1. Unit 4 Chemical Quantities and Aqueous Reactions

2. The arithmetic of equations
According to the law of conservation of matter, matter cannot be created nor destroyed.
Reactants = products!!!!
When you know one quantity in a reaction, you can calculate the other quantities. This is STOICHIOMETRY!
Stoichiometry: The calculations of quantities in chemical reactions

3. Interpreting Chemical Equations
What kind of information can you get from an equation?
N2(g) + 3H2(g) => 2NH3(g)
1. Number of Particles!
One molecule of nitrogen reacts with three molecules of hydrogen to produce 2 molecules of ammonia!
The ratio for this reaction is always 1:3:2
So if you have 10 molecules of nitrogen, you would need 30 molecules of hydrogen and you would expect 20 molecules of ammonia to be formed.
But: can you count individual molecules? NO. So, use Avogadro’s number in the same ratios

4. What else?
2. MOLES: you know that Avogadro’s number of representative particles = 1 mole. So, in the previous equation, you could relate the ratio to moles!
1 mole nitrogen molecules + 3 moles hydrogen molecules = 2 moles ammonia molecules

5. What else?
3. MASS: a balanced chemical equation MUST obey the law of conservation of mass!
Soooo: the mass of 1 mole of nitrogen gas (28.0g) + 3 mole of hydrogen gas (6.0 g) must equal 2 mole of ammonia (34.0 g). IT DOES!

6. Chemical Calculations!
Mole-Mole Calculations: use the mole ratios (COEFFICIENTS) to compare moles of different substances
Your equation MUST BE BALANCED!!!

7. Let’s Try It!
How many moles of ammonia are produced when 0.60 mol of nitrogen react with hydrogen?
WRITE A BALANCED EQUATION
Determine the mole ratio from coefficients
Solve for ammonia

8. Mass-Mass Calculations
Once you have the mole ratio, you can go from mass to mass.
1. convert starting mass to moles
2. use mole ratio to find moles of unknown
3. convert to grams of unknown
USE THE PERIODIC TABLE TO GO FROM MOLES TO GRAMS AND GRAMS TO MOLES!!!!

9. Let’s Try it!
Calculate the number of grams of NH3 produced by the reaction of 5.4 g of hydrogen with an excess of nitrogen.
1. WRITE A BALANCED EQUATION!
2. Calculate the moles of hydrogen (PERIODIC TABLE)
3. Use mole ratio (coefficients) to determine number of moles of ammonia
4. Convert to grams of ammonia (PERIODIC TABLE)

10. What else?
Once you understand mole ratios, you can perform ANY necessary conversions!
Mole to gram? Periodic Table
Mole to molecule/formula unit/atom? Avogadro’s number
Always start with what you know and convert to moles!

11. Let’s Try It!
How many molecules of oxygen are produced when a sample of 29.2 g of water is decomposed by electrolysis.
1. WRITE BALANCED EQUATION
2. Write what you know
3. convert to MOLES
4. Use Mole Ratio!
5. Convert to what you want

12. Limiting Reagent and Percent Yield
What is a limiting reagent?
Limiting reagent: limits or determines the amount of product that can be formed in a reaction.
Think of baking cookies. You need 2 eggs. If you have 3 eggs, can you bake cookies? What if you only have 1 egg? Number of eggs limits the amount of cookies you can make

13. What if you have unlimited eggs?
Excess reagent: the reactant that is not completely used up in the reaction (some will be left over)

14. What is the limiting reagent? Calculate the moles of water produced
More practice
If 2.70 mol of C2H4 is reacted with mol O2 in a combustion reaction
What is the limiting reagent?
Calculate the moles of water produced

15. Oh, there’s more!
You know how to do stoichiometry now, so you won’t always be given the starting value in moles. In fact, you usually start with grams in a laboratory setting.
1. Write what you know
2. Convert to moles
3. Compare compounds (molar ratio)
4. Determine limiting reagent

16. Let’s try it! 2 Cu + S => Cu2S
What is the limiting reagent when 80.0 g Cu react with g S?
What is the maximum number of grams of Cu2S that can be formed?
1. convert grams to moles
2. compare compound to determine limiting reagent
3. Start with limiting reagent and convert to moles
4. compare moles (molar ratio)
5. convert to grams

17. Percent Yield
Does a reaction always produce the amount of products you expect? NO. There is always some error.
Theoretical Yield: maximum amount of products that could be formed from the given amount of reactants
Actual Yield: the amount of product that is actually formed.
Percent Yield= actual yield x 100%
Theoretical yield

18. Why is actual yield less than theoretical yield?
Human error, impure reactants, transferring products, competing side reactions

19. How to Calculate
1. Using Stoichiometry, find what the theoretical yield is (if you start with grams, go to moles, compare moles, go back to grams)
2. Divide actual yield (given in problem) by theoretical yield (what you just calculated) and multiply by 100 to get a percent

20. Let’s try it!
When 84.8 g of iron (III) oxide reacts with an excess of carbon monoxide, 54.3 g of iron is produced.
1. What do you know? g iron (III) oxide
2. What do you want to know? Theoretical mass of Iron produced (STOICH)

21. Solution Concentration and Solution Stoichiometry
Solubility: how much of a solute can be dissolved in a given amount of solvent at a GIVEN TEMPERATURE
Saturated Solution: contains the maximum amount of solute for a given amount of solvent
Unsaturated Solution: less solute than a saturated solution
What will happen if you add more solute to a saturated solution??? WHY????

