Reactions and Rates

A reaction coordinate is a graph that can be used to observe the progress of a reaction as reactants are turned into products. These graphs can be used to predict whether a reaction will occur based on the activation energy for the reaction. If the reactants have a large potential energy, the activation energy will be small, meaning the reaction is likely to proceed. If the reactants have a small potential energy, the activation energy will be large, meaning the reaction will not proceed without an external energy source. So, reactions that require energy such as those with a high activation energy will have a positive delta H because they require energy to proceed, while reactions that release energy and have small activation energies will have a negative delta H.

The potential energy of the particles is dependent upon their positions and complexity, along with their activation energy. In order for the reaction to occur, the particles must collide in the correct orientation with the correct speed. In addition, concentration, temperature, and presence of a catalyst all affect reaction rates and whether a reaction will occur. As explained before, if these factors are favorable, the activation energy will be low, and the reaction will occur. By observing the activation energy on a reaction coordinate graph, the rate of a reaction can be predicted. If the reaction has a high activation energy it will not happen at all, if there is little to no activation energy, the reaction will proceed quickly, and if the amount of activation energy is somewhere between the two, the reaction will proceed slowly.

Reaction Graphs

The approximate activation energy for the forward and reverse reactions is dependent on the difference in potential energy between the products and reactants.

3 Questions 4/24/15

1. What tasks have you completed recently? 

Recently, I have completed multiple chemistry labs on buffers and acid titrations in relation to equilibrium. In my online class, I have written two biographies and analyzed a speech, and in English, I have practiced timed writing in preparation for the AP Exam.
2. What have you learned recently?

Recently, I have learned how a buffer resists a change in pH through small samples of acids and bases that neutralize small amounts of other acids and bases. In addition, I have learned about Ksp.
3. What are you struggling with &, therefore, plan to do next?

Currently I am struggling with applying the effects of a buffer to titration curves, so I plan on reviewing my labs and notes. Also, I am struggling with time management, so I plan on prioritizing my studies over other commitments.

Colorimeter pH Lab

What is the difference in pH between an acid and a base?

The difference in pH between an acid and a base can range anywhere from 0 to 14. The pH of an acid is below 7, and the pH of a base is above 7. This shows a difference in the concentration of the hydrogen ions in a substance. pH is measured by -log[H+], so high hydrogen ion concentration results in a low pH due to the inverse relationship.

What is the relationship between pH & pOH?

pH and pOH are related because they add to fourteen. In other words, pKw= pH + pOH = 14.

How are pH & pOH related to [H+] & [OH-] respectively?

pH and pOH are inversely related to [H+] and [OH-], respectively. pH and pOH are calculated by taking the negative log of the ion concentration, or in other words, taking the inverse of the concentration. This means that a high concentration of ions will result in a low pH or pOH, and a low concentration of ions will result in a high pH or pOH.

What differences did you observe between Universal Indicator and the other indicators?

I observed that the Universal indicator could be used to determine the pH of almost any reaction, while the other indicators were limited in their usefulness. These indicators would be better suited to verify a pH, rather than determine a pH.

If you know a neutralization reaction will occur at a pH of 5 and a different neutralization reaction will occur around a pH of 9, would you choose the same indicator or different indicators? Why?

I would choose different indicators because different indicators would provide more specific results. pH indicators that are specifically targeted for these two pH’s will be more accurate than using the same indicator.

What are indicators?  How do they work?  Do they have limits? (Provide specific examples from your lab.)

An indicator is a chemical compound that changes color when exposed to certain conditions. The indicators interact with H+ and OH- in a system, and give off a certain color as a result of neutralization. Indicators have limits in terms of the range of pH’s that they can qualitatively demonstrate. Universal indicator has the ability to show the entire range of pH from pink to purple, but other specific indicators can only exhibit certain changes. For example, bromothymol blue can only show from yellow to blue.

3 Questions 4/10/15

1. What tasks have you completed recently?

Recently, I have conducted endless tasks in preparation for prom. In terms of classes, I have been trying to get get ahead in my online class (unsuccessfully), and I have completed an essay in English. In chemistry, I have completed online homework assignments and labs on equilibrium.
2. What have you learned recently? 

Recently, I have learned about equilibrium and how a reaction changes in order to remain at equilibrium. In addition, I have learned how to calculate equilibrium constants. In world history, I have learned about the Chinese Revolution, and in physics, how to calculate the energy needed to unfreeze a pond in order to swim in it.

3. What are you struggling with &, therefore, plan to do next?

Currently, I am struggling with the units related to equilibrium and which direction equilibrium shifts under certain circumstances, so I plan to review my notes in order to grasp a better understanding.

Equilibrium Between NO2 and N2O4

1. What substance was in each test tube?  Provide a ball and stick model.

In each test tube, there was nitrogen dioxide and dinitrogen tetroxide.

Nitrogen Dioxide

Dinitrogen Tetroxide-Molecule Struture

Dinitrogen Tetroxide

2. What happens when you place the test tube in boiling water into the ice water? What process is favored by the changes?  What’s occurring at a faster rate?  What is the change in concentration?

When you place the test tube in boiling water into the ice water, the orangish NO2 gas disappears, and is replaced with colorless N2O4 gas. The formation of N2O4 is favored by the changes. The rate of the production of N2O4 is occurring at a faster rate than the rate of production of NO2, which is why the tube color turns from orange to clear. So, the concentration of N2O4 increases while the concentration of NO2 decreases.

3. What happens when you place the test tube in ice water into the boiling water?  What process is favored by the change?  What’s occurring at a faster rate?  What is the change in concentration?

