and cooking with chemicals Stoichiometry and cooking with chemicals

Molar Ratio Exothermic Endothermic OUTCOME QUESTION(S): C11-3-15 STOICHIOMETRY Solve stoichiometric problems involving moles, mass, and volume, given the reactants and products in a balanced chemical reaction. Include: heat of reaction Identify the limiting reactant and calculate the mass of a product, given the reaction equation and reactant data. Include: theoretical yield, experimental yield Vocabulary & Concepts  Molar Ratio Exothermic Endothermic

“The calculation of quantities in chemical reactions” STOICHIOMETRY “The calculation of quantities in chemical reactions” N2 (g) + 3 H2 (g) 2 NH3 (g) H N atomic 1 molecule + 3 molecules 2 molecules molecular There are many ways to interpret the relationships in a balanced equation…

N2 (g) + 3 H2 (g) 2 NH3 (g) 28g + 6g 34g + 1 mole + 3 moles 2 moles molar 28g + 6g 34g mass 22.4 L + Volume (gases) While the coefficients indicate the proportional relationships, only the mass and atoms are conserved in every chemical reaction

N2 (g) + 3 H2 (g) 2 NH3 (g) 1 mole + 3 moles 2 moles Molar Ratio: 1 N2 Conversion factor between parts of a chemical reaction. 1 N2 3 H2 3 H2 1 N2 1 N2 2 NH3 2 NH3 1 N2 3 H2 2 NH3 2 NH3 3 H2 There are 6 ratios that can be built from this equation to use as conversion factors depending on what is being compared

How many moles of H2 are used with 2 mol N2? N2 (g) + 3 H2 (g) 2 NH3 (g) 1 N2 3 H2 2 mol N2 = 6 mol H2 *Make sure the equation is balanced What you know Add conversion factor Units to cancel on bottom Units in need on the top Multiply top, divide bottom What about moles of NH3 made? 1 N2 2 NH3 2 mol N2 = 4.0 mol NH3 (made)

Molar ratios – used to convert between compounds in a reaction… How many moles of ammonia gas can be made from 0.350 moles of hydrogen gas? N2 (g) + 3 H2 (g) 2 NH3 (g) 0.350 mol H2 3 H2 2 NH3 = 0.233 mol NH3 (made) *Make sure the equation is balanced What you know Add conversion factor Units to cancel on bottom Units in need on the top Multiply top, divide bottom Molar ratios – used to convert between compounds in a reaction…

2 Na(s) + Cl2(g) NaCl(s) 2 3.20 mol Cl2 2 NaCl 58.5 g NaCl What mass of NaCl is formed from the reaction with 3.20 moles of chlorine gas? 2 Na(s) + Cl2(g) NaCl(s) 2 3.20 moles ? g NaCl = 58.5 g/mol mol 3.20 mol Cl2 2 NaCl 58.5 g NaCl = 374 g NaCl 1 Cl2 1 mol STOICHIOMETRY *Make sure the equation is balanced What you know Convert to moles Use molar ratios to transition Convert to desired units Multiply top, divide bottom We can skip step 2 here – it started in moles

N2 (g) + H2 (g) NH3 (g) 3 2 15.0 g NH3 1 mol NH3 3 H2 = 1.32 mol H2 Calculate the moles of hydrogen needed to make 15.0 g of ammonia? N2 (g) + H2 (g) NH3 (g) 3 2 ? mol 15.0 g NH3 = 17.0 g/mol mol 15.0 g NH3 1 mol NH3 3 H2 = 1.32 mol H2 17.0 g 2 NH3 STOICHIOMETRY *Make sure the equation is balanced What you know Convert to moles Use molar ratios to transition Convert to desired units Multiply top, divide bottom Do this (I call it the “road map”) if you are struggling – show yourself the way before you start

