Chapter 7 Chemical Reactions. . htm. htm

Chapter 7Chemical Reactions

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• Chemical Reaction

Objectives March 8, 2011

• Review what a chemical reaction is. Identify products and reactants.

• Explain how chemical reactions occur

• Discuss energy changes associated with bond breaking and bond making

• List the elements that form stable diatomic molecules

What is a Chemical Reaction

• A chemical reaction produces something new.

• One or more substances are converted into new substances that have compositions and properties that are different from the starting substances.

Reactants: The starting substances

Products: The new substances

Chemical equation: representation of the chemical reaction.

Reactants Products

“reacts to form”

Which are Chemical Reactions

1. H2O (l) H2O (g)

2. CO2 + H2O H2CO3

3. CO2 + O2 O2 + CO2

H2 + I2 2HI

How are Reactants Transformed into Products?

• It always takes energy to break a chemical bond.

• Energy is released when a chemical bond is formed.

Balancing Chemical Equations

• The Law of Conservation of Matter is obeyed.

• H2 + I2 2HI

• Adjust the coefficients only. Never change the subscripts!

Practice

• 2H2 How many Hydrogen atoms?

• 2H2O How many Hydrogen? How many Oxygen?

• 2(NH4)2S How many Nitrogen? How many Hydrogen? How many Sulfur?

Try These

H2 + O2 H2O

CH4 + O2 CO2 + H2O

H2 + Cl2 HCl

How reactants are transformed into products?

• Reactants are transformed during chemical reactions

• Energy is required (absorbed) to break a chemical bond

• Energy is released when a chemical bond forms

Balancing equations

• Obey law of conservation of matter• Chemical equations have two

parts: reactants and products

How to balance?

• The total number of atoms of each element should be the same on both sides of equation

• Use coefficients to balance equations

Example:

2H2 + O2 2 H2O

• Calcium carbonate will react with water that is saturated with carbon dioxide to form the soluble calcium bicarbonate.

• CaCO3 + CO2 + H2O → Ca(HCO3)2 This reaction is important in the erosion of carbonate rocks, forming caverns, and leads to hard water in many regions.

Ca2+ (aq) + CO32- (aq) CaCO3(s)

• Hard water is water that has high mineral content (in contrast with soft water). Hard water has high concentrations of Ca2+ and Mg2+ ions

Ca2+ (aq) + 2F- (aq) CaF2(s)

Types of chemical reactions

• Single replacement• Double replacement• Decomposition• Combination • Combustion

Synthesis (Combination) Synthesis (Combination) ReactionsReactions

Two or more substances combine to form a new compound.

A + X AX

Reaction of elements with oxygen and sulfur Reactions of metals with Halogens Synthesis Reactions with Oxides There are others not covered here!

Decomposition Decomposition ReactionsReactions

A single compound undergoes a reaction that produces two or more simpler substances

Decomposition of:   Binary compounds 2H2O(l ) 2H2(g) +

O2(g)   Metal carbonates CaCO3(s) CaO(s) +

CO2(g)   Metal hydroxides Ca(OH)2(s) CaO(s) +

H2O(g)   Metal chlorates 2KClO3(s) 2KCl(s) +

3O2(g)   Oxyacids H2CO3(aq) CO2(g) +

H2O(l )

AX A + X

Single Replacement Single Replacement ReactionsReactions

Replacement of:

Metals by another metal Hydrogen in water by a metal Hydrogen in an acid by a metal Halogens by more active halogens

A + BX AX + B

BX + Y BY + X

The Activity Series of the The Activity Series of the MetalsMetals

  Lithium   Potassium   Calcium   Sodium   Magnesium   Aluminum   Zinc   Chromium   Iron   Nickel   Lead   Hydrogen Hydrogen   Bismuth   Copper   Mercury   Silver   Platinum   Gold

Metals can replace other metals provided that they are above the metal that they are trying to replace.

Metals above hydrogen can replace hydrogen in acids.

Metals from sodium upward can replace hydrogen in water

The Activity Series of the The Activity Series of the HalogensHalogens

  Fluorine   Chlorine   Bromine   Iodine

Halogens can replace other halogens in compounds, provided that they are above the halogen that they are trying to replace.

2NaCl(s) + F2(g) 2NaF(s) + Cl2(g)

MgCl2(s) + Br2(g) ???No Reaction

???

Double Replacement ReactionsDouble Replacement Reactions

The ions of two compounds exchange places in an aqueous solution to form two new compounds.

AX + BY AY + BX

One of the compounds formed is usually a precipitate, an insoluble gas that bubbles out of solution, or a molecular compound, usually water.

Combustion ReactionsCombustion ReactionsA substance combines with oxygen, releasing a large amount of energy in the form of light and heat.

  Reactive elements combine with oxygen

P4(s) + 5O2(g) P4O10(s)

(This is also a synthesis reaction)

  The burning of natural gas, wood, gasoline

C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)

Solubility and precipitation reactions

Pb(NO3)2 (aq) + 2NaI(aq) PbI2(s) + 2NaNO3(aq)

Solubility rules

Soluble in water: • sodium, potassium, and ammonium

salts; acetates and nitrates• Halides with the exception of halides

of lead (II), silver(I), and mercury(I).• Sulfates with the exception of

sulfates of calcium, barium, lead (II) and strontium

Acid-Base reactions(neutralization reactions)

• Acid: any compound that produces hydrogen ions (H+), when added to water.

• Base: any substance that produces hydroxide ions (OH-), when added to water.

HCl(aq) + Na(OH)(aq) NaCl(aq) + H2O(l)

Acids

• Produces H+(aq)

Hydrochloric Acid

Nitric Acid

Sulfuric Acid

Hydrofluoric Acid

Acids You Know

Bases

• Produces OH-(aq)

• NaOH lye

• KOH

• Ca(OH)2

Properties of Bases

• Slimy or soapy feel on fingers

• Concentrated or strong bases are caustic on organic matter and react violently with acidic substances

• the pH is above 7

• bitter tasting[4]

Insoluble in water

• Phosphates, carbonates and sulfides except sodium, potassium, ammonium salts, and calcium sulfide

• Hydroxides except sodium, potassium, calcium, and barium hydroxides