### Richard A. Paselk

Chem 107

Fundamentals of Chemistry

Fall 2009

Lecture Notes: 24 September

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# Chemical Reactions, cont.

### Net Ionic Equations, cont.

Let's look at a couple of more examples of net ionic equations:

Consider the reaction of 50.0 mL of 0.25 M hydrochloric acid with 25.0 mL of 0.50 M sodium hydroxide.

• Write a net ionic equation for this reaction:

H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) H2O + Cl-(aq) + Na+(aq)

Giving: H+ + OH- H2O

• How much acid will be left over? Base? How much water was made (synthesized)?
• First find out the moles of each: acid = (50.0 mL)(1 L/1000 mL)(0.25 mole/L) = 1.25 x 10-2moles; base = (25.0 mL)(1 L/1000 mL)(0.50 mole/L) = 1.25 x 10-2moles.
• Since the amounts are identical they will completely neutralize each other. That is they will react completely and both acid and base will be consumed with none left over.
• Since the ratio in the equation is 1:1:1 acid:base:water, have 1.25 x 10-2moles = 1.3 x 10-2moles (or 1.2 x 10-2moles) of water produced.

Consider the reaction of calcium metal with hydrochloric acid. This is similar to the reaction of sodium and water, and hydrogen gas is given off.

• Write a net ionic equation for this reaction:

H+(aq) + Cl-(aq) + Ca0(s) H2 (g) + Ca2+(aq) + Cl-(aq)

Balancing: 2 H+ + Ca0(s) H2 (g) + Ca2+

Notice that this net ionic equation also involves electron transfer, so it is a redox equation.

## Oxidation/Reduction Reactions

In these reactions we see a transfer of electrons from one atom or molecule to another. Many of these reactions come under the description of combustion reactions in your tex. First let's look at some terms.

• Oxidation refers to taking electrons away from a substance. So to oxidize means to behave like oxygen normally does and "steal" electrons.
• Reduction refers to receiving electrons - it is the opposite of oxidation.
• Note that oxidation and reduction always go together. When oxygen oxidizes it is itself reduced (it gains electrons).
• Consider the example of burning natural gas:

CH4(g) + 2O2(g) CO2(g) + 2H2O(l)

Notice that the methane is oxidized by the oxygen. We say that the carbon and hydrogen are both oxidized to give the new covalent products, water and carbon dioxide.

Examples:

• Potassium chlorate is heated and oxygen gas is given off.

KClO3(s) + heat KCl(s) + O2(g)

Balancing: 2KClO3(s) + heat 2KCl(s) + 3O2(g)

Note that the potassium ions remain unchanged, but that there is a redox reaction where the chlorate ion is broken down to chloride ion and oxygen. Note also that a net ionic equation is NOT appropriate here as the situations of all ions/atoms have changed over the two sides of the equation. Even thepotassium ions are in a new environment/situation. We will come back later and talk about details of electron "exchange" in this situation.

• Iron filings are heated in oxygen to give "rust," an oxide of iron(III).

Fe(s) + O2(g) Fe2O3(s)

Balancing: 4Fe(s) + 3O2(g) 2Fe2O3(s)

• A piece of clean copper is placed in a solution of silver nitrate-the copper in solution becomes silver-colored and the solution goes from colorless to light blue. Assuming the copper goes to Cu(II) write a balanced equation. Notice that in this case you must balance the charge in order to get the final equation.

Ag+ + NO3- + Cu0 Ag0 + NO3- + Cu2+

Balancing: 2Ag+ + Cu0 2Ag0 + Cu2+

• A piece of clean iron is placed in a solution of copper(II) sulfate-the iron becomes copper-colored and the solution goes from colorless to light blue. Assuming iron goes to Fe(III) write a balanced equation. Again the problem is to balance the charge.

Cu2+ + SO42- + Fe0 Cu0 + Fe3+ + SO42-

Balancing: 3 Cu2+ + 2 Fe0 3 Cu0 + 2 Fe3+

 FYI As another example we can look at a key oxidation reaction in glycolysis, the central pathway of metabolism. Don't worry about these reactions - they will not be on an exam. They are presented for your interest. In this case we use a biological oxidizing agent, NAD+ to take electrons (in the form of a hydride ion, H:-) Frequently organisms need to operate this reaction under anaerobic (oxygen free) conditions. For example the maximum power (energy/sec) you can get from your muscles is anaerobically. For this to occur you need to regenerate the oxidizer (NAD+) without the presence of oxygen. For this to occur we use another reaction: The mechanism for this electron transfer is shown below:

# Stoichiometry Revisited - Quantitative Chemical Equations (Chapter 6)

### Theoretical Yield Problems

Another frequent question arising in chemical processes is the percent yield. This deals with the question of how effective was a given process in producing a product. Its an important consideration because chemical reactions rarely go completely to products. The maximum possible yield for a reaction is known as the Theoretical Yield.

• Example: How many g of water will be produced during the complete combustion of 475.5 g of natural gas (methane)?

### Limiting Problems

Asking question of what is the maximum amount of something which can be produced from a given mixture of stuff. This is a fairly straight-forward sort of problem in the day-to-day world, but seems to cause a great deal of difficulty for lots of folks in chemistry. Let's start by looking at a non-chemical problem:

Consider you have to make a bunch of sandwichs for a party. The equation for the sandwichs (in slices) is:

2 Bread + 1 Cheese + 2 Meat 1 Sandwich

You have:

• two 32 oz loaves of bread with 27 slices/loaf,
• three paks of of sliced cheese with 12 slices/pak,
• 6 paks of sliced meat with 12 slices/pak.

How many sandwichs can you make?

Look at how many sandwichs can be made from each ingredient: