Thermodynamics Problem 6.4

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Questions and Answers

What is the relationship between internal energy change (∆U) and heat at constant volume?

∆U is negative of the heat supplied.

∆U is equal to heat supplied at constant volume. (correct)

∆U is independent of heat supplied.

∆U is always greater than the heat supplied.

Under what conditions does enthalpy change (∆H) become significant compared to internal energy change (∆U)?

When gases are involved in a reaction. (correct)

When reactions occur at low temperatures.

When only solids are involved.

When only liquids are involved.

If a reaction is carried out at constant atmospheric pressure, how is the internal energy change (∆U) expressed?

∆U = qp

∆U = -qp + p∆V

∆U = qp + p∆V

∆U = qp - p∆V (correct)

What is true regarding a reaction that is endothermic?

It absorbs heat from the surroundings. Signup and view all the answers

What does the variable 'pex' represent in the context of thermodynamics?

The external pressure. Signup and view all the answers

When ∆V equals zero, how does the equation for change in internal energy (∆U) look?

∆U = qV Signup and view all the answers

What is the significance of the subscript 'V' in the equation ∆U = qV?

It refers to the volume at which heat is supplied. Signup and view all the answers

What happens to the change in enthalpy (∆H) if a reaction is performed at constant pressure?

It equals the internal energy change (∆U). Signup and view all the answers

What does a positive value of ∆rH indicate about a reaction?

Heat is absorbed during the reaction. Signup and view all the answers

According to Hess's Law, how is the standard reaction enthalpy calculated?

It is the sum of the enthalpies of the individual steps. Signup and view all the answers

When combining equations in Hess's Law, which factor is crucial to ensure correctness?

The stoichiometry of the equations must be maintained. Signup and view all the answers

What is the enthalpy of atomization of H2 in kJ mol–1?

435.0 Signup and view all the answers

What is the standard reaction enthalpy for the reaction C + O2 → CO?

$-393.5 ext{ kJ/mol}$ Signup and view all the answers

If a reaction can occur in multiple steps, what can be inferred about its enthalpy change?

It is independent of the pathway taken to form products. Signup and view all the answers

Which of the following represents a reaction for atomization?

CH4(g) → C(g) + 4H(g) Signup and view all the answers

How does the enthalpy of atomization relate to the bond dissociation enthalpy in diatomic molecules?

They are the same for diatomic molecules. Signup and view all the answers

What is the enthalpy change for the reaction 2NH3 (g) → N2 (g) + 3H2 (g)?

+91.8 kJ/mol Signup and view all the answers

Which statement accurately describes a characteristic feature of enthalpy as a state function?

The enthalpy change is dependent on the initial and final states. Signup and view all the answers

What is the enthalpy change for the atomization of methane, CH4?

1665 kJ mol–1 Signup and view all the answers

What is represented by the symbol ∆ f H 0 in thermodynamics?

Standard enthalpy of formation. Signup and view all the answers

What does the term 'bond dissociation enthalpy' refer to?

Change in enthalpy when one mole of covalent bonds is broken. Signup and view all the answers

In the process of atomization for Na(s), what is the enthalpy change value?

108.4 kJ mol–1 Signup and view all the answers

How does the bond dissociation enthalpy differ in polyatomic molecules?

Different bonds within the same molecule have different values. Signup and view all the answers

Which statement accurately describes the bond breaking process in CH4?

C-H bond enthalpies differ with each bond broken. Signup and view all the answers

What is the enthalpy change associated with the formation of glucose from its elements?

-2361 kJ/mol Signup and view all the answers

Which reaction represents the combustion of benzene at standard conditions?

C6H6 (l) + 6O2 (g) → 6CO2 (g) + 3H2O (l) Signup and view all the answers

What is the bond dissociation enthalpy of the H-H bond according to the information provided?

435.0 kJ/mol Signup and view all the answers

What is the enthalpy change (ΔH) when 6 moles of graphite react with oxygen to form carbon dioxide?

