Chem Lab Final

Oxidizing Agent

- is a substance that causes oxidation by accepting electrons; therefore, its oxidation state decreases

Reducing Agent

- is a substance that causes reduction by losing electrons; therefore, its oxidation state increases.

Oxidation

- is the gain of oxygen.

Reduction

- is the loss of oxygen.

acidus, which means sour or sharp.

Acid comes from the Latin word ___

shift to the right because decrease of excess heat

Apply Le Chatelier's principle to complete the chart below for the following reaction: H2O (l) + heat ↔ H2O (g) (Adding heat)

will shift to the left because decrease of excess water vapor

Apply Le Chatelier's principle to complete the chart below for the following reaction: H2O (l) + heat ↔ H2O (g) (Adding water vapor)

will shift to the left because Compensate by producing heat

Apply Le Chatelier's principle to complete the chart below for the following reaction: H2O (l) + heat ↔ H2O (g) (decreasing temperature)

will shift to the right because Decrease the amount of gas to decrease pressure

Apply Le Chatelier's principle to complete the chart below for the following reaction: H2O (l) + heat ↔ H2O (g) (increasing pressure)

will shift to the light because Replace lost water vapor

Apply Le Chatelier's principle to complete the chart below for the following reaction: H2O (l) + heat ↔ H2O (g) (removing water vapor)

This method of testing could not be used to determine the concentration of an NaCl solution because sodium chloride (l) is colorless. Colorless solutions do not absorb visible light, so a colorimeter or UV-Vis could not be used to analyze the solution.

Could this method of testing be used to determine the concentration of a NaCl solution?

Chemical reactions need an energy source - a "spark" - for a reaction to begin. Bonds between atoms or molecules on the reactant side of an equation must be broken so that reactants can react with one another to create products. The minimum amount of energy needed to break the existing bonds between reactants so that a chemical reaction can occur is called activation energy.

Define the term "activation energy" as it pertains to equilibrium.

Water vapor is the gaseous phase of water.

Define the term "water vapor".

The "ball and stick" model is a molecular model that is used to assist students in visualizing molecular geometry. The different colored balls are atoms, and the sticks are bonds.

Describe what a "ball-and-stick" model is, and why it is used in chemistry.

Equilibrium occurs in reversible reactions - reactions that can go forward and backwards without energy being used. Equilibrium occurs when a reversible reaction's forward process is perfectly balanced with the reaction's reverse process. Factors that would cause disturbances in a reaction's equilibrium are changes in concentration, changes in pressure, changes in temperature, and the addition of an inert gas.

Describe, in your own words, what chemical equilibrium is, and which variables can impact a chemical reaction at equilibrium.

This calibration curve shows an inverted (negative) slope, not a positive slope. The analyst inserted their solutions into the instrument from most concentrated to least concentrated instead of least concentrated to most concentrated.

During your Beer's Law experiment, you got the following standard curve plot. What is incorrect about this plot, and what error most likely occurred to get a plot like this?

The analyst should begin by preparing a calibration curve. He/she should obtain the unknown solution of interest, then insert it into the colorimeter or UV-Vis to learn the absorbance value. He/she should follow the calibration curve until they have found the unknown's absorbance point on the y axis. From this point, the analyst should draw a straight line straight into the x axis to obtain the unknown concentration. Note that you should not attempt to plot the unknown on the calibration curve!

Explain, in 2 - 3 sentences, how you would determine the concentration of an unknown solution using a Beer's Law Plot.

Peanut → System Water → Surroundings Beverage Can → Calorimeter

For each part of the experiment, match each word to its calorimetry function name: Peanut, Water, Beverage Can Surroundings , Calorimeter, System

This reaction will shift to the right, as indicated by the arrow. This is not a reversible reaction as indicated by the arrow →, so it will not reach equilibrium.

For the reaction H2O (l) → H2O (g), predict the reaction shift. Can this reaction achieve equilibrium? Explain your answer.

VSEPR theory explains molecular shape on the assumption that each atom in a molecule will achieve a geometry that minimizes the repulsion between electrons in the valence shell of that atom. This repulsion causes molecules to orient themselves into one of several specific shapes with measurable bond angles.

