Chemistry I-H Types of Reactions / Reaction Prediction / Reaction Theory

Transcription

1 Chemistry I-H Types of Reactions / Reaction Prediction / Reaction Theory I. Synthesis (combination) A + B > AB Substances that are chemically combined may be two elements or two compounds. If the synthesis reaction is a combination of two elements, the only combination that will not occur is between two metals. [Remember, though, that a physical union of two metals forms an alloy.] Theoretically then, any two nonmetals should be able to combine, as well as a combination of a metal and a nonmetal. ex. C (s) + O 2(g) > CO 2(g) Fe (s) + Cl 2(g) > FeCl 2(s) but Fe (s) + Sn (s) > N.R. (no reaction) Although the combination of two compounds could be considered as a synthesis reaction, it could also be classified as a special type of double replacement reaction. For test purposes, we will classify it as a synthesis reaction. The types of compounds that can chemically combine to form a new compound usually have acid-base properties. A metallic oxide is sometimes classified as a basic anhydride because, when it is dissolved in water, a base forms. Likewise, a nonmetallic oxide is called an acidic anhydride since dissolution of this type of compound will produce an acid. ex. BaO (s) + H 2 O (liq) Ba(OH) 2(aq) SO 2(g) + H 2 O (liq) H 2 SO 3(aq) In addition, when an acid and a base react, a salt and water are formed (see the section on double replacement reactions). Water is an amphoteric substance, and as I have already shown you, it may be written as H-OH. In an acid-base neutralization reaction, the acid is the source of the hydrogen ion while the base is the source of the hydroxide ion in the formation of the water molecule. In the synthesis reaction of an acid anhydride with a basic anhydride, only a salt is formed because the two components for the formation of water (the H+ and the OH-) are lacking. ex. BaO (s) + CO 2(g) BaCO 3(s) II. Combustion CxHy + O2 CO2 + H2O III. Decomposition AB A + B In decomposition, the chemical breakdown usually occurs by either applying direct current (D.C.) or by adding heat. In all cases, the electrolysis of a substance will occur. Often it may only be a question of how much current to apply. Substances cannot be decomposed through alternating current (A.C.). Heating of a substance may or may not produce a decomposition reaction and the following rules must be applied. When a substance is decomposed, two or more products are formed. These products may be elements, compounds, or a combination of both. A. Electrolysis: 2 H 2 O (liq) DC > 2 H 2(g) + O 2(g)

2 -2- B. Decomposition by Heating: 1. Action of heat on hydroxides All metallic hydroxides form the metallic oxide and release water, with the exception of the I-A metallic hydroxides. The I-A metallic hydroxides are so stable that heating has no effect. ex. Ba(OH) > BaO + H 2 O KOH > N.R. 2. Action of heat on carbonates All metallic carbonate form the metallic oxide and release carbon dioxide gas, with the exception of the I-A metallic hydroxides. The I-A metallic carbonates are so stable that heating has no effect. ex. BaCO > BaO + CO 2 K 2 CO > N.R. 3. Action of heat on sulfates All metallic sulfates form the metallic oxide and release sulfur trioxide gas, with the exception of the I- A and II-A metallic sulfates. The metallic sulfates of the alkali family and the alkaline earth family are so stable that heating has no effect. ex. FeSO > FeO + SO 3 K 2 SO > N.R. BaSO > N.R. 4. Action of heat on nitrates All metallic nitrates form the metallic oxide and release nitrogen dioxide gas as well as oxygen gas, with the exception of the I-A metallic nitrates. The I-A metallic nitrates decompose, but not completely. They decompose to yield the metallic nitrite as well as oxygen gas. ex. Ba(NO 3 ) > BaO + O 2 + NO 2 2 KNO > 2 KNO 2 + O 2 5. Action of heat on chlorates This group is very active, so that even the alkali metals react. All metallic chlorates form the metallic chloride and release oxygen gas. Please note that the oxide of the metal is not formed with this group of compounds. ex. Ba(ClO 3 ) > BaCl O 2 2 KClO > 2 KCl + 3 O 2 6. Action of heat on oxides Most metallic oxides are so stable that heating has no effect. The only metallic oxides that decompose upon heating are silver, mercury, platinum, palladium, and gold. ex. 2 HgO > 2 Hg + O (liq) 2(g) BaO > N.R.

