Law of Conservation of Mass

Law of Conservation of Mass Balancing Chemical Equations

What is a chemical equation? • A chemical equation is shorthand way of expressing a chemical reaction.

Anatomy of a Chemical Equation Reactants • The matter that is present before the reaction takes place. • This matter can be either elements or compounds • Reactants are always to the left of the arrow C10H8 + 12O2  10CO2 + 4H2O

Anatomy of a Chemical Equation Yield Sign • The arrow, , is read as the word, yields. • What the yield sign is saying is that the reactant(s), when combinedor dissolved, form the product(s). • (+) sign(s) to the left of the yield sign mean(s) “plus” • (+) sign(s) to the right of the yield sign mean(s) "and." C10H8 + 12O2 10CO2 + 4H2O 

Anatomy of a Chemical Equation • Products • The matter that is present after the reaction takes place. • This matter can be either elements or compounds • Products are always to the right of the arrow C10H8 + 12O2  10CO2 + 4H2O

Anatomy of a Chemical Equation • Products • The matter that is present after the reaction takes place. • This matter can be either elements or compounds • Products are always to the right of the arrow C10H8 + 12O210CO2 + 4H2O

Anatomy of a Chemical Equation DO YOU REMEMBER??? You have learned that the subscript numbers in a chemical formula represent the number of atoms in one molecule. This molecule has 10 carbon atoms and 8 hydrogen atoms C10H8

Anatomy of a Chemical Equation • Coefficient • The number that you see to the left of a chemical formula. • The coefficient number acts as a multiplier for all of the atoms in the entire compound. • As with subscripts, when no number is present then "1" is understood. 12O2 There are 24atoms of oxygen. There are 12molecules of oxygen.

Law of Conservation of Mass/Matter • States that matter or mass cannot be creatednor destroyed. • It means that the number of atoms of an element must be equalon both sides of an equation. • In other words, the equation must be balanced.

Balancing Equations • In balancing equations, you CANNOT CHANGE SUBSCRIPTS! • If you change a subscript, you change the substance. • That would mean you are no longer balancing the same chemical REACTION anymore. How do you make the numbers work out if you can’t change them? • A coefficient is added in front of a compound ONLY, and it distributes to all the elements in the compound.

Balancing Equations • Now let’s try one!!! Mg + O2 MgO

Balancing Equations • 1st Step: • Count how many atoms of each element you have on the right side of the arrow and the left side of the arrow. This equation is not balanced. The left side has more oxygen atoms than the right. Mg + O2 MgO

Balancing Equations • 2nd Step: • Find the lowest common multiple for the atom you are trying to change • That will be the number you use as the coefficient for the chemical formula Mg + O2 MgO The lowest common multiple for oxygen’s 1 & 2 is 2.

Balancing Equations • 3rd Step: • Add coefficients and check to see if it is balanced. • You must recount the atoms on each side of the arrow The oxygen atoms are balanced, but the magnesium atoms are not. Mg + O2 2MgO

Balancing Equations • 3rd Step: • Add coefficients and check to see if it is balanced. • You must recount the atoms on each side of the arrow We must add 2 in front of the Mg on the left and check to see if it’s balanced by recounting. 2Mg + O2 2MgO

Balancing Equations • 4th Step: • Place the final ratio of coefficients in brackets. • They should be placed in the respective order of reactants to products Like subscripts, the 1 is implied if there is no number where the coefficient is supposed to be. 2Mg + O2 2MgO The ratio of coefficients in this balanced equation is [2:1:2]

Law of Conservation of Mass