Electrochemistry Quiz

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Questions and Answers

What type of energy transformation occurs during photosynthesis?

Light energy into chemical energy (correct)

Chemical energy into electrical energy

Kinetic energy into heat energy

Electrical energy into light energy

Which statement best describes a voltaic cell?

It converts chemical energy into electrical energy spontaneously. (correct)

It uses electrical energy to drive a chemical reaction.

It converts thermal energy into electrical energy.

It only functions under constant temperature conditions.

The salt bridge in an electrolytic cell serves what primary purpose?

To increase the voltage output of the cell

To catalyze the chemical reactions occurring in the cell

To allow ions to flow and maintain electrical neutrality (correct)

To insulate the cell from external electromagnetic interference

In the context of electrochemistry, what does the term 'redox reaction' signify?

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In a chemical reaction, if the heat is absorbed, what type of reaction is it classified as?

Endothermic reaction Signup and view all the answers

What is the relation between electrode potential and the tendency of a species to gain electrons?

Higher electrode potential signifies a stronger tendency to gain electrons. Signup and view all the answers

Which of the following transformations would increase the efficiency of a thermodynamic system?

Minimizing energy losses through heat transfer Signup and view all the answers

Which type of reaction is characterized by the transfer of electrons between reactants?

Oxidation-reduction (redox) reaction Signup and view all the answers

What type of energy does an electric fan transform to facilitate movement?

Electrical energy into mechanical energy Signup and view all the answers

What is the most common method of oxidation?

Removal of electron Signup and view all the answers

In a galvanic (voltaic) cell, what is the role of the anode?

To oxidize compounds Signup and view all the answers

Which process occurs in an electrolytic cell?

Oxidation at the anode Signup and view all the answers

What does a higher standard electrode potential (E°’) indicate?

The species is a stronger oxidizing agent Signup and view all the answers

How is the standard electrode potential (E°’) expressed in redox reactions?

In volts Signup and view all the answers

What is the expected sign of ΔG in a spontaneous galvanic cell reaction?

Negative Signup and view all the answers

What is the primary function of a salt bridge in an electrochemical cell?

To maintain charge balance Signup and view all the answers

Which of the following redox reactions has the highest standard electrode potential?

½ O2 ↔ H2O Signup and view all the answers

What characterizes an oxidation-reduction (redox) reaction?

Involves the transfer of electrons between reactants Signup and view all the answers

In a galvanic cell, the anode is where which of the following occurs?

Oxidation of metal Signup and view all the answers

Which equation relates standard Gibbs free energy change to equilibrium constant?

ΔG° = -RT ln K Signup and view all the answers

What is the overall purpose of a voltaic cell?

To convert chemical energy into electrical energy Signup and view all the answers

In electrochemistry, what does the standard electrode potential indicate?

The tendency of a half-cell to gain or lose electrons Signup and view all the answers

Which statement best describes a redox reaction involving hydrogen?

Hydrogen can either be oxidized or reduced depending on the reaction Signup and view all the answers

How does the Nernst equation relate to a galvanic cell?

It predicts the cell potential under non-standard conditions Signup and view all the answers

Study Notes

Units

Thermodynamics

A branch of knowledge essential for understanding natural sciences.
Focuses on the system level, describing the whole system rather than individual particles.
Fundamental laws are universal and applicable to various fields.

Chemical reaction - energy change

Physical and chemical changes involve using or producing energy.
Combustion of fuels (kerosene, coal, wood, natural gas) produces heat and light.
Electrical energy comes from chemical reactions in batteries.
Photosynthesis uses light energy to form glucose.

Types of energy

Kinetic (motion)
Potential (stored)
Mechanical
Nuclear
Ionization
Chemical
Electrical/Electromagnetic
Thermal
Sonic
Gravitational

Conversion of energy

Energy can be transferred from one location to another or change forms.
Total energy is always conserved (law of conservation of energy).
The sun converts nuclear energy into heat and light energy.
Our bodies convert chemical energy in food into mechanical energy for movement.
An electric fan transforms electrical energy into kinetic energy.
Lightning converts electrical energy into light, heat, and sound.

Definitions important to thermodynamics

System: The part of the universe under study, separated from the surroundings by a boundary.
Surroundings: Everything outside the system.
Universe: System + Surroundings

Types of systems

Open system: Exchanges both energy and matter with the surroundings.
Closed system: Exchanges energy but not matter.
Isolated system: Exchanges neither energy nor matter.

Fundamental terms in thermodynamics

Energy (E)
Work (w)
Heat (q)
Temperature (T)

Energy

Capacity of a system to do work or supply heat.
Total energy equals kinetic energy + potential energy + internal energy.
Internal energy is the sum of the microscopic energies of all particles in the system.

Internal energy

Sum of kinetic and potential energies of particles in a system.
Depends on temperature, particle type, and number of particles.

Kinetic energy

Energy of motion at the particle level (translational, rotational, vibrational).
Due to the movement of molecules through space, around their center of mass and back and forth motion of atoms.

Potential energy

Energy due to particle interactions, including intermolecular forces and chemical bonds.

Law of conservation of energy

All forms of energy can transform into others but cannot be created or destroyed.
Energy exchange between the system and surroundings.
The energy of the universe remains constant.