When we placed the test tube into the boiling water after it sat in ice water, the type of gas produced changes. The color of the gas changes from clear to orangish yellow NO2 which is favored because the formation of NO2 gas is endothermic. At this point, the production of NO2 is occurring at a faster rate than the break down of N2O4, which is why the concentration of NO2 increases and the concentration of N2O4 decreases, accounting for the color change.

4. What is the balanced equation (including enthalpy)?

2NO2<—> N2O4                           Enthalpy: -58.0 kJ/mol

5. Explain your observations of the reversals in terms of LeChatelier’s principle.

Our observation makes sense based on LeChatelier’s principle that states that when a system at equilibrium is subjected to change, the system readjusts it self to attempt to counteract the effect of the change. In our lab, when the temperature was changed, the reversible reaction rates adjusted in order to accommodate the change,  as the system tried to maintain equilibrium.

6. What is happening within the test tube at room temperature?  How is it related to the hot and cold test tubes?

At room temperature, the test tube is at equilibrium. It is related to the hot and cold test tubes because the change in temperature affects which side of equilibrium the test tube is on (in regards to enthalpy). The direction of the reaction and type of gas produced is based on whether the enthalpy of the reaction is positive or negative. The positive or negative enthalpy value determines which reaction is favored at a given temperature.

IMG_2062IMG_2059

Sources:

http://en.wikipedia.org/wiki/Le_Chatelier%27s_principle

http://www.shutterstock.com/s/nitrous/search.html?page=1&inline=214524664

Three Questions 3/27/15

1. What tasks have you completed recently? 

Recently, I have completed many prom planning tasks (T-MINUS THREE WEEKS), have taken many notes for chemistry, and have worked on a synthesis essay for English.
2. What have you learned recently?

Recently, I have learned about kinetics, or more specifically, reaction rates and rate law. In addition, I have reviewed the causes of World War I in online history.
3. What are you struggling with &, therefore, plan to do next?

Currently I am struggling with Reaction Mechanisms and Catalysts because I had not taken the notes until recently, so I plan to review in preparation for my test.

Three Questions 3/13/15

1. What tasks have you completed recently? 

Recently, I have completed hours of makeup work, including physics notes, chemistry worksheets, an English essay, and numerous online history homework assignments. 
2. What have you learned recently?

Recently, I have learned that missing two weeks of school may actually be the worst thing ever. Also, I’ve learned that tonsillectomy patients need better pain medication. Regarding school, I have learned about hybridization and different types of bonding in chemistry, and in English, I have learned how to write an APA research paper.

3. What are you struggling with &, therefore, plan to do next?

Currently, I am struggling with VSEPR vs molecular geometry, and I do not have the geometries memorized, therefore I plan to review past worksheets and notes in order to distinguish the two.

Birthday Three Questions 2/27/15

1. What tasks have you completed recently? 

Recently, I have worked on and English essay and studied past chemistry notes in preparation for surgery week. Also, I have gotten my room prepped for surgery and have tried to get ahead in my online class so that I will not have so much to make up when I return.
2. What have you learned recently?

Recently, I have learned about different types of birth control while researching for my essay. In chemistry, I learned about the difference between molecular and VSEPR geometries, along with hybridization.
3. What are you struggling with &, therefore, plan to do next?

Currently, I am struggling with remembering the different types of intermolecular forces, so I plan to review those before moving on to intramolecular forces in the next sections of notes.

Solid Bonds and Properties

The four types of solids are metallic, ionic, covalent-network, and molecular. Metallic solids are held together by collectively shared delocalized valence electrons. This type of bonding is responsible for the fact that most metals are strong without the characteristic of a brittle consistency. It also allows metals to conduct electricity. Metallic solids can have a wide range of melting points. Ionic solids are held together by the mutual attraction between cations and anions. NaCl, for example, is an ionic solid that is an extended network of ions held together by ion-ion interactions. Ionic solids are conductive in aqueous solution, but not in solid form, and they tend to have high melting points due to the difference in charge between the ions. Covalent-network solids are held together by an extended network of covalent bonds, which generally results in materials that are extremely hard. These solids are non-conductive, and have high boiling points. This type of bonding also accounts for the unique properties of semiconductors. Last but not least, molecular solids are held together by intermolecular forces including dispersion forces, dipole-dipole interactions, and hydrogen bonds. Molecular solids tend to be soft with low melting points due to the fact that these forces are relatively weak. Most molecular solids are not soluble in water, and they are not conductive.

In the lab, we used the properties of conductivity, solubility in both polar and non-polar solvents, and melting point to classify the solids. Based on our definitions, metallic solids will conduct, while covalent-network, molecular, and ionic solids will not. All solids out of these four will have high melting points except for molecular solids.

Our results were as follows:

CaCl2: Ionic           Sand and Charcol: Network Covalent           Citric Acid: Molecular         Zinc: Metallic

I think that the closeness of the bonds

  • What microscopic differences do you think account (i.e., as shown by particle diagrams) for the property differences observed?

Three Questions 2/13/15

1. What tasks have you completed recently?  

Recently, I have completed multiple homework assignments for physics, I have constructed geometrical representations of molecules in Chemistry, and I have worked on a research paper for AP Language.

2. What have you learned recently?

Recently, I have learned about cosmology, equilibrium, lewis structures, and formal charge. In addition, I have learned about atomic trends and how they are related to Coulumb’s law in chemistry.
3. What are you struggling with &, therefore, plan to do next?

I am currently struggling with VSEPR and formal charge, so I plan on reviewing my notes and my online homework assignments in order to increase my understanding.