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: C 10 H O 2 ---> 10 CO H 2 O

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 + S 8 ---> 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 H 2 O ---> 2 H 2 + O 2

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 H 2 O ---> Mg(OH) 2 + H 2

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(NO 3 ) KI ---> PbI KNO 3

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 ---> H 2 O + BA One example of an acid-base reaction is the reaction of hydrobromic acid (HBr) with sodium hydroxide: HBr + NaOH ---> NaBr + H 2 O

H 2 + F 2 = 2HF H F 2 +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: H 2 → 2H+ + 2e- Substance reduced: Fluorine Reduction Reaction: F 2 + 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 + KNO 3 --> NaNO 3 + KOH 2) CH O 2 --> CO H 2 O 3) 2 Fe + 6 NaBr --> 2 FeBr Na 4) CaSO 4 + Mg(OH) 2 --> Ca(OH) 2 + MgSO 4 5) NH 4 OH + HBr --> H 2 O + NH 4 Br 6) Pb + O 2 --> PbO 2 7) Na 2 CO 3 --> Na 2 O + CO 2

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, BaSO 4, NaC 2 H 3 O 2, and CaS. Determine the solubility in water for these ionic substances. NaCl (all chlorides are soluble except...) SOLUBLE = aqueous BaSO 4 (all sulfates are soluble except...) INSOLUBLE = solid NaC 2 H 3 O 2 (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. Na 2 SO 4(aq) + CaCl 2(aq) ---> Since this is a double replacement reaction we can write the formulas of the products by exchanging the cations and anions. Na 2 SO 4(aq) + CaCl 2(aq) ---> CaSO 4(?) + 2NaCl (?) Now we'll use the Solubility Table to predict the phases of the products. According to the table CaSO 4 is INSOLUBLE and NaCl is SOLUBLE. Na 2 SO 4(aq) + CaCl 2(aq) ---> CaSO 4(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. AgNO 3(aq) + Na 2 CO 3(aq) ---> Since this is a double replacement reaction we can write the formulas of the products by exchanging the cations and anions. 2AgNO 3(aq) + Na 2 CO 3(aq) --->Ag 2 CO 3(?) + 2NaNO 3(?) Now we'll use the Solubility Table to predict the phases of the products. According to the table Ag 2 CO 3 is INSOLUBLE and NaNO 3 is SOLUBLE. 2AgNO 3(aq) + Na 2 CO 3(aq) ---> Ag 2 CO 3(s) + 2NaNO 3(aq)

Example 3: KNO 3(aq) + BaCl 2 (aq) ) ---> Example 4: Na 2 SO 4(aq) + Pb(NO 3 ) 2(aq) ) ---> Example 5: KOH (aq) + Fe(NO 3 ) 3(aq) ) --->