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Chemical change – reorganization of the atoms in one or more substances. Represented by a chemical equation with the reactants on the left side of an arrow and the products on the right side. CH 4 + O 2 → CO 2 + H 2 O ReactantsProducts Notice that the atoms have been reorganized. Bonds have been broken, and new ones have been formed.

In a chemical reaction, atoms are neither created nor destroyed. All atoms present in the reactants must be accounted for among the products. In other words, there must be the same number of each type of atom on the product side and on the reactant side of the arrow. Known as balancing a chemical equation. CH 4 + O 2 → CO 2 + H 2 O This equation is not balanced.

+ + CH 4 +O2O2 CO 2 H2OH2O+ Notice that the number of oxygen atoms (in O 2 ) on the left of the arrow is two, but on the right there are three O atoms (in CO 2 and H 2 O). Notice there are four hydrogen atoms (in CH 4 ) on the left and only two (in H 2 O) on the right. Remember atoms are not created nor destroyed in a reaction so the numbers of each type must occur in the reactants and products.

+ + By balancing each type of atom in the above representation we now have the same number of each type of atom in the reactants and the products. Written in shorthand by a chemical equation: CH 4 +2O 2 CO 2 +2H 2 O 1 C 4 H4 O1 C2 O 4 H2 O In summary: ReactantsProducts 1 C 4 H 4 O

Chemical equations give two important pieces of information: the nature of the reactants and products and the relative numbers of each. An equation usually gives the physical states of the reactants and products. Physical StateSymbol Solid(s)(s) Liquid(l)(l) Gas(g)(g) Dissolved in water (aqueous)(aq)

For example, when hydrochloric acid in aqueous solution is added to solid sodium hydrogen carbonate, the products carbon dioxide gas, liquid water, and sodium chloride (which dissolves in the water) are formed: HCl (aq) + NaHCO 3 (s) CO 2 (g) + H 2 O (l) + NaCl (aq) The relative numbers of reactants and products in a reaction are indicated by the coefficients in the balanced equation. Notice in the above reaction no coefficients are given because they are all understood to be one.

Balancing Chemical Equations When balancing a chemical equation you may add coefficients in front of the compounds to balance the equation but you may NOT change the subscripts. Changing the subscripts changes the compound.

Steps to Balancing Chemical Equations There are four basic steps to balancing a chemical equation.

Write the correct formulas for the reactants and products. DO NOT TRY TO BALANCE IT YET! You must write the correct formulas first. And most importantly, once you write them correctly DO NOT CHANGE THE FORMULAS! Aluminum sulfate and calcium hydroxide react to produce aluminum hydroxide and calcium sulfate. Al 2 (SO 4 ) 3 +Ca(OH) 2 →Al(OH) 3 +CaSO 4

Find the number of atoms of each element on the left side (reactant). Compare those against the number of atoms of the same element on the right side (product). You may find a t-chart helpful. Al 2 (SO 4 ) 3 +Ca(OH) 2 →Al(OH) 3 +CaSO 4 ReactantProduct 2 Al1 Al 3 S1 S 14 O7 O 1 Ca 2 H3 H

Al 2 (SO 4 ) 3 +Ca(OH) 2 →Al(OH) 3 +CaSO 4 ReactantProduct 2 Al1 Al 3 S1 S 14 O7 O 1 Ca 2 H3 H Determine where to place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element to balance the equation. 2 2 Al 10 O 6 H 3 3 S 3 Ca 18 O 3 3 Ca 6 H 18 O

Al 2 (SO 4 ) 3 +Ca(OH) 2 →Al(OH) 3 +CaSO 4 ReactantProduct 2 Al 3 S 2 2 Al 6 H 3 3 S 3 Ca 18 O 3 3 Ca 6 H 18 O Check your answer to see if: The number of atoms on both sides of the equation are now balanced. The coefficients are in the lowest possible whole number ratios (reduced).

Helpful Hints 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 not way to balance O with a whole number, double all the coefficients and try again. (Because O is diatomic as an element.) Polyatomic ions that exist on both sides of the equation should be balanced as independent units.