Chapter 13: Solutions Study Guide
Unsaturated solution
A solution that contains less than the maximum amount of dissolved solute at a given temperature
Saturated Solution
A solution that contains the maximum amount of dissolved solute at a given temperature
Normality
# of H+ ions or OH- ions per liter of solution # of H+ ions or OH- ions / L of solution
Miscible
Describes two liquids that are soluble in each other; they mix.
How are you able to rank the highest to lowest colligative property based on chemical equation
Determine the moles of particles of each. Remember that the identity of the compound does NOT play a role in any of the colligative properties, so they all have the same value if the have the same solvent. Multiply the arbitrary value of the property by number of moles of particles (based off dissociation and ionization rules).
Further thinking: how do you determine the most effective solvent for a solute?
Determine the prominent type of bonding a solute is capable of making, then determine the solvent that is capable of forming those same bonds. That is the best solvent.
What is the difference between strong and weak electrolyte?
Dissociation/ionization. Weak electrolytes do not dissociate/ionize completely, while strong ones dissociate AND ionize completely.
How does dilution itself happen?
Dissolving takes place when IMFs between the solvent and the solute are created. So substances dissolve in one another if the solute and the solvent are able to form intermolecular attractions.
The sum of the mole fractions in a solution is equal to?
1
ppm
1 part solute / 1,000,000 parts solution 1 mg of solute / 1 kg of solution
What would the equations look like with the Van't Hoff Factor?
1. Fsoln = Fsolvent - (i*Kf*m) 2. Bsoln = Bsolvent + (i*Kb*m) 3. pi = i*M*R*T
All concentration units are a ratio between solute and solvent or solute and solution. 1. Molarity ratio 2. Percent by mass ratio 3. Molality ratio 4. Mole Fraction Ratio
1. Molarity: Moles of solute / L of solution 2. Mass of solute / mass of solution x 100% 3. Molality: Moles of solute / kg of solvent 4. Moles of solute / sum of moles of solution
What are the two important details you need to remember about mole fractions
1. Mole fractions are derived from MOLES not grams, so if given grams of components, convert to moles using molar mass 2. There is no distinction between solvent and solute in this case, its just components. All substances have mole fraction calculated in the same way.
2 things to remember about ionization and dissociation
1. Polyatomic ions do NOT dissociate. PO4 is 1 mol, not 5. 2. Multiples of the same atom DO ionize. So MgCl2 has 3 mol of particles, Mg 2+, Cl-, Cl-
What is a supersaturated solution and how does it come to be?
A super saturated solution is a solution that contains more than the maximum amount of dissolved solute at a given temperature. This happens when temperature is decreased and the first crystal is unable to form, so the solute stays dissolved. This is a very temporary state and any physical disruptions will cause the crystallization process to begin.
In summary, what are the two factors that determine the ΔH of a solution (also known as hydration energy)?
1. The energy required to break bonds in the solute and solvent as well as the energy released when new bonds are formed between the solute and solvent in the solution. If more energy is required to break bonds, the process is endothermic. If more energy is released when new bonds are formed, the process is exothermic. 2. Entropy, a measure of randomness or disorder in a system is another factor. If a system increases entropy, it is more favorable but this is elaborated further later.
Molality with Complication Scenarios 1. How do we calculate molality if given volume of solute? 2. How do we calculate molality if given moles of solvent? 3. How do we calculate molality if given anything besides kg of solvent and moles of solute?
1. Use density to convert volume of solute to grams of solute and then molar mass to get moles of solute 2. Molar mass to determine grams of solvent then metric conversion to get kg 3. Use the appropriate conversion method/conversion factor to obtain the unit you want.
Immiscible
2 liquids with different IMG that do not dissolve in one another; dont mix.
The solubility of a substance is constant... True or False?
False, at a given temp only a certain amount of a solute may dissolve, but not all of it. All substances have a threshold of how much solute will dissolve in a solution at a given temp. There are different solubility values at different temperatures.
What is the relationship between pressure and solubility of gases
High Pressure=Increased Solubility of Gas. At higher pressures, more gas molecules are soluble in liquids. Soda manufacturers take advantage of this by preparing the drinks at high pressures; this makes the drinks supersaturated, the solution, because of high pressure, holds more gas particles than is soluble at that temperature. Excess gas is released when you open the can.
How can we use the molality of a solution as a conversion factor?
If given the mass of the solvent, you can use the molal concentration as a conversion factor to determine mols of solute. If given the moles of solute, you can use the molal concentration as a conversion factor to determine the mass of solvent.
