Section 2Chemical Reactions EQ: 〉 What is a chemical equation? 〉 What can a balanced chemical equation tell you?

Section 2Chemical Reactions Class Starter 5/8. Check EQ and MV. 1. What is the difference between reaction models A and B? 2. Why is reaction model A not fully complete as written? (Hint: Consider how many atoms of each element exist before and after the reaction.) 3. A friend tells you that an easy way to make sure the same number of atoms are on both sides of the equation is to change the subscript on the product so that H 2 O becomes H 2 O 2 instead of H 2 O. What’s wrong with this reasoning? (Hint: If you did this, would the equation still describe a reaction that forms water?)

Section 2Chemical Reactions Describing Reactions 〉 What is a chemical equation? 〉 A chemical equation uses symbols to represent a chemical reaction and shows the relationship between the reactants and products of a reaction. chemical equation: a representation of a chemical reaction that uses symbols to show the relationship between the reactants and the products

Section 2Chemical Reactions Describing Reactions, continued

Section 2Chemical Reactions Describing Reactions, continued Chemical equations show products and reactants. Balanced chemical equations account for the conservation of mass. –When the number of atoms of each element of the products matches the number of atoms of each element of the reactants, the chemical equation is said to be balanced. –A chemical equation is balanced by adding coefficients in front of one or more of the formulas.

Section 2Chemical Reactions Visual Concept: Chemical Equation

Section 2Chemical Reactions Visual Concept: Balancing a Chemical Equation by Inspection

Section 2Chemical Reactions Balancing an Equation Fe(s) + H 2 O(l)  Fe 3 O 4 (s) + H 2 (l) SubstanceReactantsProducts Fe13 H22 O14 Make a chart Count atoms Look for imbalances… what doesn’t match?

Section 2Chemical Reactions Balancing an Equation Fe(s) + H 2 O(l)  Fe 3 O 4 (s) + H 2 (l) SubstanceReactantsProducts Fe13 H22 O14 How can we fix it? COEFFICIENTS!!! But: You can never change a subscript. You can never insert a coefficient in the middle of a chemical formula.

Section 2Chemical Reactions Balancing an Equation ___Fe(s) + _4_ H 2 O(l)  ___Fe 3 O 4 (s) + ___H 2 (l) SubstanceReactantsProducts Fe13 H2 82 O14144 Balance Oxygens first because it is in compound on both sides of the equation. Put a 4 in front of water.

Section 2Chemical Reactions Balancing an Equation _3_Fe(s) + _4_ H 2 O(l)  ___Fe 3 O 4 (s) + _4_H 2 (l) SubstanceReactantsProducts Fe13133 H O14144 Next balance either Hydrogens or Iron. Put a 4 in front of H 2. Put a 3 in front of Fe. All is balanced. All is right with the world!

Section 2Chemical Reactions Class Starter 5/9. Balance these equations 1.CH 4 + O 2  CO 2 + H 2 O 2.HgO  Hg + O 2 3.Fe + H 2 O  Fe 3 O 4 + H 2 4.H 2 + Cl 2  HCl 5.Rb + S 8  Rb 2 S 6.CaO + H 2 O  Ca(OH) 2 (this subscript multiplies everything in the parentheses by 2)

Section 2Chemical Reactions Balanced Equations and Mole Ratios 〉 What can a balanced chemical equation tell you? 〉 A balanced equation tells you the mole ratio, or proportion of reactants and products, in a chemical reaction. Mole ratio: the relative number of moles of the substances required to produce a given amount of product in a chemical reaction

Section 2Chemical Reactions Balanced Equations and Mole Ratios, continued The law of definite proportions: A compound always contains the same elements in the same proportions regardless of how the compound is made or how much of the compound is formed.

Section 2Chemical Reactions Balanced Equations and Mole Ratios, continued Mole ratios tell you the relative amounts of reactants and products. 2H 2 O → 2H 2 + O 2 –The equation for the electrolysis of water shows that the mole ratio for H 2 O:H 2 :O 2 is 2:2:1. Mole ratios can be converted to masses. –Multiply the molecular mass of each substance by the mole ratio from the balanced equation. This tells you the relative masses of the substances needed to react completely.

Section 2Chemical Reactions Practice: List all available mole ratios for your balanced equations from the Class Starter: 1.CH 4 + O 2  CO 2 + H 2 O 2.HgO  Hg + O 2 3.Fe + H 2 O  Fe 3 O 4 + H 2 4.H 2 + Cl 2  HCl 5.Rb + S 8  Rb 2 S 6.CaO + H 2 O  Ca(OH) 2