How does VSEPR theory explain molecular shape?

Changing reactant concentration directly affects the rate of a chemical reaction. Increasing the concentration of one or more reactants will increase reaction rate, and decreasing the concentration of one or more reactants will decrease reaction rate. A higher concentration generally leads to more collisions, and a lower concentration generally leads to less collisions.

How does changing reactant concentrations change the rate of a reaction?

In this experiment, you will recognize the end point of the titration by the light pink color that develops in the fruit juice solution.

How will you recognize the end point of the titration? (What is observed that signals to stop adding the titrant?)

This error would decrease the mg of citric acid per mL, so the end calculation would be too small. The final calculation requires you to divide # mg calculated by the # mL analyte used. For example, in the pre-lab key, the # mg calculated was 115 mg. 115 mg/10 mL = 11.5 mg/mL. If we were supposed to use 10 mL in the denominator and we accidentally used 20 mL, the final mg/mL would decrease to 5.75 mL. Therefore, the end calculation would be too small.

How would the following errors affect the calculation of mg of citric acid present per mL of juice? Will it make the values too large, too small, or will it have no effect? Explain your answers. - Recording that 20.0 mL of juice was titrated, when actually only 10 mL of juice was present.

This error would decrease the mg of citric acid per mL, so the end calculation would be too small. Quite often, the first sign of pink in the solution is close to the end point, but it is not the end point itself. Therefore, stopping the titration prior to the endpoint would decrease the # moles NaOH, which would result in a lower # grams CA, which would result in a lower # mg CA. Therefore, the end calculation would be too small.

How would the following errors affect the calculation of mg of citric acid present per mL of juice? Will it make the values too large, too small, or will it have no effect? Explain your answers. - Stop titrating at the first sign of pink in the solution.

This error would increase the mg of citric acid per mL, so the end calculation would be too large. 0.180 M is larger than 0.109 M. A larger molarity would result in a larger # moles NaOH. This would result in a higher # grams of CA, which would result in a higher # mg CA. Therefore, the end calculation would be too large.

How would the following errors affect the calculation of mg of citric acid present per mL of juice? Will it make the values too large, too small, or will it have no effect? Explain your answers. - The molarity of NaOH was recorded as 0.180 M NaOH instead of the actual molarity of 0.109 M NaOH.

This error would increase the mg of citric acid per mL, so the end calculation would be too large. Over titrating means that you have added too much titrant - in this case, NaOH. Adding too much NaOH would result in a higher # moles NaOH, which would result in a higher # grams of CA, which would result in a higher # mg CA. Therefore, the end calculation would be too large.

How would the following errors affect the calculation of mg of citric acid present per mL of juice? Will it make the values too large, too small, or will it have no effect? Explain your answers. - Over titrating the end point.

Less light would pass through the cuvette and into the spectrometer's detector because the fingerprints would scatter some of the incoming light

How would your test results be affected if you left fingerprints on the sides of the cuvette in line with the light path of the spectrometer (or colorimeter)?

You would not be able to titrate purple grape juice using the techniques used in X1. Purple grape juice is very dark - it would be impossible to see a color change in this solution.

If the juice to be titrated was yellow, the end point color would change from the pink to more of a peach (pink of the indicator at the end point + yellow solution = pink color). Would you be able to titrate purple grape juice by this technique?

titrant

In a titration, the ____ (species being added) is added in small increments to the analyte solution until the reaction is complete

The peanut that burned for two minutes would have a higher calculated Cal value, while the peanut that burned for one minute would have a lower calculated Cal value.

In an experiment by P. Nutt, one peanut only burned for 1 minute, while the other peanut burned for 2 minutes. What effect will this have on the calculated results? (Higher or lower Cal from fat?)

Some of the heat generated by the combustion was released into the environment, absorbed by the metal beverage can, and absorbed by the metal paperclip stand.

It was assumed that all of the heat given off by the combustion was absorbed by the water, is not a good assumption. - What evidence do you have to support your answer?

This is not a good assumption.

