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Thermo Chemistry

How to make artificial snow? How to make artificial snow? A gas when expands from high pressure to low pressure cools down. If there is no supply of heat energy to the gas, then the cooling is very rapid and it can convert the moisture in the atmosphere to snow. This is according to first law of thermodynamics, where in an adiabatic expansion (no heat exchange) a system loses its internal energy to do the expansion work. Since the internal energy is the function of temperature, the temperature of the system drops abruptly.

Learning Objectives

After completing the topic, the student will be able to:

  • Define system and surroundings and the list different types of systems.
  • State the thermodynamic properties and identify the extensive and intensive properties.
  • Describe three types of thermodynamic equilibria: thermal equilibrium, mechanical equilibrium and chemical equilibrium.
  • Explain internal energy, work and heat: distinguish the state function and path function.
  • Discriminate between isobaric, isothermal, isochoric and adiabatic processes.
  • State first law of thermodynamics and express it mathematically.
  • State Hess's law and measure the change in enthalpy(ΔH) of a process.
  • State zeroth law of thermodynamics and give its application.
  • Explain entropy as a thermodynamic state function and apply it for spontaneity.
  • State second law of thermodynamics and explain the spontaneity criteria in terms of Gibbs free energy change.
  • Give the relation between free energy change, equilibrium constant of a reaction and net EMF of an electrochemical cell and solve the problems based on this relation.
  • State third law of thermodynamics and apply Nernst heat theorem to calculate transition entropies.
Steam engine and cooking food Steam engine works by burning coal. Food is cooked on stove by burning natural gas.

It is the study of heat and its transformation to mechanical energy (in the Greek, thermo means "movement of heat"). Heat is a form of energy. We utilize the energy in daily activities like cooking, burning the coal etc. In these processes, heat is transformed into another form of energy i.e., work.

According to law of conservation of energy, “Energy can neither be created nor destroyed but it can be converted from one form to another”. So, heat can be converted into work and vice‐versa. Based on the relation with heat, chemical reactions are classified into two categories: exo‐thermic (releasing heat) and endo‐thermic (consuming heat) reactions.

Energy changes in a chemical reaction

Almost all the changes are due to change in energy. For example, breaking of existing bonds and making of new bonds involve the energy change. During these changes, some amount of energy is either absorbed or released. Based on the stability of products, these changes are called by different names, like physical or reversible change, chemical or irreversible change, etc. Sometimes, a physical change is also called as temporary change and a chemical change is called as a permanent change.

Best examples for physical change are freezing, evaporation followed by condensation etc; while respiration, photosynthesis, combustion of fuels represent a chemical change. Let us study some more characteristics of these changes.

Touch your own forehead. It’s warm. All living organisms are warm to touch. When they die, their bodies are cold. Why do you think this is so? The cells of living organisms are constantly make into a new chemicals and energy is released in these changes. This makes a living organism warm to touch.

Sometimes, chemical changes and physical changes can occur at the same time. For example, when sugar is heated, it first melts (physical change) and then undergoes decomposition (chemical change).

Let us take another example. When sodium nitrate is heated, it melts (physical change), then it decomposes into a new chemical, sodium nitrite and oxygen (chemical change).

When wood is heated, gases such as carbon dioxide and water vapor are liberated. The ash that remains is mostly carbon and other non‐volatile substances.

In simple words, when a physical change occurs, inter-molecular bonds are stretched or broken. But when a chemical change occurs, inter-molecular bonds are first broken and then rearranged to form new bonds and new molecules.

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