3 -3- IV. Single Replacement (or Single Displacement) M + AB -----> MB + A or X + AY -----> AX + Y For a metallic element to replace a metal cation in a compound, the element must be more active; ie. higher on the activity series. For a nonmetal to replace an anion, it must be more active. Activity for nonmetals decreases as you move down within a group, which means that the higher it is in a family, the more active it is.. ex. Fe (s) + CuSO 4(aq) > FeSO 4(aq) + Cu (s) Cl 2(g) + 2 NaBr (aq) > Br 2(liq) + 2 NaCl (aq) Fe (s) + BaSO 4(aq) > N.R. Br 2(liq) + 2 NaF (aq) > N.R. When a metal that is above hydrogen in the activity series is treated with an acid, a salt is formed and hydrogen gas is liberated. ex. Zn (s) + 2 HCl (aq) > ZnCl 2(aq) + H 2(g) Cu (s) + 2 HCl (aq) > N.R. ALERT: FOR SINGLE REPLACEMENT REACTIONS YOU WILL NEED TO CHECK EITHER THE ACTIVITY SERIES OR THE PERIODIC TABLE. ====================================================================== V. Double Replacement (or metathesis) AX + BY > BX + AY Double replacement reactions generally fall under one of two categories - (1) acid-base neutralization reactions or (2) precipitation reactions. Many types of double replacement reactions are said to be reversible - that is, once the products are formed, they may turn back into the original reactants. The reactions that we will be considering are said to be irreversible. Two criteria must be met for a reaction to have occurred. (1) Both reactants must be soluble in water or in each other. You will need to know the solubility rules to figure out whether or not the reactants fit this criterion. (2) There must be evidence of a reaction. Evidence of a reaction may be (a) the formation of a solid (a precipitate), (b) a gas may be evolved, or (c) a molecular species may be formed. Usually when a molecule is formed, heat is evolved (an exothermic reaction!) Probably the most important molecule that will be formed will be water. [Actually when a gas is formed (b), the gas is a molecular species. IT IS IMPERATIVE THAT YOU LEARN THE SOLUBILITY RULES (ALONG WITH ALL OF THE EXCEPTIONS!) AS QUICKLY AS POSSIBLE!!! REMEMBER: BOTH CRITERIA MUST BE MET FOR A DOUBLE REPLACEMENT REACTION TO OCCUR.

4 Acid-Base Neutralization: Acid + Base > Water + Salt (Salt is the by-product of this reaction; the driving force of this reaction is the formation of water.) ex. HCl (aq) + KOH (aq) > H 2 O + KCl (aq) 2. Metallic oxide + Acid > Water + Salt ex. BaO (s) + 2 HCl (aq) > H 2 O + BaCl 2(aq) 3. Nonmetallic oxide + Base > Water + Salt ex. SO 3(g) + 2 KOH (aq) > H 2 O + K 2 SO 4(aq) Precipitation: 1. Both reactants soluble in water ex. AgNO 3(aq) + NaCl (aq) > AgCl (ppt) + NaNO 3(aq) 2. Water-insoluble reactant is soluble in acids ex. BaCO 3 + KBr (aq) > N.R. yet BaCO 3 + HCl (aq) > H 2 CO 3(aq) + BaCl 2(aq) or better H 2 O + CO 2(g) + BaCl 2(aq) There are three compounds that are commonly written in a molecular form, yet they do not exist as molecules. You need to commit these to memory. 1. ammonium hydroxide - NH 4 OH (aq) - is really an aqueous solution of ammonia gas. 2. sulfurous acid - H 2 SO 3(aq) - is really an aqueous solution of sulfur dioxide gas. 3. carbonic acid - H 2 CO 3(aq) - is really an aqueous solution of carbon dioxide gas. If any of the three substances is used as a reactant, they are considered soluble (meet the first criterion.) Yet if they are listed as a product, they meet the second criterion (release of a gas, or a molecular species.) For a reactant to meet the first criterion (solubility), it must be water-soluble, or soluble in an acid, but only if the other reactant is an acid! To be soluble in an acid solution, it must be a basic salt. That means it must contain a cation from the I-A family or the lower II-A family, and the anion must be from a weak acid. On the product side, at least one of the products must have something other than an "S". In order to show that a reaction has occurred, you must be able to write a NET IONIC EQUATION. The net ionic equation shows what ions actually reacted. Those ions that are not part of the net ionic equation are called "spectator ions" because they merely "watch" the reaction occur. molecular: AgNO 3(aq) + NaCl (aq) > AgCl (ppt) + NaNO 3(aq) - ionic: Ag + + NO + (aq) 3 (aq) Na+ + (aq) Cl > AgCl + Na (aq) (ppt) (aq) + + NO 3 -(aq) net ionic: Ag + + (aq) Cl > AgCl (aq) (ppt) In this example, the sodium and the nitrate ions were the spectator ions because they didn't change.