How does movement of solvent take place when we talk about semipermeable membranes and solutions?
If you have a solution on side of a membrane and pure solvent on the other, there will be net flow (greater movement) towards the solution side rather than the pure solvent side. This is because the solvent moves towards the high concentration of solute in order to reduce (dilute) its concentration so that both sides are closer to equilibrium. This movement will cause the side of the solution to increase in volume as pure solvent moves across the membrane towards the solution side.
What about for VP?
So with vapor pressure, we already discussed how to determine the vapor pressure when a component is volatile, though it is different from determining VP of an electrolyte. What changes is the mole fraction. When a solute dissociates or ionizes into more constituents, you must determine the moles of each part of the solute. For example, if you have 1 mole of Ca(NO3)2, you need to multiply moles of Ca(NO3)2 by 1 for Ca and by 2 for moles of NO3. Then, for your mole fraction, moles of Ca and moles of NO3 must be added independently as Mole A + Mole B in your calculations. In summary, they are treated as independent constituents whose moles must be factored into the equation.
Why does the vapor pressure lowering, ΔP only depend on concentration of solute particles and not identity?
Solids are non volatile, meaning that they do not evaporate easily. This means that when you are looking at the vapor pressure of a solution, solids do not contribute to the total vapor pressure, so all you need to know is concentration, not identity of the solute.
With all concentration measures, what are we looking at?
Some measure of the amount of solute / some measure of the total amount of solvent or solution
Percent by Mass
The mass of the solute divided by the total mass of solution x 100% Mass (g) solute / mass solution (g) x 100%
Mole Fraction
The moles of one component / sum of all moles in the solution Mole fraction for A is: moles A / Moles A + Moles B (adding the moles of C and D if there are more components)
Osmotic pressure and the relationship between concentration of a solute and osmosis
The more concentrated the solution, the more pure solvent that will move towards the solution side to equilibrate the solution. This effect can best be observed by measuring how much pressure would be required to keep the liquid level on both sides. Essentially, the difference in volume is translated to difference in pressure= osmotic pressure. So higher concentration of solution, greater volume difference, greater pressure difference = higher osmotic pressure.
At any given temperature, the solubility of a gas in a liquid is directly proportional to what?
The partial pressure of the SAME gas above the surface of the liquid; this is known as Henry's Law.
Solvation
The process of forming solute-solvent attractions
If you have a saturated solution and you decrease the temperature, what happens?
The solution cannot hold that much solute anymore because the solute threshold will decrease with a decrease in temp, so normally, the excess solute will crystalize and present itself as a solid again. This process is called recrystallization.
Usually, when you boil a liquid, if the molecules obtain enough kinetic energy, they are able to escape into the gas phase. What happens to vapor pressure when a solute is added to a liquid?
The vapor pressure decreases, the solute requires more energy to break IMF so less molecules of gas will escape into the gas phase from a solution vs a liquid. So: Solutes disrupt the ability of the surface molecules to vaporize so if you add solute (solid) to a liquid, the vapor pressure decreases.
Raoult's Law
The vapor pressure of a volatile component (solvent) of a solution is equal to the mole fraction of the component (solvent) times its vapor pressure when it is a pure liquid. (In solution)Pz = Xz x Pz (pure liquid) Here we are talking about the vapor pressure of the solvent in the solution, though this is the same thing as the vapor pressure of the solution.
IN order to convert from one concentration type to another, what must you do? 1. Converting between grams and moles? 2. Molarity to molality 3. What is a good way to make multistep calculations?
Use the appropriate concentration units with corresponding numerator and denominator values necessary to get the old unit to the new. 1. Molar Mass 2. Density to get the denominator to grams, percent by mass to get grams of solvent, metric to get kg of solvent. 3. Choose an arbitrary amount of solution
Vapor Pressure Lowering concept and equation
Vapor pressure lowering tells us the CHANGE in vapor pressure from pure solvent to solution ΔP = P(pure solvent) - P(solution) Be sure to pay attention to the difference between vapor pressure lowering and vapor pressure of a solution when calculating
We assume first that the solutes are what? Why do we assume that?
We begin by assuming that they are non-electrolytes, that they dissolve but DONT dissociate into ions/ionize. This is so that we can perform calculations without worrying about extra particles.
Freezing Point Depression
When a liquid is frozen, the intermolecular forces of the liquid need to be rearranged to create highly ordered solid; when a solute is added, it takes interferes with this process, so the liquid cannot freeze at its normal temperature and the freezing point decreases, gets more negative, because a cooler temperature is required to make the freezing happen.
