Chemical Reactions

Learning target: I can balance chemical equations. Why learn this? This concept introduces you to the idea that matter is not created or destroyed. –Law of Conservation of Mass Because you will need to know this in chemistry if you decide to take it!

Chemical Reactions A chemical reaction occurs when the atoms of the starting materials rearrange to form new substances. Bonds between the atoms in molecules or compounds break and re-form into new substances. A chemical reaction involves the valence electrons only.

All chemical reactions have two parts Reactants - the substances you start with (the ones reacting) Products- the substances you end up with (the ones produced) The reactants turn into the products. –Reactants  Products

Chemical Equations A chemical equation represents a chemical reaction. It shows the formula of each compound in a chemical reaction. The formulas of the reactants are on the left of the arrow and the formulas of the products on the right. Fe + O 2  Fe 2 O 3

Chemical Equations The arrow means: yields or to produce. The plus means: and Example: CH 4 + O 2 CO 2 + H 2 O Methane and Oxygen yields carbon dioxide and water ReactantsProducts

Law of Conservation of Mass Matter is not created or destroyed during a chemical reaction. –The bonds between the atoms in compounds or molecules break and re-form into new substances. The mass of the reactants is equal to the mass of the products.

Balanced Chemical Equations Because none of the atoms are lost or gained, chemical reactions are shown by balanced equations. A balanced equation means there is the same number of atoms for each element in the reactants and products.

Rules for balancing Remember the Law of Conservation of Mass when balancing chemical equations. 1. Count the number of atoms for each element that appear on both sides of the arrow. 2. Balance the elements one at a time by adding coefficients (the numbers in front) balance Hydrogen and Oxygen last 3. Check to make sure it is balanced.

Never Never change a subscript to balance an equation. This changes the formula you are describing and makes it different reaction. –H 2 O is a different compound than H 2 O 2 Never put a coefficient in the middle of a formula –2 NaCl is okay, Na2Cl is not.

Example This equation isn’t balanced: Fe + O 2  Fe 2 O 3 1. Count the number of atoms for each element that appear on both sides of the arrow.

Example Fe + O 2  Fe 2 O 3 Fe 1 O 2  Fe 2 O 3

Example Fe + O 2  Fe 2 O 3 Fe 1 O 2  Fe 2 O 3 2. Balance the elements one at a time by adding coefficients (the numbers in front) – balance Hydrogen and Oxygen last

Example 2Fe + O 2  Fe 2 O 3 Fe 2 O 2  Fe 2 O 3 The coefficient of 2 is added in front of Fe on the reactant side (left) to give two Fe on each side of the equation.

Example 2Fe + O 2  Fe 2 O 3 Fe 2 O 2  Fe 2 O 3 Time to balance Oxygen: – 2 are on the left and 3 on the right.

Example 2Fe + O 2  Fe 2 O 3 Fe 2 O 2  Fe 2 O 3 What coefficients could we use to get the same number of Oxygen on each side?

Example 2Fe + 3O 2  2Fe 2 O 3 Fe 2 O 6  Fe 4 O 6 That gives us 6 Oxygen on each side but now there is 2 Fe on the left and 4 on the right.

Example 4Fe + 3O 2  2Fe 2 O 3 Fe 4 O 6  Fe 4 O 6 Change the coefficient to Check to make sure it is balanced.

Example 4Fe + 3O 2  2Fe 2 O 3 Fe 4 O 6  Fe 4 O 6 There are 4 Fe and 6 O on both sides of the arrow. – The reactants and products have the same number of elements. The equation is balanced!

Practice __ H 2 + ___ O 2  ___ H 2 O __ SO 2 + ___ O 2  ___ SO 3

Answers 2 H 2 + O 2  2 H 2 O 2 SO 2 + O 2  2 SO 3