22. Molarity!
Concentration: a measure of the amount of solute dissolved in a solution
Dilute: low concentration of solute
Concentrated: high concentration of solute
MOLARITY (M)=
number of moles of solute /liters of solution

23. A saline solution contains 0. 90 g NaCl in exactly 100 ml of solution
A saline solution contains 0.90 g NaCl in exactly 100 ml of solution. What is the molarity of the solution?
What do you know?
0.90 g NaCl
100 ml solution
Molarity = moles of solute / liters solution
YOU MUST CONVERT!

24. How many moles of solute are present in 1.5L of a 0.24M Na2SO4?
What do you know?
1.5 L of solvent
Molarity = 0.24M
Molarity = moles of solute/Liters of solvent

25. You can make DILUTIONS!
Sometimes you need to dilute a solution to use it. (ex. ACID in the lab)
M1 x V1 = M2 x V2
volume must be in liters! (because molarity is moles/liter)
Stock solutions: concentrated forms of solutions

26. How many milliliters of stock solution of 2
How many milliliters of stock solution of M MgSO4 would you need to prepare ml of M MgSO4
M1 x V1 = M2 x V2
M1 = 2.00 M
V1 = ?
M2= M
V2 = mL (CONVERT)

27. Solution Stoichiometry
You can use stoichiometry to go from volume reactant to volume product
Volume reactant => mol reactant => mol product => volume product

28. 2KCL(aq) + Pb(NO3)2 (aq) => PbCl2(s) + 2 KNO3 (aq)
Let’s try it
What volume (in liters) of M KCl solution is required to completely react with L of a M Pb(NO3)2 solution according to the following balanced equation
2KCL(aq) + Pb(NO3)2 (aq) => PbCl2(s) + 2 KNO3 (aq)

29. Types of Aqueous Solutions and Solubility
Electrolyte and nonelectrolyte solutions
Non-electrolytes: substances that dissolve in water as intact molecules. Do not conduct electricity (generally molecular compounds)
Electrolytes: substances that dissolve in water to form solutions that conduct electricity (generally ionic compounds)
Dissociation: breaking into ions
Strong electrolytes = completely dissociate
Weak electrolytes = only partially dissociate

30. ACIDS
Acids are molecular compounds that will dissociate in water. ARE ELECTROLYTES!
Stong acids= dissociate completely (HCl)
Weak acids = partial dissociation (HC2H3O2)
Acid strength/weakness is DIFFERENT from acid concentration!!!!!
You can have a concentrated weak acid or a dilute strong acid

31. Solubility
There is no set way to determine if an ionic compound is going to soluble in water. We use a SOLUBILITY CHART to determine solubility.
Think of our lab: What happened when you combined aqueous calcium chloride to aqueous sodium carbonate? FLOATY STUFF!
That floaty stuff is called a Precipitate. Precipitate reactions are when a precipitate (floaty stuff) is formed by mixing two solutions.
The product is insoluble

33. How to predict if a precipitate forms?
Write the balanced equation
Use the solubility chart to determine if the products will be soluble in water
INSOLUBLE = PRECIPITATE

34. Let’s try it! Potassium carbonate reacts with Nickel (II) chloride
Determine if a precipitate will form when:
Potassium carbonate reacts with Nickel (II) chloride
Sodium nitrate reacts with Lithium sulfate

35. Net Ionic Equations
Most ionic compounds separate into their ions when dissolved in water. This is why many compounds must be dissolved in water (aq) to react!
-Complete Ionic Equation: shows all of the free ions of a reaction
Ag+ (aq) + NO3- (aq) + Na+ (aq) + Cl-(aq) => AgCl(s) + Na+ (aq) + NO3- (aq)
-We can make this easier by CANCELLING ions that appear on both sides.
-Spectator Ions: do not directly participate in the reaction (same on both sides)
-Net Ionic Equations: leaves out the spectator ions
Ag+ (aq) + Cl- (aq) => AgCl (s)
-Single and double replacement reactions can be written this way

36. Acid -Base reactions Acid = produces H+ in aqueous solutions
Base = produces OH- in aqueous solutions
There are several different definitions for what is a base and what is an acid. This is the ARRHENIUS definition
When a strong acid and a strong base are mixed together, water and a salt (ionic compound) are formed

37. REDOX
Oxidation-reduction reactions: electrons are transferred from one reactant to the other.
Many involve the reaction of a substance with oxygen.
Examples of redox: rusting of iron, bleaching of hair
Oxidation = loss of electrons
Reduction = gain of electrons

38. Oxidation Numbers
Basically, this is the same as the charge of the ion….WHY?
When we learned to determine charge, we learned that the ion will gain or lose electrons. Basically, you already learned about oxidation (losing) and reduction (gaining) but we didn’t call it that.

40. Is it REDOX? So….how can we tell if a reaction is a redox reaction?
Determine the OXIDATION NUMBERS!
If the oxidation numbers change, its REDOX!
If the oxidation number gets more positive, OXIDATION
If the oxidation number gets less positive (or more negative) REDUCTION!