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Chem.200-4.Reaction

General Chemistry Ch. 4 - Three Major Classes of Chemical Reactions

QuestionAnswer
The combined effects of its bent shapes and distribution of bonding electrons, make water a _____ Polar molecule: The O portion of the molecule is the partially negative pole, and the region midway between the H atoms on the other end is the partially positive pole.
What happens to ionic solids placed in water? In an ionic solid, the oppositely charged ions are held next to each other by electrostatic attraction. Water separates the ions by replacing that attraction with one between the water molecules and the ions.
How is water able to replace the attraction of ions with attraction to itself? The bent, polar water molecules are attracted to both the anions and cations due to its polar nature. The water molecules surround the ions and they separate and become solvated: surrounded by solvent molecules.
Why do some ionic compounds dissolve in water, while others do not? The electrostatic attraction among ions within some ionic compounds remain greater than the attraction between ions and water molecules. E.g. AgCl is “insoluble”, while NaCl is soluble.
Electrical conductivity of water vs. ionic solid vs. ionic compound dissolved in water Water shows no electrical conductivity; ionic solids show no electrical conductivity; ionic compounds dissolved in water do show electrical conductivity.
How can electric current flow through a solution of solvated ions? The current flow implied movement of charged particles. The solvated ions move toward the electrodes of the opposite charges.
A substance that conducts a current when dissolved in water is a(n) _____ Electrolyte
Strong electrolytes Soluble ionic compounds are called strong electrolytes because they dissociate completely into ions and create a large current.
How can you predict the number of moles of different ions that result when an ionic compound dissolves? The formula of the compound tells you the number: e.g. 1 mol of KBr dissociates into 2 mol of ions, 1 mol K+ and 1 mol Br-
Do covalent compounds dissolve in water? How do they behave? Many covalent compounds also dissolve, e.g. table sugar (sucrose) and ethanol. They have their own polar bonds that interact with water. But rather than solvate into ions, they remain as intact molecules and aren’t conductive.
Covalent compounds that dissolve in aqueous solutions but aren’t conductive are known as Nonelectrolytes
Three types of equations used to represent aqueous ionic reactions Molecular, total ionic, and net ionic
What does the molecular equation show? It shows all the reactants and products as if they were intact, undissociated compounds. It is the least revealing of the three types of equations.
What does the total ionic equation show? It shows the soluble ionic substances dissociated into ions
What does the net ionic equation show? The most useful ionic equation because it omits spectator ions and shows the actual chemical change taking place
Precipitation reactions Two soluble ionic compounds react to form an insoluble product, the precipitate.
Technically what happens during a precipitation reaction The electrostatic attraction between the ions outweighs the tendency of the ions to remain solvated and move randomly throughout the solution. When solutions of such ions are mixed, the ions collide and stay together.
The key event in a precipitation reaction is The formation of an insoluble product through the net removal of solvated ions from solution.
Metathesis reactions Reactions in which atoms or ions of two compounds exchange bonding partners. AKA double displacement reaction or exchange reaction.
E.g. Sodium iodide and lead nitrate dissolved in water 2NaI(aq) + Pb(NO3)2(aq)  PbI2(s) + 2NaNO3(aq). This is a precipitation reaction where the solid lead iodide forms out of the solution. Notice this is a metathesis reaction b ecause NaI and Pb(NO3)2 exchange bonding partners.
How to predict whether an ion combination is soluble or not? You have to refer to the solubility rules for ionic compounds in water.
Acid-base reactions AKA neutralization reaction, occurs when an acid reacts with a base.
Acid A substance that produces H+ ions (solvated protons) when dissolved in water, e.g. HX  H+(aq) + X-(aq)
Base A substance that produces OH- ions when dissolved in water, e.g. XOH  X+ + OH-
Acidic solutions arise from Special covalent molecules that dissociate in water. The molecules contain a bond to H in which the other atom pulls stronger than H. Thus, a polar molecule results. H2O is attracted to, and breaks apart, the molecule.
