Chem 11 UNIT 3: STOICHIOMETRY Name: Ms. Pirvu Period: Writing & Balancing Equations Chemical reactions can be described by chemical equations. Recall Law of Conservation of Mass mass cannot be nor. This is the reason that we balance equations. Atoms are what make up the mass of any chemical species. Therefore, we must show the conservation of mass in a chemical equation by it. Just like a mathematical eq n, where both sides MUST be equal to each other (ex. 7 + 2 = 9), when balancing an eq n, we have to have the same number of each type of atom on either side of the. Always balance an equation by adjusting the and NEVER by changing the subscripts of a chemical formula. Ex. Cu + O2 CuO Note: We do not write a 1 as a coefficient, much like we do not write 1 for a subscript. It is sometimes important to denote what phase the reactants or products are in, while the reaction is occurring. This is done by writing a subscript in brackets. (s) solid (l) liquid (g) gas (aq) something that is dissolved in water (aqueous) Ex. The electrolysis of water is the reaction of liquid water decomposing to form hydrogen gas and oxygen gas. Using subscripts to denote phase, the balanced equation would be: H2O (l) H2 (g) + O2 (g) For now, phases of the reactants & products are not important. Later in the course they will be. Examples: 1) NaOH + HCl NaCl + H2O 2) NaOH + H2SO4 Na2SO4 + H2O 3) CH4 + O2 CO2 + H2O 4) potassium iodide reacts with lead II nitrate to give potassium nitrate and lead II iodide
Hints in Balancing: 1. It is MUCH easier to balance an eq n using whole # s rather than fractions. Try to use whole # s at all times. If a fraction is necessary, multiply it out to get a whole number. Ex. C2H6 + O2 CO2 + H2O 2. If a polyatomic ion seems to be staying together during a chemical rxn, try to keep it together during the balancing steps. Ex. NaCl + Pb(NO3)2 NaNO3 + PbCl2 In the above example, the nitrate ion stays intact, so treat it as one whole species when balancing. Ex. CaO + P2O5 Ca3(PO4)2 In the above example, the phosphate (PO4) is not on both sides, so balance the P s and O s separately. 3. Balance free elements (species that are by themselves) LAST, since you can easily balance them at the end, without having it affect another species. Ex. Al + CuCl 2 Cu + AlCl 3
Chemical Reactions Evidence that a chemical reaction has occurred may include the following : Types of chemical reactions In general, chemical reactions can be classified as energy producers or energy consumers. A reaction that produces heat A reaction that absorbs heat Specific types of chemical reactions A) Synthesis Reaction - A reaction that occurs between two elements producing a compound. B) Decomposition Reaction - A reaction where the compound breaks down into component elements or some smaller component molecules. C) Single Replacement Reaction - When an element combines with a compound effectively trading partners with one of the ions making up the compound. A new compound is formed. D) Double Replacement Reaction - Similar to a single replacement reaction except that two compounds are mixed and then 'trade partners'. E) Acid/Base Reaction - When an acid and a base are mixed, the will neutralize each other producing a salt and water. F) Combustion Reaction - When a compound rapidly combines with oxygen, it is said to be burning. (known as ). A slow combination with oxygen is said to be oxidation ( ). A special type of combustion reaction is when a certain type of chemical called a hydrocarbon (a compound composed of ONLY carbon and hydrogen atoms) is burned. When a hydrocarbon is burned the products are ALWAYS carbon dioxide and water, no matter what hydrocarbon is burned.
Prediction of Products If you are told what type of a chemical reaction is taking place, then from the rules we learned the day before, we can predict what the products of the reaction will be. Examples: 1) Single replacement rxn one element trades places with another of the same valence type (positive or negative) in the compound. Al + CuCl2 Note: ALWAYS write out the names of the products first. Once you have the names, then PROPERLY write out the formulae. (Otherwise, if you have the wrong formula, balancing will be impossible!!) 2) Double replacement rxns both compounds change partners Al(NO3)3 + K2C2O4 3) Hydrocarbon Combustion Reactions a hydrocarbon is combined with oxygen to produce carbon dioxide and water. ALWAYS!! C34H70 + O2 Additional Notes:
STOICHIOMETRY STOICHIOMETRY: Process of calculating the amounts of elements or compounds involved in a chemical reaction. Stoichiometry problems can be solved by going through a series of 5 steps: Step 1: Find a chemical equation. Note: If an eq n is given, assume it is balanced unless otherwise stated. Step 2: Find moles of what we are information about in the problem. Note: If moles are given, step 2 is done. Step 3: Find the ratio of what we to what we are from the coefficients in the balanced chemical eq n. Express the ratio as a fraction of Step 4: Find moles of what we. That is (Step 2) x (Step 3). *Step 5: Go back and re-read the question.!!! Ex. #1 2 Na + Cl2 2 NaCl If 0.500 moles of sodium react completely with chlorine to produce sodium chloride, how many moles of chlorine are needed?
