#### Transcript General Chemistry

*Chapter 4*

*Chapter 4*

*Quantities of Reactants and Products*

*Quantities of Reactants and Products*

**Antoine Lavoisier 1743-1794. “Father**

*of modern chemistry.” Recognized true elements.*

*Used quantitative measurements in chemical reactions.*

**1**

*Chemical Equations*

*Chemical Equations*

• • •

**Lavoisier: mass is conserved in a chemical reaction.**

**Chemical equations: descriptions of chemical reactions.**

**Two parts to an equation: reactants and products : 2H 2 + O 2 2H 2 O**

*Reactants Product*

**2**

**H 2 O 2 H 2 O 3**

•

**Stoichiometric coefficients : numbers in front of the chemical formulas give numbers of molecules or atoms reacting (and numbers being produced).**

**4**

**Law of Conservation of Mass:**

*All reactions must be balanced*

**CH 4 + O 2 CO is not balanced. (Why?) 2 + H 2 O Count atoms: Reactants: Products: 1 C 1 C 4 H 2 O 2 H 3 O 5**

**Balance reactions only by changing coefficients, not by altering chemical formula 6**

**Combustion is the burning of a substance in oxygen: C 3 H 8 ( g) + 5O 2 (g) **

**3CO 2 ( g) + 4H 2 O(l) 7**

**Which is correct? (Blue=A; Red=B) a) A 2 b) A 2 + B A + 4B c) 2A + B 4 d) A + B 2 2 B 2 AB 2 AB AB 2 2 2 8**

*Atomic and Molecular Weights*

*Atomic and Molecular Weights*

**Percentage Composition from Formulas Percent composition is the atomic weight for each element divided by the formula weight of the compound multiplied by 100:**

**% Element**

**Atoms of FW of Element**

**AW**

**Compound 100**

**9**

*Percentage Composition from Formulas*

*Percentage Composition from Formulas*

**What is % O in H 2 SO 4 (by mass)?**

**FW= (2x1) + (1x32)+ (4x16)= 98 amu FW of O in H 2 SO 4 = 4 x 16 = 64 amu %O = 64 x 100 = 65.3% 98 10**

*The Mole**

***MSJ Ch 3 pp 100-104**

•

**The “amu” is an “atomic mass unit.”**

•

**O has a mass of 16 amu – but we can’t weigh out anything in amu**

•

**If we want to keep the number “16” for the mass of oxygen in some real units (like grams) then we are dealing with a whole bunch of atoms (in 16 g of oxygen).**

•

**That bunch of atoms is called a mole .**

•

**Experimentally, 1 mole = 6.02 x 10 23 things (atoms)**

•

**This number is called Avogadro’s number.**

**11**

*The mole is defined so that one mole of a substance has a mass equal to its AW or MW in grams*

**Basically, you are replacing amu with grams, e.g.**

**The mass of a P atom is 31 amu.**

**The molar mass of P is 31 grams. The mass of a Ca atom is 40 amu The molar mass of Ca is 40 grams This amount of P or Ca each contains Avogadro’s number (6.02 x 10 23 ) of atoms of P and Ca.**

**12**

*The Mole*

*The Mole*

**Experimentally, 1 mole of 12 C has a mass of exactly 12 g. (recall from Ch. 2) Molar Mass Molar mass: mass in grams of 1 mole of substance Units: g/mol or g.mol**

**-1 .**

**Mass of 1 mole of 12 C = 12 g exactly 13**

**14**

This photograph shows one mole of : solid

**NaCl**

(58.5 g), liquid

**H 2 O**

(18 g), and gaseous

**N 2**

(28 g).

**15**

*The Mole*

*The Mole*

**Molar Mass Molar mass: sum of the molar masses of the atoms: What is Molar Mass of H 2 SO 4 ?**

**16**

*The Mole*

*The Mole*

**Interconverting Masses, Moles, and Numbers of Particles 17**

**Example 1: 5.00 g of P (a) contains (b) contains mol of P atoms of P Example 2: 5.00 x 10 24 atoms of C (a) equals mol of C (b) has mass equal to grams.**

**18**

**Example 3: 3.5 mol CO 2 : (a) has what mass?**

**(b) contains how many molecules of CO 2 ?**

**(c) contains how many atoms of O?**

**19**

*Empirical Formulas from Analyses*

*Empirical Formulas from Analyses*

**Start with mass % of elements (i.e. empirical data) and calculate a formula.**

**20**

*Empirical Formulas from Analyses**

**Example: compound of N and O Given analysis: N: 25.9%; O: 74.1% Assume 100g; N: 25.9 g; O: 74.1 g Change to mol: N: 25.9 g x mol = 1.85 mol 14 g**

