sugar + flour + eggs + vanilla + salt  cake Section 2 Balancing Chemical Equations Chapter 8 Bellringer Write a word equation for baking a cake. Does the cake have the same properties as the ingredients? Answer: sugar + flour + eggs + vanilla + salt  cake The properties are different.

What evidence is there that a chemical reaction has occurred? Chapter 8 Standardized Test Preparation Read the passage below. Then answer the questions. A student places a strip of pure magnesium metal into a test tube containing a dilute solution of hydrochloric acid (hydrogen chloride dissolved in water). As the magnesium disappears, bubbles of a colorless gas form and the test tube becomes hot to the touch. If a lit match is placed near the top of the test tube, the gas that has been generated burns. What evidence is there that a chemical reaction has occurred?

Chapter 8 Standardized Test Preparation Reading Skills Answer: The gas that burns is a different chemical substance than any of the reactants, indicating a chemical reaction.

Chapter 8 Table of Contents Section 1 Describing Chemical Reactions Chemical Equations and Reactions Chapter 8 Table of Contents Section 1 Describing Chemical Reactions Section 2 Balancing Chemical Equations Section 3 Classifying Chemical Reactions Section 4 Writing Net Ionic Equations

Reactions Conserve Mass Section 2 Balancing Chemical Equations Chapter 8 Reactions Conserve Mass Mass cannot be created or destroyed by a chemical or physical change Equations must be balanced. ?Na + ?H2O  ?NaOH + ?H2

Chapter 8 Balancing Equations Section 2 Balancing Chemical Equations Chapter 8 Balancing Equations The number of atoms for each element must be the same on the reactants’ side and on the products’ side. A coefficient multiplies the number of atoms of each element in the formula that follows. H2O: 2 hydrogen atoms, 1 oxygen atom 2H2O: 4 hydrogen atoms, 2 oxygen atoms

Rules for balancing equations Chapter 8 Section 2 Balancing Chemical Equations Rules for balancing equations Identify the names of the reactants and the products, and write a word equation. Write a formula equation by substituting correct formulas for the names of the reactants and the products. Balance the formula equation according to the law of conservation of mass. • Balance the different types of atoms one at a time. • First balance the atoms of elements that are combined and that appear only once on each side of the equation. • Balance polyatomic ions that appear on both sides of the equation as single units. • Balance H atoms and O atoms after atoms of all other elements have been balanced. 4. Count atoms to be sure that the equation is balanced.

Section 2 Balancing Chemical Equations

Chapter 8 Balancing Equations Sample Problem A Section 2 Balancing Chemical Equations Chapter 8 Balancing Equations Sample Problem A Balance the equation for the reaction of iron(III) oxide with hydrogen to form iron and water.

Chapter 8 Balancing Equations Sample Problem A Solution Section 2 Balancing Chemical Equations Chapter 8 Balancing Equations Sample Problem A Solution Identify reactants and products. reactants products Fe2O3 + H2  Fe + H2O Count atoms 3 2 3 Reactants Products Balanced? Unbalanced formula equation Fe2O3 + H2 Fe + H2O Iron atoms 2 1 no Oxygen atoms 3 Hydrogen atoms yes Insert coefficients.

Reactions Conserve Mass Section 2 Balancing Chemical Equations Chapter 8 Reactions Conserve Mass Balanced equations show mass conservation ?Na + ?H2O  ?NaOH + ?H2 2Na + 2H2O  2NaOH + H2 Never change subscripts to balance equations Unbalanced: H2 + O2  H2O Incorrect: H2 + O2  H2O2 H2O  H2O2 Correct: 2H2 + O2  2H2O

Reactions Conserve Mass Section 2 Balancing Chemical Equations Chapter 8 Reactions Conserve Mass

Understanding Concepts Chapter 8 Standardized Test Preparation Understanding Concepts 6. Write a balanced equation for this reaction: iron(III) nitrate + lithium hydroxide  lithium nitrate + iron(III) hydroxide

Exit Slip - Understanding Concepts Chapter 8 Standardized Test Preparation Exit Slip - Understanding Concepts 6. Write a balanced equation for this reaction: iron(III) nitrate + lithium hydroxide  lithium nitrate + iron(III) hydroxide Fe(NO3)3 + LiOH  LiNO3 + Fe(OH)3 Answer: Fe(NO3)3 + 3LiOH  3LiNO3 + Fe(OH)3