Atomic Theory Atoms are building blocks of elements Atoms of the same element are similar Different from atoms of other elements Two or more different atoms bond in simple ratios to form compounds

Chemical Equations: An expression in which symbols and formulas are used to represent a chemical reaction. sodium metal + chlorine gas  table salt (sodium chloride)

A chemical equation identifies the starting and ending chemicals as reactants and products: reactants products Formation of water 2H2 + O2  2H20 A chemical equation is balanced when it reflects the law of conservation of mass. Graphic: http://www.chemistry.ohio-state.edu/betha/nealChemBal/

The Law of conservation of mass states that matter cannot be created or destroyed in any chemical reaction The atoms in the reactants are rearranged to form new compounds, but none of the atoms disappear, and no new atoms are formed. H O H O H H Products Reactants

Remember that atoms don’t change in a chemical reaction. The number and kinds of atoms present in the reactants of a chemical reaction are the same as those present in the products. When stated this way, it becomes the law of conservation of atoms.

So: Chemical equations are balanced, when the numbers and kinds of atoms on each side of the reaction arrow are equal. H H O H O O H O H H H H Products Reactants 4 Hydrogen 4 Hydrogen 2 Oxygen 2 Oxygen

Coefficients, the numbers placed in front of formulas to balance equations. They indicate the number of particles present in the reaction. If a number is not present it is understood that 1 is the coefficient. 2 H2 + O2  2 H2O

2H2O subscript Coefficient Coefficients are used to balance equations. NEVER change or add subscripts!

When balancing a chemical equation, first you must determine the number of each kind of atom on each side of the yield arrow. Ca + O2  CaO 1 Ca atom 2 O atoms 1 Ca atom 1 O atom Notice that there are the same number of calcium atoms on each side of the equation but the number of oxygen atoms are not the same. This equation is not balanced.

Ca + O2  CaO 2 2 2 1 Ca atom 2 O atoms 1 Ca atom 2 O atoms 2 1 Ca atom 2 1 O atom 1 Ca atom 1 O atom Coefficients are needed to make the number of oxygen atoms equal. A coefficient 2 is placed in front of CaO. This coefficient indicates 2 Ca atoms and 2 oxygen atoms. Now calcium is not equal. A coefficient of 2 in placed in front of Ca The number of each kind of atom are equal on both sides of the yield arrow. This equation is now balanced and follows the law of conservation of mass.

Ca + O2  CaO 2 2 2 1 Ca atom 2 O atoms 1 Ca atom 2 O atoms 2 1 Ca atom 2 1 O atom 1 Ca atom 1 O atom Coefficients are needed to make the number of oxygen atoms equal. A coefficient 2 is placed in front of CaO. This coefficient indicates 2 Ca atoms and 2 oxygen atoms. Now calcium is not equal. A coefficient of 2 in placed in front of Ca The number of each kind of atom are equal on both sides of the yield arrow. This equation is now balanced and follows the law of conservation of mass.

2 2 The number of oxygen atoms are not the same so the equation is balanced with coefficients. Notice that the number of hydrogen and oxygen atoms are equal

Balancing Chemical Equations The following five steps can be used as a guide to balance chemical equations. Balance this chemical reaction. Sulfuric Acid reacts with sodium hydroxide to yield sodium sulfate and water Step 1: Write an unbalanced equation, using correct formulas for all reactants and products. H2SO4 + NaOH  Na2SO4 + H2O

Step 2: Inventory all atoms found in the equation H2SO4 + NaOH  Na2SO4 + H2O 2 H + 1 H = 3H 2 Na 1 S 1 S 4 O + 1 O = 5 O 4 O + 1 O= 5 O 1 Na 2 H

Step 3: Compare the number of each atom on each side of the equation Step 3: Compare the number of each atom on each side of the equation. Add coefficients to balance the number of atoms. Remember that adding a coefficient affects all elements in the compound. H2SO4 + NaOH  Na2SO4 + H2O 2 H + 1 H = 3H 2 Na 1 S 1 S 4 O + 1 O = 5 O 4 O + 1 O= 5 O 1 Na 2 H 2 2 4H Equal Equal 6O 6 O 4H 2 Na

Step 4: Check the equation to make sure the numbers and kinds of atoms on both sides of the equation are same. H2SO4 + 2 NaOH  Na2SO4 + 2 H2O 2 H + 2 H = 4H 2 Na 1 S 1 S 4 O + 2 O = 6 O 4 O + 2 O= 6 O 2 Na 4 H

Step 5: Make sure the coefficients are reduced to their lowest whole-number value (ok here). H2SO4 + 2 NaOH Na2SO4 + 2 H2O 1 : 2: 1 : 2