Introduction Introduction to Chemical to Chemical

Reactions and Reactions and EquationsEquations

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What are the clues What are the clues that a chemical that a chemical

reaction has occurred?reaction has occurred?

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A gas bubbles off.A gas bubbles off.

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A new solid material is A new solid material is formed (a precipitate.)formed (a precipitate.)

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Color change.Color change.

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Heat and/or light are Heat and/or light are emitted.emitted.

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The key to chemical The key to chemical reactions!reactions!

The Law of Conservation of Mass

In a chemical reaction, no atoms are created or destroyed. The atoms in the reacting molecules simply re-arrange to

form new molecules.

Since atoms aren’t created or destroyed, no mass is created or destroyed either.

Mass is “conserved.”7

Count the atoms, before & Count the atoms, before & afterafter

2H2 + O2 2H2O8

In the reactionN2 + 3H2 2NH3

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• If 14 grams of N2 reacted with 3 grams of H2, how many grams of ammonia would be produced?

In the reaction2NaCl 2Na + Cl2

• If 58.5 grams of NaCl is decomposed, and 23 grams of Na is formed, how many grams of Cl2 must also be formed?

What else is What else is conserved?conserved?

• In N2 + 3H2 2NH3 besides mass (grams), what else is being conserved?

oAtoms?

oMolecules?

oMoles?

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CoefficientsCoefficients• In a chemical reaction, if more than 1 molecule of

a substance reacts or is produced, the number of molecules is shown with a coefficient.

• 6CO2 + 6H2O C6H12O6 + 6O2

• Coefficients multiply subscripts through the whole molecule that follows, so 6CO2 shows that there are 6 Carbon atoms and 12 Oxygen atoms.

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How many?How many?

• How many of each atom are shown by 4Al2(SO4)3

• How many of each atom are shown by3(NH4)3PO4

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TerminologyTerminology• In a chemical reaction, the materials to the left of

the arrow are the reactants. Reactants are consumed in the reaction.

• The arrow is like an equal sign in math. It can be read “react to produce” or “produces” or “forms”.

• The materials to the right of the arrow are the products. They form as the reaction proceeds.

• H2O + SO3 H2SO4

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Phase NotationPhase Notation• A chemical equation can also show the physical

state of the materials in the reaction. This is done with phase notations:

• (s) solid (l) liquid (g) gas• (aq) aqueous or dissolved in water• (ppt) or ( ) means “a precipitate forms”• ( ) means “a gas bubbles off”• means “is heated”• Phase notations are written to the lower right of

each formula. Clues to phase notations are often given in equations written in words.

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An exampleAn example• When pellets of sodium hydroxide are dropped

into an aqueous solution of iron(III) chloride, iron(III) hydroxide precipitates and sodium chloride stays dissolved.

• 3NaOH(s) + FeCl3(aq) Fe(OH)3 (ppt) + 3NaCl(aq)

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Now onward to Now onward to “balancing chemical “balancing chemical

equations”equations”

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