Solution = the term solution describes a system in which 1 or more substances are homogenously mixed or dissolved in another substance.

Transcription

1 1 Chemistry 047: Solution Chemistry Chemistry 047 Solution Chemistry A. Solutions and solubility B. Conductivity of aqueous solutions C. Molecular polarity and H bonding discussed in last Unit D. Polar and nonpolar solvents and the nature of solutions of ions E. Summary of bond types and how to distinguish between them F. Calculating the concentration of ions in solution G. Acids and bases and conjugate acid and base pairs H. Acids, bases, and the ph scale I. A bit about titration A. Solutions and Solubility 1. Definitions: solution Solution = the term solution describes a system in which 1 or more substances are homogenously mixed or dissolved in another substance. Solute and solvent: Simple solutions have 2 components Solute Dissolved component or least abundant component solvent dissolving agent or most abundant component example: saltwater salt water However, a solution is not just a solid dissolved in water 3 states of matter can give us 9 different types of solutions: Solid in solid Solid in gas Solid in liquid Liquid in liquid Liquid in gas Liquid in liquid Gas in solid Page 1 of 14

2 2 Chemistry 047: Solution Chemistry Gas in gas Gas in liquid 2. Properties of a true solution: 1. Mixture is homogenous, ratio of solute to solvent is variable 2. Dissolved solute, molecular or ionic in size (10-8 to 10-7 cm /0.1-1 nm) 3. Usually transparent 4. Solute won t settle out 5. Solute can be separated from solvent by physical means (i.e., evaporation, distillation etc) but can t be separated by filtering Solutions - permanent if system doesn t change (if no evaporation etc) because both solute and solvent particles are in constant random motion this is energetic enough to prevent solute from settling out. 3. Solubility a. Definitions Solubility = Amount of solute dissolved in a specified amount of solvent under standard conditions (STP = Standard Temp and Pressure) When referring to exact amount = quantitative amount - Expressed as grams/gram i.e., 36.0g NaCl in g H2O at 20 o c But often referred to in a qualitative way - i.e, substance is very soluble, slightly soluble, or insoluble Other terms for solubility of liquid in liquid: Miscible = liquids that mix and form solution alcohol and water Immiscible = do not mix and form solutions or are insoluble in each other i.e., oil and water Page 2 of 14

3 3 Chemistry 047: Solution Chemistry 4. Solubility of common ionic compounds Solubility table lists the solubilities of various compounds in each other The following table represents some General Solubility Rules Rule not a law, so lots of exceptions! Soluble in cold water Na +, K +, NH4 + Insoluble Nitrates (NO3-) Acetates (C2H3O2-) Chlorides (Cl-), Bromides (Br-), Iodides (I-) Sulfates (SO4 2- ) (Ag +, Ca 2+ are slightly soluble with sulfates) Are insoluble except with NH4 + and alkali metal cations Except with Ag +, Hg +, or Pb 2+ then they are insoluble Except with Ba 2+, Sr 2+, Pb 2+ Carbonates, phosphates, hydroxides and sulfides are insoluble except with Page 3 of 14

4 4 Chemistry 047: Solution Chemistry 4. Concentration (quantitative expression) Concentration of solution= amount of dissolved solute in a specified amount of solvent Molarity = M = mole/litre =moles solute/l of solution 5. Factors related to solubility Predicting solubility is difficult because: - Variation in : size of ions, charge of ions, interaction between ions, interaction between solvent and solute, and temperature - Therefore there are exceptions a. Nature of solute and solvent like dissolves like - Polar or ionic substances react with other polar substances - Nonpolar dissolve in nonpolar substances i.e., ionic compounds are soluble in water because water is polar Nonpolar substances form solutions easily molecules don t attract or repel each other intermingle easily Polar solutions are more complex i.e, NaCl in water Water molecules are attracted to each ion on the crystal surface and weaken attraction between Na+ and Cl- ions. Positive end of water dipole, attracted to Cl- and negative dipole end of water to Na+ ion, therefore, surface ions become surrounded by water molecules and become hydrated ions, Na+ and Cl-, and slowly diffuse away from crystal and dissolve in the solution. b. Effect of temperature on solubility Temperature affects solubility (ability to dissolve) of most substances Most have limited solubility in a specific solvent at a fixed temp For solids in liquids as temp rises, solubility usually rises (as usual no single rule for solubility of solids) However, solubility of gas in water, usually decreases as temp increases bubbles that form when water is heated are due to decreased solubility of air at higher temps (=Kinetic Molecular Theory = gas molecules form bonds with liquid as temp rises, gas energy increases therefore bubbles out) i.e, cold water has greater concentration of dissolved O2 than warm water water heating, gas bubbles form Page 4 of 14

