Introduction to Chemical Reactions Making new substances

Main Ideas Chemical Reactions are represented by Chemical Equations. Chemical Equations are balanced The Law of Conservation of Mass

How do you know when a chemical reaction takes place? Color Change Precipitate Formation Temperature Change Gas Formation Odor Change in Acidity

Representing Chemical Reactions Chemists observe chemical reactions and have come up with a way to represent or model what is happening. The chemical equations have the reactant(s) (the stuff being used) on the left side and the product(s) (the stuff being made on the right) The reactants and the products are separated by an arrow showing the direction of the reaction This is called the skeleton equation Na (s) + Cl2 (g)  NaCl

Skeleton Equations Symbol Purpose + Separates more than one reactant or product  Separates reactants from products. Indicates direction of reaction (s) Identifies a solid state (aq) Identifies that something is dissolved in water (l) Identifies liquid state (g) Identifies gaseous state Show the most basic form of the chemical equation to give you the idea of the chemicals being used and the state that the chemicals are in.

Balancing Equation The next step after writing the equation is balancing it. Since matter is neither created nor destroyed (due to the law of conservation of matter) the atoms won’t change their identity. You have to have the same number of each type of atom on each side of the chemical equation.

Balancing Equations To balance an equation, you can only change the number of atoms there are by putting a number in front of the atom or molecule. Na(s) + Cl2(g)  NaCl(s) is the skeleton equation and does not have the same number of Chlorine on both sides. 2Na(s) + Cl2(g)  2NaCl(s) is now balanced because we have placed a 2 in front of the Na solid and the product. Notice we didn’t change any subscripts, only added a number in front. This now means that 2 atoms of Na react with 1 atom of Cl2 to make 2 atoms of NaCl.

How would you balance this equation? Balancing Equations How would you balance this equation? Li + H2O  H2 + LiOH

Steps to Balancing a Chemical Equation To the white board!!!!

Another Example CH4 + O2  CO2 + H2O Reactants Products # of Carbons = 1 # of Hydrogens = 4 # of Hydrogens = 2 # of Oxygens = 2 # of Oxygens = 3 Needs oxygen Needs hydrogen

Example Continued Change the Coefficients to make the number of atoms of each element equal Balance the Hydrogens: CH4 + O2  CO2 + 2 H2O Balance the Oxygens: CH4 + 2 O2  CO2 + 2 H2O

Example Continued CH4 + 2 O2  CO2 + 2 H2O Are your coefficients in their simplest ratio? Count your atoms again to check your work: Reactants Products # of Carbons = 1 # of Hydrogens = 4 # of Oxygens = 4 Total atoms = 9

Think – Pair - Share Try These! Fe2O3 + H2SO4  Fe2(SO4)3 + H2O Hint : balance the polyatomic ion first! CaCl2 + AgNO3  AgCl + Ca(NO3)2 Think – Pair - Share

Review Matter is not destroyed or created Atoms are rearranged in chemical reactions Chemical equations represent chemical reactions You have to have the same number of each type of atom on the left and right hand side of a chemical equation

WARNING! Don’t mess with the insides of polyatomic ions – put a square around them, or label them as X – treat the WHOLE polyatomic ion as though it were an element! Don’t ever play around with subscripts (those little numbers that tell you how many atoms are in a molecule) e.g. C6H22O11