It was assumed that all of the heat given off by the combustion was absorbed by the water. - Is this a good assumption?

This situation decreases the calculated Cal value because the experimental ΔT value is smaller than it would be in a perfect environment.

It was assumed that all of the heat given off by the combustion was absorbed by the water. - What effect will this have on the calculated Cal?

The bonds should be listed as follows in order of strongest to weakest: triple bond > double bond > single bond.

List the bond types in order of strongest to weakest.

Phenolphthalein → Indicator → Solution in the flask Fruit Juice (CA) → Analyte → Solution in the flask NaOH → Titrant → In the buret

Match each chemical to its function name and where you would find it in the experiment. phenolphthalein, fruit juice, NaOH analyte, titrant, indicator in the buret, solution in the flask, solution in the flask

The buildup of soot can impact the experiment. The degree of impact would depend on the amount of soot present on the bottom of the calorimeter. It is reasonable to assume that each calorimeter could be used 2-3 times in this experiment without issue; however, 4+ uses would result a large buildup of soot. The soot would prevent some of the heat from getting through to the metal can, which would decrease the ΔT value.

P. Nutt also asks if the build up of soot affects the results. How would you answer?

This experiment should not be attempted with a Styrofoam cup. Styrofoam is highly flammable - the bottom of the calorimeter would burn up and the water would be emptied from the cup.

P. Nutt also asks if the experiment could be done using a Styrofoam cup instead of a beverage can. How would you answer and explain your answer?

A decomposition rection, or chemical breakdown, occurs when a single molecule is broken down into two or more simpler units. The most basic variable-substituted explanation for a decomposition reaction is as follows: AB→A + B; however, decompositions can present themselves in various forms as seen in the copper cycle (CHEM 1315L X3). The three types of decomposition reactions are thermal decomposition, electrolytic decomposition, and photolytic decomposition.

Recall, from previous experiments, what a decomposition reaction is.

In short, Le Chatelier's principle, or equilibrium law, states that reversible reactions with an established equilibrium will move to counteract disturbances to the reaction, thereby establishing a new equilibrium.

Research Le Chatelier's principle. Now, write 2-3 sentences summarizing this principle. Give an example of how Le Chatelier's principle works.

VSEPR is an acronym for Valence Shell Electron Pair Repulsion. VSEPR theory is a model that is used by chemists to predict molecular geometry, or shape, based on the number of paired electrons that surround a molecule's central atom.

Research and provide an explanation for the basis of VSEPR theory. Why do chemists use VSEPR theory?

Ionic materials change the colligative properties of a solution. Differences in colligative properties between electrolyte and non-electrolyte solutions are due to the dissociation of electrolyte solutes in solution. Non-electrolyte solutes will remain as whole molecules once they have been added to their solvents. For example, the non-electrolyte glucose (C6H12O6) will remain as whole C6H12O6 molecules in solution. Electrolyte solutes will behave opposite of non-electrolyte solutes - they will dissociate once they have been added to their solvents. For example, a 1 molal solution of NaCl will dissociate into 2 moles of particles: 1 mole of Na+ ions, and 1 mole of Cl- ions. The total solute particle concentration after dissociation would be 2 m. In this case, the colligative properties would depend on the total ion particle concentration (Na+, Cl-) formed after dissociation (2 m), which is not the same as the NaCl (as a whole molecule) concentration of 1 m.

The freezing point of a 1 molal aqueous solution of the nonelectrolyte ethylene glycol (the principal constituent of automotive antifreeze) is about -2 °C. The freezing point of a 1 molal aqueous solution of NaCl, a strong electrolyte, is about -4 °C. If freezing-point depression is a colligative property, it should depend only on the concentration of dissolved particles. Explain this exception

Freezing point is a colligative property. Colligative properties depend on the amount of solute molecules or ions in the solution of interest (molality), not the identity of the solute, which is different than the concept of molarity. The formula for molality is m = 𝑚𝑜𝑙 𝑠𝑜𝑙𝑢𝑡𝑒 / 𝑠𝑜𝑙𝑣𝑒𝑛𝑡. Because of this, the presence of ethylene glycol (solute) depresses the freezing 𝑘𝑔 point of the water (solvent).

