Chemical Reactions Law of Conservation of Mass Balancing Chemical Equations

Mind Jogger When an object burns, the quantity of ashes that remain is smaller than the original object that was burned. How do you account for the change in mass? Where is the matter that appears to be lost?

Law of Conservation of Mass mass is neither created nor destroyed in a chemical reaction 4 H 2 O 4 H 2 O 4 g32 g 36 g total mass stays the same atoms can only rearrange

Chemical Equations A+B  C+D REACTANTSPRODUCTS

Analyze the Problem The law of conservation of mass will allow us to determine the amount of a missing reactant or product that was involved in a chemical reaction. For example, if you were told that 4 grams of hydrogen reacted with some oxygen to make 36 grams of water then you could figure out how much oxygen was used. Write a word equationHydrogen + Oxygen Water Write the masses of each substance given in the question 4 g?36g Use the law of conservation of mass (mass of reactants = mass of products) 4 + ? = 36 Solve to find the missing value? = ? = 32 So 32 grams of oxygen were used

Learning Check In the complete reaction of g of sodium with g of chlorine, what mass of sodium chloride is formed? A 10.0 g sample of magnesium reacts with oxygen to form 16.6 g of magnesium oxide. How many grams of oxygen reacted?

Solution In the complete reaction of g of sodium with g of chlorine, what mass of sodium chloride is formed? g of Sodium chloride A 10.0 g sample of magnesium reacts with oxygen to form 16.6 g of magnesium oxide. How many grams of oxygen reacted? 6.6 g of Oxygen

Chemical Symbols

Skeletal Equation Uses chemical formulas instead of words to identify reactants and products. Important step in writing a complete equation Na (s) + Cl 2 (g) NaCl (s)

Learning Check Write skeleton equations for the following word equations. hydrogen(g) + bromine(g) hydrogen bromide(g) carbon(s) + sulfur(s) carbon disulfide(s)

Solution Write skeleton equations for the following word equations. hydrogen(g) + bromine(g) hydrogen bromide(g) H 2 (g) + Br 2 (g)HBr(g) carbon(s) + sulfur(s) carbon disulfide(s) C(s) + S(s) CS 2 (s)

Writing a Chemical Equation Chemical symbols give a “before-and-after” picture of a chemical reaction ReactantsProducts MgO + CCO + Mg magnesium oxide to form carbon monoxide reacts with carbon and magnesium

Writing Equations Identify the substances involved. Use symbols to show: 2H 2 (g) + O 2 (g)  2H 2 O(g) –How many? - coefficient –Of what? - chemical formula –In what state? - physical state Remember the diatomic elements.

Describing Equations Describing Coefficients: –individual atom = “atom” –covalent substance = “molecule” –ionic substance = “unit” 3 molecules of carbon dioxide 2 atoms of magnesium 4 units of magnesium oxide 3CO 2  2Mg  4MgO 

E. Describing Equations How many? Of what? In what state? Zn(s) + 2HCl(aq)  ZnCl 2 (aq) + H 2 (g) One atom of solid zinc reacts with two molecules of aqueous hydrochloric acid to produce one unit of aqueous zinc chloride and one molecule of hydrogen gas.

Learning Check Write the word equation for each of the following: 2Ag (s) + CuCl 2 (aq)  Cu (s) + 2AgCl (aq) 2 atoms of solid silver react with 1 unit of aqueous copper(II)chloride to yield solid copper and 2 units of aqueous silver chloride.

Chemical Reactions Balancing Chemical Equations

Balancing a Chemical Equation Balanced means that “conservation of mass” is upheld. Same numbers of each type of atom on each side of the equation Al + S Al 2 S 3 Not Balanced 2Al + 3S Al 2 S 3 Balanced

Matter Is Conserved H 2 + Cl 2 2 HCl ++ Total atoms=Total atoms 2 H, 2 Cl2H, 2 Cl Total Mass=Total Mass 2(1.0) + 2(35.5)2(36.5) 73.0 g=73.0 g

A. Balancing Steps 1.Write the unbalanced equation. 2.Count atoms on each side. 3.Add coefficients to make #s equal. Coefficient  subscript = # of atoms 4.Reduce coefficients to lowest possible ratio, if necessary. 5.Double check that atoms balance!!!

B. Helpful Tips Balance one element at a time. Update ALL atom counts after adding a coefficient. If an element appears more than once per side, balance it last. Balance polyatomic ions as single units. –“1 SO 4 ” instead of “1 S” and “4 O”

Al + CuCl 2  Cu + AlCl 3 Al Cu Cl  3  6   C. Balancing Example Aluminum and copper(II) chloride react to form copper and aluminum chloride. 2  2  3  6

Balance Equations with Coefficients Use a number before the compound/element symbol to indicate how many of them are needed called a COEFFICIENT. Coefficients in front of formulas balance each type of atom. 4 NH O 2 4 NO + 6 H 2 O 4 N = 4 N 12 H=12 H 10 O=10 O

How to write and balance Chemical Equations 1.Write the skeleton equation: Al + O 2 → Al 2 O 3 2. Count Number of atoms for each element on both sides This is not balanced because the numbers don’t match 3. Multiply coefficients until they match – multiply the entire units 2 Al + O 2 → Al 2 O 3 Go to 6 oxygens

Balancing Equations Al + O 2 2 Al 2 O Al + 2 Al 2 O 3 O2O2 Multiply each atom by 2

Learning Check Balance each equation. A. Mg + N 2 Mg 3 N 2 B. Al + Cl 2 AlCl 3

Solution A. 3 Mg + N 2 Mg 3 N 2 B. 2 Al + 3 Cl 2 2 AlCl 3

What not to do when balancing Chemical Equations Never change a subscript to balance an equation (You can only change coefficients) –If you change the subscript (formula) you are describing a different chemical. –H 2 O is a different compound than H 2 O 2 Never put a coefficient in the middle of a formula; they must go only in the front. 2 NaCl is okay, but Na 2 Cl is not.