5 5 Chemistry 047: Solution Chemistry c. Effect of pressure on solubility Small pressure changes have little effect on solids in liquids, or liquids in liquids However, there is a large effect of gas in liquid because gases are compressible Solubility of the gas is directly related to the pressure of that gas above the solution i.e, amount of gas dissolved will double if gas over solution doubles. Therefore, as pressure decreases, gas will bubble out d. Saturated vs unsaturated vs supersaturated 1) Saturated There is a limit to how much solute will dissolve in a given amount of solvent (at a standard temp). When a limit is reached the solution is saturated (If any more solute was added it would not dissolve) 2 processes happening simultaneously when a solution is saturated solid is dissolving in solution and the dissolved solute is crystallizing out Expressed as: Solute (undissolved) solute (dissolved) When these are occurring at the same rate, the amount of solute in solution is constant and equilibrium is established between dissolved and undissolved solute Therefore, a saturated solution = dissolved solute in equilibrium with undissolved. i.e, if we put 40.0g KCl in 100 g H2O at 20 o c, and 34.0g of KCl dissolved and 6.0 g is undissolved this solution would be saturated. Temperature is important because it may be saturated at one temp but not at another. 2) Unsaturated Unsaturated contains less solute per unit volume than saturated solution Meaning: there is room for more solute to dissolve in the solution 3) Supersaturated Unstable Contains more dissolved solute than needed to make a saturated solution at a certain temp Disturbances (jarring, stirring etc ) will return it to saturation level excess will crystalize out releasing heat in the process Difficult to make Page 5 of 14

6 6 Chemistry 047: Solution Chemistry Example Will a solution made by adding 2.5g of to10g of H2O be saturated or unsaturated at 20 o c given that the solubility of CuSO4 is 21g/100g H2O? (= to saying saturation is 2.1 g in 10 g) So 2.5> 2.1 so solution is saturated and 0.4g will be undissolved e. Rate (per unit time) of dissolving solids The rate at which substances dissolves depends on 4 factors: 1) Particle size - Solids dissolve only where its surface is in contact with solvent As size increases, surface area to volume ratio decreases, therefore, small crystals dissolve quicker than large ones i.e granular sugar vs sugar cube 2) Temperature- In most cases rate increases as temp increases. This is due to kinetic effects solvent molecules move faster so they hit the surface of the solid more often causes rate to increase. 3) Concentration - As concentration increases, rate of dissolving decreases When solute and solvent are first mixed, the rate of dissolving is at its max as it comes closer to saturation, rate of dissolving decreases. Dissolving becomes very slow when it gets close to saturation. 4) Agitation or stirring there is a kinetic effect. Stirring distributes the solute through the solvent If it isn t stirred, as it dissolves the more and more dissolved solute surrounds the undissolved so rate of dissolving goes down greatly if its stirred it increases exposure of undissolved solute to the solvent. B. Conductivity of aqueous solutions Remember what an ion is? a charged atom When we are looking at a solution, we can find out if it is conductive or non-conductive. It is related to the amount of ions in the solution and the phase of the solution. Phase: A solution must be melted or liquid but ions must also be present, not all melted substances break into ions. Solids do not conduct ionically (This is just talking about Ionic conduction, NOT metallic conduction!) Concentration: the more ions in solution, the greater the conductivity of the solution. Note examples in Hebden page C. Molecular polarity and Hydrogen bonds Explanations in the last unit Page 6 of 14

7 7 Chemistry 047: Solution Chemistry D. Polar and nonpolar solvents and the Nature of solutions of ions Like dissolves Like which means polar dissolves polar and nonpolar dissolves nonpolar. Solubility discussed above is talking about this. Simple version - In the case of polar dissolves polar, this makes sense, as both the molecules of the solvent and solute are polar and they will all be attracted to each other causing the atoms of the solute to be pulled apart. A nonpolar solvent would not have the pull (only has weak London forces) to be able to attract the atoms of a polar solute. Nonpolar molecules do not have positive and negative ends they only have weak London forces. However the pull of the solvents London forces are strong enough to attract the weak London forces of nonpolar solutes. Short story Like dissolves like. Some definitions: Disassociation = reaction that involves ions being separated from an ionic solid in this case, ions already exist in the ionic solid and the reaction just pulls them apart NaCl Na + + Cl - Ionization = is a reaction that involves a neutral molecule into ions that did not exist before putting the molecule in the solvent. CH3COOH CH3COO - + H + **both these reactions produce electrically conducting ionic solutions (you don t need to be able to predict which reaction will happen) E. Summary of Bond types and how to distinguish between them: (Hebden pg 208) Ionic bond compound formed from a metal and non-metal or polyatomic ions. Covalent bond compound formed between 2 nonmetals or non-metal and hydrogen. Page 7 of 14

