Types of Chemical Reactions and Solution Stoichiometry Chapter 4 of AP chemistry
4.1 Water (the universal solvent) • Water can be used to dissolve a wide variety of substances. This is in large part due to the polarity of the water molecule. • Let’s look at the structure of water
Polarity • This polarity results in water being able to “attack” the cations and the anions of ionic compounds.
4.2 Strong, weak, and non-electrolytes • Ions present in solution facilitate the conduction of electricity. • Since pure water is a covalent molecule, it is a very poor conductor of electricity. • Tap water conducts electricity because it contains dissolved ions. • If you don’t believe me, ask yourself why there’s a multi-million dollar water softening industry, or wikipedia “hard water”.
Strong electrolytes, focus on acids • A strong electrolyte is an acid, base, or ionic compound that dissolves fully. • Examples of strong acids (strong electrolytes) • Perchloric acid HClO4 • Hydroiodic acid HI • Hydrobromic acid HBr • Hydrochloric acid HCl • Sulfuric acid H2SO4 (first dissociation only) • Nitric acid HNO3 I cut and pasted this list from wikipedia.
What’s an acid • You may have notices that the strong acids were simply ionic compounds who’s cation are hydrogen. That’s an acid in a nutshell. • If you want a technical definition feel free to click here. • Acids release H+ ions (protons). These protons tend to steal electrons from other compounds, or metals and are attracted to regions of negative charge (duh).
Strong electrolytes, focus on bases • Examples of strong bases (strong electrolytes) • Potassium hydroxide (KOH) • Barium hydroxide (Ba(OH)2) • Cesium hydroxide (CsOH) • Sodium hydroxide (NaOH) • Strontium hydroxide (Sr(OH)2) • Calcium hydroxide (Ca(OH)2) • Lithium hydroxide (LiOH) • Rubidium hydroxide (RbOH) I cut and pasted this list from wikipedia.
What’s a base • A base is the counter part to the acid. A base is an ionic compound who’s anion is OH-. • If you want a technical definition feel free to click here. • Bases produce OH- ions. The OH- tend accept/steal protons (H+) from compounds and are attracted to regions of positive charge (duh).
Weak electrolytes • These compounds do not dissolve completely. • When tested use this rule of thumb: If it’s an acid or a base and it’s not strong, it’s a weak electrolyte. • Click here for a short and easy list of strong and weak electrolytes. It’s a good page!!! • For now if an ionic compound is soluble think of it as a strong electrolyte.
Non-electrolytes • Non-electrolytes do not form ions in water and hence do not permit the water to conduct electricity. • Common non-electrolytes include • Sucrose • Ethanol • Methanol • Pretty much all substances that remain covalent molecules when dissolved in water.
4.3 Composition of Solutions • The concentration of a solution is very important. • The more sugar you dissolve in sweat tea the better it will taste. • Concentration is expressed in “molarity” M • NOTE: Liters, not ml, of solution
Example • How many grams of NaOH would you need to add to 100.0 ml volumetric flask to make a 0.100 M NaOH solution? Given: V = 100.0 ml M = 0.100 M NaOH
Example Given: V = 100.0 ml M = 0.100 M NaOH Want: Grams of solute • First solve for moles of solute • Next, convert moles to grams
Examples problems • Find the molarity of a solution composed of 85.84 g of CuSO4 in 250.0 ml of solution • If 450.0 ml of 2.000 M CaCO3were evaporated, how many grams of CaCO3 would be left behind? • A 10.00 ml volumetric flask weighs 25.18 g. A NaCl solution of unknown molarity is filled to the line and then completely evaporated. If 28.62 g remain what was the molarity? Answers: 1) 2.151M 2) 90.08g CaCO3 3) 5.886M
Dilutions • This isn’t hard at all. Simply follow the equation below M1V1 = M2V2 • Example: If you take 300. ml of a 3.00 M AgNO3 and add 700. ml to it what is the new molarity?
