Counting Atoms and Balancing Chemical Equations

Elements, Compounds, Mixtures Hydrogen is an element. Hydrogen is an element. Oxygen is an element. Oxygen is an element. When 2 hydrogen atoms and one oxygen atom bond they make the compound water. When 2 hydrogen atoms and one oxygen atom bond they make the compound water. The formula for water is H 2 O The formula for water is H 2 O

Chemical Formulas Each element in a chemical formula is identified by a capital letter. If an element’s symbol contains two letters, the second letter is lower case. Each element in a chemical formula is identified by a capital letter. If an element’s symbol contains two letters, the second letter is lower case. Example: H 2 SO 4 Example: H 2 SO 4 The elements in Sulfuric Acid The elements in Sulfuric Acid Hydrogen Hydrogen Sulfur Sulfur Oxygen Oxygen

Subscripts Subscripts tell us how many atoms of each element are present. C 12 H 22 O 11 C 12 H 22 O 11 There are 12 atoms of Carbon There are 12 atoms of Carbon There are 22 atoms of Hydrogen There are 22 atoms of Hydrogen There are 11 atoms of Oxygen There are 11 atoms of Oxygen If there is not a subscript listed, it is understood to be 1. If there is not a subscript listed, it is understood to be 1. Example: NaCl Example: NaCl There is one atom of Sodium There is one atom of Sodium There is one atom of Chlorine There is one atom of Chlorine

You Practice! NaHCO 3 Sodium – 1 Hydrogen – 1 Carbon – 1 Oxygen -3 HCl Hydrogen – 1 Chlorine - 1

There are times you will see a chemical formula with parenthesis. The subscript after the parenthesis affects everything inside the parenthesis. Pb(NO 3 ) 2 So, in counting the atoms, you would have the following: Lead – 1Oxygen - 6 Nitrogen -2

You Practice!! (NH 4 ) 3 PO 4 Nitrogen – 3 Hydrogen – 12 Phosphorus – 1 Oxygen – 4 Mg(OH) 2 Magnesium – 1 Oxygen – 2 Hydrogen – 2

Coefficient A coefficient identifies how many molecules are present. 2H 2 SO 4 This means there are 2 molecules of H 2 SO 4. Counting the atoms: Hydrogen – 4 Sulfur – 2 Oxygen - 8

Coefficient A coefficient can also be used to tell us how many atoms are present. 2Mg This means there are 2 atoms of Mg. Counting the atoms: Mg = 2 Mg = 2

You Practice!! 3H 3 PO 4 Hydrogen – 9 Phosphorus – 3 Oxygen H 2 O Hydrogen – 4 Oxygen - 2

Got It????

Chemical Equations Chemical equations express what is happening in a chemical reaction using symbols. 2K + Cl 2 2KCl Reactant Product yields

Law of Conservation of Mass In a chemical reaction, matter cannot be created or destroyed. It can only be changed. Therefore, the amount of matter in the reactants must equal the amount of matter in the products.

What does that mean?? The atoms contained in the reactants must be the same as the atoms in the products. Atoms can not be created or destroyed. The atoms contained in the reactants must be the same as the atoms in the products. Atoms can not be created or destroyed. The atoms CAN be rearranged into different molecules. The atoms CAN be rearranged into different molecules.

Is the Law of Conservation of Mass supported by this chemical equation? H 2 + O 2 H 2 O ReactantsProducts H = 2 atomsH = 2 atoms O = 2 atomsO = 1 atom NO! Oxygen is not equal on each side of the equation.

So what do we do? The chemical equation must be written according to the law. To do this we change the coefficients. We NEVER change a subscript or a formula! 2H 2 + O 2 2H 2 O Reactants: Products: H = 4 H = 4 O = 2 O = 2

Do I Need to Know How to Balance? NOT YET. You need to be able to recognize if the chemical reaction follows the law of conservation of mass. Na + O 2 Na 2 O Mg + 2HCl MgCl 2 + H 2 NOT BALANCED BALANCED!

You Practice!!! HgO Hg + O 2 Hg = Hg = O =O = N 2 + 3H 2 2NH 3 Zn + 2HCl ZnCl 2 + H 2 NOT BALANCED BALANCED! BALANCED

More Practice K + Br 2 2KBr 2Fe + O 2 6Fe 2 O 3 2Na + 2H 2 O2NaOH + H 2 NOT BALANCED BALANCED

Got It???

Balancing Chemical Equations H 2 + O 2 H 2 O H 2 + O 2 H 2 O H = 2 atomsH = 2 atoms O = 2 atomsO = 1 atoms How can we use coefficients to balance this equation?

H 2 + O 2 2H 2 O Reactants: H = 2 Products: H = 4 O = 2 O = 2 O = 2 O = 2 Now the Oxygen is balanced but Hydrogen is not. What can be done to balance Hydrogen? Using a coefficient of 2 will balance oxygen.

Adding a coefficient of 2 here will balance hydrogen. 2H 2 + O 2 2H 2 O H = 4 atomsH = 4 atoms O = 2 atomsO = 2 atoms Now it is balanced!!! Now it is balanced!!!

Now You Practice ___HgO ___Hg + ___O 2 Hg =Hg = O = O = ___K + ___Br 2 ___KBr K = K = Br = Br =

Balance two more… ___N 2 + ___H 2  ___NH 3 N = N = N = N = H = H = H = H = ___C 2 H 6 + ___O 2  ___CO 2 +___H 2 O C =C= C =C= H =H = H =H = O =O = O =O =