8 8 Chemistry 047: Solution Chemistry Hydrogen bond attraction BETWEEN compounds between H on one molecule and N,O, or F of another molecule. Dipole-dipole attraction between compounds due to asymmetric molecule London Force attraction between compounds all have London forces it is the weakest if there is no Hydrogen bond or Dipole force there is London force. F. Calculating concentrations of ions in solution Reminder from mole concept unit: Solutions are a reaction medium - Many chemical reactions will not occur unless they are put into a solution -they have no way of coming into contact with each other in such a way that would allow them to react Molecules or ions must collide to react if ions are locked inside a crystal structure, they cannot react with other ions When crystal broken down when dissolved ions can move and react Solutions can also act as a diluting agent if reaction is too violent This set of square brackets means concentration [ ] When calculating concentration of ions in a solution the simplest way is to look at the relative amounts of one ion versus the other versus the original solid: Example: what is the molar concentration of chloride ions in 0.25 M AlCl3 AlCl3 (s) Al 3+ (aq) + 3 Cl - (aq) There is 1 molecule of AlCl3 to 1 molecule of Al 3+ to 3 molecules of Cl - so the concentration of Cl ions is 3 times that of AlCl3 Just have to multiply 0.25M x 3 = 0.75 M Cl - So concentration of Chloride ions is 0.75M Page 8 of 14

9 9 Chemistry 047: Solution Chemistry Example: What is the concentration of each type of ion in a solution made by mixing 50.0ml of 0.240M AlBr3 and 25.0ml of 0.300M CaBr2? First calculate new dilution: (remember M means mol/1l) mol (0.050L) = 0.012mol/0.075L = 0.160M AlBr3 1L mol (0.025L) = mol/0.075L = 0.100M CaBr2 1L Now calculate concentration of ions in reactions: AlBr3 Al Br M 0.160M 3(0.160M) = 0.480M Where concentration of bromine ions is 3 times the amount of aluminum ions CaBr2 Ca Br M 0.100M 2(0.100M)=0.200M Where concentration of bromine ions is 2 times that of calcium ions. G. Acids and bases and conjugate acid base pairs We will start out this section with an example of a reaction between an acid and water: HF (aq) + H2O H3O + + F - In this reaction, when HF is put into water, it will dissociate into a H + which will be accepted by the water to form a hydronium H3O + (H + is also referred to as a proton remember a H only has 1 proton and no neutrons and H + also has no electrons therefore, is simply a proton) and a F - ion. This reaction is reversible and will go back and forth. Therefore, these reactants and products can be paired: In one direction, there is a substance that loses a proton (H+), then when the reaction reverses, that substance can gain a proton (H+) and become an acid again; HF and F - are called a conjugate acid base pair. Page 9 of 14

10 10 Chemistry 047: Solution Chemistry The HF loses a proton (H + ) and becomes F - HF (aq) + H2O H3O + + F - And in the reverse direction, F - will gain a proton (H+) Look at the other pair in the equation, the same thing is happening except reverse: In this direction, the H2O gains a proton (H + ) and becomes a hydronium ion HF (aq) + H2O H3O + + F - And in the reverse direction, the H3O + will lose a proton (H + ) to become H2O In this case, H2O and H3O + are also called a conjugate acid-base pair. These pairs have a special relationship where you can go back and forth between them just by either gaining or losing one proton (H + ) By the name, conjugate acid-base pair, you can guess that one of the pair will be considered an acid and the other will be considered a base. Two guys, Bronsted and Lowry, both independently came up with the definition of which is considered an acid and which is a base The acid is anything that will give away (donate) one H + The base is anything that will accept one H + So in our above example: The HF is the conjugate acid because it will give away a proton (H + ) HF (aq) + H2O H3O + + F - Whereas, the F- is the conjugate base because it will accept a proton (H + ) Page 10 of 14

