CHEMISTRY November 7 th, 2011

Warm Up What is wrong with the following equation?? Can you fix it? What is wrong with the following equation?? Can you fix it? HINT: think Law of Conservation of Mass HINT: think Law of Conservation of Mass H 2 + O 2  H 2 O CHEMISTRY LAUGHS Why does hamburger have lower energy than steak?

Agenda Unit 7: Balancing Equation Unit 7: Balancing Equation Practice Practice

Balancing Equation Word Equations Describes a chemical reaction that occurs using the names of the chemicals involved. Describes a chemical reaction that occurs using the names of the chemicals involved. Reactants versus Products Reactants versus Products An arrow is written between the reactants and products to indicate that chemical change is occurring An arrow is written between the reactants and products to indicate that chemical change is occurring –For example: 2H 2 + O 2  2H 2 O

Balancing Equation Atoms involved in a chemical reaction must be balanced. Atoms involved in a chemical reaction must be balanced. –Same number of atoms of each element on both sides of a chemical equation. Unbalanced equation Unbalanced equation –Balance the equation by adding coefficients in front of the compounds  Coefficients: number of molecules of a particular compound  Subscripts: number of atoms of each element in the compound

Balancing Equations A. From the word equation, write the correct formula for all reactants and products. B. If given an unbalanced equations first, balance the equation by using coefficients: 1.NEVER change the subscripts in the formula to balance the equation 2.Elements need to be balanced from either side of the equation 3.Start with the first element and work right of the equation 4.Balance the number of oxygen and hydrogen atoms after all the other elements have been balanced. C. Go back and double check that all elements are balanced D. BOX your coefficients when finished E. USE PENCIL only! (you will be erasing A LOT!)

Balancing Equation Inspection Method Inspection Method Algebraic Method Algebraic Method

Practice and Reasoning Unbalanced Equation: Unbalanced Equation: –SnO 2 + H 2 → Sn + H 2 O Look at the equation and see which elements are not balanced. In this case, there are two oxygen atoms on the left- hand side of the equation and only one on the right-hand side. Correct this by putting a coefficient of 2 in front of water. Look at the equation and see which elements are not balanced. In this case, there are two oxygen atoms on the left- hand side of the equation and only one on the right-hand side. Correct this by putting a coefficient of 2 in front of water. –SnO 2 + H 2 → Sn + 2 H 2 O

This puts the hydrogen atoms out of balance. Now there are two hydrogen atoms on the left and four hydrogen atoms on the right. To get four hydrogen atoms on the right, add a coefficient of 2 for the hydrogen gas. Remember, coefficients are multipliers, so if we write 2 H2O it denotes 2x2=4 hydrogen atoms and 2x1=2 oxygen atoms. This puts the hydrogen atoms out of balance. Now there are two hydrogen atoms on the left and four hydrogen atoms on the right. To get four hydrogen atoms on the right, add a coefficient of 2 for the hydrogen gas. Remember, coefficients are multipliers, so if we write 2 H2O it denotes 2x2=4 hydrogen atoms and 2x1=2 oxygen atoms. –SnO H 2 → Sn + 2 H 2 O The equation is now balanced. Be sure to double-check your math! Each side of the equation has 1 atom of Sn, 2 atoms of O, and 4 atoms of H. The equation is now balanced. Be sure to double-check your math! Each side of the equation has 1 atom of Sn, 2 atoms of O, and 4 atoms of H.

Practice Problems __Fe + __Cl 2  __FeCl 3 __Fe + __Cl 2  __FeCl 3 __Cr + __O 2  __Cr 2 O 3 __Cr + __O 2  __Cr 2 O 3 __Na + __H 2 O  __NaOH + __H 2 __Na + __H 2 O  __NaOH + __H 2 __Ca(CN) 2 + __AlF 3  __CaF 2 + __Al(CN) 3 __Ca(CN) 2 + __AlF 3  __CaF 2 + __Al(CN) 3

Word Problem Potassium phosphate + Calcium hydroxide  Potassium phosphate + Calcium hydroxide  Calcium phosphate + Potassium hydroxide __K 3 (PO 4 ) + __Ca(OH) 2  __Ca 3 (PO 4 ) 2 + __KOH __K 3 (PO 4 ) + __Ca(OH) 2  __Ca 3 (PO 4 ) 2 + __KOH 2K 3 (PO 4 ) + 3Ca(OH) 2  __Ca 3 (PO 4 ) 2 + 6KOH 2K 3 (PO 4 ) + 3Ca(OH) 2  __Ca 3 (PO 4 ) 2 + 6KOH ReactantsProducts ReactantsProducts K = 6K = 6 P = 2P = 2 Ca = 3Ca = 3 O = 14O = 14 H = 6H= 6

Homework Unit 7 Balancing Equation Worksheet 1 Unit 7 Balancing Equation Worksheet 1