Stoichiometry, continued

How much N₂ and H₂ to make 7.500 g of NH₃?

We need the balanced reaction equation

N₂ + 3 H₂ ® 2 NH₃

FW N₂ = 2 ´ 14.01 = 28.02 g/mol
FW H₂ = 2 ´ 1.008 = 2.016 g/mol
FW NH₃ = 14.01 + 3 ´ 1.008 = 17.03 g/mol

grams H₂

Does mass balance?

More Stoichiometry - Variations on a Theme

Percent Yield Problems

Sometimes we don't get all of the product out of a reaction that we calculate we should.

We can use these calculations, in conjunction with actual grams obtained, to calculate percent yield.

Thus the actual yield may be less than the calculated or theoretical yield.

For example, consider the reaction:

CH₃COOH + NaOH ® Na⁺CH₃COO- + H₂O

CH₃COOH = HAc [acetic acid], and
Na⁺CH₃COO- = NaAc [sodium acetate]).

HAc + NaOH ® NaAc + H₂O

(In other words, some poor slob went into the lab, started with 100.0 g of HAc and when he was finished he measured how much NaAc he got and found that it was 115.0 g of NaAc.)

Did he get all the NaAc he could?

If not, what was the percent yield?

We know what the actual yield was 115.0 g of NaAc.

What is the calculated yield? We use stoichiometry to find out.

(Lets assume that we had more than enough NaOH.)

,

,

(The formula weight of NaAc is 82.04 g/mol.)

1.665 mol NaAc ´ 82.04 g/mol = 136.6 g NaAc

Notice that in this problem we solved a regular stoichiometry problem, but that we added an extra step – finding the percent yield.
 

Limiting reagent

Sometimes we don't have quite enough of one reagent.

New experiment

Suppose in this experiment we only had 10.00 g of NaOH to react with our 100.0 g of HAc.

Would the 10.00 g of NaOH be enough to use up all 100.0 g of HAc we have? (Recall that 100.0 g of HAc is 1.665 mol of HAc.)

NaOH = 40.00 g/mol

We say that the NaOH is the limiting reagent. That is, we can't make any more NaAc than provided by the amount of NaOH (in this case).

0.2500 mol NaAc ´ 82.04 g/mol = 20.51 g NaAc

If you are given specified amounts of more than one reagent, you have to determine which is the limiting reagent.

Notice that in this (limiting reagent) problem we had to use the stoichiometry roadmap twice.
 
 

Properties of Aqueous Solutions

Water is the most abundant liquid in the earth’s crust. It is not surprising that an enormous number of reactions and processes take place in water solution.

Before we talk about water (aqueous) solutions, which are special in many ways, let's talk a little about solutions in general.

Solutions are members of the class of substances we called mixtures. Recall that there were two categories of mixtures:

homogeneous mixtures and

inhomogeneous mixtures.

Solutions

Add sugar or salt to water - stir - the sugar or salt seems to disappear

We say the sugar or salt dissolved to form a solution.

Some definitions:

solvent = the major component

solute = the minor component

Main characteristics of a solution

1) Homogeneous

2) Stable

3) Can't be separated by filtration

4) Continuously variable composition

5) Usually transparent

6) Can be separated (distillation, etc)

Solvent             Solute             Examples

gas                   gas                  air

(We usually don't think of gas mixtures as solutions, but they satisfy all the criteria.)

liquid                gas                 carbonated water

liquid                liquid              vodka, engine coolant

liquid                solid               sea water
                                             (champagne is all three)

Solvent             Solute             Examples

solid                 gas                 H₂ in Pt or Ir

solid                 solid               alloys

Water solutions are special because water will dissolve some ionic compounds to give ions in solution.

Water soluble ionic compounds dissociate in water to give aqueous ions.

For example, in water:

HCl(aq) ® H⁺(aq) + Cl- (aq)

AgNO₃(aq) ® Ag⁺(aq) + NO₃^-(aq)

Na₂SO₄(aq) ® 2 Na⁺(aq) + SO₄^2-(aq)

NaOH(aq) ® Na⁺(aq) + OH^- (aq)

Al₂(SO₄)₃(aq) ® 2 Al³⁺(aq) + 3 SO₄^2-(aq)

Na₂CO₃(aq) ® 2 Na⁺(aq) + CO₃^2-(aq)

If you can get enough ions in a water solution the solution will conduct electricity.

Ionic compounds whose aqueous solutions conduct electric are called electrolytes.

We distinguish between strong electrolytes and weak electrolytes.

Consider the reaction of HCl and NaOH in water solution:

H⁺(aq) + Cl^- (aq) + Na⁺(aq) + OH^- (aq)® H₂O(l) + Na⁺(aq) + Cl^- (aq)

Notice that the interesting part is:

H⁺(aq) + OH- (aq) ® H₂O(l)

2 HCl(aq) + Na₂CO₃(aq) ® CO₂(g) + H₂O(l) + 2 NaCl(aq)

® CO₂(g) + H₂O(l) + 2 Na⁺(aq) + 2 Cl^- (aq)

2 H⁺(aq) + CO₃^2-(aq) ® CO₂(g) + H₂O(l)

The reaction that is left after we drop the spectator ions is called the net ionic reaction or sometimes just the net reaction.

There always has to be spectator ions in an ionic reaction, but we don't always have to know what they are.

How do we know which ions to drop and which to keep?

Drop:

ions that appear on both sides

insoluble ionic compounds (like AgCl)

gases (like CO₂)

water

nonionic compounds

There is a set of general solubility rules for ionic compounds in water on page 184 of the text.

A summary of these rules is:

Na⁺, K⁺, NH₄⁺ compounds all soluble

NO₃^-compounds all soluble

Cl^- , SO₄^2-compounds soluble except AgCl, BaSO₄, PbSO₄

CO₃^2-, PO₄^3-, S^2-, OH^- , O^2-compounds insoluble except Ba(OH)₂ (and, of course, compounds with Na⁺, K⁺, and NH₄⁺).