1/30 Thoughts

Hey everyone, so I don't know about you but it was pretty interesting to see today how there is a huge difference between the average bond enthalpy values and the change of energy of the formation of a compound at SATP. Who would have thought that the averages would cause such a large difference when conducting calculations.

What I find interesting is the fact that no matter how many steps you take in order to attain a certain product, the change in enthalpy will stay the same since any compounds formed between steps will simply cancel one another out. We should be really thankful that enthalpy is a state function since that saves us a lot of effort. This was probably in Hess's mind when he was developing his law and it is extremely useful since as chemists we tend to be lazy. What do you all think about this?

Heat of Reaction Practice

1. The heat of reaction using the heat of formation method is -137 kJ while the heat of reaction using the bond enthalpy method is -127 kJ. The results are similar but not exactly the same. This is mainly due to the error inherent to the bond enthalpy method. BE data are average bond strengths rather than specific to this reaction. 2. The standard heat of reaction is -354.8 kJ which is exothermic. 3. The standard heat of formation for Al2O3 is -1675.7 kJ/mol. So the formation of Al2O3 from Al (s) and O2 (g) is an exothermic process. 4. The heat of reaction for the Hall Process is given as endothermic at +2170.9 kJ for every 4 moles of aluminum refined. This is 2170.9 kJ/4 mol Al. If you convert 4 mol Al to grams of Al using molar mass, you get 20.12 kJ/g. So for 5.000 g, the endothermic reaction involves 100.6 kJ of energy. Much less energy is involved for 5 g Al than for 4 moles of Al!

Calorimetry Practice

Check your practice work (beige sheet)... 1.(a) deltaE = -38.9 kJ (sign is important), (b) 1.32 x 10^3 kJ released, and (c) the heat capacity (C) works out to be 27.2 kJ/oC. 2. The minimum heat is 2.64 kcal.

Nitroglycerin problem

Did you calculate 780 Liters of gas produced? All products in the detonation of nitroglycerin are gases. You need to determine the total moles of gas produced from the reaction with 1 kg of reactant...

Imagining Gas Behavior

Tiny particles so far from each other cannot interact very much. Things are less complicated then. Like most stars in space. Vast distances between them. We can imagine a gas particle as a point, traveling on and on until it bumps into something exerting a tiny pressure -- on its container or another particle. If a collision with another happens fast, then any sort of polarized interaction is insignificant. But what if the collision is a slower event? A low temperature, say lower than room temp, is a measure of slower collisions. When we all slow down, we can interact more, get to know each other, build social networks. So much interaction leads to stronger ties -- binary stars maybe, or liquid networks. Get those tiny particles to SLOW down enough and gases can even become liquids - AND/OR - PUSH those tiny particles close enough and gases can become liquids. Slow speeds measured by low temperatures, and crowding measured by high pressures and small volumes -- lead to interactions. Things get complicated. I'm not saying it's all fun and games attractions, those repulsions are important too! You can get too close. PV won't exactly predict the value of nRT. Our class experiments involved room conditions. We observed a simple relationship between P,V, n and T. How marvelous to summarize our observations on vast collections of these tiny particles designed by God with simple math! No harm in starting out simply -- we can include more complexity later if needed. Models in science -- start with simple observation, add a deeper layer when needed. The process becomes experiment, theorize, experiment again, modify the theory, repeat, repeat... No theory is without "holes".

Molecular Interpretations

For a given amount of trapped gas, we can readily observe the effect of pressure on volume with no change in temperature. If we compress a container and force gas particles into a smaller space, then they will collide more frequently with the container even though they still move with the same average speed (same temp). More collisions result in a greater internal pressure. So, if a container under this increased pressure cannot withstand it, then it will crack or maybe explode! If the "system" happens to be some air trapped behind food stuck in a choking victim's trachea, then abdominal thrusts (Heimlich maneuver) are a good way to compress the volume of this trapped air behind the food. This results in more pressure exerted by the air trapped behind the food -- hopefully enough to force the food up and out and save the person (my husband had to do this once in a public restaurant to someone we didn't know… saved her life!). This is all kind of like the motion of a piston in a cylinder...

Welcome to our class blog!

We hope the conversation here will enrich your experience in second semester general chemistry at Eastern University. When you read postings such as this one, you are very welcome to add your comments and follow up on what interests you. Here is to a great semester!