11 11 Chemistry 047: Solution Chemistry The same with the other conjugate acid-base pair: the H2O gains a proton (H + ) so it is the conjugate base HF (aq) + H2O H3O + + F - And the H3O + is the conjugate acid because it will give up a proton (H + ) Note!! If you gain or lose more than one proton (H+), these ARE NOT considered conjugate acid base pairs this reaction only applies if there is ONE proton gained or lost. H. Acids, bases and the ph scale According to our definitions above, an acid is a substance that donates a hydrogen ion and a base is a substance that accepts a hydrogen ion. Therefore, you can see that acids and bases all have to do with hydrogen ions. In water, a small number of the molecules will split apart and when they do, they become a OH - ion (hydroxide ion which is considered the base because in the reverse reaction it would accept an H + ) and the H+ ion that split off will join another H2O molecule and become a H3O + ion (called a hydronium ion which would give away a proton and is, therefore, the acid). For simplicity sake, we simplify this and refer to the H3O + ion simply as an H + ion. In pure water, there is an equal number of hydrogen ions and hydroxide ions, therefore it is considered neither acid nor base it is considered neutral. How acid (relative amount of H+ions) or base a solution is, is measured on a scale called a ph scale (ph = potential hydrogen). It is a logarithmic scale because it deals with huge numbers which are based on the H + ion concentration relative to pure water. An example of the size of numbers the ph scale deals with (and hence why it is calculated as a logarithmic scale): a strong acidic solution can have one hundred million million, or one hundred trillion (100,000,000,000,000) times more hydrogen Page 11 of 14

12 12 Chemistry 047: Solution Chemistry ions than a strong basic solution. On the flip side, a strong basic solution can have 100,000,000,000,000 times more hydroxide ions than a strong acidic solution 1. ph Scale: ph Value H + Concentration Relative to Pure Water Example battery acid gastric acid lemon juice, vinegar orange juice, soda tomato juice, acid rain black coffee, bananas 6 10 urine, milk 7 1 pure water sea water, eggs baking soda Great Salt Lake, milk of magnesia ammonia solution soapy water bleach, oven cleaner liquid drain cleaner 1 This and the ph scale from: Page 12 of 14

13 13 Chemistry 047: Solution Chemistry I. A bit about titration We have used molarities to convert back and forth between the moles of solute and the volume of their solutions. We can calculate the ion concentrations in solution as we did above. But what happens when we don t know the concentration (molarity) of a solution? We can use a process called titration to figure this out - this process will allow us to determine the molarity of a solution of an acid or base. In a titration, one solution (solution 1, called the titrant) is added from a buret to another solution (solution 2 - so, we use the titrant to titrate solution 2) until a chemical reaction between the two solutions has run to completion. The end point of this reaction is usually shown by a color change caused by an indicator (an indicator is a substance that does not contribute anything to the reaction other than to change color). A titration usually happens this way: A specific volume of a solution to be titrated (solution 2 described above that has an unknown concentration) is put into an Erlenmeyer flask. Then a solution (the titrant) with known concentration that will react with the solution to be titrated is added to a buret. This is set up like the diagram below. A buret is used because it allows the titrant to be added to the solution in the Erlenmeyer flask in a very controlled manner. An indicator is added to the solution to be titrated in the Erlenmeyer flask (a common indicator is phenolphthalein which changes from pinkish red to clear when a solution becomes acidic). The titrant is slowly added until the indicator changes color this is called the endpoint. Page 13 of 14

14 14 Chemistry 047: Solution Chemistry The volume of titrant added from the buret is then recorded. We can now use all our information to calculate the molarity of the unknown concentration of the solution that we titrated. The following example will be used to show this: Example: If we wanted to determine the concentration of a nitric acid solution we could put ml in an Erlenmeyer flask and titrate it with a 0.115M NaOH solution (in the buret). In this example we will say it took 18.3 ml of 0.115M solution. The following calculation can be used to calculate the molarity of the nitric acid solution we want to calculate: mol HNO3 1L HNO3 solution NaOH(aq) + HNO3(aq) NaNO3(aq) + H2O(l) First, remember that M is mol/l so convert ml to L (you can do this using dimensional analysis as part of the whole equation, but to simplify the process I will convert ml to L first) ml 0.115M NaOH solution = L of M solution 25.0 ml HNO3 = L of HNO3 Use your stoichiometry skills to convert from moles of NaOH to moles of HNO3: L NaOH ( mol NaOH 1 L NaOH ) ( 1 mol HNO3 1 mol NaOH ) = M HNO3 Page 14 of 14