Chapter 7 Chemical Reactions

htmhttp:// htm htmhttp:// htm htmhttp:// htm htmhttp:// htm guncotton.htmhttp:// guncotton.htm

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.H 2 O (l)H 2 O (g) 2.CO 2 + H 2 OH 2 CO 3 3.CO 2 + O 2 O 2 + CO 2

H 2 + I 2  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. H 2 + I 2 2HI Adjust the coefficients only. Never change the subscripts!

Practice 2H 2 How many Hydrogen atoms? 2H 2 O How many Hydrogen? How many Oxygen? 2(NH 4 ) 2 S How many Nitrogen? How many Hydrogen? How many Sulfur?

Try These H 2 + O 2 H 2 O CH 4 + O 2 CO 2 + H 2 O H 2 + Cl 2 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: 2H 2 + O 2  2 H 2 O

Calcium carbonate will react with water that is saturated with carbon dioxide to form the soluble calcium bicarbonate.calcium bicarbonate CaCO 3 + CO 2 + H 2 O → Ca(HCO 3 ) 2 This reaction is important in the erosion of carbonate rocks, forming caverns, and leads to hard water in many regions.erosion carbonate rockscaverns

Ca 2+ (aq) + CO 3 2- (aq)  CaCO 3 (s) Hard water is water that has high mineral content (in contrast with soft water). Hard water has high concentrations of Ca 2+ and Mg 2+ ionswatermineralsoft waterCa 2+ Mg 2+ Ca 2+ (aq) + 2F - (aq)  CaF 2 (s)

Types of chemical reactions Single replacement Double replacement Decomposition Combination Combustion

Synthesis (Combination) Reactions 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 Reactions A single compound undergoes a reaction that produces two or more simpler substances Decomposition of: Binary compounds 2H 2 O( l )  2H 2 (g) + O 2 (g) Metal carbonates CaCO 3 (s)  CaO(s) + CO 2 (g) Metal hydroxides Ca(OH) 2 (s)  CaO(s) + H 2 O(g) Metal chlorates 2KClO 3 (s)  2KCl(s) + 3O 2 (g) Oxyacids H 2 CO 3 (aq)  CO 2 (g) + H 2 O( l ) AX  A + X

Single Replacement Reactions 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 Metals Lithium Potassium Calcium Sodium Magnesium Aluminum Zinc Chromium Iron Nickel Lead 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 Halogens 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) + F 2 (g)  2NaF(s) + Cl 2 (g) MgCl 2 (s) + Br 2 (g)  ???No Reaction ???

Double 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 Reactions A substance combines with oxygen, releasing a large amount of energy in the form of light and heat. Reactive elements combine with oxygen P 4 (s) + 5O 2 (g)  P 4 O 10 (s) (This is also a synthesis reaction) The burning of natural gas, wood, gasoline C 3 H 8 (g) + 5O 2 (g)  3CO 2 (g) + 4H 2 O(g)

Solubility and precipitation reactions Pb(NO 3 ) 2 (aq) + 2NaI (aq )  PbI 2(s) + 2NaNO 3(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) + H 2 O(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] [4]

Insoluble in water Phosphates, carbonates and sulfides except sodium, potassium, ammonium salts, and calcium sulfide Hydroxides except sodium, potassium, calcium, and barium hydroxides