Thermodynamics Overview

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Questions and Answers

What is the definition of energy in a thermodynamics context?

The sum of potential and kinetic energies only

The capacity to supply heat or do work (correct)

The energy stored in chemical bonds exclusively

The ability to create matter

Which statement best describes kinetic energy?

Energy of an object at rest

Energy stored due to height

Energy of motion dependent on mass and velocity (correct)

Energy that cannot be converted to work

What does the first law of thermodynamics state?

Energy can be created or destroyed

Energy cannot be converted into another form

All forms of energy are equally convertible

The total energy of an isolated system is constant (correct)

Which unit is used to express thermal energy?

Joule Signup and view all the answers

How is heat defined in thermodynamics?

The amount of thermal energy transferred between objects Signup and view all the answers

What type of energy is stored in chemical bonds?

Potential energy Signup and view all the answers

Which form of energy is NOT considered within the definition of energy?

Sound energy Signup and view all the answers

What can the conservation of energy principle imply for energy transformations?

Energy can be transformed into various forms without loss Signup and view all the answers

What is the correct formula to calculate the standard enthalpy change for a chemical reaction?

DrxnH = nSDfH0products – nSDfH0reactants Signup and view all the answers

What is the standard heat of formation (DfH0) of O2(g)?

0 kJmol-1 Signup and view all the answers

In the reaction CO(g) + ½O2(g) → CO2(g), what is the standard enthalpy change (DrxnH0) calculated to be?

-283.5 kJmol-1 Signup and view all the answers

How do you determine the enthalpy change when DfH values are not available?

Using average bond dissociation energies Signup and view all the answers

What is the sign of bond dissociation energies?

Always positive due to energy input required Signup and view all the answers

Which is the correct approximation formula for calculating enthalpy change using bond dissociation energies?

DrxnH0 ≈ SvDdissH (bonds formed) – SvDdissH (bonds broken) Signup and view all the answers

What does Hess' Law enable in the context of enthalpy changes?

Calculation of enthalpy changes for complex reactions Signup and view all the answers

Why should coefficients of substances in a balanced equation be considered in enthalpy calculations?

They provide accuracy in calculating total energy changes Signup and view all the answers

What happens to the internal energy of a system when work is done by the system against the external pressure?

It decreases. Signup and view all the answers

How is the work done by the system mathematically expressed in relation to pressure and volume change?

w = -Pext * ΔV Signup and view all the answers

What does Δn represent in the context of the ideal gas equation?

The change in the number of moles of gas. Signup and view all the answers

In the reaction 3H2(g) + N2(g) → 2NH3(g), what is the work done when the volume decreases?

It is a negative value. Signup and view all the answers

Which of the following scenarios would result in no work being done?

A gas remains at constant volume. Signup and view all the answers

According to Avogadro’s law, what happens to the volume of gas when the number of moles increases?

Volume increases. Signup and view all the answers

What expression signifies the change in internal energy (ΔU) of a system?

ΔU = q + w Signup and view all the answers

In a chemical reaction where the moles of gaseous products are greater than those of the gaseous reactants, what effect does this have on the work?

Work done is positive. Signup and view all the answers

What is the nature of the lattice energy (U) for an ionic solid?

It is a measure of the energy required to break the ionic solid into individual gaseous ions. Signup and view all the answers

According to Coulomb's law, what factor increases the force of attraction between ions?

Decreasing the separation distance or increasing the charges. Signup and view all the answers

What happens to the lattice energy as the cation size decreases when anion size is constant?

Lattice energy increases. Signup and view all the answers

In the Born-Haber cycle, which step is typically exothermic?

Formation of NaCl(s) from its gaseous ions. Signup and view all the answers

What is the final energy result (DrxnH) when Na+(g) and Cl-(g) combine to form NaCl(s)?

-787 Signup and view all the answers

How is the total energy of the reaction calculated when forming NaCl(s)?

It requires subtracting the total energy injected into the system from the energy released. Signup and view all the answers

What is required for the stability of a crystal lattice?

An exothermic overall reaction involving the formation of the lattice. Signup and view all the answers

Which statement correctly describes the energy involved in the atomization of chlorine?

It requires energy to dissociate Cl2(g) into Cl(g). Signup and view all the answers

What characterizes an open thermodynamic system?

It can exchange matter and energy with its surroundings. Signup and view all the answers

How is total internal energy (U) defined?

U = Ek + Ep for all molecules or ions. Signup and view all the answers

What does a negative change in internal energy (ΔE) indicate?

Energy has left the system. Signup and view all the answers

Which of the following is true regarding state functions?

State functions depend solely on the present state of the system. Signup and view all the answers

What occurs during a reversible process concerning state functions?

The overall change in state functions remains unchanged. Signup and view all the answers

Which system is defined as one with no exchange of energy or matter with its surroundings?

Isolated system Signup and view all the answers

What formula represents the concept of physical work in thermodynamics?

w = F x d Signup and view all the answers

What condition does NOT affect the internal energy of a system?

The path taken to achieve those characteristics Signup and view all the answers

What is the total bond dissociation energy required to break the bonds for the reaction H2(g) + Cl2(g) → 2HCl(g)?

679 kJ Signup and view all the answers

In the enthalpy change calculation for the reaction 2NH3(g) + Cl2(g) → N2H4(g) + 2HCl, which bond dissociation energy is positive?

DdissHN—H Signup and view all the answers

Which formula correctly represents the calculation for DrxnH0 using bond dissociation energy?

DrxnH0 = DdissHH—H + DdissHCl—Cl - 2DdissHHCl Signup and view all the answers

What is the correct enthalpy change (DrxnH0) for the reaction CH4 + 3Cl2 → CHCl3 + 3HCl?

