EXP 6 - experiment 6 lab report PDF

Preview

Summary

experiment 6 lab report...

Description

CHEM 101L Pre-Lab 6

Fall 2019

Name

Grayson Garcia

UNC onyen

graygar

Experiment 6: Thermochemistry Experiment #6 Pre-Lab Assignment Instructions: 1. When answering questions, do not repeat or copy from the lab manual. You are encouraged to use the Lab Manual as a reference, but plagiarized answers will not be graded. 2. Do not ever work to answer these questions with a partner - avoid plagiarism and cheating. 3. Do not exceed the limit given for each question. 4. Do not remove instructions or any bolded text. This section will help prepare you for writing an introduction for future experiments: 1. (4) What is the objective of this lab experiment and how will you accomplish this objective? (what reactions are you monitoring; what are you measuring; how are you monitoring the reaction; what apparatus or special equipment will be essential for accurate and precise measurements in this experiment) (3-5 sentences) In this experiment we are measuring the molar enthalpy, ∆H, of the following reactions: sodium hydroxide dissolving in water to form an aqueous solution of ions, neutralizing HCl solution using NaOH solution, and the dissolution and neutralization of solid NaOH with an HCL solution. The result of summing the first two reactions is the third: therefore, Hess’s law can be used to calculate the final ∆H, or change in enthalpy. We are monitoring these thermodynamic reactions using a carefully calibrated calorimeter and calorimeter constant to measure the changes in temperature as accurately as possible given that the reactions will not be 100% insulated, and also creating a cooling curve to determine the point in the reactions that the enthalpy exchange begin.

2. (3) What are the three reactions you will conduct in this experiment? Show the balanced chemical equations below and include states of matter. Below each reaction, write the net ionic equation. o

Reaction 1: §

o

NaOH (s)

Net ionic equation:

Reaction 2:

Na + (aq) + OH -

è H2Oè NaOH (s)

NaOH (aq) + HCl (aq)

è

è H2Oè

(aq)

Na + (aq) + OH - (aq)

H2O (l) + NaCl (aq)

CHEM 101L Pre-Lab 6 §

o

Fall 2019 Net ionic equation:

Reaction 3:

§

OH- (aq) + H+ (aq)

è

H2O (l)

NaOH (s) + HCl (aq) è H2O (l) + NaCl (aq)

Net ionic equation:

NaOH (s) + H + (aq)

è

H2O (l) + Na + (aq)

3. (3) Generate a hypothesis: What do you hypothesize will be the difference between the three reactions? Will each reaction be endo- or exothermic? How did you formulate this hypothesis? (use chemical concepts to explain the origin of this hypothesis) (4 sentences maximum) Since Reaction 1 is only creating ions in solution, there will be no observable reaction and because the ions come from breaking a bond, it will be exothermic with a negative ∆𝐻. Reactions 2 and 3 will differ in that the heat change from a neutralization reaction happens from NaOH solution in Reaction 2 and solid NaOH in Reaction 3, though they both yield the same products. I hypothesize that Reactions 2 and 3 will be exothermic because new bonds are formed (exothermic), and though bonds are broken (endothermic), the bonds created typically have higher values of ∆H, making the final ∆H negative for Reactions 2 and 3.

4. (2) Summarize this experiment: Provide a bulleted list of tasks for this experiment, using 1 phrase or sentence per bullet. You can use a maximum of 4 bullet points to describe Experiment 4. •

• • •

Calibrate calorimeter and place into 50mL water in Styrofoam cup on a stir plate, collecting temperature every 30s for 3min, then add 2g sodium hydroxide and collect temp for 7 more min (10min total). Empty, clean, and dry Styrofoam cup. Repeat step 1 using 50mL 1.0M HCl instead of 50mL water. Add 55mL 1.0M HCl to 45mL water (100mL total). Repeat Step 1 for 100mL HCl solution instead of 50mL water.

This next section is designed to prepare you for the experimental portion of the experiment, as well as your discussion and analysis after the experiment. Answer each question carefully. You may need to reference the lab manual or textbook to answer these questions. For some questions, you will likely need to think carefully about the reasoning (the answer won't be exactly written in a book!).