Why is that "like dissolves like?"
When two molecules that share similar intermolecular forces are combined in a solution, the new intermolecular forces formed between the solute and the solvent are strong enough to offset the intermolecular forces that must be broken in the pure solute and solvent to cause dissolving. So basically, like IMF between solvent and solute means strong IMF formation between solute and solvent in solution = large release of energy which is energetically favorable
What are the only 2 occasions during which we use the terms regarding saturation?
When we are talking about solutions made of solids and liquids or gases and liquids.
Boiling Point Elevation Concept and equation
When you add a solute to a liquid and you are boiling it, more energy is required to break the IMF between the solute so the boiling point increases. Tb (solution) = Tb (solvent) + ΔTb (boiling point elevation). ΔTb represents the difference in boiling point between the solvent in its pure state and the solution
What are the rules for like dissolves like
Perliminary step is to simply look at polarity. If asked which solvent is more likely to dissolve some solute: Ionic solutes (which form ion-dipole bonds and can basically be considered SUPER polar) are dissolved by strongly polar solvents (because those polar solvents are able to form those ion dipole bonds). So ionic or polar solutes are dissolved by polar solvents. Non-polar solutes are only capable of forming dispersion forces and can only be dissolved by non-polar solvents (which too are only capable of forming dispersion forces). Non polar solutes are dissolved by non polar solvents.
If a solution is made of two volatile substances, when we must take into consideration the partial pressure of both substances. What is the equation for vapor pressure of above a solution with two volatile components (like 2 liquids)
Ptotal = PaXa + PbXb The total pressure of the solution is equal to the first component's pure pressure times its mole fraction, plus the second component's pure pressure times its mole fraction.
Be careful not to mix up a electrolyte vs non electrolyte and solublity of like dissolves like.
Remember that ALL aqueous solutions dissolve, ofc. When we talk about like dissolves like, we are talking about what is actually going to dissolve in what. When we are deciding between electrolytic and non-electrolytic solutions, we are talking about already dissolved solutes, no need to decide if like will dissolve like. What we are distinguishing between is dissociation/ionization (electrolytic solution) and lack there of (non-electrolytic solution). Electrolyte: Strong acids, strong bases, ALL ionic compounds Weak Electrolyte: Weak acid, weak base Non-Electrolyte: Any covalent compound that is not an acid (H plus something else).
Van't Hoff Factor
i = moles of particles in solution (dissociated) / moles of solute (total compound)
Equation for osmotic pressure
pi = MRT M=molarity R=0.08206 T= Degrees Kelvin Make sure to remember that osmotic pressure needs to be in ATM because R is in atm!!!!!!!!!!!!!!!!!!
Molality (m)
the number of moles of a solute per kilogram of solvent m = moles solute / kg solvent
How to determine the two ΔT values:
ΔTf = Kf x m (molality) ΔTb = Kb x m (molality)
Semipermeable membrane and how it works
A semipermeable membrane allows solvent (typically water) to pass from one side to the other, however, solute is unable to pass.
A solution that follows Raoult's law is called what? What does it mean? What happens if you stray from this?
An ideal solution; it means that the IMF between solutes, solvents, and solute-solvents are all of equal strength. If the energy required to break bonds is greater than energy released by new bond formation (endothermic), the vapor pressure will be higher than Roult's Law because the low IMF in products makes it easy to break them=more molecules in the gas phase=higher VP. If the energy released by bond formation is greater than energy breaking, then the pressure will be lower than predicted by Roult's law because it is harder for the molecules to escape into the gas phase.
A solution is not necessarily liquid + liquid, what is the actual definition of a solution:
Any homogenous mixture between two states of matter in which one is the solute and the other is the solvent
What is the constant and accurate relationship between temperature and solubility of a gas in a liquid?
As temperature increases, the solubility of a gas will decrease. This is because when temperature increases, the kinetic energy of the molecules in the gas phase will increase and more of them can escape the liquid into a gas phase, which means that less gas particles are dissolved (decreased solubility). This is why soda goes flat are room temp and is more carbonated fresh out of the fridge.
How do colligative properties change with electrolyte solutions?
Colligative properties depend on the number of particles in a solution. With electrolytes, the dissociation of ionic compounds and ionization of strong acids and bases results in more than 1 mol of particles per compound. For example, 1 mol NaCl has 2 mols of particles, Na+ and Cl-, whereas sucrose would have 2 mol of particles bc it doesnt dissociate. So when calculating colligative properties of NaCl, it would be double that of sucrose.
