1.
What is meant by an electron volt (eV) ? Express this energy in (i) J, (ii) KJ mol
and (ii) Kcal moll
Calculate the force of attraction between an electron (charge = -16 Y 10-19
a body with a charge + 10
Answers
Answer:
Thermochemistry studies the contribution of chemical processes to thermodynamics, the science of energy transfer. Energy is often (unsatisfyingly) defined as the ability to do work, and can be classified as one of two types. Kinetic Energy is energy of motion, such as that possessed by a baseball thrown by a pitcher, a bullet shot from a gun, or a translating H2 gas molecule. Potential Energy is energy of position (technically position in a gradient "field"). The most familiar form of potential energy is gravitational, such as Newton's apple as it hung in the tree. More relevant to chemistry is the potential energy due to position in an electric or magnetic field, such as solvated ions, or atoms transferring charge when forming compounds or molecules. Another important form of potential energy is that found in a coiled spring. Bonded atoms exhibit strikingly similar behavior (vibrational motions) to the action of springs. Potential energy is often thought of as "stored" kinetic energy, meaning that bodies remain stationary in a potential field while held in place by some force, and upon change in this force (such as breaking the twig holding an apple, or breaking the bond between two atoms), potential energy is converted to kinetic form (the apple "falls" or the molecule "dissociates"). The energy unit is a derived physical quantity having dimension energy = mass × length2 × time-2. The SI unit of energy is the Joule (J):
Energy is often measured in other units, specific to particular applications. Some common examples are shown to the left.
Energy manifests itself in many forms:
Nuclear Energy - The potential energy required to bind nucleons in the nucleus
Light Energy - The potential energy possessed by the oscillating electric and magnetic fields that make up electromagnetic radiation
Chemical Energy - The potential energy stored in the electrostatic bonding relationships among atoms in a molecule
Electrical Energy - The potential energy involved in initiating and maintaining electron flow
Mechanical Energy - The energy generated (or stored) by machines which induces (or results from) concerted motion processes in a system
Heat Energy - The kinetic energy associated with random motion of matter including the vibratory and rotatory action of molecules
This lesson will focus on heat and mechanical energy. Subsequent lessons will separately deal with the implications of chemical, electrical, and light energy in chemistry.
The First Law of Thermodynamics
We begin by defining the Universe, or all-encompassing environment. The universe is partitioned into two sections- the system, consisting of the chemical or physical process of interest, and the surroundings which includes everything (and I do mean everything) else. This partitioning at first seems ridiculous, as the system is obviously an infinitessimal component of the universe. However in the realm of thermodynamics, or energy transfer, this definition makes it possible to solve many practical problems. We now categorize systems according to the following definitions.
A closed system does not allow matter in or out
An open system allows matter in or out
An adiabatic system does not allow energy (in this context heat energy) in or out
A diathermic system allows energy in or out.
In addition, we define an isolated system to be closed and adiabatic, or one that allows no heat or matter in or out.
We are now positioned to state the First Law of Thermodynamics:
"The universe is an isolated system"
Since no energy is allowed in or out we say that energy is conserved in the universe, or that there is a fixed amount of energy present and that it is forever interconverted among its various forms. It should be noted that energy can also be converted to matter according to Einstein's relationship:
where E is the energy, m the mass, and c the speed of light. Mass-energy interconversion is an important mechanism in the incredibly large potential field required to bind protons in a nucleus. Our study of the first law will focus on heat and mechanical energy transfer between a system and its surroundings.
An electrostatic potential map for water shows where electron density congregates. Regions in blue are electron-poor, varying to regions in red which are electron-rich.