are contained in 10 g of H 2 O can one get from 1 mole of O 2 O are produced from 10 g of H 2

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1 Lecture 8 Review of Stoichiometry 1) g to mol, mol to g - molar mass (g/mol) 2) mol to mol - Stoichiometric (moles/moles) Ratio of atoms (mole/mole) 3) g to g - simply a combination of the two. The key is to convert everything to moles! s How many moles of H 2 are contained in 10 g of H 2? How many moles of H 2 can one get from 1 mole of 2? 2H > 2H 2 gram A mole A mole B gram B How many grams of H 2 are produced from 10 g of H 2? ne reacts 23 g of Na with 71 g of Cl 2 in water and gets 60 g of NaCl. What is the percentage yield? Is this possible? What would you predict is going on?

2 WRKING WITH SLUTINS Reactions of solids occur quite slowly. A common way to improve their reactivity is to dissolve them in a solution. Here we develop the methodology for how to deal with reactions in solutions. Two terms commonly used to describe the components of solutions are the solute and solvent. Solute --- the substance dissolved Solvent --- the liquid the substance is dissolved in. When table salt is dissolved in water, we call the salt, the solute, and the water, the solvent. Concentration (Molarity) Perhaps the most important property when talking about solutions is the concentration. This reflects the amount of substance dissolved in a particular volume of solvent. Qualitative description When the amount of the substance dissolved is low, we call the solution dilute. When the amount of substance is high, the solution is called concentrated. Quantitative description Concentration can also be expressed quantitatively in terms of a molar concentration. Molar concentration or molarity(m) --- the moles of solute dissolved in one liter(cubic decimeter) of solution. Molarity(M) = moles of solute liters of solution To determine the molarity of a given mass of compound in a given volume of solution, simply 1) Convert the mass to moles, 2) Convert the volumes to liters 3) Divide the number of moles by the number of liters.

3 1: ne dissolves moles of NaH in 1 liter of H 2. What is the molarity of the solution? 2: ne dissolves 40.0 g of NaH (MW 40.0 g/mol) in 100 ml of water. What is the molarity of the solution? ne major advantage of expressing concentrations in terms of molarity(moles/l) is that it is easy to determine the amount(moles) of compound in a given volume of solution. (i.e. the molarity can be used as a conversion factor) 1: If you have a 1.5 M HCl solution, how many moles are in 10 ml of solution? Molarity is also useful for calculations involving the dilution of concentrated solutions. These calculations take advantage of the fact that the number of moles of solute (or salt) before dilution equals the number of moles after dilution. : moles initial = moles final I have a solution that contains one mole of NaCl.

4 I add water to the solution. The water still contains one mole of NaCl. molarity =M= moles V moles = M V moles initial = moles final M i V i = M f V f init. molarity init. vol. = final molarity final vol. Note: Dilutions are one of the few cases that we have that isn t solved with just dimensional analysis Here its best to modify the equation to get what you want and then solve for the solution. 1: You have 1 L of 1.5 M HCl solution and you add 2 L of water to it, what is the new concentration? 2: g of CuS 4 is dissolved in 1 L of H ml of this solution is added to a beaker, and then is used to prepare 1 L of a diluted CuS 4 solution. What is the concentration of the final dilute solution? Electrolytes Water is a particularly good solvent for ionic compounds. As such, many studies have been performed to understand and classify the properties of ions in water.

5 Recall, ionic substances consist of a lattice of alternating cations and anions. Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + This changes however when you dissolve the substance in water. (Salt into water) H 2 Na + Cl - H 2 H 2 Na + Cl - H 2 H 2 Na + Cl - The Na + and Cl - ions separate and interact with water. H 2 Compounds that form ions when dissolved in water are called electrolytes. Experimentally, the definition is: An electrolyte is a substance that dissolves in water to give an electrically conducting solution. (NaCl) A non-electrolyte a substance that dissolves in water to give a non-conducting or poorly conducting solution (sugar) Cl Cl + + Na Na - When a battery is connected to two electrodes in a solution, it generates positively and negatively charged electrodes. Battery The (+) charged electrode attracts the (-) charged ions The (-) charged electrode attracts (+) ions. Since the (-) charged ions can give up an electron, and the (+) charged ions can get an electron, one can get conduction. Most ionic compounds dissolve in water to give ions, while most molecules do not.

6 Types of electrolytes. 1. Strong electrolyte 2. Weak electrolyte Strong electrolyte A strong electrolyte is an electrolyte that exists in solution almost entirely as ions. s Strong electrolyte NaCl(s) --> Na + (aq) + Cl - (aq) HCl(g) --> H + (aq) + Cl - (aq) Weak electrolyte A weak electrolyte is an electrolyte that dissolves in water, but does not separate completely into ions. s Weak electrolyte NH 3 + H 2 NH 4 + (aq) + H - (aq) Another way to distinguish strong and weak electrolytes is by their reversibility. A reversible reaction is one which goes both forwards and backwards. We use the double arrows to indicate this. NH 3 + H 2 NH + 4 (aq) + H - (aq) A single arrow is used to indicate that it is irreversible. NaCl(s) Na + (aq) + Cl - (aq) What would the products of the following reactions be? NH + 4 (aq) + H - (aq) Na + (aq) + Cl - (aq)

7 Dissociation versus Ionization ne important point is that ionic compounds are not the only chemicals that form electrolytes. Some molecules also form electrolytes. When a molecule separates into ions it is called ionization. HCl - a molecular compound Ionization (molecular compound) HCl(g) --> H + (aq) + Cl - (aq) Dissociation (ionic compound) NaCl(s) --> Na + (aq) + Cl - (aq) Acids and Bases Acids and Bases are perhaps the most important electrolyte. s of acids hydrochloric acid (in stomach), HCl vinegar - acetic acid aspirin - acetylsalicylic acid vitamin C - ascorbic acid s of bases Drano, sodium hydroxide, NaH Tums, calcium hydroxide,ca(h) 2 Rolaids, magnesium hydroxide, Mg(H) 2 Definitions of Acids, Bases, and Salts An acid is a substance that produces H + ions. A base is a substance that accepts H + ions. Many, but not all, bases consist of a cation + H - (NaH). Here the hydroxide ion (H - ) can accept a proton (H + ) to produce water. Since H + ions and H - ions react to give water, H + (aq) + H - (aq) --> H 2 (l) acids and bases are said to neutralize each other.