Ex. #2 2 Na + Cl2 2 NaCl If 20.0 grams of sodium react completely with chlorine to produce sodium chloride, what mass of chlorine is needed? Ex. #3: How many molecules of BaSO4 are produced by reacting 18.2 g of Ba(OH)2 with sufficient H2SO4? (water is also produced.)
Ex. #4: What mass of carbon dioxide is produced when 27.4 grams of pentane (C5H12) are burned? Additional Notes:
Stoichiometric Calculations with One Reactant In Excess In most cases, when we have a rxn, we add too much of one reactant or another. Because of this, only one of the reactants is completely used up and the other will have some remaining. To solve stoichiometric problems, the reactant that we run out of first (the ) is the one we MUST use in the calculations, since this one the amount of product(s) produced. The other reactant is the one in excess ( ). INXS Calculations Ex. #1 Ba + S BaS If we have 10.0 grams of each reactant a) which will be used up first? b) what mass BaS will be produced? Sol n:
Ex #2 A double replacement rxn occurs when 60.0 g of zinc chloride is mixed with 40.0 g of hydrogen phosphate. When the rxn is complete, determine the mass of each remaining particle. Summary Answer:
Percent Yield Calculations All the rxns so far have assumed one thing that is that we make all of the products possible. In reality however, we can t always obtain perfection. In fact, most rxns do not go to but to some what lesser degree. The percent yield is an indication as to how much product we get how much we should have gotten if everything worked out properly. Ex.1 CaCO3 + 2HCl CaCl2 + H2CO3 When 36.4 g of calcium carbonate react with sufficient HCl according to the above eq n, it is found that 38.2 g of CaCl2 are produced. Calculate the percent yield in this rxn. Sol n
Ex.2 Bromine was made according to the rxn: HBrO3 + 5 HBr 3 H2O + 3 Br2 10.0 g or HBrO3 was reacted with an excess amount of HBr. It was found that 26.3 g of Br2 was produced. What was the percentage yield for this rxn? What mass of HBr was used? Additional Notes:
Moles & Volume of a Gas Avogadro s Hypothesis: equal of different gases at the same temperature and pressure, contain the same number of particles. Molar volume of a gas is the volume occupied by of the gas. Standard Temperature and Pressure = 0 o C and 101.3 kpa Re-stating Avogadro s hypothesis: all gas samples with the same pressure, temperature, and number of particles occupy identical volumes. OR 1 mol of ANY GAS at STP has a volume of 22.4 L The molar volume of any gas at STP is 22.4 L Ex. What is the volume occupied by 0.350 mol of SO2 (g) at STP Ex. How many moles of gas are contained in a balloon with a volume of 10.0 L at STP? Problems: 1. Calculate the volume at STP occupied by the following: a) 12.5 mol of NH3 (g) b) 0.350 mol of O2 (g) c) 4.25 mol of HCL (g)
2. Calculate the number of moles in the following gases at STP a) 85.9 L of H2 (g) b) 375 ml of SO3 (g) c) 5.00 ml of OCl2 (g) 3. A 2.50 L bulb contains 4.91g of a gas at STP. What is the molar mass of the gas? 4. What is the density of O2 (g) at STP? Answers: 1.a) 2.80 x 10 2 L b) 7.84 L c) 95.2 L 2.a) 3.83 mol b) 0.0167 mol c) 2.23 x 10-4 mol 3. 44.0 g 4. 1.43 g/l
Stoichiometric Problems involving Volumes & Energy Ex. 5C + 2SO2 CS2 + 4CO If 13.4L of gaseous SO2 are reacted to completion, what volume of CO will be produced? Sol n: Energy in a chemical reaction Recall: rxns that release energy are exothermic; rxns that absorb energy are endothermic. We symbolize the energy in a chemical eq n by showing the component just like a reactant or product. Ex.1 Sol n: 2 NO2 N2O4 + 56 kj If we use 25 g of NO2, what amount of energy will be produced? Ex.2 Sol n: 2 NO2 N2O4 + 56 kj If 126 kj of energy are released from the above reaction, what mass of N2O4 is also produced?