Reactions in Solution

Reactions in Solution Name: ______AP Chemistry Lecture Outline solution: a homogeneous mixture of two or more substances -- The ______is present in greatest quantity. -- Any other substance present is called a ______. aqueous solutions: solutions in which water is the dissolving medium (i.e., the solvent) electrolyte: any substance whose aqueous solution will conduct electricity e.g., -- as opposed to a nonelectrolyte, e.g.,

As a general rule, ionic solids dissociate into ions in aqueous solution. The partial negative charge on the O and the partial positive charge on the H atoms allow H2O to interact strongly with – and “pull out” – ions in the crystal lattice. Thus, ionic compounds are usually electrolytes.

For molecular compounds, structural integrity of molecules is maintained. Substance may dissolve, but it doesn’t dissociate. Thus, molecular compounds are usually nonelectrolytes. -- major exception:

Strong electrolytes exist almost completely as ions in aqueous solution. e.g.,

Weak electrolytes produce only a small concentration of ions that reach equilibrium. e.g.,

Some of the strong electrolytes are the strong acids and strong bases. STRONG ACIDS STRONG BASES

hydrochloric, HCl chloric, HCiO3 the hydroxides of...

hydrobromic, HBr perchloric, HClO4

hydroiodic, HI nitric, HNO3

sulfuric, H2SO4

Be careful to distinguish between dissolution and dissociation in regard to strong or weak electrolytes. For example, CH3COOH dissolves completely, but dissociates only slightly; it is therefore a weak electrolyte. On the other hand, Ba(OH)2 dissolves very little, but the amount that does dissolve dissociates almost completely. Ba(OH)2 is a strong electrolyte. The question is: Of the amount that dissolves, what fraction dissociates?

Precipitation reactions are reactions in solution that form an insoluble product. The insoluble product is called a...

Solubility Guidelines for Selected Ions in Aqueous Solution

– – Soluble NO3 , CH3COO no exceptions

– – – + 2+ 2+ Cl , Br , I except with Ag , Hg2 , and Pb

2– 2+ 2+ 2+ 2+ SO4 except with Sr , Ba , Hg2 , and Pb

2– 3– + Insoluble CO3 , PO4 except with NH4 and alkali metal cations

2– + S except with NH4 and “strong base cations” OH– except with “strong base cations”

EX. Suppose you mix solutions of lead (II) nitrate and sodium iodide.

The ions present are...

The reaction is…

Balance to get the overall ionic equation…

Cancel the spectator ions to get the net ionic equation…

2 Acids and Bases For now, acids ionize in aqueous solutions to form a hydrogen ion (H+). -- “proton donors” -- monoprotic acids e.g., -- diprotic acids e.g., (only first ionization is complete)

Weak acids are only partially ionized. When reactivity depends only on the amount of H+, then weak acids are less reactive than strong acids. However, if the anion is very reactive, that reactivity trend goes out the window. e.g.,

For now, bases are substances that accept hydrogen ions (i.e., protons). -- OH– is very basic -- equation:

The most common weak base is ammonia, NH3; it only ionizes about 1%. -- equation:

EX. Classify the following as a strong electrolyte, weak electrolyte, or nonelectrolyte.

CH3CH2OH HCOOH KOH

EX. If 0.10 mol of each of the following are dissolved in water, rank them from least to greatest electrical conductivity.

Ca(NO3)2

C6H12O6

NaCH3COO 3 4

CH3COOH

A neutralization reaction has the form: -- recall that “salt” means an ionic compound

EX. Write the balanced molecular equation when acetic acid reacts w/barium hydroxide.

EX. Write the net ionic equation for the above reaction.

EX. Write the balanced molecular eq. when carbonic acid reacts w/potassium hydroxide.

EX. Write the net ionic equation for the above reaction.

Three other common anions that act as bases are the sulfide ion (S2–), the carbonate ion

2– – (CO3 ), and the bicarbonate ion (HCO3 ). All react with acids to form gases.

-- sulfide ion: reacts w/acids to form H2S(g)

e.g., __ HCl(aq) + __ Na2S(aq)

-- carbonate ion: reacts w/acids to form CO2(g)

e.g., __ HNO3(aq) + __ CaCO3(s)

-- bicarbonate ion: reacts w/acids to form CO2(g), but yields unstable H2CO3 first

e.g., __ HCl(aq) + NaHCO3(aq) then...

