Chemistry B2A Chapter 6 & 7 Chemical reactions

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1 Chemistry B2A Chapter 6 & 7 Chemical reactions Chemical change (Chemical reaction): when the substances are used up (disappear) and others are formed to take their place (for example: burning a paper or cooking an egg). Evidence for a chemical reaction: 1- The color changes. 2- A solid forms (precipitation). 3- Bubbles form (formation of a gas). 4- Heat and/or flame is produced, or heat is absorbed. Reactants: starting materials. Products: the materials that are created in a reaction. Chemical Equation: we represent a chemical reaction in the form of a chemical equation, using chemical formulas for the reactants and products, and an arrow to indicate the direction in which the reaction proceeds. Note: It is important to show the state of each reactant and product in a chemical equation (immediately following each reactant and product). We use the symbol (g) for gas, (l) for liquid, (s) for solid, and (aq) for a substance dissolved in water (aqueous). 6CO 2 (g) + 6H 2 O(l) C 6 H 12 O 6 (aq) + 6O 2 (g) Chemical reactions are classified into five groups: A + B AB Synthesis reactions (Combination) 2H 2 + O 2 2H 2 O AB A + B Decomposition reactions (Analysis) 2NaCl 2Na +Cl 2 A + BC AC + B Single replacement reactions Fe + CuSO 4 FeSO 4 +Cu AB + CD AD + CB Double replacement reactions NaCl + AgNO 3 NaNO 3 +AgCl AB + xo 2 yco 2 + zh 2 O Combustion C 3 H 8 + 5O 2 3CO 2 + 4H 2 O Balance a chemical equation: in a balanced equation, there must be the same number of atoms of each element on both sides (the law of conservation of mass: atoms are neither destroyed nor created in chemical reactions; they merely shift from one substance to another). How to balance a chemical equation: 1. We begin with atoms that appear in only one compound on the left and only one compound on the right. 2. If an atom occurs as a free element, we balance this element last.

2 3. We can change only the coefficients in balancing an equation (we cannot change chemical formulas). C 3 H 8 (g) + O 2 (g) CO 2 (g) + H 2 O(g) We begin with either carbon or hydrogen and we balance oxygen last (free element): C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O(g) Note: the convention is to have integer (whole-number) coefficients. Therefore, sometimes, we should multiply the entire equation by a number. C 4 H 10 (g) O 2 (g) 4CO 2 (g) + 5H 2 O(g) 13 2 [C 4 H 10 (g) + O 2 (g) 4CO 2 (g) + 5H 2 O(g)] 2 2C 4 H 10 (g) + 13O 2 (g) 8CO 2 (g) + 10H 2 O(g) Note: Chemists do not normally write equations with coefficients that are all divisible by a common number. A correctly balanced equation is almost always written with the coefficients expressed in the lowest set of whole numbers. Correct C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O(g) 2 2C 3 H 8 (g) + 10O 2 (g) 6CO 2 (g) + 8H 2 O(g) 3 3C 3 H 8 (g) + 15O 2 (g) 9CO 2 (g) + 12H 2 O(g) Driving forces for a chemical reaction: there are several driving forces that pull reactants toward products-changes that tend to make reactions go in the direction of the arrow: 1. Formation of a solid (precipitation) 2. Formation of water (acid-base reactions) 3. Transfer of electrons 4. Formation of a gas 1. Formation of solid (Precipitation): Aqueous solution: a solution in which the solvent is water. Many ionic compounds are soluble in water. Water molecules separate the positive and negative ions from each other (dissociation reactions). NaCl(s) Na + (aq) + Cl - (aq) Dissolving: when a solid ionic compound is added to water, water molecules surround the ions at the surface of the crystal. The negative ions (anions) attract the positive poles of water molecules, and the positive ions (cations) attract the negative poles of water molecules. Each ion attracts multiple water molecules. We say ions are hydrated and this phenomenon is called hydration (a more general term, covering all solvents, is solvated).

