## Thursday, July 18, 2019

### Unizor - Physics4Teens - Energy - Atoms

Notes to a video lecture on http://www.unizor.com

Atoms and Chemical Reaction

Discussing mechanical energy, we analyzed the movement of objects.

When talking about thermal energy (heat), we had to go deeper inside the objects and analyzed the movement of molecules, the smallest parts of objects that retained the properties of objects themselves.

Now we go even deeper, inside the molecules, in search of new kinds of energy.

The components of molecules are called atoms. Currently there are more than 100 kinds of atoms, classified in the Mendeleev's Periodic Table.

Different combinations of these atoms in different quantities make up all kinds of molecules, each with its own properties.

In some cases a single atom makes up a molecule. For example, a single atom of iron (denoted by symbol Fe) makes up a molecule of iron.

In some other cases a pair of atoms of the same type makes up a molecule. For example, two atoms of oxygen (denoted by symbol O) make up a molecule of oxygen (denoted by symbol O2).

In more complicated cases a few atoms of different types make up a molecule. For example, two atoms of hydrogen (denoted by symbol H) and one atom of oxygen (O) make up a molecule of water (H2O).

One of the most complicated molecules that contains many elements in
different quantities is a molecule of protein that has about half a
million of atoms.

Chemical energy is a potential energy of bonds between atoms that hold them together in a molecule.

Chemical reaction is a process of re-arranging of atoms in a group of molecules, getting, as a result, a group of other molecules.

During chemical reactions some bonds between atoms are broken and some are created. Therefore, the energy might be either released or consumed in the process of chemical reaction.
This energy, stored in the molecules as potential energy of atomic
bonds and released or consumed during chemical reaction, is classified
as chemical energy.

Let's consider a few examples of chemical energy.

1. Coal burning

One molecule of carbon, that consists of one carbon atom C, and one molecule of oxygen, that consists of two oxygen atoms O2, when brought together and lit up, will join into one molecule of carbon dioxide CO2
in the process of burning. After the chemical reaction of burning is
initiated, it will maintain itself, as the process of burning produces a
flame that lights up new molecules of carbon, joining them with oxygen.

The chemical reaction

C + O2 = CO2

is endothermic (consumes heat energy) in the very beginning, when we
have to light up the carbon, but, as soon as the reaction started, it
becomes exothermic, that is it produces heat energy, because the
potential energy of atoms inside molecules of carbon and oxygen together
is greater than potential energy of atoms inside a molecule of carbon
dioxide.

2. Making water from hydrogen and oxygen

Two molecules of hydrogen, each consisting of two hydrogen atom H2, and one molecule of oxygen, that consists of two oxygen atoms O2, when brought together and lit up, will join into two molecules of water H2O
in the process of hydrogen burning. After the chemical reaction of
burning is initiated, it will maintain itself, as the process of burning
produces a flame that lights up new molecules of hydrogen, joining them
with oxygen.

The chemical reaction

2H2 + O2 = 2H2O

is endothermic (consumes heat energy) in the very beginning, when we
have to light up the hydrogen, but, as soon as the reaction started, it
becomes exothermic, that is it produces heat energy, because the
potential energy of atoms inside two molecules of hydrogen and one
molecule of oxygen together is greater than potential energy of atoms
inside two molecules of water.

3. Photosynthesis

This is a complicated process, during which the light from sun, air
components (such as carbon dioxide, nitrogen and oxygen), water and
whatever is in the soil are converted by the plants into chemical energy
that maintains their life. This is an endothermic process, and, as its
result, plants grow. In most cases they consume carbon dioxide from the
air, break it into carbon and oxygen, consume the water from the soil,
break it into hydrogen and oxygen (they need sun's radiation energy to
break the molecules of CO2 and H2O),
use the carbon, hydrogen and part of oxygen to produce new organic
molecules they consist of and release the unused oxygen back into
atmosphere.

4. Battery

Battery consists of three major components: anode, cathode and electrolyte
in-between anode and cathode. As a result of a chemical and
electro-magnetic reaction between the molecules of anode and
electrolyte, some electrons are transferred from anode to electrolyte.
Then, as a result of a chemical and electro-magnetic reaction between
the molecules of cathode and electrolyte, some electrons are transferred
from electrolyte to cathode. As the result, there are extra electrons
on the cathode, which were taken from the anode, thus creating
electrical potential.

These simple examples explain the general mechanism of chemical energy,
released or consumed in the course of chemical reaction, that transforms
molecules by rearranging their atoms' composition. As a result of a
chemical reaction and change in the atomic composition of molecules,
potential energy of bonds between atoms in molecules is changing. If the
total potential energy of the resulting molecules is greater than the
potential energy of the bonds inside original molecules, the process is
endothermic, it consumes energy. In an opposite case the process is
exothermic, it produces energy.

The exothermic process of extracting chemical energy using chemical
reaction is the key to getting energy from gasoline in the car engine,
producing heat and light in the fireplace by burning wood, it's the
source of energy in all living organisms, including humans. We exist
because our body knows how to extract chemical energy from the food.