Chemical Reaction Notes Chapter 9

Chemical Reaction Notes Why do atoms (either individual or polyatomic) join together to form compounds? Answer: Lower potential energy and increase stability.

Chemical Reaction Notes Sometimes compounds can come together and they can become more stable still by rearranging atoms. These changes are called chemical reactions.

Chemical Reaction Notes How do we know if a chemical reaction has taken place? Temperature change Color change Odor changes, gas bubbles, or formation of a solid can also indicate chemical change ***Not during a phase change (melting, boiling, etc) of the same substance.

Chemical Reaction Notes Chemists represent chemical reactions with “equations”

Chemical Reaction Notes Equations consist of a list of formulas for all of the reactants (compounds or elements added together to begin a chemical reaction) on the left side and a list of all of the products (compounds or elements which form during a chemical reaction) on the right side.

Chemical Reaction Notes Reactants and products are separated by an arrow Reactants are separated from one another by plus signs (as are products)

Chemical Reaction Notes Each formula may be followed by a subscript to show what state each compound is in. Solid (s) Liquid (l) Gas (g) Aqueous (aq) ***This means in water.

Chemical Reaction Notes Sometimes, energy is added into the equation to show that the chemical reaction either requires energy to occur (reactants side) or releases energy (products side).

Chemical Reaction Notes Law of Conservation of Mass: In a chemical reaction the arrangement of atoms will change, but the total number of each type of atom, and therefore the total mass, will remain constant.

Continue on Balancing Act Notes…

Chemical Reaction Notes In order to ensure that our chemical equations obey the law of conservation of mass, we must balance all of our equations so that each side (reactants and products) has the same number of atoms of each element.

Example: Al2S3 + Cl2  AlCl3 + S

Chemical Reaction Notes Things to remember: 1. You must make sure that all of the individual reactant and product formulas are correct. Balance any ionic formulas considering their charge, also factoring in polyatomic ions, etc. Hydrogen, oxygen, nitrogen, and the halogens are all diatomic elements, meaning that whenever they exist as pure substances (are not found in a compound) they form a molecule with themselves and have a formula with a subscript 2

Chemical Reaction Notes 2. Pick any element that is unbalanced on both sides of the equation and place a number (a coefficient) in front of the compound formula to balance out the amount of that element on both sides of the equation. 3. You should then balance other elements, one at a time, from one side of the equation to the other only by placing coefficients in front of the remaining formulas

Chemical Reaction Notes 4. Multiply coefficients to get rid of fractions if necessary 5. Reduce coefficients if necessary

Chemical Reaction Notes Other Special hints: 1. Never change your subscripts 2. Balance elements that only appear in one formula on each side first 3. After your first “free choice” of a coefficient, you must only use coefficients that balance the elements 4. For polyatomic ions, balance them as though they are single “groups”

Practice problems p. 287