What mass of carbon dioxide is produced from burning 50 What mass of carbon dioxide is produced from burning 50.0 g of ethane, C2H6? C2H6 (g) + O2 (g) CO2 (g) + H2O (g) 2 7 4 6 50.0 g ? g mol mol C2H6 = 30.0 g/mol CO2 = 44.0 g/mol 50.0 g C2H6 1 mole C2H6 4 CO2 44.0 g CO2 30.0 g C2H6 2 C2H6 1 mole CO2 STOICHIOMETRY *Make sure the equation is balanced What you know Convert to moles Use molar ratios to transition Convert to desired units Multiply top, divide bottom = 147 g CO2

FeS(s) + O2 (g) Fe2O3 (s) + SO2 (g) 7 2 4 How many grams of FeS are needed to make 36.3 g of Fe2O3? 4 FeS(s) + O2 (g) Fe2O3 (s) + SO2 (g) 7 2 4 ? g 36.3 g mol mol FeS = 87.9 g/mol Fe2O3 = 159.6 g/mol 36.3 g Fe2O3 1 mole Fe2O3 4 FeS 87.9 g FeS 159.6 g Fe2O3 2 Fe2O3 1 mole FeS = 40.0 g FeS

What volume of O2 is needed to make 19.8 L SO3? SO2(g) + O2 (g) SO3 (g) 2 ? L 19.8 L mol mol 19.8 L SO3 1 mole SO3 1 O2 22.4 L O2 22.4 L SO3 2 SO3 1 mole O2 = 9.90 L O2 Maybe you have caught the trick here: since ALL gases at STP behave the same, we can just use the molar ratio to convert between litres of gas…

2 Na(s) + Cl2 (g) NaCl (s) 2 = 4.87 L Cl2 What volume of chlorine gas is needed to react with 10.0 g of sodium? 2 Na(s) + Cl2 (g) NaCl (s) 2 10.0 g ? L Na = 23.0 g/mol mol mol 10.0 g Na 1 mole Na 1 Cl2 22.4 L Cl2 23.0 g Na 2 Na 1 mole Cl2 = 4.87 L Cl2 STOICHIOMETRY *Make sure the equation is balanced What you know Convert to moles Use molar ratios to transition Convert to desired units Multiply top, divide bottom

The energy change that occurs during a chemical reaction is called the Heat of Reaction: Endothermic reactions – absorption of energy is required for the reaction to occur (reactant). Exothermic reactions – energy is released as part of a spontaneous reaction (product). A + energy B A B + energy

Heat of reaction can be used just like ANY conversion factor… What heat is produced from burning 60.0 g of ethane (C2H6), if the heat of combustion is 1560 kJ/mol of ethane? C2H6 (g) + O2 (g) CO2 (g) + H2O (g) 2 7 4 6 60.0 g mol C2H6 = 30.0 g/mol heat 60.0 g C2H6 1 mole C2H6 1560 kJ = 3120 kJ (energy released) 1 mole C2H6 30.0 g C2H6

4 NH3 + 3 O2 2 N2 + 6 H2O + 300 kJ = 3000 g H2O 8000 kJ 6 mole H2O What mass of water is produced in the reaction when 8000 kJ of energy is released? 4 NH3 + 3 O2 2 N2 + 6 H2O + 300 kJ ? g 8000 kJ The heat is in the equation – you have to build a conversion factor… mol H2O = 18.0 g/mol 8000 kJ 6 mole H2O 18.0 g = 3000 g H2O 300 kJ 1 mole H2O IF you can find a relationship between ideas, you can solve!

Molar Ratio Exothermic Endothermic CAN YOU / HAVE YOU? C11-3-15 STOICHIOMETRY Solve stoichiometric problems involving moles, mass, and volume, given the reactants and products in a balanced chemical reaction. Include: heat of reaction Identify the limiting reactant and calculate the mass of a product, given the reaction equation and reactant data. Include: theoretical yield, experimental yield Vocabulary & Concepts  Molar Ratio Exothermic Endothermic