Chemical Reactions. Chapter 4

Transcription

1 Chemical Reactions Chapter 4

2 Chemical Equations A balanced chemical equation gives us a variety of information about the relative amounts of substances reacting and forming. Law of Conservation of Mass - of atoms of given element on reactant side must equal of atoms of that element on product side. coefficients of balanced equation tell how many of each species are involved in reaction when balancing equation - identities (subscripts) cannot be changed; only change amounts (coefficients)

3 Patterns of Chemical Reactions Four general types of chemical reactions combination decomposition displacement exchange

4 Combination Reactions often occur with O or halogens metal + oxygen metal oxide 4 Fe(s) + 3 O 2 (g) 2 Fe 2 O 3 (s) metal + halogen ionic compound 2 K(s) + F 2 (g) 2 KF (s) 2 Fe(s) + 3 C 2 (g) 2 FeCl 3 (s) nonmetal + oxygen (or halogen) molecular compound P 4 (s) + 5 O 2 (g) 2 P 2 O 5 (s)

5 Decomposition Reactions thermal decomposition metal oxides 2 HgO(s) 2 Hg(l) + O 2 (g) metal carbonates MgCO 3 (s) MgO(s) + CO 2 (g) unstable compounds 4 C 3 H 5 (NO 3 ) 3 (l) 12 CO 2 (g) + 10 H 2 O(g) + 6 N 2 (g) + O 2 (g)

6 Displacement Reactions examples: 3 C(s) + Fe 2 O 3 (g) 3 CO(g) + 2 Fe(l) 3 Fe(s) + 4 H 2 O(g) Fe 3 O 4 (s) + 4 H 2 (g)

7 Exchange Reactions most often seen in precipitation reactions AgNO 3 (aq) + NaCl(aq) AgCl(s) + NaNO 3 (aq)

8 Steps for Balancing Chemical Equations 1. write an unbalanced equation with correct reactant and product formulas 2. balance atoms of one element (start with non- H or O if possible) 3. balance atoms of remaining elements 4. check equation to verify that it is balanced

9 Example 1 Iron metal reacts with chlorine gas to form iron(iii) chloride. Write a balanced equation for this reaction. 1. Fe(s) + Cl 2 (g) FeCl 3 (s) 2. Fe(s) + 3 Cl 2 (g) 2 FeCl 3 (s) 3. 2 Fe(s) + 3 Cl 2 (g) 2 FeCl 3 (s) 4. 2 Fe, 6 Cl 2 Fe, 6 Cl

10 Example 2 Write and balance the chemical equation for the combustion of C 7 H 14 O 2. 2 C 7 H 14 O O 2 14CO H 2 O 14 C, 28 H, 42 O 14 C, 28 H, 42 O

11 Example 3 Balance the following chemical equation: NaBr + H 3 PO 4 HBr + Na 3 PO 4 3 NaBr + H 3 PO 4 3HBr + Na 3 PO 4

12 Moles and Chemical Reactions N H 2 2 NH 3 1 molecule N 2 3 molecules H 2 2 molecules NH 3 1 mol N 2 3 mol H 2 2 mol NH 3 28 g N 2 6 g H 2 34 g NH 3 34 g total 34 g total

13 Example 4 How many moles of ammonia are formed from the reaction of 0.30 mol nitrogen gas with excess hydrogen gas? N 2 + 3H 2 2NH 3 ( ) 2 mol NH 3 mol NH 3 = 0.30 mol N 2 1 mol N 2 % ' & = 0.60 mol NH 3

14 Example 5 Propane, C 3 H 8, is a common fuel used for cooking and home heating. What mass of O 2 is consumed in the combustion of 1.00 g of propane? 2 C 3 H O 2 6 CO H 2 O 1 mol % mol C 3 H 8 = ( 1.00 g) ' = mol C 3 H g& mol O 2 = ( mol C 3 H 8 ) 10 mol O % 2 ' = mol O 2 2 mol C 3 H 8 & mass O 2 = mol ( ) g 1 mol % ' = 3.63 g &

15 Example 6 When heated in dry air, sodium reacts with oxygen to form sodium peroxide: Na(s) + O 2 (g) Na 2 O 2 (s) What masses (in kilograms) of Na and O 2 are required to prepare 457 kg of Na 2 O 2 by this reaction? 2 Na(s) + O 2 (g) Na 2 O 2 (s) 1 kmol % kmol Na 2 O 2 = ( 457 kg) ' = 5.86 kmol kg& ( ) mass of Na = 5.86 kmol Na 2 O 2 ( ) mass of O 2 = 5.86 kmol Na 2 O 2 2 kmol Na % ' 1 kmol Na 2 O 2 & kg Na 1 kmol % ' = 269 kg Na & 1 kmol O 2 % ' kg O % 2 ' = 188 kg O 1 kmol Na 2 O 2 & 1 kmol & 2

