Chemistry B11 Chapter 4 Chemical reactions

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1 Chemistry B11 Chapter 4 Chemical reactions Chemical reactions are classified into five groups: A + B AB Synthesis reactions (Combination) H + O H O AB A + B Decomposition reactions (Analysis) NaCl Na +Cl 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 yco + zh O Combustion C 3 H 8 + 5O 3CO + 4H O 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 (g) + 6H O(l) C 6 H 1 O 6 (aq) + 6O (g) Note: the term Formula can be used for both ionic and covalent compounds (formula of NaCl). However; the term Molecule is strictly correct only when used covalent compound (molecule of H O). Formula and molecular weight: formula weight (FW) of a compound is the sum of the atomic weights in atomic mass units (amu) of all atoms in the compound s formula (for both ionic and covalent compounds). The molecular weight (MW) is the same as the formula weight; however, it is only used for the covalent compounds. MW of H SO 4 : MW or FW of AlCl 3 : (1 amu) for H + 1(3 amu) for S + 4(16 amu) for O 98 amu 1(7 amu) for Al + 3(35.5 amu) for Cl amu Mole (mol): is the amount of substance that contains as many atoms, molecules, or ions as there are atoms in exactly 1g of carbon-1. Mole is the formula weight of a substance expressed in grams. FW of NaCl 58.5 amu 58.5g of NaCl 1 mole of NaCl MW or FW of AlCl amu 133.5g of AlCl 3 1 mole of AlCl 3

2 Avogadro s number ( ): number of formula units in a mole. 1 mole of hydrogen atoms atoms of hydrogen 1 mole of water molecules molecules of water 1 mole of Na + ions ions of Na + Molar mass: is the mass of one mole of the substance expressed in grams. We can say that it is the formula weight of a compound expressed in grams. Formula weight of H O 18 amu molar mass 18 g (mass of 1 mole H O) Formula weight of NaCl 58.5 amu molar mass 58.5 g (mass of 1 mole NaCl) 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.. If an atom occurs as a free element, we balance this element last. 3. We can change only the coefficients in balancing an equation (we cannot change chemical formulas). C 3 H 8 (g) + O (g) CO (g) + H O(g) We begin with either carbon or hydrogen and we balance oxygen last (free element): C 3 H 8 (g) + 5O (g) 3CO (g) + 4H O(g) Stoichiometry: the study of mass relationships in chemical reactions. The coefficients in a balanced equation refer to the relative numbers of moles, particles (atoms, molecules, ions), and volume, not grams. H O(g) O (g) + H (g) 1 moles 1 mole moles liters 1 liter liters molecules 1 molecule molecules grams 1 gram grams Note: We use the coefficients of a balanced chemical equation for the following conversions. Therefore, there is only one step. mole A mole B volume A volume B # of particles A # of particles B H + O H O How many moles of H O are produced from the reaction of 56 moles of hydrogen?

3 moles HO 56 mole 56 moles HO molesh Note: For other conversions, we use the following diagram: volume volume mass A mole B mole mass Particle (atom) (molecule) Particle (atom) (molecule) H + O H O 1. We have a sample of 74 grams of oxygen. Find the volume (in liter) that this sample can occupy. Note: at STP condition (0ºC as the standard temperature and 1 atm as the standard pressure), one mole of any gas occupies a volume of.4 L (or.4 dm 3 or 400 cc). 1 mole O.4 L O 74g O 3 g O 1mole O 5 L O. A sample of 3.0 grams of hydrogen is reacted with oxygen. How many grams of H O are produced? 1 mole H moles HO 18 grams HO 3.0 grams H 07 grams HO grams H moles H 1 mole H Limiting reagent: is the reactant that is used up first, leaving an excess of another reagent(s) unreacted. Note: The limiting reagent can control a reaction. Whenever, the limiting reagent is used up the reaction will be stopped. Therefore, to determine how much product can be formed a given mixture of reactants, we have to look for the reactant that is limiting. How to find the limiting reagent: 1. Write and balance the equation for the reaction.. Convert known masses of reactants to moles. 3. Using the numbers of moles of reactants to determine which reactant is limiting. Note: If the coefficients of reactants in a balanced reaction are not the same, we need to divide the number of the moles of each reactant by its coefficient.

4 Note: Always use the amounts of a limiting reagent to find the amount of the product. H + O H O If 39 grams oxygen reacts with 87 grams hydrogen, which reactant is limiting. How many grams of H O will be formed? 1 mole O 39g O 3 g O 1. moles O 1 mole H 87g O g H 44 moles H Because the coefficients of reactants (hydrogen and oxygen) are not the same, we have to divide the number of moles of each reactant by its coefficient: 1. mole O 1 1. moles O 44 mole H moles H Therefore, oxygen is the limiting reagent. To find the amount of the product, we need to use the amount of the limiting reagent (which is oxygen): 1mole O moles H O 18 grams H O 39 grams O 44 grams H O 3 grams O 1 moles O 1mole H O Percent Yield: actual yield precent yield 100 theoretical yield Actual yield: the mass of product formed in a chemical reaction (experimental). Theoretical yield: the mass of product that should form in a chemical reaction according to the stoichiometry of the balance equation (it is always more than actual yield). 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). HO NaCl(s) Na+ (aq) + Cl -(aq) HO + AgNO 3(s) Ag (aq) + NO - 3 (aq)

5 If we mix the two solutions together: 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) + Cl - (aq) AgCl(s) Net ionic equation Oxidation and reduction reactions (redox reactions): Oxidation is the loss of electrons and Reduction is the gain of electrons. Zn(s) + Cu + (aq) Zn + (aq) + Cu(s) Zn(s) Zn + + e - Cu + + e - Cu(s) Zn is oxidized (loses electrons) so it is a reducing agent. Cu + 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) + O (g) CO (g) + H O(g) CH 4 gains oxygen (loses hydrogen), therefore it is oxidized (a reducing agent). O 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. Exothermic reaction: a chemical reaction that gives off heat. C(s) + O (g) CO (g) + heat (energy) Note: All combustion reactions are exothermic. Endothermic reaction: a chemical reaction that absorbs heat (needs heat to accomplish). HgO(s) + heat (energy) Hg(l) + O (g)