Stoichiometry Chemical reactions and the quantitative relationships (often mass) between: Reactant and Reactant Reactant and Product Product and Product

Describing a Chemical Reaction Indications of a Chemical Reaction Evolution of heat, light, and/or sound Production of a gas Formation of a precipitate Color change

Chemical Equations The numbers in the front are called aluminum oxide Chemical equations represent chemical change or chemical reactions. They depict the kind of reactants and products and their relative amounts in a reaction. 4 Al(s) + 3 O2(g) 2 Al2O3(s) A chemical equation is an expression that gives the identities and quantities of the substances in a chemical reaction Chemical formulas and other symbols are used to indicate the starting material(s) or reactant(s), which are written on the left side of the equation, and the final compound(s) or product(s), which are written on the right side. An arrow, read as yields or reacts to form, points from the reactants to the products. Abbreviations are added in parentheses as subscripts to indicate the physical state of each species:—(s) for solid, (l) for liquid, (g) for gas, and (aq) for an aqueous solution. A balanced chemical equation is when both the numbers of each type of atom and the total charge are the same on both sides. A chemical reaction represents a change in the distribution of atoms but not in the number of atoms. http://img.alibaba.com/photo/10784694/Aluminum_Oxide_Abrasive_Fine_Grit_120_220_.jpg http://upload.wikimedia.org/wikipedia/commons/thumb/f/f2/Corundum_Aluminum_Oxide_Gallantin_Co_MontanaDSC03201.jpg/600px-Corundum_Aluminum_Oxide_Gallantin_Co_MontanaDSC03201.jpg http://www.germes-online.com/direct/dbimage/50327532/Aluminum_Alloy_Pipe.jpg The letters (s), (g), and (l) are the physical states of compounds. Also used are (v), (c), (aq). The numbers in the front are called stoichiometric coefficients.

Chemical Equations 4 Al(s) + 3 O2(g) 2 Al2O3(s) aluminum oxide sandpaper 4 Al(s) + 3 O2(g) 2 Al2O3(s) 4 g Al + 3 g O2 yield 2 g Al2O3 This equation means: 4 Al atoms + 3 O2 molecules yield 2 f. u. ‘s of Al2O3 or 4 Al moles + 3 O2 moles yield 2 moles of Al2O3 4 mol Al@27g/mol 3 mol O2@32g/mol 2 mol Al2O3@102g/mol 108 g + 96 g = 204 g

Chemical Equations Because the same atoms are present in a reaction at the beginning (reactants) and at the end (products), the amount of matter in a system does not change. Remember Dalton’s atomic theory! The Law of Conservation of Matter What does the law state? Law of Conservation of Matter means that nothing is “lost” or “stolen”. In the real world retail theft costs business lots (much of the theft is actually employee theft)! 20% 100% Chemical Factory 100% 80%

Chemical Equations Because of the principle of the conservation of matter, An equation must be balanced. It must have the same number of atoms of the same kind on both sides. Conservation of atoms… NOTE: not conservation of molecules!! Lavoisier, 1788

Characteristics of Chemical Equations The equation must represent known facts. The equation must contain the correct chemical formulas or chemical symbols for the reactants and products. The law of conservation of mass must be satisfied for formula equations. Conservation of atoms

Chemical Equations Reactants – the substances that exist before a chemical change (or reaction) takes place. Enter the chemical change Products – the new substance(s) that are formed during the chemical change (or reaction). Exit the chemical change CHEMICAL EQUATION indicates the reactants and products of a reaction. Reactants ---------------> Products Objectives: To identify seven elements that occur naturally as diatomic molecules, H2, N2, O2, F2, Cl2, Br2, I2. [HOBrFINCL twins or BrINClHOF brothers] To write a chemical equation from the description of a chemical reaction.

Word Equations A WORD EQUATION describes chemical change using the names of the reactants and products. Write the word equation for the reaction of methane gas with oxygen gas to form carbon dioxide and water. methane + oxygen carbon dioxide + water Reactant Product CH4 + 2 O2 CO2 + 2 H2O

Word Equations Word equations may include the phases of the substances entering or exiting a chemical change. hydrogen gas + oxygen gas -----> water vapor However, one shortcoming of word equations is their FAILURE to provide or include quantitative information about the reactants and products.

Balancing Chemical Equations Balanced Equation – one in which the number of atoms of each element as a reactant is equal to the number of atoms of that element as a product What is the relationship between conservation of mass and the fact that a balanced equation will always have the same number of atoms of each element on both sides of an equation? Objective: To write balanced chemical equations by inspection. Balanced chemical equation – Provides qualitative information about the identities and physical states of the reactants and products – Provides quantitative information because it tells the relative amounts of reactants and products consumed or produced in the reaction – The number of atoms, molecules, or formula units of a reactant or product in a balanced chemical equation is the coefficient of that species – Mole ratio of two substances in a chemical reaction is the ratio of their coefficients in the balanced chemical equation Determine whether the following equation is balanced. 2 Na + H2O  2 NaOH + H2 2 Na + 2 H2O  2 NaOH + H2

Balancing Chemical Equations Write a word equation for the chemical reaction. Write the correct chemical formulas/symbols for all reactants and products. Determine the stoichiometric coefficients that make the equation balance.

