4.2 Classifying Chemical Reactions

• Humans interact with one another in many different ways and we classify them according to their behavior.
• We say that two humans are communicating.
• We say they are fighting when they hit each other with their fists or feet.
• Scientists have found it convenient to classify chemical interactions by identifying common patterns of reactivity.
• The module will give an introduction to three types of chemical reactions: precipitation, acid-base, and oxidation-reduction.

• These reactions are common in nature and are responsible for the formation of coral reefs in ocean waters.
• They are used in the production of a number of chemicals.

• There is more information in the text chapter on solutions.

• PbI2 forms when solutions containing Pb2+ and I- are mixed.

• When solutions of ionic compounds are mixed together, the guidelines in Table 4.2 may be used to predict a precipitation reaction.
• If possible cation/anion pairs could result in an insoluble compound, one needs to identify all the ion present in the solution.
• A solution containing Ag+, NO - 3, Na+, and F- ion can be created by mixing solutions of silver nitrate and sodium fluoride.
• Two additional ionic compounds, NaNO3 and AgF, may be derived from this collection of ion compounds.
• AgF is one of the exceptions to the general solubility of salts according to the guidelines.

• Predict the solution of the ionic compounds.
• Write a balanced net ionic equation if there is precipitation.

• BaSO4 is insoluble and so a precipitation reaction is expected.
• The precipitation reaction is expected because AgCl is insoluble.
• The precipitation reaction is expected because PbCO3 is insoluble.

• Such reactions are important to many natural and technological processes, ranging from the chemical transformations that take place within cells and the lakes and oceans to the industrial-scale production of fertilizers, pharmaceuticals, and other substances essential to society.
• The subject of acid-base chemistry is worthy of thorough discussion, and a full chapter is devoted to this topic later in the text.

• We will only consider the more common types of acid-base reactions that take place in a solution.

• When hydrogen chloride gas is dissolved in water, it reacts as an acid, transferring protons to water molecule to yield hydronium ion and solvated chloride ion.

• Virtually every HCl molecule that is dissolved in water will undergo this reaction because of the nature of HCl.

• The stinging sensation of insect bites and the unpleasant smells associated with body odor are caused by weak acids.

• The most common bases are ionic compounds composed of alkali or alkaline earth metal cations.
• hydroxide ion are released into the solution when these compounds are dissolved in water.
• The cations K+ and Ba2+ are created by the dissolution of KOH and Ba( OH)2 in water.

• NaOHs can yield Na+ and OH- ion when dissolved in water.
• This is true for any other ionic compound.
• When ionic compounds are dissolved in water under typical conditions, NaOH and other ionic hydroxides are classified as strong bases.

• Some compounds produce hydroxide ion when dissolved by water.

• The stonichiometry of Chemical Reactions compounds is abundant in nature and important commodities.

• Ammonia is used in many applications.

• Acid-base reactions are the chemical reactions described in which acids and bases are dissolved in water.

• In addition to water, this reaction produces magnesium chloride.

• Since the substance is reported to be an acid, its reaction with water will involve the transfer of H+ from HOCl to H2O.

• The two reactants are provided in the OpenStax book.
• Since this is a neutralization reaction, the two products will be water and a salt composed of the cation of the ionic hydroxide.

• The neutralization of any strong acid with an ionic hydroxide is represented by the net ionic equation.

• The metabolism of aerobic organisms and many environmental processes that shape the world are dependent on the amount of O2 in the atmosphere.
• A clear picture of this classification will be developed using a few examples of such reactions.

• They are present in the form of a solid ionic compound.

• Some processes do not involve the transfer of electrons.

• The oxidation numbers are assigned to each element in a molecule.

• The oxidation number of an atom is zero.

• The oxidation number of a monatomic ion is the same as the ion's charge.

• The charge on the molecule or ion is equal to the sum of oxidation numbers for all atoms.

• The convention for reporting charge is to write the number first, followed by the sign, and oxidation number is written with the reversed sequence, sign followed by number.
• The distinction between these two properties is emphasized by this convention.

• The oxidation number for oxygen is -2.

• It's convenient to assign oxidation numbers for the cation and anion separately for ionic compounds.

• The oxidation number for sodium is +.

• The reactions used to introduce this topic may now both be identified as redox processes.
• The oxidation number of sodium increases and the oxidation number of chlorine decreases in the reaction between the two.
• The oxidation number of hydrogen increases and the oxidation number of chlorine decreases in the reaction between hydrogen and chlorine.

• The one depicted in the picture is a solid rocket-fuel reaction.

• The new Space Launch System being developed by NASA will use a small-scale, prototype, hybrid rocket engine.
• The first engines firing at 3 s use a liquid fuel/oxidant mixture, and the second engines firing at 4 s use a solid mixture.

• The oxidation of a metallic element replaced the stonichiometry of Chemical Reactions.
• When silver is reduced to silver at the surface of the copper wire, Cu2+ is dissolved in the solution.

• Provide a name for the reaction if appropriate, and identify which equations represent it.
• The oxidant and reductant are identified as redox reactions.

• If one or more elements undergo a change in oxidation number, redox reactions are identified.

This OpenStax book is free and can be found at http://cnx.org/content/col11760/1.9

• Provide a more specific name for the reaction and identify the oxidant and reductant.

• Water, hydronium ion, and hydroxide ion are often used as reactants in redox reactions.
• These species do participate in chemical change by providing the elements required to form oxyanions.
• Systematic approaches have been developed to assist in balancing the equations that represent these reactions.

• The two half-reactions represent the redox process.

• Add H2O molecule to balance oxygen atoms.

• Add H+ ion to balance hydrogen atoms.

• Adding electrons balance charge.

• To get equal numbers of electrons in each half-reaction, divide the coefficients by the smallest possible number.

• Remove species that appear on both sides of the equation to make it simpler to add the balanced half-reactions together.

• There are additional steps for reactions occurring in basic media.

• On the side of the equation, combine the H+ and OH- ion to yield water.

• Check to see that both the number of atoms and the total charges are balanced.

• A balanced equation for the reaction between dichromate ion and iron is needed.

• Mass balance is a type of charge balance in which the species in question are electrons.
• The reactant and product sides must have equal numbers of electrons in an equation.

• Mass balance is a type of charge balance in which the species in question are electrons.
• The reactant and product sides must have equal numbers of electrons in an equation.

• One reactant and one product with one element in common will be contained in each half-reaction.

• The iron half-reaction shows two atoms on the left and one on the right, but the chromium half-reaction shows only one atom on the right.
• Changing the coefficients on the right side of the equation will balance the equation.

• The iron half-reaction does not contain O atoms.
• Seven water molecule are added to the right side after the half-reaction shows seven O atoms on the left.

• The iron half-reaction does not contain H atoms.
• Fourteen hydrogen ion are added to the left side of the half-reaction because there are no H atoms on the right.

• The iron half-reaction shows a charge of 2 on the left side and 1 on the right side.
• Charge balance is achieved when one electron is added to the right side.

• There is a total charge on the right side.

• The left side's total charge is achieved if six electrons are added to it.

• The iron half-reaction's coefficients must be increased by 6 to be in line with the idea that redox reactions involve the transfer of electrons.

• The six electrons are redundant.