4.5 Precipitation reactions • A precipitation reaction occurs when two soluble ionic solutions are mixed and a precipitate is formed. • Example: Both NaCl and AgNO3 are soluble in water. However, if you mix the two a precipitate of AgCl will form. This is express as: • The (aq) means aqueous (dissolved, still in solution) • The (s) stands for solid (precipitate)
Double replacement • Virtually all precipitation reactions are double replacement reactions. • This is when cations and anions simply swap partners.
4.6 Net ionic equations • Take the example below: • The complete ionic equation is below (it’s long):
4.6 Net ionic equations • Writing the complete ionic equation is awful! • Note that NH4+ and Cl- appear on both sides of the reaction and do nothing. Because they do nothing they’re called spectator ions. • In the net ionic equation you simply leave out the spectator ions: • This is much easier to read and write.
Solubility rules • It’s awful, but you’re just going to need to memorize the solubility rules. • Check out this matching game. • To help this concept sink in, we’ll make nearly 100 mixtures. • YOU NEED TO HAVE THESE RULE MEMORIZED!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! • Play the game, do whatever, but memorize them! (pg 152 has the rules)
You also need to do stoichiometry • Example if you mix 325 ml 3.00M Ba(OH)2 and 600. ml of 1.00 M Al(NO3)3 what precipitate would form? What mass? • First, write the net ionic equation • Next, determine the moles of each ion
# of moles • Multiply the OH- by three because each mole of Al(OH)3 has three moles of OH-bonded to it. • Now find the limiting reagent. Recall the net ionic equation:
Limiting regent • YOU STILL NEED TO DIVIDE BY THE COEFECENT • Aluminum is the limiting reagent. Only 0.600 moles of Al(OH)3 will be produced.
Finished • If you know that 0.600 mole of Al(OH)3 will be produced simply go from moles to grams. • And you’re done
Sample problems • If you mix 10.00 ml 18.00M H2SO4 and 600. ml of 1.00 M Ba(NO3)2 what precipitate would form? What mass? • If you mix 1.25 L 0.50M FeCl3 and 100. ml of 6.00 M Ba(OH)2 what precipitate would form? What mass?
4.8 Acid-Base reactions • Acids produce H+ ions • They donate protons • Bases produce OH- ions • They accept protons • When an acid and base are mixed they produce water. See below:
Strong bases and acids • A strong base, like NaOH, will strip a strong or weak acid of all it’s protons. • Example: If H3PO4 (weak acid) is mixed with an excess of NaOH what will occur? • Under normal circumstances H3PO4 would not disassociate completely into H+ and H2PO4-. Only a small % would disassociate. • However, as soon as a proton leaves H3PO4 it immediately is attacked by the OH- from NaOH.
Neutralization • The OH- ions will continue to grab up H+ ions until the OH- ions run out or until the H+ ions run out. • The second an H+ ion breaks off of the H3PO4 an OH- ion gets it and becomes water.
Neutralization • Example: How much 1.00 standard NaOH would be required to neutralize 50.0 ml of 18.00 M sulfuric acid? • First write the equation: • Next, solve for moles of H2SO4:
Neutralization • Now solve the stoichiometry for mole NaOH: • Finally, solve for the volume of NaOH:
Measuring acid content, pH • The standard way that acidity is measured is on the pH scale. • A pH of 7 in neutral. • 1 is very acidic • 14 is very basic • pH is measured with litmus paper or with a pH meter. • We’re going to use a pH meter in lab today
Measuring acid content, indicators • pH can also be measure using indicators. • An indicator is a chemical that will change color when a specific pH is reached. • The indicator that we are using in lab today is called phenolphthalein. • When the pH exceeds 8.2 (more or less neutral) the color will change from clear to fuchsia.
Measuring acid content, indicators • When your phenolphthalein turns this color you know that you’ve arrived. • Watch out, it comes quick
We’re going to lab!!! • It has been said that soft drinks contain a lot of acid. But exactly how much acid is in there? • Ever wondered? Well, regardless, you’re about to find out. Because… We're going to lab!!!