Ch 7: Reactions

• Predicting Whether a Reaction Will Occur• Why does a reaction occur? What

causes a reaction to “want” to form products?• Scientists have recognized several tendencies

in reactants the DRIVE them to form products.

There are several driving forces:

• Formation of a solid• Formation of water• Formation of a gas• Transfer of electrons

• When two or more chemicals are brought together, if any of these things can occur, a chemical change is likely to take place.

• *IF NONE OF THESE FACTORS TAKE PLACE NO REACTION WILL TAKE PLACE

• Our goal is to determine whether a reaction will occur and what products might form.

All chemical reactions can be placed into one of six categories:

• 1) Combustion: A combustion reaction is when oxygen combines with a hydrocarbon (a combination of C and H) to form water and carbon dioxide. These reactions are exothermic, meaning they produce heat. An example of this kind of reaction is the burning of napthalene:

• C10H8 + 12 O2 ---> 10 CO2 + 4 H2O

• 2) Synthesis: A synthesis reaction is when two or more simple compounds combine to form a more complicated one. These reactions come in the general form of:

• A + B ---> AB• One example of a synthesis reaction is the

combination of iron and sulfur to form iron (II) sulfide:

• 8 Fe + S8 ---> 8 FeS

• 3) Decomposition: A decomposition reaction is the opposite of a synthesis reaction - a complex molecule breaks down to make simpler ones. These reactions come in the general form:

• AB ---> A + B

• One example of a decomposition reaction is the electrolysis of water to make oxygen and hydrogen gas:

• 2 H2O ---> 2 H2 + O2

• 4) Single displacement: This is when one element trades places with another element in a compound. These reactions come in the general form of:

• A + BC ---> AC + B• One example of a single displacement reaction is

when magnesium replaces hydrogen in water to make magnesium hydroxide and hydrogen gas:

• Mg + 2 H2O ---> Mg(OH)2 + H2

• 5) Double displacement: This is when the anions and cations of two different molecules switch places, forming two entirely different compounds. These reactions are in the general form:

• AB + CD ---> AD + CB• One example of a double displacement reaction is the

reaction of lead (II) nitrate with potassium iodide to form lead (II) iodide and potassium nitrate:

• • Pb(NO3)2 + 2 KI ---> PbI2 + 2 KNO3

• 6) Acid-base: This is a special kind of double displacement reaction that takes place when an acid and base react with each other. The H+ ion in the acid reacts with the OH- ion in the base, causing the formation of water. Generally, the product of this reaction is some ionic salt and water: Term “salt” is used to mean ionic compound

• HA + BOH ---> H2O + BA

• One example of an acid-base reaction is the reaction of

hydrobromic acid (HBr) with sodium hydroxide: • HBr + NaOH ---> NaBr + H2O

• H2 + F2 = 2HF

• H2

+0 + F2 +0 = H +1 F -1

• hydrogen lost an electron to become positively charged ………..OIL

• Fluorine gained an electron to become negatively charged……RIG

• Substance oxidized: Hydrogen Oxidation Reaction: H2 → 2H+ + 2e-Substance reduced: FluorineReduction Reaction: F2 + 2e- → 2F-

• ****A lot of reactions are redox and another type of reaction from the above list.

• 7*) Special Reaction Type: Redox: or oxidation-reduction reactions, primarily involve the transfer of electrons between two chemical species. The compound that loses an electron is said to be oxidized (OIL = oxidization is loss), the one that gains an electron is said to be reduced (RIG = reduced is gained).

• *If the element stands alone its net charge is ZERO. • *If the element is contained in a compound then you must

look at their individual oxidation numbers (periodic table) • *If it is a transition metal you must split the compound apart

and figure out the charge on the transition metal based off of its partner (like you did in chapter 5 when naming)

• ***YOU MUST FIRST FIGURE OUT THE REACTION TYPE 1-6 AND THEN DECIDE IF IT IS ALSO A REDOX REACTION

• Questions to ask yourself if you cannot figure out the type of reaction:

• Follow this series of questions. When you can answer "yes" to a question, then stop!

