chapter six chemical reactions: classification and mass relationships fundamentals of general,...

Chapter SixChemical Reactions:

Classification and Mass Relationships

Fundamentals of General, Organic and Biological Chemistry

6th Edition

James E. Mayhugh

Copyright © 2010 Pearson Education, Inc.

Copyright © 2010 Pearson Education, Inc. Chapter Six 2

Outline► 6.1 Chemical Equations► 6.2 Balancing Chemical Equations► 6.3 Avogadro’s Number and the Mole► 6.4 Gram–Mole Conversions► 6.5 Mole Relationships and Chemical Equations► 6.6 Mass Relationships and Chemical Equations► 6.7 Limiting Reagent and Percent Yield► 6.8 Classes of Chemical Reactions► 6.9 Precipitation Reactions and Solubility Guidelines► 6.10 Acids, Bases, and Neutralization Reactions► 6.11 Redox Reactions► 6.12 Recognizing Redox Reactions► 6.13 Net Ionic Equations


Goals

►1. How are chemical reactions written? Given the identities of reactants and products, be able to write a balanced chemical equation or net ionic equation.

►2. What is the mole, and why is it useful in chemistry? Be able to explain the meaning and uses of the mole and Avogadro’s number.

►3. How are molar quantities and mass quantities related? Be able to convert between molar and mass quantities of an element or compound.


Goals Contd.

► 4. What are the limiting reagent, theoretical yield, and percent yield of a reaction? Be able to take the amount of product actually formed in a reaction, calculate the amount that could form theoretically, and express the results as a percent yield.

► 5. How are chemical reactions of ionic compounds classified? Be able to recognize precipitation, acid–base neutralization, and redox reactions.

► 6. What are oxidation numbers, and how are they used? Be able to assign oxidation numbers to atoms in compounds and identify the substances oxidized and reduced in a given reaction.


6.1 Chemical Equations► Chemical equation: An expression in which

symbols are used to represent a chemical reaction.► Reactant: A substance that undergoes change in a

chemical reaction and is written on the left side of the reaction arrow in a chemical equation.

► Product: A substance that is formed in a chemical reaction and is written on the right side of the reaction arrow in a chemical equation.


► The numbers and kinds of atoms must be the same on both sides of the reaction arrow.

► Numbers in front of formulas are called coefficients, they multiply all the atoms in a formula.

► The symbol 2 NaHCO3 indicates two units of sodium bicarbonate, which contains 2 Na,2 H, 2 C, and 6 O.

► Substances involved in chemical reactions may be solids, liquids, gases, or they may be in solution.

► This information is added to an equation by placing the appropriate symbols after the formulas:

► Solid=(s) Liquid=(l) Gas=(g) Aqueous solution=(aq)


6.2 Balancing Chemical Equations

► Balancing chemical equations can be done using four basic steps:

► STEP 1: Write an unbalanced equation, using the correct formulas for all reactants and products.

► STEP 2: Add appropriate coefficients to balance the numbers of atoms of each element.


► A polyatomic ion appearing on both sides of an equation can be treated as a single unit.

► STEP 3: Check the equation to make sure the numbers and kinds of atoms on both sides of the equation are the same.


► STEP 4: Make sure the coefficients are reduced to their lowest whole-number values.

► The equation:

2 H2SO4 + 4 NaOH 2 Na2SO4 + 4 H2O

is balanced, but can be simplified by dividing all coefficients by 2:

H2SO4 + 2 NaOH Na2SO4 + 2 H2O

► Hint: If an equation contains a pure element as a product or reactant it helps to assign that element’s coefficient last.


6.3 Avogadro’s Number and the Mole

► Molecular weight: The sum of atomic weights of all atoms in a molecule.

► Formula weight: The sum of atomic weights of all atoms in one formula unit of any compound.

► Mole: One mole of any substance is the amount whose mass in grams (molar mass) is numerically equal to its molecular or formula weight.

► Avogadro’s number: The number of molecules or formula units in a mole. NA = 6.022 x 1023


6.4 Gram – Mole Conversions

► Molar mass = Mass of 1 mole of a substance.

