Balancing Equations & Types of Reactions

Parts of an Equation Reactants Yield Products –Reactants react to form products –The arrow, —> means yield –Coefficients are the large numbers in front of molecules or compounds –Subscripts are the little numbers the the bottom right of an element or compound

Coefficients and Subscripts Use coefficients to balance equations. They tell you how many of an entire molecule there are. 2H 2 O There are two molecules of water … the blue 2 indicates this information You learned to use subscripts to balance molecules 2H 2 O Since O has a –2 charge you have to have 2 H+ to balance the molecule … the red 2 indicates this information In two molecules of water there are 4 atoms of Hydrogen and 2 atoms of oxygen

5 Types of Reactions Single Replacement Double Replacement Combination or Synthesis Decomposition Combustion

Single Replacement Chemical change which one element replaces a second element in a compound. Zn(s) + Cu(NO 3 ) 2 (aq) —> Cu(s) + Zn(NO 3 ) 2 (aq) The zinc and the copper change places … there is a single element on both sides of the reaction. (s) Means solid (aq) means dissolved in water

Double Replacement A chemical change involving an exchange of positive ions between two compounds Na 2 S (aq) + Cd(NO 3 ) 2 (aq) —> CdS (s) + 2NaNO 3 (aq) Two compounds are on both sides of the reaction – they switch positive ions.

Combination Reaction (also called Synthesis ) A chemical change in which two or more substances react to form a single new substance. Fe (s) + S (s) —> FeS (s)

Decomposition Chemical change in which a single compound breaks down into two or more simpler products 2HgO (s) —> 2Hg (l) + O 2 (g) (l) Means liquid (g) Means gas

Combustion A chemical change in which an element or compound reacts with oxygen often producing energy in the form of heat and light. 2C 8 H 18 (l) + 25O 2 (g) —> 16CO 2 (g) + 18H 2 O(l) The above equation shows the complete combustion of gasoline in a car engine. The reactants are gasoline and oxygen, the products are carbon dioxide and water. You will do a project later this trimester that calculated the amount of kg of CO 2 your dream car produces in a year given a certain amount of miles traveled.

Balancing Equations Chemical Reactions obey the Law of Conservation of Mass. In other words, the number of atoms of each element on both sides of the yield sign must be equal – you use coefficients to accomplish this!!!! H 2 + O 2 —> H 2 O Inventory:Reactants / Products 2 H 2 2 O 1

Balance the Equation H 2 + O 2 —> H 2 O Put a 2 in front of the H 2 O Put a 2 in front of the H 2 2H 2 + O 2 —> 2H 2 O (the bold 2’s are coefficients) Re-do the inventory Reactants / Products 4 H 4 2 O 2 There are the same number of atoms of each element on both sides of the equation – it is balanced.