3 Electrolyte: substances that conduct an electric current when dissolved in water or when in the molten state are called electrolytes. These substances can be ionized and produce ions. The positively charged ions (cations) migrate to the negative electrode (cathode) and the negatively charged ions (anions) migrate to the positive electrode (anode). The movement of ions constitutes an electric current. Note: compounds that dissociate (ionize) completely are called strong electrolytes (most of the ionic compounds and some acids). Compounds that dissociate partially are called weak electrolytes (such as CH 3 COOH). Compounds that do not dissociate (do not conduct electricity) are called nonectrolytes (such as distilled water). Ionic and net ionic equations: imagine we have two ionic compounds and both of them are soluble in water: H2O + NaCl(s) Na (aq) + Cl- (aq) AgNO 3(s) If we mix the two solutions together: H2O + - Ag (aq) + NO3 (aq) Ag + (aq) + NO - 3 (aq) + Na + (aq) + Cl - (aq) AgCl(s) + Na + (aq) + NO - 3 (aq) This equation is called Ionic equation. Na + - and NO 3 ions do not participate in this reaction. They are called Spectator ions and we can eliminate them from both sides: Ag + (aq) + NO - 3 (aq) + Na + (aq) + Cl - (aq) AgCl(s) + Na + (aq) + NO - 3 (aq) Ag + (aq) + Cl - (aq) AgCl(s) Net ionic equation

4 2. Formation of water (acid-base reactions): Arrhenius definitions: an acid is a substance that produces H 3 O + ions in aqueous solution. A base is a substance that produces OH - ions in aqueous solution. CH 3 COOH(aq) + H 2 O(l) CH 3 COO - (aq) + H 3 O + (aq) CH 3 COOH is an acid. NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH - (aq) NH 3 is a base. H 3 O + (hydronium ion): an H+ ion in water immediately combines with an H 2 O molecule (H + cannot exist in water. Because an H + ion is a bare proton, and a charge of +1 is too concentrated to exist on such a tiny particle). H + (aq) + H 2 O(l) H 3 O + (aq) Weak acid and base: an acid or a base that is only partially ionized in aqueous solution. CH 3 COOH(aq) + H 2 O(l) NH 3 (aq) + H 2 O(l) CH 3 COO - (aq) + H 3 O + (aq) NH 4 + (aq) + OH - (aq) Strong acid and base: an acid or a base that ionizes completely in aqueous solution. HCl(aq) + H 2 O(l) NaOH(aq) + H 2 O(l) Cl - (aq) + H 3 O + (aq) Na + (aq) + OH - (aq) 3. Neutralization: acids and bases react with each other to produce a salt and water. HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(l) Note: when a strong acid reacts with a strong base, the product has neither acidic nor basic properties. We call such a solution neutral. Na + (aq) + OH - (aq) + H + (aq) + Cl - (aq) + Na + (aq) + Cl - (aq) + H 2 O(l) Na + and Cl - are spectator ions (they undergo no changes), so we can eliminate them. The only chemical change that occurs for these kind of reactions is that water is formed from H + and OH Transfer of electrons: OH - (aq) + H + (aq) H 2 O(l) Metal-Nonmetal reactions: Metals lose electron(s) and nonmetals gain electron(s). Therefore, electrons are transferred in a metal-nonmetal reaction. These reactions are called oxidation-reduction reactions: Na Na + + e - Cl + e - Cl -

5 Oxidation and reduction reactions (redox reactions): Oxidation is the loss of electrons and Reduction is the gain of electrons. Zn(s) + Cu 2+ (aq) Zn 2+ (aq) + Cu(s) Zn(s) Zn e - Cu e - Cu(s) Zn is oxidized (loses electrons) so it is a reducing agent. Cu 2+ is reduced (gains electrons) so it is an oxidizing agent. In some reactions, it is not easy to see the electron loss and gain, so chemists developed another definition of oxidation and reduction: Oxidation is the gain of oxygen atoms and/or the loss of hydrogen atoms. Reduction is the loss of oxygen atoms and/or the gain of hydrogen atoms. CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) CH 4 gains oxygen (loses hydrogen), therefore it is oxidized (a reducing agent). O 2 gains hydrogen, so it is reduced (an oxidizing agent). Note: all single replacement reactions and all combustion reactions are redox reactions. All double replacement reactions are non-redox reactions.