Chemical Reactions

Chemical and Physical Changes

Chemical reactions are fundamental processes in chemistry where substances are transformed into new substances. It is important to distinguish between chemical changes and physical changes:

• Physical Change: The chemical composition remains constant. Examples include melting ice or boiling water.

• Chemical Change: The chemical composition changes, resulting in the formation of new substances.

Evidence for Chemical Reactions

There are four main observations that indicate a chemical reaction is occurring:

1. Gas Production

• Gas formation can be observed as fizzing or bubbling during a reaction.

• Example: The release of hydrogen gas when magnesium reacts with acid.

2. Formation of an Insoluble Solid (Precipitate)

• When two aqueous solutions are mixed, a solid may form, called a precipitate.

• This is evidence of a new substance being produced.

3. Permanent Color Change

• A lasting color change often signals the formation of a new substance.

• Example: Mixing chemicals that result in a color shift.

4. Energy Change

• Energy changes are observed as heat release (exothermic) or heat absorption (endothermic).

• Example: Combustion reactions release heat and light.

Writing Chemical Equations

Structure of Chemical Equations

A chemical equation uses formulas and symbols to describe a chemical reaction. The general form is:

• Reactants: Substances present at the start (A and B).

• Products: Substances formed (C and D).

States of Matter in Equations

Physical states are specified in chemical equations:

• (s): Solid

• (l): Liquid

• (g): Gas

• (aq): Aqueous solution (dissolved in water)

Example of a Chemical Reaction

Consider the reaction:

• Aqueous acetic acid + solid sodium bicarbonate → aqueous sodium acetate + liquid water + carbon dioxide gas

Diatomic Molecules

Seven Diatomic Elements

Some elements naturally exist as molecules composed of two atoms. The seven diatomic nonmetals are:

• Hydrogen (H2)

• Nitrogen (N2)

• Oxygen (O2)

• Fluorine (F2)

• Chlorine (Cl2)

• Bromine (Br2)

• Iodine (I2)

These elements are written as two-atom molecules in chemical equations. On the periodic table, they form a "7" shape.

Subscripts and Coefficients in Chemical Equations

Understanding Subscripts and Coefficients

Subscripts and coefficients in chemical formulas convey different information:

• Subscripts: Indicate the number of atoms of each element in a molecule.

• Coefficients: Indicate the number of molecules.

For example:

• means 2 molecules of water (each with 2 H and 1 O atoms).

• means 1 molecule of hydrogen peroxide (with 2 H and 2 O atoms).

Balancing Chemical Equations

Principles of Balancing

Balancing chemical equations ensures the number of atoms of each element is the same on both sides. The steps are:

1. Check that all formulas are correct before adding coefficients.

2. Never change subscripts to balance an equation.

3. Balance each element, starting with the most complex formula.

4. If polyatomic ions appear on both sides, balance them as a unit.

5. Coefficients must be whole numbers. If fractions appear, multiply the equation by the denominator.

6. After balancing, verify that the number of atoms for each element is equal on both sides.

7. Ensure the smallest whole number ratio of coefficients.

Example equation:

This equation is balanced because there are 4 hydrogen atoms and 2 oxygen atoms on both sides.

Additional info: Balancing equations is essential for quantitative chemical calculations and reflects the law of conservation of mass.