What happens when HCl is dissolved into water The Cl end of the molecule is partially negative, the H is partially positive. When dissolved, the H2O breaks apart the H—Cl bond, which the H becoming the solvated cation H+ and the Cl becoming the solvated anion Cl-.
Water interacts strongly with many ions, most strong with _____ The hydrogen cation, H+
How does H+ behave when in water? It is unique because it is a full positive charge (proton) concentrated in such a small volume. As result it attracts the negative pole of the surrounding water molecules so strongly that it actually forms a covalent bond (H3O+).
How does H3O+ behave in water? H3O+ interacts and associates tightly with other H2O molecules, forming species such as H5O2+ or H7O3+.
Does any other ions act like H+ when dissolved in water? Water interacts covalently with many metal ions as well. For example, Fe3+ exists in water as Fe(H2O)6^3+
Strong acids/bases (definition) Strong acids and strong bases dissociate completely into ions. Therefore, like soluble ionic compounds, they are strong electrolytes and conduct a current as well.
Weak acids/bases (definition) Dissociate into ions very little, and most of their molecules remain intact. As a result, they conduct only a small current and are weak electrolytes.
Strong acids, list HCl, HBr, HI, HNO3, H2SO4, HClO4
Weak acids, list HF, H3PO4, CH3COOH
Strong bases, list NaOH, KOH, Ca(OH)2, Sr(OH)2, Ba(OH)2
Weak bases, list NH3
As part of their structure, strong and weak acids have one or more __ atoms H
Other strong bases include… Soluble ionic oxides (containing O^2-), e.g. K2O because the oxide ion is not stable in water and reacts immediately to form OH- ion when dissolved. (K2O + H2O  2K+ + 2OH-)
How do weak bases, such as ammonia (NH3), behave in water? They don’t contain OH- ions but they produce them in a reaction with water that occurs to a small extent: NH3 + h2O  NH4+ + OH-
Note: what is a bicarbonate? A salt containing the anion HCO3-
Note: what happens when you mix baking soda and vinegar? Baking soda (sodium bicarbonate) and vinegar (acetic acid) swap partners creating sodium acetate NaCH3COO and carbonic acid H2CO3. H2CO3 is unstable and decomposes into CO2.
Note: Baking soda + vinegar full formula NaHCO3 + CH3COOH  NaCH3COO + CO2 + H2O
The essential change in all aqueous reactions between a strong acid and a strong base is… That an H+ ion from the acid and an OH- ion from the base form a water molecule.
Salt An ionic compound that results from the reaction of an acid and a base, e.g. HX(acid) + MOH(base)  MX(salt) + H2O(water)
Example of an acid-base reaction and a resulting salt: antacid tablets Antacid tablets are Al(OH)3 (aluminum hydroxide). Reaction: 3HCl + Al(OH)3  AlCl3 + 3H2O. The salt, aluminum chloride, is created.
Titration One solution of known concentration is used to determine the concentration of another solution through a monitored reaction.
In a typical acid-base titration… A standardized solution of base, one whose concentration is known, is added slowly to an acid solution of unknown concentration until the end point is reached, which is theoretically very close to the equivalence point.
End point Occurs when a tiny excess of OH- ions changes the indicator permanently to its color in base. In calculations we assume this tiny excess is insignificant, and therefore regarded close enough to the equiv. point.
Acid-base indicator A substance whose color is different in acid than in base
Equivalence point When all of the moles of H+ ions present in the original volume of acid solution have reacted with an equivalent number of moles of OH- ions added from the burette.
What ‘actually’ happens to the hydrogen proton when an acid is dissolved in water? E.g. HCl It is transferred from the acid to H2O to form a Hydronium ion: HCl + H2O  H3O+ + Cl-, thus aqueous hydrochloric acid actually consists of solvated H3O+ and Cl- ions.
