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For an ensemble consisting of a mole of particles having two energy levels separated by \(1000 . \mathrm{cm}^{-1},\) at what temperature will the internal energy equal \(3.00 \mathrm{kJ} ?\)

Short Answer

Expert verified
Using the Boltzmann distribution and the internal energy formula, we can set up an equation to find the temperature at which the internal energy of a mole of particles with two energy levels separated by 1000 cm鈦宦 equals 3.00 kJ. Convert the energy difference to joules, calculate the partition function, and use a numerical method to solve the equation: \[3000 \,\text{J} = 6.022 \times 10^{23} \frac{E_1}{1 + e^{-\frac{E_1}{k_{B}T}}} e^{-\frac{E_1}{k_{B}T}}\] Once the temperature, T, is found, it will be the temperature at which the internal energy equals 3.00 kJ.

Step by step solution

01

Convert the energy difference into joules

First, let's convert the energy difference between the two levels from wavenumbers (cm鈦宦) to joules (J). We can use the conversion factor: 1 cm鈦宦 鈮 1.986 x 10鈦宦猜 J So, the energy difference in joules is: \[E = 1000 \,\text{cm}^{-1} \times 1.986 \times 10^{-23} \,\text{J}\]
02

State the Boltzmann distribution

The Boltzmann distribution provides us with the probability of finding particles in different energy states at a given temperature. The distribution is given by the formula: \[P(E) = \frac{1}{Z} e^{-\frac{E}{k_{B}T}}\] where - \(P(E)\) is the probability of finding a particle with energy E, - \(Z\) is the partition function, - \(E\) is the energy of the state, - \(k_B\) is the Boltzmann constant (鈮 1.381 x 10鈦宦猜 J/K), - \(T\) is the temperature in kelvins.
03

Calculate the partition function

The partition function \(Z\) is given by the sum of probabilities for each energy state. Since we have two energy levels, one ground state with energy \(E_0 = 0\) and the excited state with energy \(E_1\): \[Z = e^{-\frac{E_0}{k_{B}T}} + e^{-\frac{E_1}{k_{B}T}}\] Since the ground state energy level is 0, this simplifies to: \[Z = 1 + e^{-\frac{E_1}{k_{B}T}}\]
04

Calculate the internal energy

The internal energy of the system is given by the weighted average of the energy levels and their probabilities: \[U = N\sum_{i} P(E_i)E_i\] In our case, there are two energy levels, so: \[U = N \left[P(E_0)E_0 + P(E_1)E_1\right]\] where - \(U\) is the internal energy (3.00 kJ = 3000 J), - \(N\) is the number of particles (1 mole, N 鈮 6.022 x 10虏鲁 particles). Since the ground state energy level is 0, this simplifies to: \[U = NP(E_1)E_1\]
05