5 -5- Example #2 Write the net ionic equation for the following situation: Mixing solutions of lead(ii) chlorate and sodium sulfide Example #3 Write the net ionic equation for the following situation: ammonium perchlorate and sodium bromide are mixed Evidences of Chemical Reactions How do chemists know if and when a reaction has taken place? There are a variety of different ways which range from using different pieces of instrumentation and the powers of observation. Generally, however, chemists agree upon four pieces of evidence to indicate a reaction has taken place. These evidences are based on the powers of observation and are as follow: 1. Formation or consumption of a precipitate 2. Heat is either given off or absorbed (exothermic or endothermic) 3. Evolution of a gas (bubbles form in the absence of heat) 4. Color change Kinetics and Collision Theory Kinetics is the study of the rate of a reaction. This rate may be measured in terms of appearance of products or in terms of disappearance of reactants. These rates may be measured by using some of the following: - a colorimeter or a spectrophotometer (which measure color changes), changes in ph, changes in volume and the amount of precipitate formed. Factors that affect the rate of reaction: 1. temperature 2. concentration 3. catalyst 4. surface area 5. volume / pressure (if gases are involved) 6. nature of reactants Factors that speed up the rate of a reaction: 1. Increased concentration of solvent 2. Increase collisions (stirring, shaking, adding heat makes molecules move faster) 3. Increase surface area (crush the substance) Kinetics has its foundations in Collision Theory. Collision Theory explains how and why reactions take place. Essentially, the Collision Theory says that in order for a reaction to take place, the substances must come in contact with one another by by collision. The reaction will only take place if the collisions are effective. For a collision to be considered effective, two requirements must be met. First, the species must collide in a very particular spatial alignment (we re talking a 3-D spatial collision not a 2-D chalkboard collision). Second, each of the species must collide with enough energy to overcome an energy barrier known as the activation energy (EA). Potential Energy Diagrams Potential Energy (PE) Diagrams detail the total amount of energy is stored in between the bonds of molecules before a reaction occurs (reactants), while the reaction is occurring (activated complex), and after the reaction is complete (products).

6 -6- Essential Vocabulary for PE Diagrams: Enthalpy (H): heat of the reaction heat that goes into or comes out of the reaction (it s the difference in energy between the reactants & the amount of energy in the products) Activation Energy (Ea): Amount of energy needed to make the reaction happen (collision theory) Activated Complex: amount of energy during the transitional period between reactants and products Catalyst: speeds up the reaction by lowering Ea (catalyst is written above the yields sign) 1. Heat () 2. Sunlight 3. electricity 4. Compounds Forward Reaction: reactants products Reverse Reaction: products reactants means both heat & change in. Seen over the yields sign or written as a product or reactant means heat. Seen in front of letters like H, T, E, q, etc means change in Exothermic (- H) Endothermic P or R More energy in reactants more energy in products Energy Energy is released to the surroundings energy is absorbed from the surroundings Heat the reaction produces heat the rxn consumes heat (heat is absorbed in bonds) Eqn heat is shown on the product sign of the eqn heat is shown on the reactant side of the eqn Ex. HCl + Zn ZnCl2 + H2 + heat 6CO2 + 6H2O + heat C6H12O6 + 6O2 (photosynthesis) Use the diagram below to indicate where the Essential Vocabulary falls on the diagram.

7 -7- Classifying and Balancing Chemical Reactions Anton Lavoisier (1774) Law of Conservation of Matter (Mass): For any system closed to all transfers of matter and energy (both of which have mass), the mass of the system must remain constant over time, as system mass cannot change quantity if it is not added or removed Simply put Matter can be neither created nor destroyed but it may change its form. To balance chemical reactions, the Law of Conservation of Matter must always be obeyed. Use the following steps to balance any chemical reaction. Steps (Sight Inspection Method): 1. Take inventory. 2. Obey the Law of Conservation of Matter by placing coefficients in front of the species (compound, element, ion, etc.) 3. Sight Inspect (or Check) the equation to ensure there are equal amounts of the species on both the reactant and product sides of the reaction. In-class Sample Problems: For each of these problems, identify the type of chemical reaction using the key below. Balance the reaction using the steps above. Then, justify the reasoning of the type of chemical reaction. Comb (Combustion), Syn (synthesis), Dec (decomposition), SR (single replacement), DR (double replacement) 1. H2 + O2 H2O 2. Ba(ClO3)2 BaCl2 + O2 3. Cu + AgNO3 Cu(NO3)2 + Ag 4. Pb(NO3)2 + AlCl3 PbCl2 + Al(NO3)3 5. C2H6 + O2 CO2 + H2O