Week 10 Lesson 1: Types of Chemical Reactions PDF

WEEK 10 TYPES OF CHEMICAL REACTIONS LESSON ONE  CHEMICAL EQUATION A chemical reaction is represented by a chemical equation. The substances on the left side of the equation are called the reactants and the substances on the right side are called the products. Wr...

WEEK 10 TYPES OF CHEMICAL REACTIONS LESSON ONE  CHEMICAL EQUATION A chemical reaction is represented by a chemical equation. The substances on the left side of the equation are called the reactants and the substances on the right side are called the products. Write a balanced equation for the reaction between solid cobalt (III) oxide and solid carbon to produce solid cobalt and carbon dioxide gas. Write a balanced equation for the combustion of gaseous butane (C4H10), a fuel used in portable stoves and grills, in which it combines with gaseous oxygen to form gaseous carbon dioxide and gaseous water. Write a balanced equation for the reaction between solid silicon dioxide and solid carbon to produce solid silicon carbide and carbon monoxide gas. Write a balanced equation for the combustion of gaseous ethane (C2H6), a minority component of natural gas, in which it combines with gaseous oxygen to form gaseous carbon dioxide and gaseous water.  Solutions and Solubility When a solid is put into a liquid solvent, the attractive forces that hold the solid together (the solute- solute interactions) come into competition with the attractive forces between the solvent molecules and the particles that compose the solid (the solvent-solute interactions) For example, when sodium chloride is put into water, there is a competition between the attraction of Na+ cations and Cl- anions to each other (due to their opposite charges) and the attraction of Na+ and Cl- to water molecules. The attraction of Na+ and Cl- to water is based on the polar nature of the water molecule.  Solutions and Solubility The oxygen atom in water is electron-rich, giving it a partial negative charge (8- ). The hydrogen atoms, in contrast, are electron-poor, giving them a partial positive charge (8+). As a result, the positively charged sodium ions are strongly attracted to the oxygen side of the water molecule, and the negatively charged chloride ions are attracted to the hydrogen side of the water molecule. In the case of NaCl, the attraction between the separated ions and many water molecules overcomes the attraction of sodium and chloride ions to each other, and the sodium chloride dissolves in the water Sugar dissolves because the attraction between sugar molecules and water molecules overcomes the attraction of sugar molecules to each other. So, unlike a sodium chloride solution (which is composed of dissociated ions), a sugar solution is composed of intact C12H22O11 molecules homogeneously mixed with the water molecules.  Solutions and Solubility Substances such as sodium chloride that completely dissociate into ions when they dissolve in water are strong electrolytes, and the resulting solutions are strong electrolyte solutions. Compounds such as sugar that do not dissociate into ions when dissolved in water are called nonelectrolytes, and the resulting solutions-called nonelectrolyte solutions-do not conduct electricity. Acids are molecular compounds, but they do ionize-form ions- when they dissolve in water. Hydrochloric acid (HCI) is a molecular compound that ionizes into H+ and Cl- when it dissolves in water. HCl is an example of a strong acid, which means that it completely ionizes in solution. Since strong acids completely ionize in solution, they are also strong electrolytes.  The Solubility of Ionic Compounds Many acids are weak acids; they do not completely ionize in water. For example, acetic acid CH3COOH, the acid present in vinegar, is a weak acid. A solution of a weak acid is composed mostly of the non-ionized form of the acid molecules-only a small percentage of the acid molecules ionize. Not all ionic compounds dissolve completely in water. If we add AgCl to water, for example, it remains solid and appears as a white powder at the bottom of the water. In general, a compound is termed soluble if it dissolves in water and insoluble if it does not. There is no easy way to tell whether a particular compound is soluble or insoluble just by looking at its formula. For now, we can follow a set of empirical rules that chemists have inferred from observations on many ionic compounds.  Types of Chemical Reactions Aqueous chemical reactions are those that occur in water. Because of its molecular shape and uneven distribution of electrons, water dissolves many ionic and covalent substances. In water, many ionic compounds and a few simple, H-containing covalent compounds, such as HCl, dissociate into ions. A molecular equation shows all substances intact and undissociated into ions.  