Describing Chemical Reactions Prentice-Hall Chapter 11.1 Dr. Yager

Objectives Describe how to write a word equation Describe how to write a skeleton equation Describe the steps for writing a balanced chemical equation

Reactant + Reactant  Product + Product Word Equations To write a word equation, write the names of the reactants to the left of the arrow separated by plus signs. Write the names of the products to the right of the arrow, also separated by plus signs. Reactant + Reactant  Product + Product

Methane + Oxygen  Carbon dioxide + Water

iron + oxygen  iron(III) oxide

The Word Equation (Not generally used) Reactants → Products Example: Iron + Oxygen → Iron (III) oxide or Iron + Oxygen → Iron (II) oxide How do you tell the difference?

Hydrogen Peroxide  Water and Oxygen

Chemical Equations A chemical equation is a representation of a chemical reaction where the formulas of the reactants (on the left) are connected by an arrow with the formulas of the products (on the right).

Here is the equation for rusting: Skeletal Equations A skeleton equation is a chemical equation that does not indicate the relative amounts of the reactants and products. Here is the equation for rusting: Fe + O2  Fe2O3

Skeleton Equation Reactants → Products Fe + O2 → Fe2O3 How do you get Fe2 from Fe and O3 from O2? We do this by balancing equations. 4Fe + 3O2 → 2Fe2O3 You cannot make or destroy matter! It also helps to know if it is solid, liquid or gas. 4Fe(s) + 3O2(g)→ 2Fe2O3(s)

Catalyst A catalyst helps or speeds up a reaction, but is not used up in the reaction. Example: H2O2 → H2O + O2 This happens faster in the presence of MgO2: MgO2

Without Catalyst With Catalyst

Balancing the Equation To write a balanced chemical equation: First, write the skeleton equation. Then use coefficients to balance the equation so that it obeys the law of conservation of mass.

Balancing the Equation 4Fe + 3O2 → 2Fe2O3 The coefficients in yellow balance the equation. Law of the Conservation of Matter: There are the same number and types of atoms on the left side as on the right side. 4Fe = 2Fe2 3O2 = 2O3

The Reaction to Make Water

Balanced chemical equation: 2 H2 (g) + O2 (g) → 2 H2O (l) 2 x 2 “H” = 4 “H” 2 x 2 “H” = 4 “H” 1 x 2 “O” = 2 “O” 2 x 1 “O” = 2 “O” Steps: Write the unbalanced equation. Inventory the atoms in reactants and products. Balance the equation. Strategies: Balance elements left-to-right on the Periodic Table (metals first, then nonmetals) Leave H and O until last.

__Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g) 1. Write the unbalanced equation. __Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g)

__Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g) 2. Inventory reactants and products. __Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g) Zn Cl H 1 Zn Cl H 1 2

__Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g) 3. Balance the equation by changing coefficients. __Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g) 2 Zn Cl H 1 Zn Cl H 1 2 BALANCED ! 2 BALANCED !

__CH4 (g) + __O2 (g)  __CO2 (g) + __H2O (l) Balance this reaction: __CH4 (g) + __O2 (g)  __CO2 (g) + __H2O (l)

C H O 1 4 2 C H O 1 2 3 Balance this reaction: __CH4 (g) + __O2 (g)  __CO2 (g) + __H2O (l) C H O 1 4 2 C H O 1 2 3

__CH4 (g) + __O2 (g)  __CO2 (g) + __H2O (l) 1 4 2 C H O 1 2 3 4 4 4

Balance This Equation K Cl O 1 3 2 6 K Cl O 1 2 2 6 __KClO3 (s)  __KCl (s) + __O2 (g) 2 2 3 K Cl O 1 3 2 6 K Cl O 1 2 2 6

Al(OH)3(s) + H2SO4(aq)  Al2(SO4)3(aq) + H2O(l) 6 Al S H O 1 5 7 2 2 Al S H O 2 3 13 2 1 3 3 8 12 6x2=12 10 18 12+6=18

Practice Problems _AgNO3 + _H2S → _Ag2S + _HNO3 _Zn(OH)2 + _H3PO4 → _Zn3(PO4)2 + _H2O _FeCl3 + _Ca(OH)2 → _Fe(OH)3 + _CaCl2 _CuS + _O2 → _Cu + _SO2 _Fe2O3 + _H2 → _Fe + _H2O

Practice Problems 2AgNO3 + 1H2S → 1Ag2S + 2HNO3 _Zn(OH)2 + _H3PO4 → _Zn3(PO4)2 + _H2O _FeCl3 + _Ca(OH)2 → _Fe(OH)3 + _CaCl2 _CuS + _O2 → _Cu + _SO2 _Fe2O3 + _H2 → _Fe + _H2O

Practice Problems 2AgNO3 + 1H2S → 1Ag2S + 2HNO3 3Zn(OH)2 + 2H3PO4 → 1Zn3(PO4)2 + 6H2O _FeCl3 + _Ca(OH)2 → _Fe(OH)3 + _CaCl2 _CuS + _O2 → _Cu + _SO2 _Fe2O3 + _H2 → _Fe + _H2O

Practice Problems 2AgNO3 + 1H2S → 1Ag2S + 2HNO3 3Zn(OH)2 + 2H3PO4 → 1Zn3(PO4)2 + 6H2O 2FeCl3 + 3Ca(OH)2 → 2Fe(OH)3 + 3CaCl2 _CuS + _O2 → _Cu + _SO2 _Fe2O3 + _H2 → _Fe + _H2O

Practice Problems 2AgNO3 + 1H2S → 1Ag2S + 2HNO3 3Zn(OH)2 + 2H3PO4 → 1Zn3(PO4)2 + 6H2O 2FeCl3 + 3Ca(OH)2 → 2Fe(OH)3 + 3CaCl2 1CuS + 1O2 → 1Cu + 1SO2 _Fe2O3 + _H2 → _Fe + _H2O

Practice Problems 2AgNO3 + 1H2S → 1Ag2S + 2HNO3 3Zn(OH)2 + 2H3PO4 → 1Zn3(PO4)2 + 6H2O 2FeCl3 + 3Ca(OH)2 → 2Fe(OH)3 + 3CaCl2 1CuS + 1O2 → 1Cu + 1SO2 1Fe2O3 + 3H2 → 2Fe + 3H2O

1. Propane gas reacts with oxygen to produce water vapor and carbon dioxide. Choose the correct word equation for this reaction. propane + carbon dioxide  water + oxygen propane + oxygen + water  carbon dioxide propane + oxygen + water + carbon dioxide propane + oxygen  water + carbon dioxide

1. Propane gas reacts with oxygen to produce water vapor and carbon dioxide. Choose the correct word equation for this reaction. propane + carbon dioxide  water + oxygen propane + oxygen + water  carbon dioxide propane + oxygen + water + carbon dioxide propane + oxygen  water + carbon dioxide

2. Which of the following is a skeleton equation? H2 + CO  CH3OH 2H2 + CO  CH3OH 2H2 + CO2  CH3OH hydrogen + carbon monoxide  methanol

2. Which of the following is a skeleton equation? H2 + CO  CH3OH 2H2 + CO  CH3OH 2H2 + CO2  CH3OH hydrogen + carbon monoxide  methanol

3. What coefficient for H2SO4 is required to balance the following equation? Ca3(PO4)2 + ____ H2SO4  3CaSO4 + 2H3PO4 1 2 3 4

3. What coefficient for H2SO4 is required to balance the following equation? Ca3(PO4)2 + ____ H2SO4  3CaSO4 + 2H3PO4 1 2 3 4