= Mass of 6.022 x 1023 molecules of a substance.

= Molecular (formula) weight of substance in grams.

► Molar mass serves as a conversion factor between numbers of moles and mass. If you know how many moles you have, you can calculate their mass; if you know the mass of a sample, you can calculate the number of moles.


The molar mass of water is 18.0 g. The conversion factor between moles of water and mass of water is 18.0 g/mol and the conversion factor between mass of water and moles of water is 1 mol/18.0 g:


6.5 Mole Relationships and Chemical Equations

The coefficients in a balanced chemical equation tell how many molecules, and thus how many moles, of each reactant are needed and how many molecules, and thus moles, of each product are formed. See the example below:


► The coefficients can be put in the form of mole ratios, which act as conversion factors when setting up factor-label calculations.

► In the ammonia synthesis the mole ratio of H2 to N2 is 3:1, the mole ratio of H2 to NH3 is 3:2, and the mole ratio of N2 to NH3 is 1:2 leading to the following conversion factors:

(3 mol H2)/(1 mol N2)

(3 mol H2)/(2 mol NH3)

(1 mol N2)/(2 mol NH3)


6.6 Mass Relationships and Chemical Equations

Mole to mole conversions are carried out using mole ratios as conversion factors.


► Mole to mass and mass to mole conversions are carried out using molar mass as a conversion factor.

► Mass to mass conversions are frequently needed, but cannot be carried out directly.

► Overall, there are four steps for determining mass relationships among reactants and products.


Mass to mass conversions:►STEP 1: Write the

balanced chemical equation.►STEP 2: Choose molar

masses and mole ratios to convert known information into needed information.

►STEP 3: Set up the factor-label expression, and calculate the answer.

►STEP 4: Estimate or check the answer using a ballpark solution.


6.7 Limiting Reagent and Percent Yield

► The amount of product actually formed in a chemical reaction is somewhat less than the amount predicted by theory.

► Unwanted side reactions and loss of product during handling prevent one from obtaining a perfect conversion of all the reactants to desired products.

► The amount of product actually obtained in a chemical reaction is usually expressed as a percent yield.


► Percent yield is defined as:

(Actual yield ÷ Theoretical yield) x 100%► The actual yield is found by weighing the product

obtained.► The theoretical yield is found by a mass-to-mass

calculation.► When running a chemical reaction without the

exact amounts of reagents to allow all of them to react completely, the reactant that is exhausted first is called the limiting reagent.


6.8 Classes of Chemical Reactions

► When learning about chemical reactions it is helpful to group the reactions of ionic compounds into three general classes: precipitation reactions, acid–base neutralization reactions, and oxidation–reduction reactions.

► Precipitation reactions are processes in which an insoluble solid called a precipitate forms when reactants are combined in aqueous solution.


► Acid–base neutralization reactions are processes in which H+ ions from an acid react with OH- ions from a base to yield water. An ionic compound called a salt is also produced. The “salt” produced need not be common table salt. Any ionic compound produced in an acid–base reaction is called a salt.

► Oxidation–reduction reactions, or redox reactions, are processes in which one or more electrons are transferred between reaction partners (atoms, molecules, or ions). As a result of this transfer, the charges on atoms in the various reactants change.


6.9 Precipitation Reactions and Solubility Guidelines

► Reaction of aqueous Pb(NO3)2

with aqueous KI gives a yellow precipitate of PbI2.

► To predict whether a precipitation reaction will occur on mixing aqueous solutions of two ionic compounds, you must know the solubility of the potential products.


If a potential product does not contain at least one of the ions listed below, it is probably not soluble and will precipitate from solution when formed.


6.10 Acids, Bases, and Neutralization Reactions

► When acids and bases are mixed together in correct proportion acidic and basic properties disappear.

► A neutralization reaction produces water and a salt. HA(aq) + MOH(aq) H2O(l) + MA(aq)

acid + base water + salt► The reaction of hydrochloric acid with potassium

hydroxide to produce potassium chloride is an example:

► HCl(aq) + KOH(aq) H2O(l) + KCl(aq)


6.11 Redox Reactions

► Oxidation– reduction (redox) reaction: A reaction in which electrons transfer from one atom to another.

