Chemistry notes

Topics: Energy, Thermodynamics, Enthalpy Pages: 10 (2259 words) Published: June 19, 2014
Lecture 12 Chapter 6

6.1 Thermochemistry: Energy & units (p.227)

From observation we know, that
- some chemical reactions begin as soon as the reactants come into contact with each other (precipitation reactions)
- some reactions are slow or even so slow at room temperature that even lifetime is not enough to observe a measurable change (rusting of iron, tarnishing silver) Also, almost all chemical reactions involve exchange of heat (or energy): in combustion reactions great deal of heat is released

in neutralization reactions and others.

Transfer of heat is a major theme of thermodynamics, the science of heat exchange (or energy exchange).

Q: Why it is important to know the quantity of heat (energy) involved in a chemical reaction?
Answer: when chemical reaction takes place the rearrangement of atoms takes place and some bonds have to be broken (energy is needed) and new bonds form (energy is released) we can learn about the strength of the bonds, we can also predict whether the reaction will occur.

Q: How to define energy? or What is energy?

Energy is capacity to do work. (or ability to do work)

Unlike matter, energy is known and recognized by its effect. It cannot be seen, touched, smelled, or weighed.

Different forms of energy:
Kinetic energy, Ek: energy associated with the moving object, it can be calculated, if we know the mass of an object and it speed or velocity Ek = m v2 units of energy: kg x m2/s2 is called joule (J) Examples of kinetic energy:

energy of moving baseball
electric energy of electrons moving through a conductor
radiant energy from the sun
thermal energy of atoms, molecules ,or ions at the submicroscopic level Kinetic energy is often expressed by common unit of energy, that is by calorie (cal)

1 calorie is the amount of energy needed to raise the temperature of one gram of water, H2O, by one oC.
1 cal = 4.184 J

Potential energy, Ep: energy that results from an object’s position in a field of force. Is often referred to as stored energy. Examples:
chemical potential energy of electrons and atomic nuclei in molecules gravitational energy (Ep = m x g x h) as for a ball held above the floor or table electrostatic energy between positive and negative ions a small distance apart chemical energy stored within the structured units of chemical substances (when the chemical reaction occurs, chemical energy is released, stored, or converted to other forms of energy

Internal energy, (U): the sum of kinetic and potential energy of the subatomic particles (electrons and nuclei) making up the substance. So, the total energy of a certain quantity of substance is

Etot = Ek + Ep + U
When a substance is at rest (not moving): Ek = 0 When a substance is at constant level (on the table) Ep = 0 So, Etot = U internal energy (any material object has internal energy)

Energy can neither be created nor destroyed; the total energy of the universe is constant.

Law of conservation of energy
Energy can be converted from one form to another, but the total quantity of energy remains constant.

6.2 First Law of Thermodynamics; Work & Heat (p. 230)

In thermodynamic processes (processes that involve the heat or energy flow) we deal with the system and the surrounding. The system is the substance or mixture of substances in which a change occurs. The surrounding is everything outside the system.

For example: if we study the reaction between A and B (in a solution) the reaction mixture is the system, and the beaker (reaction vessel), the air above the solution and outside the beaker is a surrounding. ( see Fig. 6.4 p.230)

Almost all chemical reactions absorb or release (produce) energy in the form of heat.

Heat is the transfer of thermal energy between two bodies that are at different temperatures.

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