Units of energy

SI unit of energy is the joule (J) = 1 kg m² / s².
Calorie (cal) raises the temperature of 1.00 g of water by 1°C.
Calorie (nutritional) is a unit of energy provided by food/drinks.

Energy flow to/from a system

Internal energy (at given state variables) cannot be determined, only changes can be calculated or measured.
ΔE = E₂ - E₁ (final - initial)
When a system changes from reactants to products, the internal energy changes: • ΔE = E(products) - E(reactants).

Entropy

The microscopic randomness, or disorder, of a system; the spread of energy.
A measure of how dispersed energy is amongst possible energy states in a system.
Greater disorder = higher entropy.

Enthalpy

Heat content of a reaction at constant pressure.
ΔH = qp
ΔH is a state function and extensive property.
Positive ΔH = endothermic (heat absorption).
Negative ΔH = exothermic (heat release).

Heat of reaction

Total heat evolved or absorbed in a chemical reaction at constant pressure (qp).
Exothermic: Energy released (q < 0, ΔH negative); system warms.
Endothermic: Energy absorbed (q > 0, ΔH positive); system cools.

Enthalpies of reaction

Energy changes resulting from bond breaking and formation.
ΔHrxn = ΣH(products) - ΣH(reactants).
Thermochemical equations show reactions with enthalpy changes.
Coefficients in balanced equations give the number of moles of reactant/product producing a certain ΔH.

Standard enthalpy changes

Data based on enthalpy changes with all materials in their standard states.
Standard state = most stable physical form at 0.1 MPa and 298K.

Standard enthalpies of formation (△H°f)

Change in enthalpy when 1 mole of a substance forms from its elements in standard states.
Standard enthalpy of formation of elements in their most stable form = 0.

Enthalpies of reaction under standard conditions

Can be calculated from standard enthalpies of formation values (a table).
To find standard enthalpy change for a given reaction sum up the enthalpies of formation of the products and subtract from this value the sum of the enthalpy changes of the reactants.

Hess's law (law of heat summation)

Total enthalpy change of a reaction is independent of the reaction pathway.
The heat exchange is independent of the pathway.

Changing of enthalpy at phase transfer

Finding total enthalpy change for a solid to gas conversion can be broken down to finding changes for solid to liquid and then liquid to gas.
ΔH°sub = ΔH°melt + ΔH°vap

Spontaneity

Whether a reaction occurs or not when substances are brought together.
Spontaneous processes: Physical or chemical changes that occur without external intervention.
Temperature affects spontaneity.

Entropy and Disorder

Order is less probable than disorder.
Work is generally required to produce order out of disorder; therefore, energy must be used to produce a highly ordered state.

Entropy is a state function

Its value depends on the exact state of the system.
Its change (ΔS) does not depend on the path, only on the initial and final states.
ΔS = S₂ - S₁
ΔS = q/T

Processes where entropy increases (ΔS > 0)

As energy is transferred between systems or from one form to another, it becomes dispersed from localized to more spread out states. – solid < liquid < gas.

Standard molar entropy (S°)

Entropy gained when 1 mole of a substance changes from a perfect crystal at 0K to 0.1 MPa and 298K.

Entropy change for a reaction

Increase in entropy: Molecule breaking down into smaller molecules; increase in the moles of gases; solid changing to liquid or liquid to gas.
ΔS°rxn = ΣnS°(products) - ΣmS°(reactants)

Predicting spontaneity of reactions

Reactions that have a large and negative ΔH tend to be spontaneous.
Spontaneous reactions lead to an increase in the entropy of the universe.
Spontaneity depends on enthalpy, entropy, and temperature.

Gibbs free energy (G)

Unifies the first and second laws of thermodynamics for constant temp and pressure.
G = H – TS
ΔG = ΔH – TΔS (change in Gibbs free energy)
G is a state function, extensive property, maximum work done at constant temp and pressure.

Gibbs free energy (G) and spontaneity

Negative ΔG = spontaneous reaction in the forward direction (exergonic process).
Positive ΔG = non-spontaneous reaction in the forward direction (endergonic process).
Zero ΔG = system is at equilibrium.

Standard Gibbs free energy

Free energy change when reactants in standard states are converted to products in standard states.
ΔG° = ΔH° - TΔS°

Standard Gibbs free energy of formation (△G°f)

Free energy change when 1 mol of a substance forms from its elements in their reference states at 0.1 MPa and 298K.

Gibbs free energy and temperature

When enthalpy change (△H) is negative and entropy change (△S) is positive, the reaction is exergonic at all temperatures.
When △H is positive and △S is negative, the reaction is endergonic at all temperatures.
When △H is positive and △S is positive, the reaction is spontaneous at high temperatures and nonspontaneous at low temperatures.
When △H is negative and △S is negative, the reaction is spontaneous at low temperatures and nonspontaneous at high temperatures.

Free energy and eqilibrium

Q<K → ΔG<0, reactants
Q=K → ΔG=0 equilibrium q>K → ΔG >0, products

△G° and the chemical equilibrium

To find △G, use the equation △G = △G° + RT In Q
At equilibrium △G=0