Solve for the temperature

Now we can insert Boltzmann distribution for \(P(E_1)\) and solve for the temperature: \[U = N\frac{E_1}{Z} e^{-\frac{E_1}{k_{B}T}}\] Substitute \(Z = 1 + e^{-\frac{E_1}{k_{B}T}}\): \[3000 \,\text{J} = 6.022 \times 10^{23} \frac{E_1}{1 + e^{-\frac{E_1}{k_{B}T}}} e^{-\frac{E_1}{k_{B}T}}\] This equation cannot be solved analytically, but it can be solved numerically using computational methods (such as the bisection method or the Newton-Raphson method) to find the temperature T. Once you solve for T, you will have the temperature at which the internal energy of the given ensemble equals 3.00 kJ.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Boltzmann distribution
The Boltzmann distribution is a statistical way to describe how particles are distributed among various energy levels at a specific temperature. It explains that particles at higher energy states are less likely to be occupied than those at lower energy states. This distribution is essential for understanding thermodynamics and statistical mechanics because it links temperature with how energy is distributed within a system. For any energy level, the probability of finding a particle with that specific energy, \(P(E)\), is given by the formula: \[P(E) = \frac{1}{Z} e^{-\frac{E}{k_{B}T}}\]Here, \(Z\) is the partition function, \(E\) is the energy of the state, \(k_B\) is the Boltzmann constant, and \(T\) is the temperature in kelvins. Understanding this formula is fundamental in predicting the behavior of molecules at different temperatures, thus providing insight into phenomena such as chemical reactions and phase transitions. The Boltzmann distribution helps chemists and physicists explain why some reactions occur more readily at elevated temperatures.
Partition function
The partition function, denoted as \(Z\), is a crucial concept in statistical mechanics. It acts as a normalization factor that ensures all probabilities sum to one, thus serving as a key to understanding the macroscopic properties of a system based on its microscopic states. In the context of a system with multiple energy levels, the partition function is calculated as:\[Z = \sum_{i} e^{-\frac{E_i}{k_{B}T}}\]For our specific problem with two energy levels, it simplifies to:\[Z = 1 + e^{-\frac{E_1}{k_{B}T}}\]The knowledge of \(Z\) allows us to determine numerous thermodynamic properties, such as internal energy and entropy, directly from the microscopic details of a system. As temperature rises, the partition function tends to increase since higher energy states become more accessible. Understanding the partition function is essential for predicting how systems behave under different thermal conditions, as it links the microstate energy levels to macroscopic observations.
Energy levels
Energy levels represent quantized states of a system in which particles exist. These discrete states are fundamental in quantum mechanics and in understanding the behavior of particles at the atomic and molecular levels. In any given system, particles such as electrons can only occupy certain allowed energy levels. The lowest energy state is often referred to as the ground state, while higher energy levels are known as excited states. The separation between these energy levels determines a plethora of physical and chemical properties, including spectral lines and the system's stability.For our given ensemble with energy levels \(E_0 = 0\) and \(E_1\), understanding how energy levels are populated according to the Boltzmann distribution provides insight into the thermal behavior and internal energy of the system. Energy levels are central to determining reactions, transitions, and many more chemical processes.
Numerical methods
Numerical methods are powerful tools used when analytical solutions to equations cannot be achieved. They involve computational algorithms to approximate solutions to mathematical problems. In thermodynamics and statistical mechanics, such methods are indispensable for solving complex equations derived from experimental data or theoretical models. In the problem we examined, solving for the temperature at which the internal energy equals 3.00 kJ involves a transcendental equation that cannot be solved with traditional algebra. Instead, numerical approaches like the bisection method or the Newton-Raphson method are employed to iteratively find an approximate solution. These techniques help scientists and engineers model complex systems numerically, ensuring accuracy and efficiency while addressing real-world questions that arise in chemical physics and various other fields. Hence, learning numerical methods is essential for students and professionals dealing with computations in physical sciences.

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Most popular questions from this chapter

Using the Helmholtz energy, demonstrate that the pressure for an ideal polyatomic gas is identical to that derived for an ideal monatomic gas in the text.

Consider an ensemble of units in which the first excited electronic state at energy \(\varepsilon_{1}\) is \(m_{1}\) -fold degenerate, and the energy of the ground state is \(m_{o}\) -fold degenerate with energy \(\varepsilon_{0}\) a. Demonstrate that if \(\varepsilon_{0}=0,\) the expression for the electronic partition function is \\[ q_{E}=m_{o}\left(1+\frac{m_{1}}{m_{o}} e^{-\varepsilon_{1} / k T}\right) \\]\ b. Determine the expression for the internal energy \(U\) of an ensemble of \(N\) such units. What is the limiting value of \(U\) as the temperature approaches zero and infinity?

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Calculate the internal energy of He, Ne, and Ar under standard thermodynamic conditions. Do you need to redo the entire calculation for each species?

The three lowest energy levels for atomic carbo (C) have the following energies and degeneracies: $$\begin{array}{ccc} \text { Level }(\boldsymbol{n}) & \text { Energy }\left(\mathrm{cm}^{-1}\right) & \text {Degeneracy } \\ \hline 0 & 0 & 1 \\ 1 & 16.4 & 3 \\ 2 & 43.5 & 5 \end{array}$$ Determine the electronic contribution to \(C_{V}\) for atomic \(\mathrm{C}\) at \(100 . \mathrm{K}\)

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