Introduction Chemical reactions occur when bonds between the outermost parts of atoms are formed or broken Chemical reactions involve changes in matter, the making of new materials with new properties, and energy changes. Symbols represent elements, formulas describe compounds, chemical equations describe a chemical reaction

Chemical Equations Lavoisier, 1788 Because of the principle of the conservation of matter, an equation must be balanced. It must have the same number of atoms of the same kind on both sides.

Scientists know that there must be the same number of atoms on each side of the arrow.

To balance the chemical equation, you must add COEFFICIENTS to the different parts of the equation.

Step # 1 Determine the number of atoms for each element.

Determine the number of atoms for each element. Mg + O2 MgO

Determine the number of atoms for each element. Mg + O2 MgO 1 Magnesium 1 Magnesium 2 Oxygen 1 Oxygen

Try to add coefficients to the equation to get the same number of atoms on each side. Mg + O2 MgO 1 Magnesium 1 Magnesium 2 Oxygen 1 Oxygen 2

Recount the atoms after adding a coefficient. Mg + O2 MgO 1 Magnesium 2 Magnesium 2 Oxygen 2 Oxygen Still out of balance… 2

Add a coefficient to the other side Mg + O2 MgO 1 Magnesium 2 Magnesium 2 Oxygen 2 Oxygen Recount again… 2 2

Recount the atoms… Mg + O2 MgO 2 2 2 Magnesium 2 Magnesium 2 Oxygen 2 Oxygen It’s balanced!!! 2

Try these… Ca + O2 CaO

Count atoms… Ca + O2 CaO calcium 1 calcium 1 oxygen 2 oxygen 1 not in balance – so…

Add coefficients… then recount atoms 2 Ca + O2 CaO calcium 2 calcium 2 oxygen 2 oxygen 2 Now it’s balanced 2

What type of reaction is this? Synthesis Decomposition Single replacement Double replacement Combustion

synthesis

#3 Try this one on your own H2O2 H2O + O2

#3 counting atoms H2O2 H2O + O2 hydrogen 2 hydrogen 2 oxygen 2 oxygen 3

#3 adding coefficients and recount 2 H2O2 H2O + O2 hydrogen 4 hydrogen 4 oxygen 4 oxygen 4 2

What type of reaction? decomposition

#4 counting atoms N2 + H2 NH3 nitrogen 2 nitrogen 1 hydrogen 2 hydrogen 3

#4 adding coefficients N2 + H2 NH3 2 4 nitrogen 2 4 nitrogen 1 4 hydrogen 2 hydrogen 3 12 Still out of balance…. 2 4

#4 adding more coefficients N2 + H2 NH3 nitrogen 2 nitrogen 1 2 hydrogen 2 12 hydrogen 3 12 Sometimes you have to be creative …. 6 2

What type of reaction? synthesis

#5 try this one on your own Cu2O + C Cu + CO2 copper copper oxygen oxygen carbon carbon

#5 counting atoms Cu2O + C Cu + CO2 copper 2 copper 1 oxygen 1 oxygen 2 carbon 1 carbon 1

#5 adding coefficients Cu2O + C Cu + CO2 2 copper 2 4 copper 1 oxygen 1 2 oxygen 2 carbon 1 carbon 1 2

#5 still adding coefficients Cu2O + C Cu + CO2 copper 2 4 copper 1 4 oxygen 1 2 oxygen 2 carbon 1 carbon 1 2 4

What type of reaction is this? Single replacement

Problems 1-7 on the back Remember, count the atoms first Add coefficients to balance one of the atoms first Recount atoms Keep working the coefficients until atoms are balanced

Problems Na + MgF2 NaF + Mg Mg + HCl MgCl2 + H2 Cl2 + KI KCl + I2 NaCl Na + Cl2 Na + O2 Na2O Na + HCl H2 + NaCl K + Cl2 KCl CHALLENGE!! CO2 + H2O C6H12O6 + O2

Solutions 2 Na + MgF2 2 NaF + Mg 2 Mg + 4 HCl 2 MgCl2 + 2 H2 Cl2 + 2KI 2 KCl + I2 2 NaCl 2 Na + Cl2 4 Na + O2 2 Na2O 2 Na + 2 HCl H2 + 2 NaCl 2 K + Cl2 2 KCl

Challenge solution 6 CO2 + 6 H2O C6H12O6 + 6 O2