Stoichiometry & Chemical Reactions in Aqueous Solution PDF

# Stoichiometry & Chemical Reactions in Aqueous Solution ## Objectives At the end of this lecture you must be able to: 1. Solve Stoichiometric problem 2. Understand Limiting Reactants 3. Calculate Percent Composition 4. Understand heat of reaction 5. Can predict if ions react with each other in a...

# Stoichiometry & Chemical Reactions in Aqueous Solution ## Objectives At the end of this lecture you must be able to: 1. Solve Stoichiometric problem 2. Understand Limiting Reactants 3. Calculate Percent Composition 4. Understand heat of reaction 5. Can predict if ions react with each other in aqueous solution. ## Moles Moles provide a bridge from the molecular scale to the real-world scale. The number of moles correspond to the number of molecules. 1 mole of any substance has the same number of molecules. * Grams * Use molar mass * Moles * Use Avogadro's number * Formula units **Example:** 2 H2(g) + O2(g) --> 2 H2O(l) * **Molecules**: 2 molecules H2 + 1 molecule O2 --> 2 molecules H₂O * **Mass (amu)**: 4.0 amu H2 + 32.0 amu O2 --> 36.0 amu H₂O * **Amount (mol)**: 2 mol H2 + 1 mol O2 --> 2 mol H2O * **Mass (g)**: 4.0 g H2 + 32.0 g O2 --> 36.0 g H2O **Example:** 4NH3 + 5O2 --> 6H2O + 4 NO How many moles of NO are produced in the reaction if 15 mol of H2O are also produced? 6 mole H₂O --> 4mole NO 15mole H₂O --> ? mole NO mole NO = 15mol x 4 mol/6mol = 10 mol NO **Example:** 4NH3 + 5O2 --> 6H2O + 4 NO How many grams of H2O are produced if 2.2 mol of NH3 are combined with excess oxygen? 4 mole NH3 --> 6 mole H₂O 2.2 mole NH3 --> ? mole H₂O mole H2O = 2.2mol x 6 mol / 4mol = 3.3 mol H2O gram= mole x molecular weight gram= 3.3 × 18.02= 59.466 gm ## Molecular Weight **Molecular Weight (MWt.)** Sum of the atomic weights of the atoms in a molecule. For the molecule ethane, C2H6, the molecular weight would be * C: 2(12.0 amu) * + H: 6(1.0 amu) **= 30.0 amu** **Example:** 4NH3 + 5O2 --> 6H2O + 4 NO How many grams of O2 are required to produce 0.3 mol of H2O? 5 mole O2 --> 6 mole H₂O ? mole O2 --> 0.3 mole H₂O mole O2 = 0.3mol x 5 mol / 6mol = 0.25 mol O2 mass in (gram) = molex molecular weight mass = 0.25 × 32 = 8 gm **Example:** 4NH3 + 5O2 --> 6H2O + 4 NO How many grams of NO is produced if 12 g of O2 is combined with excess ammonia? mole = gram + molecular weight mole O2 = 12gm + 32g/mol = 0.375mol O2 5 mole O2 --> 4 mole NO 0.375mole O2 --> ? mole NO = 0.3mol 0.3mol × 30.01 = 9.003 gm ## Stoichiometric Calculations From the mass of Substance A you can use the ratio of the coefficients of A and B to calculate the mass of Substance B formed (if it's a product) or used (if it's a reactant). | Given: | Find: | |---|---| | Grams of substance A | Grams of substance B | | Use molar mass of A | Use molar mass of B | | Moles of substance A | Moles of substance B | | Use coefficients of A and B from balanced equation | | **Example:** 10 grams of glucose (C6H12O6) react in a combustion reaction. How many grams of each product are produced, M.wt=180 C6H12O6(s) + 6 O2(g) --> 6 CO2(g) + 6 H2O (l) 10g --> ? + ? 10 - 180 = 0.055 mol **FOR CO2** 1 mole C6H12O6 --> 6 mole CO2 0.055 mole C6H12O6 --> ? mole CO2 X=0.33 mol gm= 0.33 x 44 = 14.52 gm **Example:** 10 grams of glucose (C6H12O6) react in a combustion reaction. How many grams of each product are produced, M.wt=180 C6H12O6(s) + 6 O2(g) --> 6 CO2(g) + 6 H2O (l) 10g --> ? + ? 