|Lecture Notes: 24 September||
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.
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.
Notice that this net ionic equation also involves electron transfer, so it is a redox equation.
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.
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.
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.
Fe(s) + O2(g) Fe2O3(s)
Balancing: 4Fe(s) + 3O2(g) 2Fe2O3(s)
Balancing: 3 Cu2+ + 2 Fe0 3 Cu0 + 2 Fe3+
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.
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.
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
- 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:
- If bread limits: (2 loaves Bread)(27 slices bread/loaf)(1 sandwich/2slices bread) = 27 sandwichs
- If cheese limits: (3 paks cheese)(12 slices cheese/pak cheese)(1 sandwich/slice cheese) = 36 sandwiches
- If meat limits: (6 paks meat)(12 slice meat/pak meat)(1 sandwich/2 slice meat) = 36 sandwiches
Bread limits and we can make 27 sandwichs.
© R A Paselk
Last modified 24 September 2009