Question

2 chemistry questions

Answer 1

A cold pack is placed on an injured leg. Indicate the direction of the flow of energy between the leg and the cold pack:
Energy flows from the injured leg to the cold pack. Heat flows from the higher temp to the lower temp. the cold pack absorbs heat energy from the injured leg.


LAW:Energy cannot be created or destroyed, only changed from one form to another.

Answer 2

$$\text{We always start calorimetry problems with}\ q=mC\Delta t\\ \text{Then we plug in what we know.}\\$$\[q=(500g)\cdot \left( \frac{4.184J}{g{\ \cdot {^\circ C}}}\right)\cdot(9.3 {^\circ C} -25.0 {^\circ C}) \]
That gives us the amount of energy that flowed from the leg to the cold pack. The amount of energy that the ammonium nitrate(cold pack) gained is the exact opposite that the leg gave away. Therefore, using the q=mHf you gave \[-q(\text{from the previous answer})=(100g)\cdot(H_f)\] just rearrange and solve for the Hf. So the answer should at least be positive since you've got a negative times a negative. Let me know what you get!

Answer 3

Correct!

Answer 4

When you rearrange the equation to solve for the enthalpy you would divide the heat by the amount of weight. This should make sense since enthalpies of fusion are usually in energy per amount like cal/mole or joules/gram or joules/mole. Also remember that the energy you calculated in the beginning is the amount that left the water hence the negative sign. When you calculate the amount gained for the ammonium nitrate the sign changes to a positive since it's gaining it not releasing it. So\[\frac{-(\text{energy taken from the water})}{100g} = \text{Enthalpy of fusion}\]

Answer 5

Does the negative sign make sense intuitively to you? The ammonium nitrate is gaining energy not realising it. When you calculated the energy the first time you calculated the amount given off by the water therefore the sign changes to a positive when you use it to describe how much the ammonium nitrate gained. The answer should be positive and what are the units? You're sooooo close!

Answer 6

Woot woot! yeah! and the units?

Answer 7

The unit's are super important. It would be in joules per gram. 328J of energy are used to dissolve one gram of ammonium nitrate. \[\frac{328.554J}{g}\] So a little re-cap. You dumped some ammonium nitrate solid into a glass of water. The water gave some energy to the ammonium nitrate so it could break it's solid bonds and dissolve. Since we first calculated the amount of energy being released by the water q's sign is negative(being release). So the amount of energy gained by the ammonium nitrate solid is the exact same but the sign is positive since it's being gained. Then we just divide the amount of energy gained by the amount that we have(100grams) to find out the enthalpy of fusion. That's just a fancy way of saying the amount of energy it takes to convert it from a solid to a liquid pergram.

Hope this helps!

Answer 8

Perfectomundo! Do you actually understand it though?