10 - 180 = 0.055 mol **For H2O** 1 mole C6H12O6 --> 6 mole H2O 0.055 mole C6H12O6 --> ? mole H2O X=0.33 mol gm= 0.33 x 18 = 5.94 gm ## Limiting Reactants If the reactants are not mixed in the same proportions as in the balanced reaction equation, there will still be excess of one reagent left when all the other has been used up. The limiting reagent will determine the maximum yield of the product. The limiting reactant is the reactant present in the smallest stoichiometric amount. **Limiting Reactants:** HCl + NaOH --> H2O + NaCl If 0.5 mole of HCl react with 0.7 gm NaOH how many moles NaCl will formed ? In any chemical reaction, the limiting reactant is a substance which is fully absorbed when the chemical reaction is complete. ## Percent Composition One can find the percentage of the mass of a compound that comes from each of the elements in the compound by using this equation: % element = (number of atoms)(atomic weight) / (molecular weight of the compound) x 100 **Percent Composition** the percentage of carbon and hydrogen in ethane (C2H6) molecular mass = 30.0is: * %C = (2)(12.0 amu) / 30.0 amu x 100 = 80.0% * %H = (6)(1.01 amu) / 30.0 amu x 100 = 20.0% **Q: What is % of Li in Li2O (atomic mass: Li = 7, O = 16)** (number of atoms)(atomic weight) / (M.WT of the compound) x 100 %Li = (7 x 2) / 30 x 100% = 46.7% **Q: Calculate the percentages of oxygen and hydrogen in water, H2O** * %H = 2/18 x 100 = 11.1% * %O = 16/18 x 100 = 88.9% ## How Can We Predict if Ions In Aqueous Solution Will React With Each Other? Many ionic compounds are soluble in water. Ionic compounds always consist of both positive and negative ions. When they dissolve in water, the positive and negative ions become separated from each other by water molecules. We call such a separation a dissociation. NaOH + H2O --> Na+(aq.) + OH-(aq.) In many cases, no reaction takes place when we mix solutions of ionic compounds because none of these situations holds. For example, if we mix solutions of copper (II) nitrate, Cu(NO3)2, and potassium sulfate, K2SO4, we merely have a mixture containing Cu2+, K+1, NO3-1, and S04-2 ions dissolved in water. In general, ions in solution react with each other only when one of these four things can happen: 1. Two ions form a solid that is insoluble in water. AgCl is one example. **Precipitation Reaction** 2. Two ions form a gas that escapes from the reaction mixture as bubbles. **Combustion Reaction** 3. An acid neutralizes a base. **Acid-base reactions. Neutralization Reaction** 4. One of the ions can oxidize another. **Redox Reaction** ## Soluble and Insoluble Ionic Solids There is no simple way to remember which ionic solids are soluble in water and which are not, but we can make some useful generalizations: 1. All compounds containing Na+, K+ or NH4+ ions are soluble in water. 2. All nitrates (NO3¯) and acetates (CH3COO) are soluble in water. 3. Most chlorides (Cl-) and sulfates (S04-2) are soluble in water. Important exceptions include silver chloride (AgCl), lead (II) chloride (PbCl2), mercury(I) chloride (HgCl), barium sulfate (BaSO4), and lead sulfate (PbSO4), which are insoluble in water. 4. Most carbonates (C03-2), phosphates (PO4-3), sulfides (S-2), and hydroxides (OH-) are insoluble in water. Important exceptions include LiOH, NaOH, KOH, and aqueous NH3, which are soluble in water. ## Heat of Reaction The heat given off or gained in a reaction is called the heat of reaction. * A reaction that gives off heat is **exothermic**. * A reaction that absorbs heat is **endothermic**. * The amount of heat given off or absorbed is proportional to the amount of material. * All combustions are exothermic. The heat given off in a combustion reaction is called the **heat of combustion** ## Wisdom "If you tend to the difficult door, you will break the side of the gate." - An Egyptian proverb. **A chemical wisdom:** "Don't worry if your dream persists as a vapor. Surely you will find a surface to condense on. One day your dreams will be on you."