Permanent room change for the Tuesday Chem 121 Study Group held from 2-4. Biology 234 instead of Environmental Sciences 418

Chemical Equations identifies all reactants & products shows molar ratios between all chemical species in the reaction (“stoichiometry”) proper stoichiometry is represented ONLY when the chemical equation is balanced

Chapter 4 3 general classes of chemical reactions: 1)Precipitation reactions Ex: geology, heavy metal analysis Solid formation Solid formation from ionic compounds 2) Acid/base reactions Ex: many biochemical reactions Proton transfer Proton transfer in polar covalent compounds 3) Oxidation/Reduction (“redox”) reactions Ex: batteries, metabolic energy production Electron transfer Electron transfer in ionic & molecular compounds

Classifying Chemical Reactions Similarities in written equations 1)Combination reactions (CR) often redox 2)Decomposition reactions (DR) often redox 3)Single replacement (SR) often redox 4)Double displacement (DD) Acid/base, precipitation, redox Lab Fundamental rxn. similarities 1)Precipitation 2)Acid/base 3)Redox Textbook

Chemical Equation C 2 H 5 OH + 3O 2  2CO 2 + 3H 2 O Check that the equation is balanced. 1 mole of ethanol reacts with ► 3 moles of oxygen 1 mole of ethanol reacts to produce ► 2 moles of carbon dioxide ► 3 moles of water

__ NH 3 + __ O 2 → __ NO + __ H 2 O 1.Balance the chemical equation 2.For every 1 mole of NH 3 reacted, how many moles of NO are produced? 3.How many moles of O 2 react with 2.0 moles of NH 3

Mole ratios: relating moles within chemical equations If we had 2 moles of C 3 H 8 - how many moles of CO 2 would be produced? - how many moles of H 2 O would be produced? If we had 0.5 moles of C 3 H 8 - how many moles of O 2 would be needed?

Solving Mass  Moles Reaction Problems molesmass molar mass mass A (g) Generic Reaction: aA + bB → cC + dD moles A moles D mass D (g) M m = grams mole dada mole ratio

Stoichiometry Problem If you burned 4.4 grams of propane in a plentiful supply of oxygen - how many grams of O 2 would be consumed? - how many grams of carbon dioxide would be produced?

Ethanol combusts to form CO 2 & water. __ C 2 H 5 OH + __ O 2  __ CO 2 + __ H 2 O 1.Balance the chemical equation 2.Identify as many mole ratios as you can 3.How many moles of oxygen (O 2 ) react with 15.0 moles of ethanol 4.How many grams of O 2 react with 15.0 moles of ethanol 5.How many g of CO 2 are formed when 1.00 kg of ethanol is burned with 1.00 kg of O 2

Sardine & Swiss on Rye Sandwich 20 sardines 12 slices 20 slices

An Ice Cream Sundae Analogy for Limiting Reactions Fig. 3.10

Ethanol combusts to form CO 2 & water. __ C 2 H 5 OH + __ O 2  __ CO 2 + __ H 2 O 1.Balance the chemical equation 2.Identify as many mole ratios as you can 3.How many moles of oxygen (O 2 ) react with 15.0 moles of ethanol 4.How many grams of O 2 react with 15.0 moles of ethanol 5.How many g of CO 2 are formed when 1.00 kg of ethanol is burned with 1.00 kg of O 2

Limiting Reactant (or reagent) The limiting reactant is the reactant in a chemical reaction which limits the amount of products that can be formed. The limiting reactant in a chemical reaction is present in insufficient quantity to consume the other reactant(s). This situation arises when reactants are mixed in non-stoichiometric ratios.

76.15 g/mol g/mol Limiting Reactant Example 1

Add: 14 mol 20 mol Limiting Reactant Example 3 4NH 3 + 5O 2 → 4NO + 6H 2 O Could make 14 mol NO Could make 16 mol NO NH 3 is the limiting reagent. (Use this as basis for all further calculations)

Using Stoichiometry Stoichiometry is used to answer two fundamental questions in chemical analysis: What is the theoretical yield? What is the limiting reactant? REMEMBER: stoichiometry shows molar ratios not mass ratios

Percent yield actual yield % yield = actual yield: observed yield of product theoretical yield: calculated assuming 100% conversion of the LIMITING REAGENT Both yields can be in moles or grams theoretical yield x 100

Theoretical Yield: Which Reactant is Limiting? 1) calculate moles (or mass) of product formed by complete reaction of each reactant. 2) the reactant that yields the least product is the limiting reactant (or limiting reagent). 3)the theoretical yield for a reaction is the maximum amount of product that could be generated by complete consumption of the limiting reagent.

When 66.6 g of O 2 gas is mixed with 27.8 g of NH 3 gas and 25.1 g of CH 4 gas, 36.4 g of HCN gas is produced by the following reaction: g/mol 2CH 4 + 2NH 3 + 3O 2 → 2HCN + 6H 2 O 1.What is the % yield of HCN in this reaction? 2.How many grams of NH 3 remain? Limiting Reactant Example 2

Mass to moles 66.6 g of O 2 → 2.08 mol O g of NH3 → 1.63 mol NH g of CH 4 → 1.56 mol CH 4 Which reactant is limiting? 2.08 mol O 2 can yield 1.39 mol HCN (or 37.5 g) 1.63 mol NH 3 can yield 1.63 mol HCN (or 44.1 g) 1.56 mol CH 4 can yield 1.56 mol HCN (or 42.2 g) Conclusion? O 2 is the limiting reagent.

moles could also be used % yield = actual yield theoretical yield x 100 % yield = 36.4 g HCN 37.5 g HCN x 100 = 97.1% O 2 is the limiting reagent. Thus, theoretical yield is based on 100% consumption of O mol O 2 can yield 1.39 mol (or 37.5 g) HCN

2. How many grams of NH 3 remain? 36.4 g (or 1.35 mol) of HCN gas is produced 2CH 4 + 2NH 3 + 3O 2 → 2HCN + 6H 2 O Since the reaction stoichiometry is 1:1, 1.35 mol of NH 3 is consumed: 1.63 mol NH 3 initially present – 1.35 mol NH 3 consumed 0.28 mol NH 3 remaining 0.28 mol NH 3 x (17.03 g NH 3 /mol) = 4.8 g NH 3 remain