat required to melt 10 g of ice?

Answer 18

3340 J

Use Lf = Q/m

Lf = 334 and m is 10 g

Question 19

What occurs during the freezing process in terms of heat of fusion?

Answer 19

Heat is released, resulting in a phase change from liquid to solid at the freezing point.

The freezing point is equivalent to the melting point.

Question 20

What is the heat released when 5 g of water freezes?

Answer 20

-1670 J

Freezing releases energy equal to the heat of fusion but with a negative sign.

Question 21

True or False:

Heat of fusion is zero for gases.

Answer 21

True

Gases do not undergo melting; they directly condense or freeze.

Question 22

Define:

heat of vaporization

Answer 22

The heat needed to convert a liquid to vapor at a fixed temperature (boiling point).

It requires more energy than melting a solid.

Vaporization requires breaking more intermolecular forces.

Question 23

What is the formula for heat of vaporization?

Answer 23

LH = Q/m

Where LH is the heat of vaporization, Q is the heat needed to evaporate a liquid, and m is the mass of the evaporated substance.

Question 24

What is the heat required to vaporize 250 grams of iron if its heat of vaporization is 6340 J/g?

Answer 24

1585000 J

Calculated using Q = LH × m, where LH = 6340 J/g and m = 250 g.

Question 25

# Define: thermal expansion

Answer 25

The **change in the dimensions** of objects due to the application of heat and subsequent rise in temperature. ## Footnote It can be categorized into linear, area, or volume expansion.

Question 26

What happens to **mercury in a thermometer** when it is heated?

Answer 26

It **expands and rises** in the column. ## Footnote Thermal expansion of mercury allows temperature measurement.

Question 27

State the **difference** between *heat of fusion* and *specific heat*.

Answer 27

* **Heat of fusion** relates to **phase changes**. * **Specific heat** relates to **temperature changes**. ## Footnote Specific heat involves heating without phase changes.

Question 28

List the *two* **kinds of molar specific heat** capacities of gases.

Answer 28

1. Molar specific heat capacity at **constant pressure (Cp)**. 1. Molar specific heat capacity at **constant volume (Cv)**. ## Footnote Heat required at constant pressure is greater than at constant volume.

Question 29

What is the **specific heat ratio** or **adiabatic index**?

Answer 29

The ratio γ=Cp/Cv ## Footnote γ is always greater than 1 because Cp > Cv.

Question 30

What happens to the temperature of a substance during **boiling, melting, or freezing**?

Answer 30

The temperature **remains constant** as heat is used for phase change. ## Footnote This energy is stored as potential energy once the molecules move apart. These processes cannot be observed directly using thermometers.

Question 31

List *two* factors that **affect the boiling point** of water.

Answer 31

1. Altitude 1. Impurities ## Footnote Water boils at a lower temperature at higher altitudes. Impurities increase the boiling temperature but decrease the freezing temperature.

Question 32

Why does water **heat up more slowly** than land?

Answer 32

Water has a **higher specific heat capacity** than land. ## Footnote Water absorbs more heat without a significant rise in temperature.

Question 33

What does a **heating curve** illustrate?

Answer 33

The **temperature change** of a substance as heat is applied over time. ## Footnote It shows increases and plateaus during phase transitions. At the plateaus, melting or vaporization occurs.