► Oxidation: Loss of one or more electrons by an atom.► Reduction: Gain of one or more electrons by an atom.


► Oxidation and reduction always occur together.► A substance that is oxidized gives up an electron,

causes reduction, and is called a reducing agent. ► A substance that is reduced gains an electron, causes

oxidation, and is called an oxidizing agent. ► The charge on the reducing agent increases during

the reaction, and the charge on the oxidizing agent decreases.


Reducing agent: ► Loses one or more electrons► Causes reduction► Undergoes oxidation► Becomes more positive (or less negative)

Oxidizing agent: ► Gains one or more electrons► Causes oxidation► Undergoes reduction► Becomes more negative (or less positive)


6.12 Recognizing Redox Reactions

► One can determine whether atoms are oxidized or reduced in a reaction by keeping track of changes in electron sharing by the atoms. Each atom in a substance is assigned a value called an oxidation number or oxidation state.

► The oxidation number indicates whether the atom is neutral, electron rich, or electron poor.

► By comparing the oxidation state of an atom before and after reaction, we can tell whether the atom has gained or lost electrons.


► Rules for assigning oxidation numbers:► An atom in its elemental state has an oxidation

number of zero.

► A monatomic ion has an oxidation number equal to its charge.


► In a molecular compound, an atom usually has the same oxidation number it would have if it were a monatomic ion.

► Examples: H often has an oxidation number of +1, oxygen often has an oxidation number of -2, halogens often have an oxidation number of -1.


► For compounds with more than one nonmetal element, such as SO2, NO, and CO2, the more electronegative element—oxygen in these examples—has its preferred negative oxidation number.

► The less electronegative element is assigned a positive oxidation number so that the sum of the oxidation numbers in a neutral compound is 0.


6.13 Net Ionic Equations

► Ionic equation: An equation in which ions are explicitly shown.

► Spectator ion: An ion that appears unchanged on both sides of a reaction arrow.

► Net ionic equation: An equation that does not include spectator ions.


Chapter Summary

► Chemical equations must be balanced; the numbers and kinds of atoms must be the same in both the reactants and the products.

► To balance an equation, coefficients are placed before formulas but the formulas themselves cannot be changed.

► A mole refers to Avogadro’s number of formula units of a substance. One mole of any substance has a mass equal to its formula weight in grams.

► Molar masses act as conversion factors between numbers of molecules and masses in grams.


Chapter Summary Contd.► The coefficients in a balanced chemical equation

represent the numbers of moles of reactants and products in a reaction.

► Mole ratios relate amounts of reactants and/or products. Using molar masses and mole ratios in factor-label calculations relates unknown masses to known masses or molar amounts.

► The yield is the amount of product obtained. ► The percent yield is the amount of product obtained

divided by the amount theoretically possible and multiplied by 100%.


Chapter Summary Contd.► Precipitation reactions are processes in which an

insoluble solid called a precipitate is formed. ► In acid–base neutralization reactions an acid reacts

with a base to yield water plus a salt. ► Oxidation–reduction (redox) reactions are processes

in which one or more electrons are transferred between reaction partners.

► Oxidation is the loss of electrons by an atom, and reduction is the gain of electrons by an atom.

► Oxidation numbers are assigned to provide a measure of whether an atom is neutral, electron-rich, or electron-poor.


Key Words

►Actual yield►Avogadro’s number►Balanced equation►Chemical equation►Coefficient►Formula weight►Ionic equation

►Law of conservation of mass►Limiting reagent►Molar mass►Mole►Molecular weight (MW)►Net ionic equation►Neutralization reaction


Key Words Contd.

►Oxidation►Oxidation number►Oxidation–reduction (redox) reaction►Oxidizing agent►Percent yield►Precipitate►Product

►Reactant►Reducing agent►Reduction►Salt►Solubility►Spectator ion►Theoretical yield


End of Chapter Six

chapter six chemical reactions: classification and mass relationships fundamentals of general,...

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