Types of Chemical Equations The molecular equation reveals the least about the species that are actually in solution because it shows all the reactants and products as if they were intact, undissociated compounds. Only the designation for solid, (s), tells that a change has occurred: A total ionic equation shows all soluble ionic compounds as separate, solvated ions. Spectator ions appear unchanged on both sides of the equation.  Types of Chemical Equations The total ionic equation is much more accurate because it shows all the soluble ionic substances dissociated into ions. The Ag2CrO4(s) stands out as the only undissociated substance: The charges balance: four positive and four negative for a net zero charge on the left, and two positive and two negative for a net zero charge on the right.  Types of Chemical Equations Notice that Na+(aq) and NO3-(aq) appear unchanged on both sides of the equation. These are called spectator ions. They are not involved in the actual chemical change but are present only as part of the reactants; that is, we can’t add an Ag+ ion without also adding an anion, in this case, the NO3- ion. A net ionic equation eliminates the spectator ions and, thus, shows only the actual chemical change. The net ionic equation is very useful because it eliminates the spectator ions and shows only the actual chemical change:  Types of Chemical Equations The formation of solid silver chromate from silver ions and chromate ions is the only change. To make that point clearly, suppose we mixed solutions of potassium chromate, K2CrO4(aq), and silver acetate, AgC2H3O2(aq), instead of sodium chromate and silver nitrate. In that case, only the spectator ions would differ—K+ (aq) and C2H3O2- (aq) instead of Na+ (aq) and NO3-(aq).  Precipitation In a precipitation reaction, two soluble ionic compounds react to form an insoluble product, a precipitate. A precipitation reaction occurs in aqueous solution because one product is insoluble. A precipitate is an insoluble solid compound formed during a chemical reaction in solution. Precipitation reactions occur when soluble ionic compounds exchange ions (metathesis) and form an insoluble product (precipitate), in which the ions attract each other so strongly that their attraction to water molecules cannot pull them apart.  Precipitation Precipitates form for the same reason that some ionic compounds don’t dissolve: the electrostatic attraction between the ions outweighs the tendency of the ions to remain solvated and move throughout the solution. When the two solutions are mixed, the ions collide and stay together, and a solid product “comes out of solution.” Thus, the key event in a precipitation reaction is the formation of an insoluble product through the net removal of ions from solution.  Predicting Solubility Three steps help us predict if a precipitate forms: 1. Note the ions in the reactants. 2. Consider all possible cation-anion combinations. Decide whether any combination is insoluble.  Solubility Rules for Ionic Compounds Soluble Ionic Compounds in water 1. All common compounds of Group 1A (1) ions (Li+, Na+, K+, etc.) and ammonium ion (NH4+) are soluble. 2. All common nitrates (NO3-), acetates (CH3COO- or C2H3O2-), and most perchlorates (ClO4-) are soluble. 3. All common chlorides (Cl-), bromides (Br-), and iodides (I-) are soluble, except those of Ag+, Pb2+, Cu+, and Hg22+. All common fluorides (F-) are soluble, except those of Pb2+ and Group 2A (2). 4. All common sulfates (SO42-) are soluble, except those of Ca2+, Sr2+, Ba2+, Ag+, and Pb2+.  Insoluble Ionic Compounds 1. All common metal hydroxides are insoluble, except those of Group 1A(1) and the larger members of Group 2A(2) (beginning with Ca2+). 2. All common carbonates (CO32-) and phosphates (PO43-) are insoluble, except those of Group 1A(1) and NH4+. 3. All common sulfides are insoluble except those of Group 1A(1),Group 2A(2), and NH4+. QUESTIONS  QUESTIONS Write a net ionic equation for the precipitation reaction that occurs (if any) when solutions of potassium carbonate and nickel(II) chloride are mixed. Write a net ionic equation for the precipitation reaction that occurs (if any) when solutions of ammonium chloride and iron(III) nitrate are mixed. Write a net ionic equation for the precipitation reaction that occurs (if any) when solutions of sodium hydroxide and copper(II) bromide are mixed. Predict whether each compound is soluble or insoluble: (a) NiS (b) Mg3(PO4)3 (c) Li2CO3 (d) NH4Cl