Oxidation-Reduction (a.k.a., Redox) Reactions In redox reactions, electrons are transferred between ions. Oxidation is the loss of electrons, so when a substance is oxidized, its charge increases. Reduction is the gain of electrons, so when a substance is reduced, its charge decreases.

+ 2+ Mg(s) + 2 H (aq) Mg (aq) + H2(g) “OIL RIG” 4 Whenever one substance is oxidized, another is reduced. Rules for Assigning Oxidation Numbers 1. Atoms in their elemental form have an oxidation number of zero. 2. For a monatomic ion, the oxidation number is the charge on the ion. 3. Nonmetals can have variable oxidation numbers.

2– a. Oxygen is usually 2–, but in the peroxide ion (O2 ) it is 1–. b. Hydrogen is 1+ when bonded to nonmetals, 1– when bonded to metals. c. Fluorine is 1–. The other halogens are usually 1– in binary compounds, but are + whenever combined with oxygen. 4. The sum of the oxidation numbers in a neutral compound is zero.

EX. Determine the oxidation number for sulfur in each of the following.

2– H2S S8 SCl2 Na2SO3 SO4

EX. Determine the oxidation number for nitrogen in each of the following.

– N2O4 NH3 N2 NO3

Displacement reactions have the following form:

EX. Write balanced molecular and net ionic equations for the reaction between calcium and hydrochloric acid.

The activity series is list of metals. At the top of the list are the active metals, so called because they are highly reactive. At the bottom are the not-so-reactive noble metals. -- Any metal on the list can be oxidized by ions of the elements below it. -- An ion of any metal on the list can be reduced by the elements above it. The activity series is readily available in standard references.

EX. Which of the following metals will reduce PbCO3? Ag Mg Hg

EX. Which of the following metals will be reduced by PbCl4? Pt Fe Mn Molarity A solution’s concentration tells us the amount of solute per solvent. A common unit of concentration is molarity. -- equation:

EX. What mass of magnesium nitrite is needed to make 3.25 L of a 0.35 M solution?

Steps for Properly Mixing an Aqueous Solution 1. Fill an appropriate container (e.g., graduated cylinder or volumetric flask) mostly full of water (~80% full). This is an approximate technique and should take very little time. 2. Weigh out the proper amount of solute and mix it into the water from Step 1. 3. “Top off” the solution to the proper volume and mix. DONE.

EX. What is the concentration of Na+ ions in a 0.025 M solution of sodium phosphate?

Dilutions Acids (and sometimes bases) are often purchased in concentrated form and are easily diluted to any desired concentration. A purchased bottle of acid is sometimes called a concentrate or a stock solution. -- **Safety Tip: When diluting, add acid or base to water, not the other way around.

Dilution Equation:

EX. Concentrated phosphoric acid is 14.8 M. What volume of concentrate is required to

make 25.0 L of 0.500 M H3PO4? 6 Solution Stoichiometry

EX. What volume of 0.150 M sulfuric acid is needed to neutralize 26 g sodium hydroxide?

EX. What mass of lead (II) nitrate will completely react with 85.0 mL of 0.45 M NaI?

Titrations If we don’t know a solution’s concentration, we can use a second solution of known concentration – called a standard solution – to react with the first. If we know the stoichiometry of the reaction, we can determine the initial solution’s concentration. -- This procedure is called a titration.

The equivalence point of a titration occurs when stoichiometrically equivalent quantities are brought together in a titration. In an acid-base titration, this point is identified by using 7 8 indicators, which are chemicals that change color based on the pH. A color transition indicates the end point of the titration, which usually coincides very closely with the equivalence point, and is usually considered to be “good enough.” EX. If 56.0 mL of sodium hydroxide neutralize 19.0 mL of 0.235 M nitric acid, find the concentration of the base.

EX. A sample of an iron ore is dissolved in acid, and the iron is converted to Fe2+. The

– sample is then titrated with 47.20 mL of 0.02240 M MnO4 solution. The reaction during titration is:

– 2+ + 2+ 3+ MnO4 (aq) + 5 Fe (aq) + 8 H (aq) Mn (aq) + 5 Fe (aq) + 4 H2O(l)

(a) How many moles of permanganate ion were added to the solution?

(b) How many moles of the ferrous ion were in the sample?

(c) How many grams of iron were in the sample?

(d) The sample’s mass was 0.8890 g. What was the percent iron in the sample?