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CHEMICAL REACTIONSCHEMICAL REACTIONS

Reactants: Zn + IReactants: Zn + I22 Product: Zn IProduct: Zn I22

5th Grade Physical Science

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Chemical Chemical EquationsEquations

Their Job: Depict the kind of Their Job: Depict the kind of reactantsreactants and and productsproducts and and their relative amounts in a reaction.their relative amounts in a reaction.

44 Al Al (s)(s) + + 33 OO2 (g)2 (g) ---> ---> 22 AlAl22OO3 (s)3 (s)

The The numbers in the frontnumbers in the front are called are called

stoichiometric ____________stoichiometric ____________

The letters (s), (g), and (l) are the The letters (s), (g), and (l) are the physical states of compounds.physical states of compounds.

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– Chemical equations show the conversion of reactantsreactants (the molecules shown on the left of the arrow) into productsproducts (the molecules shown on the right of the arrow).• A + sign separates molecules on the same

side• The arrow is read as “yields”• Example

C + O2 CO2

• This reads “carbon plus oxygen react to yield carbon dioxide”

Parts of a Reaction Parts of a Reaction EquationEquation

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• The charcoal used in a grill is basically carbon. The carbon reacts with oxygen to yield carbon dioxide. The chemical equation for this reaction, C + O2 CO2, contains the same information as the English sentence but has quantitative meaning as well.

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Because of the principle of the Because of the principle of the

conservation of matterconservation of matter, ,

an an equation must be equation must be balancedbalanced..

It must have the same It must have the same number of atoms of the number of atoms of the same kind on both sides.same kind on both sides.

Lavoisier, 1788Lavoisier, 1788

Chemical EquationsChemical Equations

66Subscripts vs. Subscripts vs. CoefficientsCoefficients

• The subscripts The subscripts tell you how tell you how many atoms of many atoms of a particular a particular element are in a element are in a compound. The compound. The coefficient tells coefficient tells you about the you about the quantity, or quantity, or number, of number, of molecules of molecules of the compound.the compound.

77Some Suggestions to Help Some Suggestions to Help YouYou

Some of Mrs. von Buelow’s Helpful Hints for balancing equations:• Take one element at a time, working left to

right except for H and O. Save H for next to last, and O until last.

• IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again. (Because O is diatomic as an element)

• (Shortcut) Polyatomic ions that appear on both sides of the equation should be balanced as independent units

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