General Chemistry II Lab Quiz 2 Labs 13, 14, 15
Helpful comments regarding rate law experiment
1. according to the procedure of this experiment, the solution will turn blue black when exactly 2 x 10^-4 mol mol of S2O8^2- has reacted. 2. The purpose of the KNO_3 solution in this reaction is to keep the reaction medium the same in each run in terms of the concentration of ions. it does not enter in the reaction in any way. 3. The reaction studied in this experiment is catalyzed by metal ions. The purpose of the drop of the EDTA solution is to minimize the effects of trace quantities of metal ion impurities that would cause spurious effects on the reaction.
How do calculate molar mass if g of solute, and grams of solvent, and freezing point or boiling point is known?
1. calculate difference in temperature from normal freezing or boiling point from measured temperature. 2. divide change in temperature by Kfp or Kbp to obtain molality. 3. Find the amount of grams over kg, and divide by molality from 2 to get molar mass of the solute.
how do you calculate change in freezing point or boiling point?
1. use gram amount of solute divided by molar mass of element to find number of moles of solute. 2. convert grams or liters of solvent to kg. 3. divide molarity of solute by kg amount to find molality. 4. multiply Kfp or Kbp by molality to get the change in temperature. 5. add or subtract from the normal freezing point to get the new freezing or boiling point.
What is the order of reaction with respect to A and B for a reaction that obeys the rate law rate = k[A]^2[B]^3?
2nd order in A, 3rd order for B, and 5th order overall.
If 2 x 10^-4 moles of S2O8^2- in 50 ml of solution is consumed in 188 seconds, what is the rate of consumption of S2O8^2-?
2x 10^-4 moles divided by .050 L multiplied by 1/188 seconds to get answer.
Helpful comments regarding rate law experiment continued
4. You will perform a few preliminary experiments to become acquainted with the observations in this experiment so that you will know what to expect in the reactions. 5. the initial concentrations of the reactants have been provided for you on the report sheet. 6. cumulative times were noted for each reappearance of blue black color.
Define volatile substance.
A volatile substance has a high vapor pressure, and a nonvolatile substance has a low vapor pressure at room temperature. Obviously, volatility is a relative term and depends upon temperature and pressure. Volatility increases with increasing temperature and decreases with increasing temperature.
Lab 13 - Colligative Properties We may give you a hypothetical experiment with a different solvent/solute system.
After dissolving a carefully measured unknown into a solvent sample of known freezing point, you then allowed the sample to cool until both solid and liquid were present and the temperature was stable. Note that the solution first cooled below the freezing point of the system, supercooled, prior to reaching equilibrium.
Know the purpose and fate of every reactant in the process. .
Also, the reaction should be 1st order in both S2O8^2- and I- though some of you got different results, that's life.
reaction rate based on temperature
As a rule of thumb, for each 10 degrees Celsius increase in temperature, the rate of reaction doubles.
how did we determine the freezing point of glacial acetic acid?
As soon as the acid began to solidif we removed the test tube from the ice water bath, and continued to stir for 5 minutes until the temperature reading was constant to determine the freezing point of glacial acetic acid.
How can the rate of reaction be determined?
Can be measured by observing the rate of disappearance of either of the reactants A and B or the rate of appearance of either of the products C and D. can measure for any species
Experiment 15 Neutralization Reaction
H3O+ (aq) + OH^- (aq) -->2 H2O (l)
Lab 13 - Colligative Properties - Part 2 We may give you a hypothetical experiment with a different solvent/solute system.
It wasnecessary to use the density of solvent (0.7726g/mL for cyclohexane 1.05g/mol for acetic acid) to calculate the mass of solvent used (denominator in molality). You determined molality via ∆T = kbm and your solvent mass. Once you have molality, you can calculate molecular weight by converting from your measured grams to your measured molality. Possible answers were benzoic acid and salicylamide (122.1 and 137.1 g/mol respectively). Remember the effect of "i" on freezing point - it's not the molality of solute, but the molality of "stuff" in the solute that counts. Your unknowns had an "i" (Van't Hoff factor) of 1. Know how to calculate molality frontwards and backwards.
balance equation of neutralization of KHP
KHC8H4O4 (aq) + NaOH (aq) --> H2O (l) + KNaC8H4O4 (aq) at equivalence point moles NaOH = moles KHP.
where does the constant value come from?
