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Hess's Law

Hess's Law

There are two ways to determine the amount of heat involved in a chemical change: measure it experimentally, or calculate it from other experimentally determined enthalpy changes. Some reactions are difficult, if not impossible, to investigate and make accurate measurements for experimentally. And even when a reaction is not hard to perform or measure, it is convenient to be able to determine the heat involved in a reaction without having to perform an experiment.

Spin–Spin Coupling Constant: Overview

Spin–Spin Coupling Constant: Overview

In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must...

Electrophilic Addition of HX to 1,3-Butadiene: Thermodynamic vs Kinetic Control

Electrophilic Addition of HX to 1,3-Butadiene: Thermodynamic vs Kinetic Control

The addition of a hydrogen halide to 1,3-butadiene gives a mixture of 1,2- and 1,4-adducts. Since more substituted alkenes are more stable, the 1,4-adduct is expected to be the major product. However, the product distribution is strongly influenced by temperature; low temperature favors the 1,2-adduct, whereas the 1,4-adduct is predominant at high temperature.

Atomic Nuclei: Nuclear Spin State Population Distribution

Atomic Nuclei: Nuclear Spin State Population Distribution

Near absolute zero temperatures, in the presence of a magnetic field, the majority of nuclei prefer the lower energy spin-up state to the higher energy spin-down state. As temperatures increase, the energy from thermal collisions distributes the spins more equally between the two states. The Boltzmann distribution equation gives the ratio of the number of spins predicted in the spin −½ (N−) and spin +½ (N+) states.

IR Spectroscopy: Hooke's Law Approximation of Molecular Vibration

IR Spectroscopy: Hooke's Law Approximation of Molecular Vibration

A covalently bonded heteronuclear diatomic molecule can be modeled as two vibrating masses connected by a spring. The vibrational frequency of the bond can be expressed using an equation derived from Hooke's law, which describes how the force applied to stretch or compress a spring is proportional to the displacement of the spring. In this case, the atoms behave like masses, and the bond acts like a spring.
According to Hooke's law, the vibrational frequency is directly proportional to...

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

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At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...

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Related Experiment Video

Updated: Sep 28, 2025

Non-equilibrium Microwave Plasma for Efficient High Temperature Chemistry

Non-equilibrium Microwave Plasma for Efficient High Temperature Chemistry

Published on: August 1, 2017

HeH⁺ Collisions with H₂: Rotationally Inelastic Cross Sections and Rate Coefficients from Quantum Dynamics at

K Giri¹, L González-Sánchez², Rupayan Biswas³

¹Department of Computational Sciences, Central University of Punjab, Bathinda, Punjab 151401, India.

The Journal of Physical Chemistry. A

|April 1, 2022

Summary

We developed an accurate potential energy surface for HeH+-H2 interactions. Hydrogen molecules (H2) are highly effective at exciting helium hydride ions (HeH+) in interstellar conditions.

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Direct Imaging of Laser-driven Ultrafast Molecular Rotation

Direct Imaging of Laser-driven Ultrafast Molecular Rotation

Published on: February 4, 2017

Area of Science:

Physical Chemistry
Chemical Physics
Computational Chemistry

Background:

Helium hydride ion (HeH+) is a key species in interstellar chemistry.
Understanding HeH+ interactions with other molecules is crucial for modeling interstellar environments.

Purpose of the Study:

To compute an accurate four-dimensional potential energy surface for the HeH+-H2 system.
To investigate the rotational excitation of HeH+ by H2 collisions at interstellar temperatures.

Main Methods:

Ab initio electronic structure calculations were performed.
An artificial neural network was used to fit the potential energy surface.
Quantum close coupling calculations were employed to determine state-to-state cross sections and rate coefficients.

Main Results:

Accurate four-dimensional potential energy surface for HeH+-H2 was generated.
Rate coefficients for HeH+ rotational excitation by H2 were computed for interstellar conditions.
H2 was identified as a highly efficient collision partner for exciting HeH+.

Conclusions:

Reduced-dimensionality calculations agree well with four-dimensional results for non-rotationally excited HeH+.
H2 is more effective than H or He in inducing rotational excitation of HeH+ at interstellar temperatures.
The findings highlight the importance of HeH+-H2 interactions in interstellar chemical kinetics.