• 1) Does your reaction have oxygen as one of it's reactants and carbon dioxide and water as products? If yes, then it's a combustion reaction

• 2) Does your reaction have two (or more) chemicals combining to form one chemical? If yes, then it's a synthesis reaction

• 3) Does your reaction have one large molecule falling apart to make several small ones? If yes, then it's a decomposition reaction

• 4) Does your reaction have any molecules that contain only one element? If yes, then it's a single displacement reaction

• 5) Does your reaction have water as one of the products? If yes, then it's an acid-base reaction

• 6) If you haven't answered "yes" to any of the questions above, then you've got a double displacement reaction

• Examples to figure out: • 1) NaOH + KNO3 --> NaNO3 + KOH

• 2) CH4 + 2 O2 --> CO2 + 2 H2O

• 3) 2 Fe + 6 NaBr --> 2 FeBr3 + 6 Na

• 4) CaSO4 + Mg(OH)2 --> Ca(OH)2 + MgSO4

• 5) NH4OH + HBr --> H2O + NH4Br

• 6) Pb + O2 --> PbO2

• 7) Na2CO3 --> Na2O + CO2

• Answers:• 1) double displacement

2) combustion 3) single displacement 4) double displacement 5) acid-base 6) synthesis 7) decomposition

• Solubility Tables • A Solubility Table summarizes the solubility

behavior of a large group of ionic substances. How to interpret a Solubility Table?

Example 1:

Example 2:

• We can use the Solubility Table to determine whether an ionic compound exist as ions in aqueous solution (soluble) or as a solid (insoluble). Once we know the compound we use the Solubility Table to determine its solubility.

• For example, consider the following compounds; NaCl, BaSO4, NaC2H3O2, and CaS. Determine the solubility in water for these ionic substances.

• NaCl (all chlorides are soluble except...) SOLUBLE = aqueous

• BaSO4 (all sulfates are soluble except...) INSOLUBLE = solid

• NaC2H3O2 (all sodium compounds are soluble) SOLUBLE = aqueous

• CaS (all sulfides are insoluble...) INSOLUBLE = solid

• We'll also use the information in a Solubility Table to help identify the phase of ionic substance in a chemical equation. The chemical reaction types where the Solubility Table is important are;

• Double Replacement reactions • Neutralization reactions • Single Replacement reactions

• Example 1: double displacement reaction problem;• Write the formula and identify the phase for the

product(s) and balance the following reaction.• Na2SO4(aq) + CaCl2(aq) --->• Since this is a double replacement reaction we can write

the formulas of the products by exchanging the cations and anions.

• Na2SO4(aq) + CaCl2(aq) ---> CaSO4(?) + 2NaCl(?)

• Now we'll use the Solubility Table to predict the phases of the products. According to the table CaSO4 is INSOLUBLE and NaCl is SOLUBLE.

• Na2SO4(aq) + CaCl2(aq) ---> CaSO4(s) + 2NaCl(aq)

• Example.2: double displacement reaction problem• Write the formula and identify the phase for the

product(s) and balance the following reaction.• AgNO3(aq) + Na2CO3(aq) --->• Since this is a double replacement reaction we can write

the formulas of the products by exchanging the cations and anions.

• 2AgNO3(aq) + Na2CO3(aq) --->Ag2CO3(?) + 2NaNO3(?)

• Now we'll use the Solubility Table to predict the phases of the products. According to the table Ag2CO3 is INSOLUBLE and NaNO3 is SOLUBLE.

• 2AgNO3(aq) + Na2CO3(aq) ---> Ag2CO3(s) + 2NaNO3(aq)

• Example 3: • KNO3(aq) + BaCl2 (aq) ) --->

• Example 4:• Na2SO 4(aq) + Pb(NO3)2(aq) ) --->

• Example 5:• KOH (aq) + Fe(NO3) 3(aq) ) --->