What happens when a base (e.g. NaOH) is added to an acid (e.g. HCl dissolved in water). Formula: [H3O+ + Cl-] + [Na+ OH-]  2H2O + Cl- + Na+. Cl- and Na+ are spectator ions so the product is just 2H2O. Thus, at this point, if you evaporate the water, what is left is crystallized salt.
Johannes Bronsted and Thomas Lowry, in the early 20th century, defined acids/bases as the following Acid = a molecule that donates a proton. Base = a molecule that accepts a proton.
Gas-forming acid-base reactions When an ionic carbonate (or sulfite) reacts with an acid, the molecules exchange pairs and H2CO3 is created (carbonic acid) which is very unstable and decomposes into CO2(g) + water. E.g. Sodium bicarbonate + Acetic acid
Note: what is meant by “carbonate”? It is a salt of a carbonic acid, characterized by the presence of the ion CO3^2-
Salt Ionic compounds that result from neutralization of an acid and a base.
Carbonic Acid H2CO3
Sulfite Compounds that contain SO3^2-
Describe the reactions between weak acids and strong bases The weak acid barely dissociates. E.g. CH3COOH + NaOH  CH3COO- + H2O (net ionic; exclude spectator ions). Since it’s a weak acid it appears dissociated on both ionic equations, rather than appearing dissolved.
What types of reactions are Redox (oxidation-reduction) reactions? They include: formation of a compound from its elements, all combustion reactions, the reactions that generate electricity in batteries, and the reactions that produce cellular energy.
Redox reactions Net movement of electrons from one reactant to another. This movement of electrons occurs from the reactant (or atom in the reactant) with less attraction for electrons to the reactant (or atom) with more attraction for electrons.
Example of redox reaction: formation of magnesium oxide 2Mg + O2  2MgO. Each Mg loses two electrons, and each O atom gains them.
The change from 2Mg + O2  2MgO represents… A transfer of electron charge away from each Mg atom toward each O atom, resulting in the formation of Mg^2+ and O^2- ions.
Does the following equation represent a redox reaction? H2 + Cl2  2HCl It’s not an ionic bond, it’s covalent. Thus none of the elements are “losing” their electrons (i.e. there is no transfer), rather they’re SHIFTING. However, this is still regarded as a redox reaction.
Oxidation The loss of electrons.
Reduction Gain of electrons
Oxidizing agent The species doing the oxidizing (taking electrons).
Reducing agent The species doing the reducing (giving electrons).
Oxidizing and reducing agents in 2Mg + O2 Mg = reducing agent (gives electrons), O2 = oxidizing agent (takes electrons).
Oxidizing and reducing agents in H2 + Cl2  HCl H2 = reducing agent, Cl2 = oxidizing agent. H gives away from negative charge, Cl2 takes some negative charge
Oxidation number AKA oxidation state. The charge the atom would have if electrons were not shared but were transferred completely. Thus, the oxidation state for each element in a binary ionic compound equals the ionic charge.
Oxidation rules for Group 1A +1 in all compounds (except hydrogen)
Oxidation rules for Group 2A +2 in all compounds
Oxidation rules for hydrogen +1 in combination with nonmetals; -1 in combination with metals and boron
Oxidation rules for fluorine -1 in all compounds
Oxidation rules for oxygen -1 in peroxides; -2 in all other compounds
Oxidation rules for group 7A -1 in combination with metals, nonmetals (except O), and other halogens lower in the group.
Oxidation rules for atoms in their elemental form (O2, Cl2, etc) Oxidation number is zero.
Oxidation number for monatomic ion Equal to the ion charge
The sum of the oxidation values for the atoms in a polyatomic ion equals The ion’s charge
E.g. list the oxidation states for every element in nitric acid H = +1; NO3 = -1 (N = +5; O3 = -6).
How to identify a redox reaction Mark the oxidation states for every reactant atom. Compare the states to those of the products. If they are different, then you have a redox reaction.
