Chemical Equations Putting chemical reactions into words

A chemical equation is a symbolic representation of a chemical reaction. Chemical Equation for Photosynthesis

Equation Example: The burning of methane gas in oxygen is: CH O 2 → CO H 2 O Methane + Oxygen makes Carbon Dioxide and Water

Review: Element Symbols  All elements are represented by a 1 or 2 letter symbol For example For example C = Carbon C = Carbon Ne = Neon Ne = Neon O = Oxygen O = Oxygen  The symbols are shown on the periodic table

Chemical Formulas  Shows the elements & number of atoms of each element in a molecule or compound  H 2 SO 4 Elements Elements Hydrogen: 2 atoms Hydrogen: 2 atoms Sulfur: 1 atom Sulfur: 1 atom Oxygen: 4 atoms Oxygen: 4 atoms 7 atoms total 7 atoms total Subscript

Coefficients  A chemical formula may begin with a number. CH O 2 → CO H 2 O  If there is no number, then “1” is understood to be in front of the formula. This number is called the coefficient. This number is called the coefficient.

Coefficients The coefficient represents the number of molecules of that compound or atom needed in the reaction. The coefficient represents the number of molecules of that compound or atom needed in the reaction. For example: For example: 2H 2 SO 4 = 2 molecules of Sulfuric Acid2H 2 SO 4 = 2 molecules of Sulfuric Acid

Coefficients  2H 2 SO 4 = 2 molecules of Sulfuric Acid A coefficient is distributed to ALL elements in a compound (like parenthesis in math!) A coefficient is distributed to ALL elements in a compound (like parenthesis in math!) Think of it like this: 2(H 2 SO 4 ) Think of it like this: 2(H 2 SO 4 ) 2 – H 2 (for a total of 4 H atoms) white2 – H 2 (for a total of 4 H atoms) white 2 – S (for a total of 2 S atoms) yellow2 – S (for a total of 2 S atoms) yellow 2 – O 4 (for a total of 8 O atoms) red2 – O 4 (for a total of 8 O atoms) red

Chemical Reactions are…  When one or more substances are changed into new substances.  Reactants- the stuff you start with  Products- what you make  The products will have NEW PROPERTIES different from the reactants you started with  Arrow points from the reactants to the new products

Reading Chemical Equations  Each side of an equation represents a combination of chemicals.  The combination is written as a set of chemical formulas, separated by + symbols. CH O 2 → CO H 2 O

Reading Chemical Equations  The two sides of the equation are separated by an arrow. The combination of chemicals before the reaction are on the left side of the arrow, called the reactants The combination of chemicals before the reaction are on the left side of the arrow, called the reactants The right side indicates the combination of chemicals after the reaction, called the product. The right side indicates the combination of chemicals after the reaction, called the product.

Arrow always points from reactants to products Reactants Products

For Example:  In this reaction, sodium (Na) and oxygen (O 2 ) react to make a single molecule, Na 2 O  The reactants in this equation are the elements Na + O 2  The product in this equation is the compound Na 2 O Na + O 2 → Na 2 O Reactant Product

Conservation of Mass  During any chemical reaction, the mass of the products is always equal to the mass of the reactants.

 All the mass can be accounted for: Burning of wood results in products that appear to have less mass as ashes; where is the rest? Burning of wood results in products that appear to have less mass as ashes; where is the rest?  Law of conservation of mass Gases and smoke.

Matter is conserved  type and amount of atoms does not change Nothing is created or destroyed

Does this reaction work? C + O 2 = CO 2 1 carbon on each side (reactants and products) 2 Oxygen on each side (reactants and products) No atoms were lost or gained in this reaction!

Balancing Equations  The Law of Conservation of Mass states that in a chemical reaction, the quantity or amount of each element does not change.

 This means that each side of the equation must represent the same quantity of each element; in other words have the same number of each kind of atom

Burning Methane CH 4 in Oxygen O 2  CH O 2 -> CO 2 + 2H 2 O  Is this balanced?  Reactants: 1 C, 4 H, 4 O  Products: 1 C, 4 H, 4 O  Have I destroyed Hydrogen atoms and created Oxygen atoms?  No! Law of conservation of matter says I can’t! the atoms are just rearranged

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So How do I balance this? CH 4 + O 2 -> CO 2 + H 2 O CH 4 + O 2 -> CO 2 + H 2 O  I need to add more molecules to my original reaction.  If I add two molecules of oxygen (use twice as much oxygen as methane)  I will end up with more product, but now its balanced! (1-C, 4- H, 4-O on each side!) 22

Balancing Equations Na + O 2 → Na 2 O ReactantsProducts Na 1Na 2 O 2 O 1 In order for this equation to be balanced, there must be equal amount of Na on the left hand side and on the right hand side.

Balancing Equations Na + O 2 → Na 2 O Right now, there is 1 Na atom on the left, but 2 Na atoms on the right. We solve this problem by putting a 2 in front of the Na on the left hand side, Like this: 2Na + O 2 → Na 2 O

Balancing Equations 2Na + O 2 → Na 2 O  ReactantsProducts  Na 2 Na 2  O 2 O 1

Balancing Equations 2Na + O 2 → Na 2 O  There are 2 Na's on the left and 2 Na's on the right. But what about the O?  We now must check to see if the O's are balanced on both sides of the equation.  On the left hand side there are 2 O's and the right hand side only has one. This is still an unbalanced equation.

Balancing Equations 2Na + O 2 → Na 2 O  To fix this we must put a coefficient of 2 in front of the Na 2 O on the right hand side. Now our equation reads: 2Na + O 2 → 2Na 2 O

ReactantsProducts Na 2Na 4 O 2 O 2  Notice that the 2 on the right hand side is "distributed" to both the Na 2 and the O.  Currently the left hand side of the equation has 2 Na's and 2 O's.  The right hand side has 4 Na's total and 2 O's.

Balancing Equations 2Na + O 2 → 2Na 2 O Again, this is a problem, there must be an equal amount of each chemical on both sides. To fix this let's add 2 more Na's on the left side. The equation will now look like this: 4Na + O 2 → 2Na 2 O

 Is this balanced? ReactantsProducts Na 4 Na 4 O2 O2 Same number of each atom on each side of the equation- YES IT IS BALANCED

Law of Conservation of Mass

Practice  Practice balancing equations: ncing-chemical-equations ncing-chemical-equations  Khan academy- balancing equations: O2h74 O2h74 O2h74