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

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 Word Equations

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? How do you tell the difference?

Hydrogen Peroxide  Water and Oxygen

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). Chemical 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 + O 2  Fe 2 O 3 Skeletal Equations

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

CatalystExample: H 2 O 2 → H 2 O + O 2 H 2 O 2 → H 2 O + O 2 This happens faster in the presence of MgO 2 : H 2 O 2 → H 2 O + O 2 MgO 2 A catalyst helps or speeds up a reaction, but is not used up in the reaction.

Without Catalyst With Catalyst

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 + 3O 2 → 2Fe 2 O 3 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 = 2Fe 2 3O 2 = 2O 3

The Reaction to Make Water

Balanced chemical equation: 2 H 2 (g) + O 2 (g) → 2 H 2 O (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: 1. Write the unbalanced equation. 2. Inventory the atoms in reactants and products. 3. Balance the equation. Strategies: o Balance elements left-to-right on the Periodic Table (metals first, then nonmetals) o Leave H and O until last.

1. Write the unbalanced equation. __Zn (s) + __HCl (aq) → __ZnCl 2 (aq) + __ H 2 (g)

Zn Cl H Zn Cl H Inventory reactants and products.

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

Balance this reaction: __CH 4 (g) + __O 2 (g)  __CO 2 (g) + __H 2 O (l)

Balance this reaction: CHOCHO CHOCHO __CH 4 (g) + __O 2 (g)  __CO 2 (g) + __H 2 O (l)

CHOCHO CHOCHO

Balance This Equation __KClO 3 (s)  __KCl (s) + __O 2 (g) K Cl O K Cl O

Al(OH) 3 (s) + H 2 SO 4 (aq)  Al 2 (SO 4 ) 3 (aq) + H 2 O(l) Al(OH) 3 (s) + H 2 SO 4 (aq)  Al 2 (SO 4 ) 3 (aq) + H 2 O(l) Al S H O Al S H O x 2= =18 36

Practice Problems 1. _AgNO 3 + _H 2 S → _Ag 2 S + _HNO 3 2. _Zn(OH) 2 + _H 3 PO 4 → _Zn 3 (PO 4 ) 2 + _H 2 O 3. _FeCl 3 + _Ca(OH) 2 → _Fe(OH) 3 + _CaCl 2 4. _CuS + _O 2 → _Cu + _SO 2 5. _Fe 2 O 3 + _H 2 → _Fe + _H 2 O

Practice Problems 1. 2AgNO 3 + 1H 2 S → 1Ag 2 S + 2HNO 3 2. _Zn(OH) 2 + _H 3 PO 4 → _Zn 3 (PO 4 ) 2 + _H 2 O 3. _FeCl 3 + _Ca(OH) 2 → _Fe(OH) 3 + _CaCl 2 4. _CuS + _O 2 → _Cu + _SO 2 5. _Fe 2 O 3 + _H 2 → _Fe + _H 2 O

Practice Problems 1. 2AgNO 3 + 1H 2 S → 1Ag 2 S + 2HNO Zn(OH) 2 + 2H 3 PO 4 → 1Zn 3 (PO 4 ) 2 + 6H 2 O 3. _FeCl 3 + _Ca(OH) 2 → _Fe(OH) 3 + _CaCl 2 4. _CuS + _O 2 → _Cu + _SO 2 5. _Fe 2 O 3 + _H 2 → _Fe + _H 2 O

Practice Problems 1. 2AgNO 3 + 1H 2 S → 1Ag 2 S + 2HNO Zn(OH) 2 + 2H 3 PO 4 → 1Zn 3 (PO 4 ) 2 + 6H 2 O 3. 2FeCl 3 + 3Ca(OH) 2 → 2Fe(OH) 3 + 3CaCl 2 4. _CuS + _O 2 → _Cu + _SO 2 5. _Fe 2 O 3 + _H 2 → _Fe + _H 2 O

Practice Problems 1. 2AgNO 3 + 1H 2 S → 1Ag 2 S + 2HNO Zn(OH) 2 + 2H 3 PO 4 → 1Zn 3 (PO 4 ) 2 + 6H 2 O 3. 2FeCl 3 + 3Ca(OH) 2 → 2Fe(OH) 3 + 3CaCl CuS + 1O 2 → 1Cu + 1SO 2 5. _Fe 2 O 3 + _H 2 → _Fe + _H 2 O

Practice Problems 1. 2AgNO 3 + 1H 2 S → 1Ag 2 S + 2HNO Zn(OH) 2 + 2H 3 PO 4 → 1Zn 3 (PO 4 ) 2 + 6H 2 O 3. 2FeCl 3 + 3Ca(OH) 2 → 2Fe(OH) 3 + 3CaCl CuS + 1O 2 → 1Cu + 1SO Fe 2 O 3 + 3H 2 → 2Fe + 3H 2 O

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

2. Which of the following is a skeleton equation? a)H 2 + CO  CH 3 OH b)2H 2 + CO  CH 3 OH c)2H 2 + CO 2  CH 3 OH d)hydrogen + carbon monoxide  methanol

2. Which of the following is a skeleton equation? a)H 2 + CO  CH 3 OH b)2H 2 + CO  CH 3 OH c)2H 2 + CO 2  CH 3 OH d)hydrogen + carbon monoxide  methanol

3. What coefficient for H 2 SO 4 is required to balance the following equation? Ca 3 (PO 4 ) 2 + ____ H 2 SO 4  3CaSO 4 + 2H 3 PO 4 Ca 3 (PO 4 ) 2 + ____ H 2 SO 4  3CaSO 4 + 2H 3 PO 4 a)1 b)2 c)3 d)4

3. What coefficient for H 2 SO 4 is required to balance the following equation? Ca 3 (PO 4 ) 2 + ____ H 2 SO 4  3CaSO 4 + 2H 3 PO 4 Ca 3 (PO 4 ) 2 + ____ H 2 SO 4  3CaSO 4 + 2H 3 PO 4 a)1 b)2 c)3 d)4