Are acid-base reactions redox reactions? No, the oxidation numbers don’t change. E.g. H+ + OH- -> H2O
How to determine the oxidizing agent or reducing agent when given an equation? Compare the reactant elements with the product elements. If the oxidation number goes up, it lost electrons, and thus is the reducing agent. If the oxidation number went down, it gained electrons, and thus is the oxidizing agent.
Can a reaction just be an oxidizing or reducing reaction? No, wherever an oxidation occurs, a reduction simultaneously occurs.
Special redox behavior of carbon Carbon can have either a +2 (CO), +4 (CO2), or -4 oxidation number
Two methods to balance redox reactions The oxidation number method and the half reaction method
Using the oxidation number method to balance redox equations 1. Write un-balanced equation. 2. Assign all oxidation states. 3. Identify oxidized and reduced species and compute electrons gained/loss and draw lines between them. 4. Make them equal, and then balance manually.
Redox titrations Same principle as acid-base titration. A known concentration of oxidizing agent is used to find an unknown concentration of reducing agent.
Commonly used compound in redox titrations Permanganate ion, MnO4-, because it is strongly colored and thus serves as a good indicator.
Whenever atoms appear in the form of a free element on one side of an equation and as part of a compound on the other… There must have been a change in oxidation state and the reaction is a redox process
Three types of redox reactions Combination reactions, decomposition reactions, and displacement reactions
Combination reactions X + Y -> Z
Decomposition reactions Z -> X + Y
Displacement reactions X + YZ -> XZ + Y
Examples of combination reactions 1. Metals and nonmetals form an ionic compound (2K + Cl2 -> 2KCl). 2. Two nonmetals form a covalent compound (N2 + 3H2 -> 2NH3) 3. Combining a compound with an element to create a larger compound.
How do decomposition reactions occur? These reactions occur when a reactant absorbs enough energy for one or more of its bonds to break. E.g. heat, electricity, light, mechanical, etc.
Thermal decomposition One type of decomposition reaction. Decomposition due to absorbing heat energy. A delta sign hanging above the reaction arrow indicates significant heating.
What type of molecules release oxygen when heated? Many metal oxides, chlorates, and perchlorates. E.g. 2KClO3 -> 2KCl + 3O2
Electrolytic decomposition In the process of electrolysis, a compound absorbs electrical energy and decomposes into its elements. E.g. 2H2O -> 2H2 + O2
Examples of electrolysis used commercially Many active metals, such as Na, Mg, and Ca, are produced industrially by electrolysis of their molten halides. (e.g. MgCl2 -> Mg + Cl2)
Halides A binary compound of a halogen with another element or group
Displacement reactions have the same number of _____ as _____ Reactants; products
Double displacement reactions Metatheses, discussed earlier
Single displacement reactions All oxidation-reduction processes. They occur when one atom displaces the ion of a different atom from the solution.
Single displacement reactions in metals vs. nonmetals In metals: the displaced atom reduces the ion, in nonmetals (specifically halogens): the atom oxidizes the ion.
How can you predict whether an atom will displace an element? The activity series of the metals ranks them based on how strong they are as a reducing agent, e.g. displace H from water.
Other than displacing H2 from water or acids, what else do single displacement reactions include? A metal displacing another metal ion from solution. Direct comparisons of metal reactivity can be used to predict this behavior.
Dynamic equilibrium The reaction has not stopped but the reverse reaction is occurring at the same rate. On the macroscopic scale, the reaction is static, but is dynamic at the molecular scale.
In principle all reactions are _____ Reversible.
Example of reversible reaction acetic acid + water. The H dissociates from acetic acid but only about 1.3% is dissociated at any given moment because the H bonds with acetic acid re-form simultaneously as they dissociate.
Reversible reactions: gas escaping causing reactions to complete e.g. Soda: once you open the can, the CO2 (decomposed from the carbonic acid) escapes and the reaction is no longer reversible. As long as the CO2 can’t escape, the chemicals in the can remain in dynamic equilibrium
Created by: Intellex_