Balancing Chemical Equations An important point to remember 2 NO(g) + O2(g)  2 NO2(g) The 2 to the left of NO(g) and NO2(g) refers to the number of molecules present in the balanced equation. It is a “multiplier” for every atom in the molecule. http://images.google.com/imgres?imgurl=http://www.uwec.edu/Images/UWECWeb/uwec8.gif&imgrefurl=http://www.uwec.edu/carneymj/Chem103.htm&h=67&w=359&sz=10&hl=en&start=101&tbnid=7T7IE0FudtwTnM:&tbnh=23&tbnw=121&prev=/images%3Fq%3Dgeneral%2Bchemistry%2Bpowerpoint%26start%3D90%26gbv%3D2%26ndsp%3D18%26hl%3Den%26sa%3DN The subscript 2 in O2 (g) and NO2(g) refers to the number of atoms of this type that are present in each molecule (molecular compound) or formula unit (ionic compound).

Balancing By Inspection Balance the most complex substance in the equation first Balance “free” elements, H, and O last if possible Use stoichiometric coefficients to adjust quantities, not subscripts Subscripts are set by atomic valences Chem FAQ: How do I balance a chemical equation?

Balancing By Inspection Some equations may be balanced using fractions, but the most common approach allows only for integer coefficients In addition, the coefficients MUST represent the simplest ratio between the substances (reactants and products)

Balancing by Inspection If polyatomic ions remain intact in a reaction balance them as a group or unit, not individual atoms If NO3- appears on the reactant side and the product side, balance as NO3 units not N’s and O’s If you have an even/odd problem dilemma, multiply all previously balanced moieties by 2

Balancing by Inspection Never express a coefficient of one If the symbol or formula is present there is at least one of them; hence it would be redundant.

What Does The Balanced Equation Mean? 2 CO(g) + O2(g) →2 CO2(g) For every 2 CO reacted, 1 O2 is also reacted and 2 CO2 are also formed 3.4 Chemical equations link amounts of substances in a reaction

Stoichiometry of Reactions Stoichiometry of reactions - Quantitative relationships between the reactants and products in a chemical reaction What does quantitative mean? Relating to number or quantity; involves measurement

Chemical Reactions Understanding chemical reactions helps us to predict what will be produced by a reaction. It is also important to know how much will be produced by a reaction. We also can determine how much reactant is required to produce a desired amount of product.

Using The Balanced Equation: The balanced equation gives the relationship between amounts of reactants used and amounts of products likely to be formed The numeric coefficient tells: - how many individual particles are needed in the reaction on the submicroscopic level - how many moles are necessary on the macroscopic level - The stoichiometric coefficient Chem FAQs How can I obtain mole-to-mole conversion factors form a chemical equation? How can I relate moles of substances involved in a chemical reaction? How can I relate grams of substances involved in chemical reactions? 3.4 Chemical equations link amounts of substances in a reaction

Learning Check For the reaction N2(g) + 3 H2(g) → 2 NH3(v), How many moles of N2 are used when 2.3 moles of NH3 are produced? 2.3 mol NH3 x 1 mol N2 = 2 mol NH3 = 1.2 mol N2

Learning Check If 0.575 mole of CO2 is produced by the combustion of propane, C3H8, how many moles of oxygen are consumed? The balanced equation is; C3H8 + 5 O2 → 3 CO2 + 4 H2O 0.575 mol CO2 x 5 mol O2 = 3 mol CO2 = 0.958 mol O2

Step Wise Example STEP 2 Convert mass reactant (454 g NH4NO3) into moles. - use molar mass 454 g NH4NO3 x 1 mol NH4NO3 = 80.04 g NH4NO3 = 5.672 163 9 mol NH4NO3

Step Wise Example STEP 3 Convert moles reactant (5.672 163 9 mol) into moles product using the “mole ratio” from the stoichiometric coefficients of the balanced chemical equation 5.672 163 9 mol NH4NO3 x 1 mol N2O = 1 mol NH4NO3 = 5.672 163 9 mol N2O

Step Wise Example STEP 4 Convert moles product (5.672 163 9 mol N2O) into mass product - use molar mass 5.672 163 9 mol N2O x 44.02 g N2O = 1 mol N2O = 249.688 655 6 g N2O = 250. g N2O

All together now……. 454 g NH4NO3 x 1 mol NH4NO3 x 1 mol N2O __ x 80.04 g NH4NO3 1 mol NH4NO3 44.02 g N2O = 250. g N2O 1 mol N2O

Step Wise Example STEP 2 Convert mass reactant (454 g NH4NO3) into moles. - use molar mass 454 g NH4NO3 x 1 mol NH4NO3 = 80.04 g NH4NO3 = 5.672 163 9 mol NH4NO3