8 s HCl --> NaH --> HCl + NaH --> Not all bases contain a hydroxide ion. NH 3 Quantitative Measure of Acidity or Basicity When an aqueous solution has a lot of H + ions it is said to be acidic. When an aqueous solution has a lot of H - ions it is said to be basic. How does one determine the acidicity of a solution? ne way is to use an acid-base indicator - a dye that changes color at different concentrations of H + (aq) and H - (aq) ions. : phenolphthalein H H H - + H + acidic - clear basic - pink Strong or Weak Acids/Bases Acids and bases are electrolytes and dissociate to different extents in water. Like other electrolytes we describe this using the terms strong and weak. Strong acid/bases ionize completely in water. HCl(aq) + H 2 (l) --> H 3 + (aq) + Cl - (aq) NaH(s) --> Na + (aq) + H - (aq)

9 Weak acid/bases partially ionized in water. HCN(aq) + H 2 (l) H 3 + (aq) + CN - (aq) NH 3 + H 2 NH + 4 (aq) + H - (aq) Below are listed several strong acids/bases and weak acids/bases. Strong Acids Weak Bases - HCl 4 Cl 4-2 H 2 S 4 S 4 - HN 3 N 3 HCl Cl - HI I - Note that the conjugate base of a strong acid is a weak base. Strong Bases LiH NaH Ca(H) 2 Ba(H) 2 Weak Acids H 2 Conversely, the conjugate acid of a strong base is a weak acid. Weak Acid Weak Base + NH 4 NH 3 CH 3 CH CH 3 C - Just because an acid/base is weak doesn t mean that its conjugate base/base is strong though. How do you know whether an acid is strong or weak. Memorize the following common strong acids and bases. Strong Acids Hydrochloric acid, HCl Hydrobromic acid, HBr Hydroiodic acid, HI - Chloric acid, HCl 3 - Perchloric acid, HCl 4 Nitric acid, HN 3 Sulfuric Acid, H 2 S 4 Strong Bases Group 1A metal hydroxides (LiH, NaH, KH, RbH, CsH) Heavy group 2A metal hydroxides [(Ca(H) 2, Sr(H) 2, Ba(H) 2 Assume to a first approximation that the rest are weak.

10 Writing Equations with Electrolytes There are three different ways to write an equation for an electrolyte. 1) Formula Unit equation 2) Total Ionic equation 3) Ionic equation Formula Unit equation The formula unit equation equation shows the complete formula of all compounds, even though they may actually exist in solution as ions. Ca(H) 2 (aq) + Na 2 C 3 (aq) --> CaC 3 (s) + 2NaCl(aq) Total Ionic equation A total ionic equation or ionic equation shows soluble substances by the formulas of the predominant species in that solution. Ca +2 (aq) + 2 H - (aq) + 2 Na + (aq) + C 3 2- (aq) --> CaC 3 (s) + 2 Na + (aq) + 2 H - (aq) Here CaC 3 precipitates and is thus listed as a solid. Rules for converting a formula unit equation to an total ionic equation: 1. Ionic compounds that dissolve into solution are normally written as ions. NaCl would be written Na + (aq) and Cl - (aq). 2. Ionic compounds that are insoluble in solution are represented by the full formula followed by (s). CaC 3 would be represented as CaC 3 (s). 3. Molecular substances that are strong electrolytes are written as ions. Thus, HCl dissolved in water is written as H + (aq) + Cl - (aq). 4. Molecular substances that are weak electrolytes or non-electrolytes are represented by their molecular formulas. CH 3 CH would be represented by CH 3 CH.

11 1. Write the total ionic equation for: HCl(aq) + NaCH 3 C(aq) NaCl(aq) + CH 3 CH(aq) H + (aq) + Cl - (aq) + Na + (aq) + CH 3 C - (aq) --> Na + (aq) + Cl - (aq) + CH 3 CH(aq) Net Ionic Equation The net ionic equation shows only the ions that change between reactants and products. The ions that do not take part in the reaction, or spectator ions, are cancelled out. Ionic Equation Ca 2+ (aq) + 2 H - (aq) + 2 Na + (aq) + C 2-3 (aq) --> CaC 3 (s) + 2 Na + (aq) + 2 H - (aq) Net Ionic Equation Ca 2+ (aq) + C 2-3 (aq) --> CaC 3 (s) The Na + and H - ions are on both sides and so are omitted. s. Given the formula unit equations, give the ionic equation and net ionic Formula unit equation: Ca(N 3 ) 2 (aq) + K 2 C 3 (aq) --> CaC 3 (s) + 2 KN 3 Ionic equation: Net ionic equation:

12 Formula Unit equation: 2HCl 4 (aq) + Na 2 C 3 (aq) --> H 2 + C 2 (g) + 2NaCl 4 (aq) Ionic equation: Net ionic equation: Note that although the molecular equations are different, the net ionic equations of the two examples are the same.