Kfb C/m or Kbp C/m comes from temperature constant of solvent NOT solute.
Labs 15, acid base #1)
Know what an equivalence point is and how you used it to both standardize an NaOH solution and check the purity of an unknown acid. You're comparing the known moles to unknown volumes to get unknown moles. The standardization is known moles of acid by weight to unknown volume of NaOH to measure its concentration. The next experiment is known NaOH that you just measuredagainst unknown acid. Y
Experiment 14 rates of chemical reactions
Purpose: to measure the effect of concentration upon the rate of the reaction of peroxydisulgate ion with iodide ion. to determine the order of the reaction with respect to the reactant concentrations and to obtain the rate law for the chemical reaction.
Lab 14: Kinetics Know all of the balanced reactions involved in the experiment:
S2O8^2- + 2I- → I_2 + 2SO4^2- persulfate oxidizes iodide 2S2O3^2- + I_2 → S4O6^2- + 2I- thiosulfate reduces iodine. I_2 + starch → blue/black complex iodine reacts with starch.
Reaction of Peroxydisulfate Ion with Iodide Ion
S2O8^2- + 2I^- --> I_2 + 2SO4^2- Iodide was oxidized. rate law was determined by measuring the amount of peroxydisulfate S2O8^2- that reacts as a function of time. rate of disappearance of S2O8^2- = k [S2O8^2-]^x [I^-]^y = delta [S2O8^2-]/ delta time you will add to the solution a small amount of another reagent sodium thiosulfate Na2S2O3 which cause the solution to change clear.
order of reaction
The powers to which the concentrations in the rate law are raised are termed the order of the reaction. in previous case, the reaction is said to be second order in A and first order in B. The overall order of the reaction is the sum of the exponents, 2+1 = 3, or a third order reaction.
what are three colligative properties?
boiling point, freezing point, and vapor pressure. they are called colligative properties because they are related to to the number and energy of collisions between particles and not to what the particles are.
rate of disappearance of a or rate of appearance of c
calculate change in concentration of a or c/time required for change
phenolphtalein indicator solution
causes solution to change color at end point, in theory, equivalence point and end point should coincide.
rate law
determining values of x and y. suppose we found that x= 2 and y = 1 for this reaction. doubling the concentration of B while keeping A the same would cause the reaction rate to double. On the other hand, doubling the concentration of A, keeping B the same would cause the rate to increase by a factor of 4, because the rate of the reaction is proportion to the square of the concentration of A.
vapor pressure
escaping tendency of the solvent molecules.
colligative-property law
freezing point and boiling point of a solution differ from those of the pure solvent by amounts that are directly proportional to the molal concentration of the solute.
Van't Hoff factor
how many ions one unit of a substance will dissolve in solution
Supercooling
involves the lowering of the temperature of a substance below its normal freezing point without the solidification of the substance. Supercooling can be minimized by cooling slowly with rapid stirring.
define rate
k [A]^x[B}^y where A and B are the molar concentrations of A and B, x and y are the powers to which the respective concentrations must be raised to describe the rate, and k is the specific rate constant.
value of k
k is the specific rate constant, has a definite value that is independent of the concentration. It is characteristic of a given reaction and depends only on temperature. Once the rate is known, the value of k can be calculated.
It is found for the reaction Z+B --> C that doubling the concentration of either A or B quadruples the rate of the reaction. Write the rate law for this reaction.
k[A]^2[B]^2
What is the general form of a rate law?
k[A]^x[B]^y
how can you calculate mass of cyclohexane?
multiple ml amount by density g/ml
Labs 15, acid base #1 part 2
ou then assume that moles of acid = moles of base at the equivalence point watch out for polyprotic species, wherein normality does not equal molarity! Given a certain concentration and volume of acid or base, know how much volume you need of acid or base needed to reach the equivalence point. For the unknown, your indicated moles of acid is compared against the weight of the acid to determine %purity.