Step Wise Example STEP 3 Convert moles reactant (5.672 163 9 mol) into moles product using the “mole ratio” from the stoichiometric coefficients of the balanced chemical equation 5.672 163 9 mol NH4NO3 x 2 mol H2O = 1 mol NH4NO3 = 11.344 327 8 mol H2O

Step Wise Example STEP 4 Convert moles product (11.344 327 8 mol H2O) into mass product - use molar mass 11.344 327 8 mol H2O x 18.02 g H2O = 1 mol H2O = 204.424 787 0 g H2O = 204 g H2O

All together now…… 454 g NH4NO3 x 1 mol NH4NO3 x 2 mol H2O __ x 80.04 g NH4NO3 1 mol NH4NO3 18.02 g H2O = 204 g H2O 1 mol H2O

Summary FIRST STEP – Write a balanced chemical equation SECOND STEP – Convert “given” into moles if necessary - Avogadro’s number - molar mass - molar volume

Summary cont’d STEP THREE – Apply the “mole ratio” between the “given” and the “desired” from the stoichiometric coefficients of the balanced chemical equation STEP FOUR – Convert the moles of the “desired” into the proper units for the problem - Avogadro’s number - molar mass - molar volume

Limiting Reactants If the amounts of two or more reactants are given, the reactant used up first determines the amount of product formed. It is called the “limiting” reactant.

Think about….. Suppose you are preparing cheese sandwiches. Each sandwich requires 2 pieces of bread and 1 slice of cheese. If you have 4 slices of cheese and 10 pieces of bread, how many cheese sandwiches can you make?

Learning Check 6 How many sandwiches can you make? ____ slices of bread + ____ slices of cheese = ____ sandwiches What is left over? ________________ What is the limiting reactant?

Solution 6 How many sandwiches can you make? __10__ slices of bread + __4__ slices of cheese = __4__ sandwiches What is left over? _2 slices of bread What is the limiting reactant? cheese

Limiting Reactants The limiting reactant is the reactant present in the smallest stoichiometric amount

Limiting Reactants The limiting reactant is the reactant present in the smallest stoichiometric amount - In other words, it’s the reactant you’ll run out of first (in this case, the H2)

Limiting Reactants In the example below, the O2 would be the excess reagent 2 H2 + O2 ------------> 2 H2O

Limiting Reagent Consider the reaction of N2 with H2 to form NH3: N2(g) + 3 H2(g) → 2 NH3(g) The stoichiometry suggests that for every mole of N2 we will need 3 moles of H2 to form 2 moles of NH3. So what happens if these proportions are not met? The reaction proceeds, to use up one of the reactants (the limiting reagent) and will not use all of the other reactant (it is in excess) Imagine a simple bolt assembly : 1 nut + 1 bolt = 1 bolt assembly. If we don’t have the same number of nuts as bolts, there will be leftover parts and not entirely product. 3.5 The reactant in shortest supply limits the amount of product that can form

Limiting Reagents Note that in this reaction, some of the O2 is not consumed. This is because there is not enough CO to continue consuming the O2. Thus, CO is the limiting reagent. 3.5 The reactant in shortest supply limits the amount of product that can form

Determining The Limiting Reagent (LR) There are several approaches to this. One method is to compare the quantities available to the quantities required. Any substance present in excess of the requirement cannot be limiting. 3.5 The reactant in shortest supply limits the amount of product that can form

What should we compare? Using moles of reactants: Convert the “given” amount of each reactant into moles; if not given as moles. Apply the mole ratio to a reactant(s) in order to compare to the moles available of one of the reactants to establish the limiting reactant.

What should we compare? Use moles or actual amounts of products obtained. Run the stoichiometric calculation for each of the reactants based upon their “given” amounts. The amount of common product that is the least is the limiting reactant and will provide the correct answer.

Finding Excess In order to find the amount of excess reactant, subtract the amount of reactant consumed by the limiting reactant from the amount of “excess” reactant. Convert to desired units as needed.

Actual Yield Often we do not obtain the quantity expected This may be due to errors, mistakes, side reactions, contamination or a host of other events Thus we describe the actual yield, the amount obtained experimentally or industrially

Percentage Yield The amount of product, predicted by the limiting reagent is termed the theoretical yield Percent yield relates the actual yield to the theoretical yield It is calculated as:

Example In the production of methanol, 68.5 kg of carbon monoxide is reacted with 8.60 kg of hydrogen. The reaction produces 37.4 kg of methanol. What is the percent yield of the process? CO(g) + 2 H2(g) ---------> CH3OH(l)

Hints for Limiting Reactant Problems 1. For each reactant amount given, calculate the moles (or grams) of a product it could produce. 2.The reactant that produces the smaller amount of product is the limiting reactant. 3. The number of moles of product produced by the limiting reactant is ALL the product possible. There is no more limiting reactant left.