The freezing point of antifreeze, a solution of ethylene glycol and water, has a lower freezing point than pure water. Explain why this solution has a lower freezing point that 0 °C.

analyte

Titration is a method to analyze how much ___ (species of interest) is present.

The molar ratio between CA and NaOH is a 1:3 ratio.

Titrations are a type of stoichiometry calculation. In this experiment, what is the molar ratio between citric acid and NaOH?

NaOH

To determine how much citric acid is present, the citric acid is neutralized by a solution of ___ in a titration.

acid-base indicator

To signal the end of the reaction, an ____ (a species that changes colors at different pHs) is used.

Way Boat should obtain a 200+ mL beaker since the desired volume is 100 mL (your beaker should have the capacity to measure more volume than what the problem requires, or the liquid will spill over the beaker). Way should add approximately 50 mL of DI H2O to the beaker, add 27.39 g CoCl2 · H2O to the beaker, and then stir until the CoCl2 · H2O is well dissolved. Finally, way should dilute the solution to the 100 mL mark on the selected beaker.

Way Boat needs to make 100 mL of a 1 M solution of CoCl2. He looks in the lab cabinet and finds CoCl2 · 6 H2O. Calculate the amount of CoCl2 · 6 H2O needed to make 100 mL of 1M CoCl2. Write 2 - 3 sentences explaining how to make this solution.

A reversible reaction is noted by reversible arrows ↔ and occurs when a chemical reaction produces a forward and reverse reaction simultaneously.

What does a scientist mean when describing a reaction as reversible?

A scientist that has applied stress to a chemical reaction at equilibrium has disturbed the reaction by changing concentration, pressure, or temperature. The scientist may have also added an inert gas to the reaction. These stressors disturb a reaction's equilibrium, which causes the reaction to shift in either a left or right direction to establish a new equilibrium.

What does a scientist mean when stating that they have applied stress to a reaction at equilibrium?

The formula for Beer's Law is A = Ɛbc. A = absorbance, Ɛ = molar absorptivity, b = path length, and c = solution concentration. These factors will impact how much light is able to pass through a colored, liquid solution.

What factors are included in the Beer's law expression for determining how much light passes through a liquid solution?

A single bond consists of one pair of electrons, or two bonding electrons. A double bond consists of two pairs of electrons, or four bonding electrons. A triple bond consists of three pairs of electrons, or six bonding electrons.

What is a "single bond", a "double bond", and a "triple bond"?

A bond angle is the angle between two bonds in a compound.

What is a bond angle?

Hybrids exist in many places within our world. Some common examples are flowers, animals, and vehicles. A hybrid is a mixture of two different things that combine to form something new. The new thing created from the mixture retains some properties of both "parents" that created it. In chemistry, a hybridized orbital is a combination of two or more atomic orbitals.

What is meant when a chemist says that an orbital is "hybridized"?

Both freezing point and melting point are colligative properties; however, freezing point is the point at which a liquid becomes a solid, and melting point is the point at which a solid becomes a liquid.

What is the difference between freezing point and melting point?

The freezing point will be depressed further than the experiment calls for since colligative properties depend on the amount of solute added to the solvent, not the identity of the solute

Your laboratory partner accidentally mislabeled their test tubes. The test contained more benzoic acid than it should have. Will this affect the observed experimental freezing point? If so, will this freezing point be higher or lower than expected? Explain your answer.

You should respond to your laboratory partner explaining that he/she is incorrect, and that temperature directly affects the rate of a chemical reaction. As seen in the X6 simulation, heating the reaction chamber caused molecules to move around more quickly, which resulted in molecules colliding with one another more frequently. Particles can only react when they collide, so quicker collisions result in a faster reaction, when timed.

Your laboratory partner suggests that temperature does not affect the rate of a reaction. How do you respond to this suggestion?

phenolphthalein, colorless, bright pink

____ is the acid-base indicator. It is ____ at acidic pHs (beginning of the reaction) and turns ____ at basic pHs (end of the reaction).