-2802.0 kJ/mol Signup and view all the answers

Which equation correctly describes the formation of water from hydrogen and oxygen?

2H2 (g) + O2 (g) → 2H2O (l) Signup and view all the answers

What does the overall process of energy generation from food mimic?

Combustion processes Signup and view all the answers

What process is described by the equation H2(g) → 2H(g)?

Dissociation of hydrogen gas Signup and view all the answers

How many moles of oxygen are involved in the combustion of one mole of benzene?

6 moles Signup and view all the answers

What does the equation $orall H = U + pV$ represent?

The enthalpy of a system Signup and view all the answers

Which equation relates the change in enthalpy $orall H$ to the change in internal energy $orall U$ and the change in volume at constant pressure?

$orall H = orall U + p orall V$ Signup and view all the answers

What does the term $orall n_g$ refer to in the context of gaseous reactions?

The change in the number of moles of gaseous products Signup and view all the answers

When heat is absorbed by the system at constant pressure, which thermodynamic quantity is being measured?

The change in enthalpy Signup and view all the answers

Which of the following statements is true regarding the function $orall q$ and $orall H$?

$orall H$ is a state function and independent of path Signup and view all the answers

In an ideal gas, how is the total volume of gaseous products ($orall V_B$) expressed in terms of moles?

$pV_B = n_B RT$ Signup and view all the answers

What is the correct relationship expressed by the equation $orall riangle H = orall riangle U + n_g RT$?

The change in enthalpy is equal to the change in internal energy plus the change in moles times temperature Signup and view all the answers

How is the moles of gaseous reactants ($n_A$) defined in the context of the gaseous product equation?

The number of moles of gaseous reactants Signup and view all the answers

Study Notes

Thermodynamic Principles

The internal energy change (∆U) in a system relates to heat (q) and expansion work (w) by the equation: ∆U = q - p_ex∆V.
For constant volume processes (∆V = 0), the equation simplifies to ∆U = q_V, indicating heat change equals the internal energy change.
Enthalpy (H) is a state function defined as H = U + pV, crucial for processes under constant atmospheric pressure.

Heat Transfer and Enthalpy Changes

Positive ∆H indicates endothermic reactions, where the system absorbs heat.
The relationship between ∆H and ∆U is given by ∆H = ∆U + p∆V, facilitating calculations in reactions involving gases.
For gases, changes in volume significantly affect energy calculations due to p∆V.

Hess's Law

Hess's Law states the total enthalpy change of a reaction is the sum of the enthalpy changes for individual steps, regardless of pathway, reinforcing the principle that enthalpy is a state function.
Example of Hess’s Law application includes calculating the overall reaction enthalpy by summing enthalpy changes of intermediate reactions.

Bond Dissociation and Atomization Enthalpy

Bond dissociation enthalpy measures energy required to break a bond in a molecule, like the H–H bond in dihydrogen (H2).
The enthalpy of atomization (∆aH⁰) is the energy required to separate a compound into its gaseous atoms, significant for diatomic and polyatomic molecules.

Thermodynamic Equations for Gases

Ideal gas laws relate pressure, volume, and temperature to moles of gas in reactions, where changes in gaseous reactants/products affect enthalpy calculations: ∆n_g = n_B - n_A.
For reactions involving gases, the equation ∆H = ∆U + ∆n_gRT helps to relate internal energy and enthalpy changes effectively.

Practical Applications

The enthalpy of various reactions can be quantified, including common combustion reactions, enabling calculations of energy changes in biochemical processes.
Understanding these concepts is essential for predicting energy changes in chemical reactions, particularly in industrial and laboratory settings.

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Description

This quiz focuses on applying the principles of thermodynamics in solving hypothetical problems involving changes in internal energy. You will explore the relationships between heat, work, and volume during reversible processes. Test your understanding of the equations and conditions provided throughout the problem.