16 Limiting Reactants The reactant that is completely comsumed in a chemical reaction is called the limiting reactant. 2 H 2 (g) + O 2 (g) 2 H 2 O (l) If you start with 10 mol H 2 and 7 mol O 2, which reactant is used up first? Try H 2 : Try O 2 : mol H 2 O formed = 10 mol H 2 mol H 2 O formed = 7 mol O 2 ( ) 2 mol H 2O 2 mol H 2 ( ) 2 mol H 2O 1 mol O 2 % ' = 10 mol H 2 O & % ' = 14 mol H 2 O & smallest amount formed: H 2 is limiting reactant

17 Example 7 Suppose 184 g of Al 2 O 3, 75 g of C, and 92 g of Cl 2 are mixed and the reaction proceeds until one of the reactants is used up. Which is the limiting reactant? How many grams of AlCl 3 are produced, and how many grams of the other reactants remain? g AlCl 3 = ( 184 g) 1 mol Al O 2 3 from Al 2 O g 2 Al 2 O C + 6 Cl 2 4 AlCl CO 2 % ' 4 mol AlCl % 3 ' g % ' = 481 g AlCl 3 & 2 mol Al 2 O 3 & 1 mol AlCl 3 & g AlCl 3 from C = ( 75 g ) 1 mol C % ' 4 mol AlCl % 3 ' g % ' = 1110 g AlCl g& 3 mol C & 1 mol AlCl 3 & Cl 2 limiting reactant g AlCl 3 = ( 92 g) 1 mol Cl 2 from Cl g % ' 4 mol AlCl % 3 ' g % ' = 115 g AlCl 3 & 6 mol Cl 2 & 1 mol AlCl 3 & mass of AlCl 3 produced

18 Example 7 (cont d) mass of Al 2 O 3 remaining: mass Al 2 O 3 used = 92 g Cl 2 % ' 2 mol Al 2O 3 % ' g % ' = 44 g Al 2 O g& 6 mol Cl 2 & 1 mol & ( ) 1 mol Cl 2 mass Al 2 O 3 remaining = 184 g - 44 g = 140. g mass of C remaining: mass C used ( ) 1 mol Cl 2 = 92 g Cl 2 % 3 mol C % ' ' g % ' = 7.8 g C g& 6 mol Cl 2 & 1 mol & mass C remaining = 75 g g = 67 g

19 Percent Yield Amount of product calculated to form when all the limiting reactant is consumed is the theoretical yield. Actual yield is the amount actually obtained from the reaction. % yield = actual yield theoretical yield x 100%

20 Example 8 Calculate the percent yield of C 2 H 5 Cl if the reaction of 125 g of C 2 H 6 with 255 g of Cl 2 produced 206 g of C 2 H 5 Cl. C 2 H 6 + Cl 2 C 2 H 5 Cl + HCl determine limiting reactant: mass of C 2 H 5 Cl from C 2 H 6 mass of C 2 H 5 Cl from Cl 2 = 125 g ( ) 1 mol C 2H g % ' 1 mol C 2H 5 Cl% ' g % ' = 268 g & 1 mol C 2 H 6 & 1 mol & = ( 255 g) 1 mol Cl % 2 ' 1 mol C 2H 5 Cl% ' g % ' = 232 g g & 1 mol Cl 2 H 6 & 1 mol & % yield = (206 g / 232 g) x 100 % = 88.8 % Cl 2 limiting reactant theoretical yield of C 2 H 5 Cl

21 Combustion Analysis During combustion, a compound is burned in oxygen. The carbon in the compound is converted to CO 2 and the hydrogen is converted to H 2 O. Many compounds also contain oxygen. Mass of oxygen = mass of sample - (mass of C + mass of H) Combustion analysis will tell us how much CO 2 and H 2 O were formed during the reaction. This information can be used to determine the amounts of C, H and O in the sample. From this we can get the empirical formula.

22 Example 9 Propionic acid, an organic acid, contains only C, H, and O. If g of the acid burns completely in O 2 and gives g of CO 2 and g of H 2 O, what is the empirical formula of the acid? C x H y O z + O 2 CO 2 + H 2 O determine masses of C and H in original sample: ( ) 1 mol CO 2 mass of C = g CO 2! g! 1 mol C! % & 1 mol CO 2 % & g 1 mol C % & = g C! mass of H = ( g H 2 O) 1 mol H O 2! 2 mol H! g g % & 1 mol H 2 O% & 1 mol H % & = g H mass of O in compound = g - (0.115 g g) = g O determine masses of C and H in original sample:

23 Example 9 (cont d) Propionic acid, an organic acid, contains only C, H, and O. If g of the acid burns completely in O 2 and gives g of CO 2 and g of H 2 O, what is the empirical formula of the acid? C x H y O z + O 2 CO 2 + H 2 O determine mole ratios and empirical formula as usual: ( ) mol of C = g C ( ) mol of H = g H ( ) mol of O = g O! 1 mol C g% &! 1 mol H g% &! 1 mol O g% & = mol C = mol H = mol O divide by smallest 6 2 C 3 H 6 O 2