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 43 general classes of chemical reactions:

1)1) Precipitation reactionsPrecipitation reactionsEx: geology, heavy metal analysisSolid formationSolid formation from ionic compounds

2) Acid/base reactions2) Acid/base reactionsEx: many biochemical reactionsProtonProton transfertransfer in polar covalent compounds

3) Oxidation/Reduction (“redox”) reactions3) Oxidation/Reduction (“redox”) reactionsEx: batteries, metabolic energy production ElectronElectron transfertransfer 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

C2H5OH + 3O2 2CO2 + 3H2O

Check that the equation is balanced.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

__ NH3 + __ O2 → __ NO + __ H2O

1. Balance the chemical equation

2. For every 1 mole of NH3 reacted, how many moles of NO are produced?

3. How many moles of O2 react with 2.0 moles of NH3

Mole ratios:relating moles within chemical

equationsC3H8 + O2 CO2 + H2O1 5 3 4

• If we had 2 moles of C3H8

- how many moles of CO2 would be produced?- how many moles of H2O would be produced?

• If we had 0.5 moles of C3H8

- how many moles of O2 would be needed?

Solving Mass Moles Reaction Problems

moles massmolar mass

massA (g)

Generic Reaction: aA + bB → cC + dD

molesA

molesD

massD (g)

Mm = grams

mole

da

mole ratio

Stoichiometry Problem

C3H8 + O2 CO2 + H2O1 5 3 4

If you burned 4.4 grams of propane in a plentiful supply of oxygen

- how many grams of O2 would be consumed?

- how many grams of carbon dioxide would be produced?

Ethanol combusts to form CO2 & water.

__ C2H5OH + __ O2 __ CO2 + __ H2O

1. Balance the chemical equation

2. Identify as many mole ratios as you can

3. How many moles of oxygen (O2) react with 15.0 moles of ethanol

4. How many grams of O2 react with 15.0 moles of ethanol

5. How many g of CO2 are formed when 1.00 kg of ethanol is burned with 1.00 kg of O2

Sardine & Swiss on Rye Sandwich

20sardines

12slices

20slices

An Ice Cream Sundae Analogy for Limiting Reactions

Fig. 3.10

Ethanol combusts to form CO2 & water.

__ C2H5OH + __ O2 __ CO2 + __ H2O

1.Balance the chemical equation

2. Identify as many mole ratios as you can

3.How many moles of oxygen (O2) react with 15.0 moles of ethanol

4.How many grams of O2 react with 15.0 moles of ethanol

5.How many g of CO2 are formed when 1.00 kg of ethanol is burned with 1.00 kg of O2

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 32.00 g/mol

Limiting Reactant Example 1

Add: 14 mol 20 mol

Limiting Reactant Example 3

4NH3 + 5O2 → 4NO + 6H2O

Could make14 mol NO

Could make16 mol NO

NH3 is the limiting reagent.(Use this as basis for all

further calculations)

Using StoichiometryStoichiometry 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 O2 gas is mixed with 27.8 g of NH3 gas and 25.1 g of CH4 gas, 36.4 g of HCN gas is produced by the following reaction:

16.04 17.03 32.00 27.03 g/mol

2CH4 + 2NH3 + 3O2 → 2HCN + 6H2O

1.What is the % yield of HCN in this reaction?

2.How many grams of NH3 remain?

Limiting Reactant Example 2

Mass to moles66.6 g of O2 → 2.08 mol O2

27.8 g of NH3 → 1.63 mol NH3

25.1 g of CH4 → 1.56 mol CH4

Which reactant is limiting? 2.08 mol O2 can yield 1.39 mol HCN (or 37.5 g) 1.63 mol NH3 can yield 1.63 mol HCN (or 44.1 g) 1.56 mol CH4 can yield 1.56 mol HCN (or 42.2 g)

Conclusion?O2 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%

O2 is the limiting reagent. Thus, theoretical yield is based on 100% consumption of O2.

2.08 mol O2 can yield 1.39 mol (or 37.5 g) HCN

2. How many grams of NH3 remain?

36.4 g (or 1.35 mol) of HCN gas is produced

2CH4 + 2NH3 + 3O2 → 2HCN + 6H2O

Since the reaction stoichiometry is 1:1, 1.35 mol of NH3 is consumed:

1.63 mol NH3 initially present – 1.35 mol NH3 consumed

0.28 mol NH3 remaining

0.28 mol NH3 x (17.03 g NH3/mol) = 4.8 g NH3 remain