Why are chemists concerned with the rates of chemical reactions? What possible practical value does this type of information have?
practical value of synthesis of new compounds.
standardization
process of determining concentration of solution.
define colligative properties
properties are independent of the nature of the solute and depend only upon the solute concentration. include vapor pressure lowering, boiling point elevation, freezing point lowering, and osmotic pressure.
Reaction of Peroxydisulfate Ion with Iodide Ion part 2
reaction is such that the color change will occur when 2 x 10^-4 mol of S2O8^- has reacted. solution will turn blue balck when 2 x10^-4 mol of S2O8^2- has reacted. You will quickly add another portion of Na2S2O3 after the appearance of the blue black color and the blue black color will disappear. when the blue black color reappears a second time another 2x10^-4 has reacted making the total 2(2x10^-4) By changing the initial concentrations of S2O8^2- and I ^- and observing the effects upon the rate of the reaction, we should have determined that the reaction was first order in both.
what is difference between solute and solvent?
solute is lesser component and solvent is the greater component of the solution.
Reaction of Peroxydisulfate Ion with Iodide Ion part 3
the blue black color that will appear in the sreaction is due to the presence of a starch iodine complex that is formed from iodine I_2 and starch in the solution. Color will not appear until S2O8^2 - +2I^- --> I_2 and 2SO_4^2- sufficient amount of I_2 has been produced by being oxidized. The thiosulfate that is added to the solution reacts extremely rapidly with the iodine. I_2 +2S_2O3^2- -->2I^- +S_4O6^2- An aliquot of 4x10^-4 mol of S_2O3^2- , when this amount reacts, an 2x10^-4 of of S_2O8^2- has reacted. Note that althought I^- is consumed according to equation 4, it is rapidly regenerated and therefore its concentration does not change in experiement.
chemical kinetics
the branch of chemistry that is concerned with the rates of reactions. rates of homogeneous reactions depend on the nature of the reactants, the concentration of the reactants, the temperature, and catalysis.
Define term supercooled.
the point at which the solution first cooled below the freezing point of the system.
titration
the technique of accurately measuring the volume of a solution required to reaction with another reagent. In this experiemnt your solution of NaOH will be standarized by titrating it against a very pure sample of KHP.
What were the next steps after determining the freezing point of glacial acetic acid?
we put the glacial acetic acid in warmer water to dissolve the solid and the unknown sample was dissolved in the glacial acetic acid. We placed the solution in cold water bath and took temperature measurements ever 20 seconds as the solution cooled to determine freeezing point, we took 10-12 readings after first crystals were noted. You will observe supercooling where the solution solidifies, a dilute solution will have the solvent freeze creating a gradual cooling caused by increased concentration of solute rather than reaching plateau.
if doubling the concentration causes rate to go from 3.0 x 10^-4 to 6.0 x10^-4 what does this determine?
we then know that doubling the concentration doubles the rate of the reaction and the reaction is first order with regard to the species that was doubled in concentration.
graphical determination of rate
y axis = moles of S2O8^2- reacted x 10 ^-4 x axis = time in seconds slope = (difference in y values) x 10^-4 mol divided by difference in time in seconds to obtain rate in mol/seconds
Lab 13 Colligative Properties how solute depresses freezing point
∆T = kbm, where ∆T is the drop in temperature, kb is the freezing-point constant for your particular solvent, and m is the molality of your solute). You will be given any kb values you need - the one used in the experiment was 3.9 /m for glacial (pure) acetic acid ℃ .
Describe temperature lab.
We calculated molar mass of unknown by determining the freezing point depression of a cyclohexane solution having a known concentration of unknown.
Cooling curve
a diagram that illustrates temperature changes and changes of state for a substance as heat is removed. the inflection point is the freezing point of solvent.
