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Standard 4 notes
Evidence that a chemical reaction has taken place might include a change
in temperature, production of bubbles, production of light, the formation of a
precipitate, etc. Rusting of metals, cooking food, bleaching, and cleaning
are examples of everyday chemical reactions. Boiling water is not a
chemical reaction. It is a physical reaction because the water molecules do
not split into hydrogen and oxygen. It is still water—its just water in the
form of a gas.
Reactants are the chemicals before the reaction takes place and products
are produced after the reaction has taken place. Usually the reactants are
written to the left of the arrow and the products to the right.
The chemical and physical properties of the reactants are often very different
from the chemical and physical properties of the products. A good example
of this is salt (NaCl). Na is a soft, solid, shiny, metal that reacts violently
with water. Chlorine is a poisonous gas. Put them together and you get a
product which is white, solid, and tastes great on fries. Another example is
water (H2O). Hydrogen is a flammable gas, and oxygen is a gas that causes
things to burn. But bond them together and you get a product that is a liquid
at room temperature that puts out fires. The point is that the properties of
the reactants are often very different from the properties of the products even
though they are made of the same atoms.
Just changing the ratio of atoms in a compound can drastically change the
properties of a compound. For example, water (H2O) has properties that are
very different from hydrogen peroxide (H2O2) even though they are both
made only of hydrogen and oxygen.
A chemical equation describes the molar proportions of reactants and
products in a chemical reaction. Consider this equation.
2H2 + O2  2H2O
This means that we need twice as many moles of hydrogen gas as oxygen
gas in order to make water. The molar ratio of hydrogen gas to oxygen gas
is 2:1. The molar ratio of oxygen gas to water is 1:2.
Conservation of Mass—Atoms are not created or destroyed in chemical
reactions. They just change partners. If we could mass the reactants in a
sealed, airtight container and make them react without opening the
container, we should find that the total mass before the reaction is the same
as the total mass after the reaction.
Conservation of Energy- Energy is not created or destroyed in a chemical
reaction. It just changes into different forms of energy. For example, there
is chemical energy stored in the bonds between hydrogen atoms in hydrogen
gas and there is chemical energy stored between the oxygen atoms in oxygen
gas. When they combine to make water some of the energy is turned into
heat, light, sound, and motion energy and some of the energy is now found
in the new bonds between oxygen and hydrogen in the water molecules.
The heat, light, sound, and motion energy is still somewhere in the universe,
so the total amount of energy before the reaction is the same as the total
amount after the reaction. If a reaction gives off heat energy it is called an
exothermic reaction. If the reaction takes in energy from the environment it
is called an endothermic reaction. An endothermic reaction will feel cold
because heat energy is leaving your hands and going into the reaction.
Chemical energy can also be changed to electrical energy. When an ionic
compound forms, one element donates its electrons to another element. If
we separate the element that is donating from the element that is accepting
and connect them with a wire, then the electrons have to go through the wire
to get from one to the other. Electrons moving through a wire make
electricity! This is how a battery works. Two different metals (electrodes)
are placed in an electrolyte solution (a solution containing ions) and
connected by a wire. The two electrodes are separated by a porous barrier.
Electrons will flow from one metal to the other through the wire. When the
electrons get to the other electrode they are taken up by positive ions in the
solution. Ions in the solution can move across the barrier to equalize the
difference in charge so that the electrons can keep flowing through the wire.
(page 655-658)