2B5 Energy Concepts and Calculations Flashcards
Analyze concepts relating to cell potentials, heat of fusion, and heat capacity.
Define:
cell potential
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.
List the two types of electrochemical cells.
- Galvanic (voltaic) cells
- Electrolytic cells
Galvanic cells generate energy while electrolytic cells require energy input.
What is a galvanic cell?
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.
What is a redox reaction?
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.
What is the standard cell potential?
The cell potential measured under standard conditions (1atm, 25°C, 1M).
Standard conditions ensure consistent measurement.
Define:
standard reduction potential
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.
State the formula for calculating cell potential.
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.
What does a positive standard potential value indicate?
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.
Define:
oxidation potential
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.
Which equation relates cell potential to temperature and concentration?
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.
True or False:
Cell potential depends on the path of the reaction.
False
Cell potential is a state function; it depends only on the initial and final states.
Define:
latent heat
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.
Define:
heat of fusion
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.
State the formula for heat of fusion.
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.
List three units commonly used for heat of fusion.
- Joules per gram (J/g)
- Kilojoules per kilogram (kJ/kg)
- Calories per gram (cal/g)
True or False:
Heat of fusion depends on the path of the reaction.
False
Heat of fusion is a state function, depending only on the initial and final states.
List two substances with high heat of fusion.
- Water
- 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.
What is the heat required to melt 10 g of ice?
3340 J
Use Lf = Q/m
Lf = 334 and m is 10 g
What occurs during the freezing process in terms of heat of fusion?
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.
What is the heat released when 5 g of water freezes?
-1670 J
Freezing releases energy equal to the heat of fusion but with a negative sign.
True or False:
Heat of fusion is zero for gases.
True
Gases do not undergo melting; they directly condense or freeze.
Define:
heat of vaporization
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.
What is the formula for heat of vaporization?
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.
What is the heat required to vaporize 250 grams of iron if its heat of vaporization is 6340 J/g?
1585000 J
Calculated using Q = LH × m, where LH = 6340 J/g and m = 250 g.
Define:
thermal expansion
The change in the dimensions of objects due to the application of heat and subsequent rise in temperature.
It can be categorized into linear, area, or volume expansion.
What happens to mercury in a thermometer when it is heated?
It expands and rises in the column.
Thermal expansion of mercury allows temperature measurement.
State the difference between heat of fusion and specific heat.
- Heat of fusion relates to phase changes.
- Specific heat relates to temperature changes.
Specific heat involves heating without phase changes.
List the two kinds of molar specific heat capacities of gases.
- Molar specific heat capacity at constant pressure (Cp).
- Molar specific heat capacity at constant volume (Cv).
Heat required at constant pressure is greater than at constant volume.
What is the specific heat ratio or adiabatic index?
The ratio γ=Cp/Cv
γ is always greater than 1 because Cp > Cv.
What happens to the temperature of a substance during boiling, melting, or freezing?
The temperature remains constant as heat is used for phase change.
This energy is stored as potential energy once the molecules move apart.
These processes cannot be observed directly using thermometers.
List two factors that affect the boiling point of water.
- Altitude
- Impurities
Water boils at a lower temperature at higher altitudes.
Impurities increase the boiling temperature but decrease the freezing temperature.
Why does water heat up more slowly than land?
Water has a higher specific heat capacity than land.
Water absorbs more heat without a significant rise in temperature.
What does a heating curve illustrate?
The temperature change of a substance as heat is applied over time.
It shows increases and plateaus during phase transitions.
At the plateaus, melting or vaporization occurs.