CHEM 101L Pre-Lab 6

Fall 2019

1. (2) Look up (use an internet search engine) and reference the SDS (Safety Data Sheet) for sodium hydroxide pellets and 1M sodium hydroxide. Report the health hazard ratings, one potential hazard and one prevention method for both compounds below. o Sodium hydroxide pellets § § § §

o

Health Hazard Information: Health: 3, Flammability: 0, Instability: 1 Potential health hazard: may cause respiratory irritation if inhaled Hazard Statement: causes severe skin burns and eye damage. Prevention: wear protective clothing and wash hands and exposed skin thoroughly after handling—do not breathe in fumes

1 M sodium hydroxide § § § §

Hazard Identification: Health: 2, Flammability: 0, Physical Hazard: 0 Hazard statement: causes severe skin burns and eye damage Precautionary statement: wash hands/exposed skin thoroughly after handling Prevention: Wear protective clothing and eye protection and do not breathe in dust of fumes

2. (1) Why do you weigh the solid NaOH by difference instead of directly weighing it on a weigh boat? (1 sentence) Weighing by difference is the most accurate method the measure the mass of a solid sample of NaOH because it is highly hygroscopic, meaning it absorbs water and carbon dioxide from the air quickly, making it impossible to measure in an open environment (weigh boat). 3. (1) In reaction 2, why do you use a different graduated cylinder to measure HCl and NaOH solutions? (1 sentence) The HCl and NaOH solutions must be measured in separate graduated cylinders so that the second solution measured is not at risk of partially reacting with the first solution, as they form a neutralization reaction.

4. (4) Answer the next two questions using complete sentences and a complete thought. (maximum 3 sentences for each question) o In an endothermic reaction, what will you expect to see in your data? How is this related to enthalpy? (explain this on a molecular level) In an endothermic reaction, there is a decrease in temperature measured and the ∆ H will be positive, meaning that the final temperature/heat energy of the system will be higher than the initial. The data plot will look like a bell curve, but the left side (initial values) will be lower than the right side (final product values) and the top of the curve (highest value) will be in the middle of the plot. This is because the total potential energy increases as the system absorbs energy from its surroundings.

CHEM 101L Pre-Lab 6 o

Fall 2019

In an exothermic reaction, what will you expect to see in your data? In an exothermic reaction, there is an increase in temperature measured and the ∆H will be negative, meaning that the final temperature/heat energy of the system will be lower than the initial. The data plot will look like a bell curve, but the left side (initial values) will be higher than the right side (final product values) and the top of the curve (highest value) will be in the middle of the plot. This is because the total potential energy decreases as the surroundings absorb energy from the system.

5. (2) What is qcal and why do you need to calculate it? How do you account for qcal in this experiment? (2-3 sentences) qCAL is the amount of heat energy that is transferred to or from the calorimeter or calorimeter cup. It is important to calculate the qCAL for this experiment because there is heat loss to the calorimeter due to initial temperature differences and the Styrofoam calorimeter cup not being a perfect insulator to the reactions. The qCAL is accounted for in the calculation of the calorimeter constant (cCAL = ∆qCAL(/ ∆𝑇𝑐𝑜𝑙𝑑) and change in temperature of the temperature probe.

6. (6) Calculate ∆Hrxn for the reaction between NaOH(s) and HCl(aq) using the appropriate ∆Hf values which can be found in the Experiment 6 lab manual. You will need to look up the ∆Hf value for each component of the first two reactions. Use the equation below to help you solve for each individual If needed, also refer to the lab manual for an example calculation of the ∆Hrxn for the decomposition of water H°rxn = ∑ [n∆H°f (products)] – ∑ [n∆H°f (reactants)] •

NaOH(s) → Na+(aq) + OH-(aq)

•

calculation:

•

Na+(aq) + OH-(aq) + H+(aq) + Cl-(aq) → H2O(l) + Na+(aq) + Cl-(aq)

•

calculation:

•

NaOH(s) + H+(aq) + Cl-(aq) → Na+(aq) + Cl-(aq) + H2O(l)

•

calculation:

∆Hrxn1 =

-44.50 kJ

[-240.1 + -230.0 ]kJ - [-425.6]kJ = -44.50 kJ

∆Hrxn2 =

-55.83 kJ

[-285.83 + -240.1 + -167.2]kJ – [-240.1 + -230.0 + 0 + -167.2]kJ = -55.83 kJ

∆Hrxn3 =

-100.33 kJ

[-240.1 + -167.2 + -285.83]kJ - [-425.6 + 0 + -167.2]kJ = -100.33 kJ

CHEM 101L Pre-Lab 6

Fall 2019

Note: If reactions 1 and 2 are summed, the resulting reaction is equivalent to reaction 3. Comparison of the measured ∆Hrxn for the three reactions can therefore be used to verify Hess’s Law. These values can also be compared with ∆Hrxn values calculated from ∆H°f for all reactants and products. 1. Reproduce the calculations shown in Section IV.A of the lab manual for determination of the calorimeter constant using numbers provided. Use the temperatures provided in the example. Set up this calculation in Excel with well-defined labels. If the calorimeter was perfect, and no heat was transferred to the calorimeter, what would Tmixed be? 30.89

o

(3) Tmixed:

o

(4) Calculations:

TA ONLY: Late Penalty

Formatting...