What is a colligative property and what are the four described in this chapter?
Colligative property is a property of a solution that depends on the concentration of the solute but NOT the identity of those particles 1. Vapor Pressure Lowering 2. Freezing/Melting Point Depression 3. Boiling Point Elevation 4. Osmotic Pressure
What are the three steps for dissolving and what are the corresponding energy properties; how do we determine the ΔH of a solution?
In order to form a solution 1. The intermolecular forces between the solute must be broken (requires energy, endothermic) 2. The intermolecular forces between the solvent must be broken (requires energy, endothermic) 3. New intermolecular forces between the solvent and solute must be formed (exothermic) If the energy required to break IMF bonds in the solvent and solute is greater than the energy released when new interactions are made in the solution, then the solution is a positive enthalpy, endothermic. If the energy required to break IMF bonds in the pure solvent and solute is less than the energy released when the new IMF are formed in the solution, then the process is exothermic, more energy is released.
There is high similarity in molality and molarity at what?
Low concentration. In general, you should be getting numbers above 1 for molality and morality, or close to that. Like no crazy minuscule decimals, that probably means you divided with grams instead of kg for solvent in molality calculation. Be very watchful of that, the denominator is KG of solvent not grams.
So, the Concentration of a Dissolved Gas (MOLARITY)=
M (of gas)= kP k= Henry's constant for that gas, given P= the partial pressure of the same gas that is dissolved in the drink above the surface of the liquid Remember that the pressure being asked for is the pressure of the same gas above the surface of the liquid in which the gas is dissolved, not just in the air. Pay attention to words like "at atmospheric pressure," in which case the gas is in the air...
What are the six main concentration units we will pay attention to in Chem 1081?
Molarity, percent by mass, molality, mole fraction, ppm, and Normality
VAPOR COMPOSITION: Relationship in ratios between vapor pressure of components and moles of components:
Moles of A in Vapor Phase = Vapor Pressure of A Moles of B in Vapor Phase. Vapor pressure of B Xa = Pa/Total P (moles of A over total moles is equal to pressure of A over total pressure).
The book states that solids get _____ soluble as temp increases, but Perkins says what?
More, but Perkins says that this can only be determined by experimentation or by using a graph.
How can we modify the boiling point elevation, freezing point depression, and osmotic pressure equations to account for ionization of strong acids and dissociation of ionic solutes?
Multiply by the van't Hoff factor
With percent by mass, does identity of the solute or solvent matter?
No! All that matters is the mass of the solute and the mass of the total solution
What is the equation for the freezing point of a solution?
Tf (solution)= Tf solv - ΔTf (freezing point depression) ΔTf, the freezing point depression, shows how different the freezing point is between the pure solvent and solution Be VERY CAREFUL: make sure you pay attention to what you are being asked for: FREEZING POINT DEPRESSION (which measures the change) VS FREEZING POINT OF SOLUTION.
What is another way to think about percent mass? What can you do with this other way of thinking about percent mass?
The amount (in grams) of solute dissolved in 100 total grams of solution 7.2% percent by mass = 7.2 grams of solute/100 g solution It is important to think of percent by mass this way because you can then use the fraction as a conversion factor.
When calculating the boiling point or freezing point of a solution, it is essential that you pay attention what what?
The boiling or freezing point of the SOLVENT! is what you subtract ΔTf or add ΔTb to. Make sure you identify the correct substance from the equation as the solvent.
Osmosis
The flow of solvent from a solution of low concentration to a solution of higher concentration through a semipermeable membrane.
How do we determine if a solution will have positive or negative deviation given that it is not an ideal solution?
You need to look at what bonds are stronger: Between the solute-solute or solvent-solvent, OR those between solvent-solute in the solution. Positive Deviation of Vapor Pressure: IF the IMF between the solute-solute and solvent-solvent are greater than the intermolecular forces of the solvent-solute in the solution, then the energy required to break the bonds in the solution is low (weak IMF), more molecules will escape into the gas phase=Increase in Vapor Pressure. The process is endothermic bc more energy required to break bonds. Endothermic=Increase VP=Positive deviation. Negative Deviation of Vapor Pressure: The IMF between the solute-solvent in the solution is greater than the IMF between the solute-solute and solvent-solvent in the reactants. Because the IMF of the solution are stronger than the ideal, they're harder to break, less vaporization, lower VP. The process is exothermic=decrease VP=negative deviation.
