CHEMISTRY – Chapter 8

Chemical Equations and Reactions

Chapter 8 – Section 1

Objectives:1. List three observations that suggest that

a chemical reaction has taken place.2. List three requirements for a correctly

written chemical equation.3. Write a word equation and a formula

equation for a given chemical reaction.

Chemical Reactions

A process by which one or more substances are changed into one or more different substances

Reactants – the original substances Products – the resulting substances

Mass is always conserved

Chemical Equation Represents the identities and

relative amounts of reactants and products in a chemical rx

Chemical Reactions

A new substance must be produced The following changes indicate a

chemical reaction: Heat and light – release of E Production of gas – bubbles Formation of a precipitate – a solid

produced in a solution that settles Color change

Characteristics of Chemical Equations

Chemical Equations must do the following: Represent known facts Contain the correct formulas for the

reactants and products Diatomic molecules, etc.

Law of conservation of mass must be satisfied Atoms are not created or destroyed, just

rearranged

Word Equations Equations where products and reactants

are represented by words Qualitative Does not give the whole story No quantities are given

ex. methane + oxygen → carbon dioxide + water

Formula equation

Represents reactants and products of a chemical rx with symbols or formulas Use coefficients to balance

Small whole number that appears in front of a formula in an equation

ex. CH4 + 2 O2 → CO2 + 2 H2O

Symbols in Chemical Equations → - yields ↔ - reversible reaction – chem rx in which the products

reform the originals reactants

(s) – solid state ↓ - precipitate (l) – liquid (aq) – aqueous solution (dissolved in water) (g) – gas heat→ - reactants are heated 0°→ - temperature at which reaction is carried out MnO

2→ - catalyst – substance used to alter the rate of rx

Review and Assignment

1. List three observations that suggest that a chemical reaction has taken place.

2. List three requirements for a correctly written chemical equation.

3. Write a word equation and a formula equation for a given chemical reaction.

Assignment: HW 8-1 and HW 8-2

Chapter 8 - Section 2Objectives:1. Balance a chemical formulas by

inspection.

Chemical Equations Give us quantitative information

Coefficients indicate relative, not absolute amounts of reactants and products H2 + Cl2 → 2 HCl

1 mole of H2 and 1 mole of Cl2 Yields 2 moles HCl

Relative masses of the reactants and products of a chemical rx can be determined from the coefficients H2 + Cl2 → 2 HCl

1 mole H2 = 2 g H2 1 mole Cl2 = 71 g Cl2 Yields 2 moles HCl = 73 g HCl

Reverse reaction for a chemical equation has the same relative amounts of substances as the forward reaction

Balancing Chemical Reactions 1. Identify the names of the reactants and the

products, and write a word equation. 2. Write a formula equation by substituting correct

formulas for the names of reactants and products. Don’t forget diatomic molecules (H2, N2, O2, F2, Cl2, Br2, I2)

3. Balance the formula equation. Balance the different types of atoms one at a time 1st balance atoms of elements that are combined and appear only

once on each side Balance polyatomic ions that appear on both sides Balance H and O atoms

4. Count atoms to be sure that the equation is balanced.

Example 1. zinc + hydrochloric acid → zinc chloride + hydrogen

2. Zn(s) + HCl(aq) → ZnCl2(aq) + H2(g)

3. Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)

4. 1 Zn + 2H and 2 Cl → 1 Zn and 2 Cl + 2 H

Demo. Zinc + hydrochloric acid → zinc chloride + hydrogen gas

Aluminum sulfate + calcium chloride → aluminum chloride + calcium sulfate

Balance the equations from the previous demonstrations

Zn + HCl → ZnCl2 + H2

Zn + 2 HCl → ZnCl2 + H2

Al2(SO4)3 + CaCl2 → AlCl3 + CaSO4

Al2(SO4)3 + 3 CaCl2 → 2AlCl3 + 3 CaSO4

Review and Assignment

1. Balance a formula equation by inspection.

Assignment: HW 8-3 and HW 8-4

Chapter 8 - Section 2Objectives:1. Define and give general equations for

synthesis, decomposition, single-replacement, and double-replacement reactions.

2. Classify a reaction as synthesis, decomposition, single-replacement, and double-replacement, or combustion.

Types of Chemical Reactions

1. Synthesis reaction A + X → AX

2. Decomposition AX → A + X

3. Single-Replacement A + BX → AX + B Y + BX → BY + X

4. Double-Replacement AX + BY → AY + BX

Synthesis

2 or more substances combine to produce a new compound Also known as composition reaction Forms oxides with oxygen and metals

Decomposition

A single compound undergoes a reaction that produces two or more simpler substances Most are endothermic when energy is

supplied by electricity or heat ex. electrolysis – decomposition of a

substance by an electric current

Single-Replacement

One element replaces a similar element in a compound Also called displacement Many are in aqueous solutions Less E required The more active element replaces the

less active one Most active metals (group 1) react

w/water and produce metal hydroxides

Double-Replacement

Ions of two compounds exchange places in an aqueous solution to form two new compounds One compound formed is usually a

precipitate, an insoluble gas, or a molecular compound (usually water)

Other compound remains dissolved

Combustion Reaction

A substance combines with oxygen, releasing a large amount of E in the form of light and heat Always have oxygen as a reactant

Review and Assignment

1. Define and give general equations for synthesis, decomposition, single-replacement, and double-replacement reactions.

2. Classify a reaction as synthesis, decomposition, single-replacement, and double-replacement, or combustion.

Assignment: WS, HW 8-5, HW 8-7

Chapter 8 - Section 3Objectives:1. Explain the significance of an activity

series. 2. Use the activity series to predict whether

a given reaction will occur and what the products will be.

Chemical Activity

The ability of an element to react Metals lose e- easily Non-metals gain e- easily

Activity Series A list of elements organized according to

ease with which the elements undergo certain chemical reactions Metals – greater activity = greater ease to lose e- Non-metals – greater activity = greater ease to

gain e- Order is determined by single-replacement

reactions The metal at the top can replace anything

beneath it Predicts if a reaction will actually take place d/o what they are reacting with (water vs. acid)

Examples

2 Al (s) + 3 ZnCl2(ag) → 3 Zn(s) + 2AlCl3(aq)

Al will replace Zn This reaction will occur

Co(s) + 2 NaCl(s) → no reaction

Cr(s) + H2O(l) → no reaction (b/c water, not steam)

Examples Zn(s) + H2O(l) 50°C→

No reaction (b/c water is not steam)

Sn(s) + O2(g) → yes, Sn reacts w/oxygen

Cd(s) + Pb(NO3)2(aq) → yes, Pb is below Cd

Cu(s) + HCl(aq) → no, Cu is below H2

Review and Assignment

1. Explain the significance of an activity series.

2. Use the activity series to predict whether a given reaction will occur and what the products will be.

Assignment: HW 8-8