Chemical Reactions Chapter 7 Section 1

Chemical Reactions Occur when substances go through chemical changes to form new substances… You can often tell because you can see the changes A chemical reaction is rearrangement of atoms in the reactants to make new substances in the products.

What is a chemical reaction Reactants- Starting materials in a reaction Products- Ending materials in a reaction The products of a reaction are made up of the same number and type of atoms as were present in the reactants (just in a different arrangement)

For example 2 H2 + O2 2 H2O Same number and kind of atoms on each side They are just in different arrangements Both reactants are gases, the product is a liquid It has changed into a different molecule.

Evidence for a chemical change Some observations that suggest a chemical change has taken place… Evolution of energy as heat, light or sound Production of a gas

More Evidence… Formation of a precipitate (“stuff forming”) Change in color This is not conclusive proof though, it needs to be proven that some new compound is present.

Activation Energy In order to start a chemical reaction you must break bonds (covalent, ionic or metallic) To do this takes energy The energy required to break enough bonds for a reaction to start is the activation energy of a reaction

Exothermic Exothermic reactions give off energy.. When the bonds reform they will release energy If more energy is released than is used to break more bonds (to continue) the reaction is exothermic

Exothermic energy graph Activation energy energy reactants products time It ended up giving energy To get to the products More energy came out, than was put in

Endothermic If more energy is required to make the reaction than is released the reaction is endothermic Endothermic take in energy.

Endothermic energy graph Activation energy Energy products reactants time Overall the reaction still took energy. It required Energy to get to the products

Balancing Equations Chapter 7 section 2

Reminder: All MATTER is conserved, but it can change forms.. What goes in must come out, but it can be rearranged! You have to account for all the “stuff” that goes into the reaction

What is balancing an equation? Balancing an equation is changing the ratios of molecules/atoms to make the lowest possible same whole number and type of atoms on each side of the equation. Na + Cl2  NaCl What goes in must come out Na + Cl2  NaCl 2 2

Describing Reactions A chemical equation uses symbols to represent a chemical reaction and shows the relationship between the reactants and products of a reaction….

What you can not do! You can NOT change the formula of the molecules I give you. 2H2 + O2  2H2O If the number is one, you don’t have to write it!

What you can not do! you cannot change water to H2O2 (hydrogen peroxide) So, you can change COEFFICIENTS, not SUBSCRIPTS! you have to increase the amount of H on this side and this will make 2 water molecules H H O O H O  H H H O H H

What you are allowed to do change the coefficient in front of any molecule/atom (changing the ratio of atoms) ___ Ag + ___ Cl2  ___ AgCl you can only write on those lines 2 2

Balancing Equations The coefficient (the number in front) is used to make the number of atoms on each side of the arrow equal (you do not use the subscript for this)

Problem solving strategies It helps to make a list of all atoms present HCl + Ba(OH)2  BaCl2 + H2O 2 Ba – 1 H – 3 Cl – 1 O – 2 Ba – 1 H – 2 Cl – 2 O – 1 4 2 4 2 2

Problem solving strategies It helps to make a list of all atoms present HCl + Ba(OH)2  BaCl2 + H2O 2 2 Ba – 1 H – 3 Cl – 1 O – 2 Start making the numbers agree one at a time (pick something, that occurs in only one molecule 1st) ~not H in this case Ba – 1 H – 2 Cl – 2 O – 1 Now going to O 4 2 4 2 Now everything is balanced! starting w/ Cl

Another to try CH4 + O2  H2O + CO2 2 2 C- 1 C- 1 H- 4 H- 2 O- 2 O- 3 Everything is balanced now! This takes a lot of trial and error. Start with H now O