*MSJ Ch 3 pp 104-108

**O: 74.1 g x mol = 4.63 mol 16 g Preliminary emp. Formula: N 1.85**

**O 4.63**

**Clean it up: divide both by 1.85: Get N 1 O 2.5**

**; get rid of fractions, multiply both by 2: Get N 2 O 5 which is the empirical (simplest) formula**

*What are some possible molecular formulas?*

**21**

*Empirical Formulas from Analyses*

*Empirical Formulas from Analyses*

**Molecular Formula from Empirical Formula Once we know the empirical formula, we need the MW to find the molecular formula.**

**Subscripts in the molecular formula are always whole number multiples of subscripts in the empirical formula.**

**Example: suppose compound of C and H has empirical formula of C 3 H 8 and a MW = 176 g/mol. What is the molecular formula?**

**22**

*Quantitative Information from Balanced Equations*

*Quantitative Information from Balanced Equations*

**Balanced chemical equation gives number of molecules (or moles) that react to form products.**

**Interpretation: balanced equation gives us the ratio of number of moles of reactant to product (or v.v.) .These ratios are called stoichiometric ratios . Example: 2 H 2 + O 2 2 H 2 O Molecules: 2 1 2 Moles: 2 1 2 Ratio of O 2 :H 2 O = 1:2 (either molecules or moles) 23**

*Quantitative Information from Balanced Equations*

*Quantitative Information from Balanced Equations*

**The ratio of grams of reactant cannot be directly related to the grams of product.**

**24**

*Stoichiometry*

*Stoichiometry*

**Problem: aluminum sulfide + water aluminum hydroxide + hydrogen sulfide (a) Write balanced reaction: (b) How many g aluminum hydroxide obtained from 10.5 g of aluminum sulfide?**

**25**

*Stoichiometry*

*Stoichiometry*

**Problem: 2 NaN 3 (s) 2Na(s) + 3 N 2 (g) (a) how many mol N 2 produced from 2.50 mol NaN 3?**

**(b) how many g NaN 3 needed to form 6.00 g N 2 (c) how many g NaN 3 (1.00 ft 3 needed to produce 10.0 ft = 28.3 L; density of N 2 = 1.25 g/L) 3 of N 2 ?**

**26**

*Limiting Reactants*

*Limiting Reactants*

**If the reactants are not present in stoichiometric amounts, at end of reaction some reactants are still present (in excess).**

**Limiting Reactant : one reactant that is consumed.**

**H 2 O 2 O 2 INXS 27**

**RECIPE: 3 cups flour + 4 eggs + 2 cups sugar cake ------------------------------------------------------- On Hand: cups flour 20 eggs 20 cups sugar 20 How many cakes can be made from these amounts?**

**What is the “limiting reactant” (LR) What’s left over and how much of it is left over?**

**28**

**RECIPE: 3 cups flour + 4 eggs + 2 cups sugar cake ------------------------------------------------------- On Hand: cups flour 24 eggs 28 cups sugar 10 How many cakes can be made from these amounts?**

**What is the “limiting reactant” (LR) What’s left over and how much of it is left over?**

**29**

*Limiting Reagent*

*Limiting Reagent*

**(a)Assume a reactant (any one) is LR.**

**Calculate stoichiometric amount of product (any product) formed.**

**(b) Pick another reactant and make it the LR.**

**Calculate the stoichiometric amount of same product formed.**

**(c) Whichever reactant gives the smaller amount of product is the Limiting Reagent 30**

*Limiting Reagent The LR is always used up in a chemical reaction. Everything else is in excess (INXS)*

*Limiting Reagent The LR is always used up in a chemical reaction. Everything else is in excess (INXS)*

*.*

**31**

*Limiting Reagent*

*Limiting Reagent*

**Problem: 4 NH 3 + 5 O 2 4 NO + 6 H 2 O 2.25 g NH 3 mixed with 3.75 g O 2 and allowed to react (a) which is LR?**

**(b) how many grams NO formed?**

**(c) how much of excess reactant remains?**

**32**

*Limiting Reactants*

*Limiting Reactants*

**Theoretical Yields The amount of product predicted from stoichiometry taking into account limiting reagents is called the theoretical yield.**

**The percent yield relates the actual yield (amount of material recovered in the laboratory) to the theoretical yield:**

**% Yield**

**Actual yield Theoretica l yield**

**100**

**33**

*Theoretical Yield*

*Theoretical Yield*

**Problem: C 6 H 6 + Br 2 C 6 H 5 Br + HBr (a) theoretical yield of C 6 H 5 Br when 30.0 g of C 6 H 6 reacts with 65.0 g of Br 2 ?**

**(b) if actual yield of C 6 H 5 Br is 56.7 g, calculate %yield 34**