Chemical reactions

• A chemical reaction occurs when substances react to form a new substance with different properties.
• Example: combustion (burning) reactions

Reactants vs. products

• Reactants: substances that are used up during a reaction; starting substances

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• Products: substances that are produced during a reaction; ending substances

Reactants

Products

Describing chemical reactions

• Chemists use equations to describe chemical reactions.
• There are 2 different kinds of equations:
• Word Equations

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• Chemical Equations

Word Equations

• Word Equations: names of the reactants (starting substances) and products (ending substances) are written out in full.

• Example:

Iron + sulfur

🡪

iron(II) sulfide

+ energy

reactants

products

Chemical Equations

• Chemical Equations: chemical formulas of the reactants and products are used.

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• Example:

Fe (s) +

S (s)

🡪

FeS (s) + energy

Similarities between word and chemical equations

• An arrow indicates the direction in which the chemical reaction is going. The arrow is read as “yields,” “forms,” or “produces.”
• Substances to the left of the arrow are called

reactants.

• Substances to the right of the arrow are called

products.

• “+” signs are placed between the reactants or products if more than one are involved or formed.

State symbols

Tell us the state, or form, of each substance in a chemical equation

Example:

• Write the word and chemical equation for the following:

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Magnesium metal burns in oxygen gas with a bright white light to make a white powder called magnesium oxide.