-672 kJ Signup and view all the answers

For the bonds formed in the reaction CH4 + 3Cl2 → CHCl3 + 3HCl, which bond is formed the most times?

H-Cl Signup and view all the answers

What does Coulomb's law describe regarding ionic interactions?

The force of attraction between opposite charges Signup and view all the answers

What is the bond dissociation energy of H-Cl as given in the content?

243 kJ/mol Signup and view all the answers

Which statement about bond dissociation energy and reactions is true?

Higher bond dissociation energy indicates more stable bonds. Signup and view all the answers

Study Notes

Thermodynamics

Thermodynamics is the study of energy and its transformations.
Chemical reactions either require or release energy.
Reactions occur due to a change in relative stability of reactants and products. Lower energy signifies higher stability.
Higher energy (less stable) indicates greater reactivity.

Energy

Energy is the capacity to do work or supply heat.
It can be kinetic (energy of motion) or potential (stored energy).
Kinetic energy (Ek) is calculated as 1/2 * mass * velocity².
Potential energy (Ep) is stored due to position or chemical bonds.

Energy Changes & The Laws of Thermodynamics

Energy is conserved; it cannot be created or destroyed, only transformed.
Thermal energy (heat) is the kinetic energy of molecular motion. Measured by temperature.
Heat is the transfer of thermal energy due to temperature differences.
Chemical energy is the potential energy stored in chemical bonds.

Important Concepts and Terms

A thermodynamic system is the specific part of the universe being studied.
Surroundings are everything outside the system.
Boundary separates the system from the surroundings.
Isolated systems neither exchange energy nor matter with surroundings.
Closed system exchange energy but not matter.
Open system exchanges both energy and matter.

Internal Energy and P-V Work

Internal energy (U) is the total kinetic and potential energy of molecules/ions in a system.
Change in internal energy (ΔU) = final internal energy - initial internal energy.
ΔU = q + w (where q = heat and w = work).
P-V work occurs due to volume changes within a system.
W = −PextΔV (where Pext is external pressure).

Internal Energy and P-V Work Continued

Energy loss (ΔU is negative) if energy leaves the system.
Energy gain (ΔU is positive) if energy enters the system.
State functions are independent of the path taken.
Internal energy is a state function (it only depends on the initial and final states of the system).

Internal Energy and P-V Work Continued 2

Change in a state function (e.g., ΔX) for a process is -ΔX for the reverse process.
Overall change in a state function is zero when returning to the initial state.
Physical work (W) = force x displacement (W = F x d).
In chemical systems, P-V work is common; it's related to volume changes.
Examples include chemical reactions producing gases (e.g., combustion of C₃H₈ + 5O₂ → 4H₂O + 3CO₂).

Internal Energy and P-V Work Continued 3

In a chemical reaction, work can be calculated as W = -P_extΔV
If there is no change in volume (ΔV=0), no P-V work is done.

Internal Energy and P-V Work Continued 4

At constant volume, q = ΔU and no P-V work is done.
At constant pressure, q = ΔH (enthalpy).
ΔH = ΔU + PΔV
Enthalpy (H) is a state function related to heat changes.
H = U + PV

Enthalpy

Enthalpy (ΔH) is a thermodynamic quantity representing the enthalpy change for a reaction at constant pressure. Important in reacting systems.
ΔH = ΣΔH_products - ΣΔH_reactants
Enthalpy change (ΔH) is a state function. Independent of pathway.
Endothermic process: ΔH is positive (system absorbs heat). Examples: ice melting.
Exothermic process: ΔH is negative (system releases heat). Examples: combustion of fuels.

Hess' Law

Hess' law states that the overall enthalpy change for a reaction is independent of the pathway, and it equals the sum of enthalpy changes for individual steps involved in the process.
Useful for calculating enthalpy values for complicated reactions from known individual enthalpy changes.

Calculating Enthalpy Changes from Heats of Formation

Heats of formation (Δ_fH^o) are the enthalpy changes when one mole of a substance is formed from its elements in their standard states. The standard state is 298K and 1 atm pressure).
We can calculate ΔH^o_rxn (standard enthalpy changes for a given reaction) by considering the heats of formation of the reactants and products, along with the stoichiometric amounts.
ΔH^o_rxn = Σ n Δ_fH^o(products) – Σ n Δ_fH^o(reactants) [n are stoichiometric coefficients]

Calculating Enthalpy Changes from Bond Dissociation Energies

Bond dissociation energies (BDE) are the enthalpy changes required to break a particular bond in one mole of gaseous molecules.
To calculate enthalpy changes of a reaction using Average Bond Dissociation Energies, consider the sum of the energies required to break bonds (reactants) and the product bonds. Adiss H ≈Σ bonds broken – Σ bonds formed

Coulombs Forces and Lattice Energies

Opposite charges attract. The strength of attraction depends on the magnitudes of the charges and the distance between them.
Lattice energy (U) is the energy needed to separate one mole of a crystalline ionic solid into its gaseous ions. It results from the electrostatic attraction between ions. Important in ionic compounds).
Larger charges and smaller distances between ions lead to larger lattice energies. For compounds with the same anion, compounds with smaller cations will have larger lattice energies because the smaller cations are closer to the anion and have a larger force of attraction.

Born-Haber Cycle

Hess's Law can be used to find Lattice Energy. A cycle used to evaluate multiple steps together to get the overall enthalpy of a reaction.

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Description

Explore the fundamental principles of thermodynamics, including energy transformations and the laws governing energy changes. Understand concepts such as kinetic and potential energy, as well as the relationship between heat and temperature. This quiz is essential for grasping the core ideas in thermodynamics.