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# Change of state example

Specific heat capacity and enthalpy of vaporization example: calculating how much energy it takes to vaporize 1.00 kg of ethanol starting from 20 degrees C.  Created by Sal Khan.

## Want to join the conversation?

• hey guys do you know were I can get this type of question and the other examples we did from? I want to practice so that I'm comfortable in doing them , thanks
• At Sal mentioned Kotz, Treichel, and Townsend 'Chemistry and Chemical Reactivity' as his source, so you can probably rent that from a library. Perdue hosts a chemistry site with practice problems for every section, too: http://chemed.chem.purdue.edu/genchem/topicreview/index.php
• Why does it take more time for liquids to turn into a gas than solids to turn into liquids?
Could you make a video or could some one answer this question?
and why does it take that much energy?
• latent heat of fusion is less than latent heat of vaporization for an object.
That is the reason of more time taken by solids to transform into liquids than liquid to transform into gas.
• what is a joule?
• An SI unit of energy. N (Newtons) x m (meters) are equal to J (Joules). Kg (Kilograms) x m/s² (seconds²) are equal to Joules.The Joule is the SI unit for energy and is very closely related to work.
• Why do you write C2H5OH, rather than C2H6O?
• Because there two chemicals that have the molecular formula C₂H₆O.
Ethanol can be written as CH₃CH₂OH or as C₂H₅OH
The other chemical with the molecular formula C₂H₆O is methoxymethane, which has the structural formula CH₃OCH₃

Different chemicals that have the same atoms in the same quantities, but connected to each other in different ways are called isomers. Organic chemicals quite often have isomers, so it is necessary to use the structural formula rather than the molecular formula to specify which chemical you are talking about.
• Shouldn't the total grams and the final number of Joules be in Scientific Notation because it is difficult to tell how many significant figures the grams have
• Yes, that would make it easier to see the sig figs. But it's not uncommon for smallish numbers -- up to 10^4 or so, maybe -- to be written normally rather than in scientific notation. We're used to seeing ordinary numbers of about that size and it makes more sense at a glance.
• How do u know the freezing point of something if u are only given the boiling and melting point ?
• The melting point is the same thing as the freezing point.
• Does the joules system work the same as the metric system?
• Joules is the unit of energy in the metric system (or the SI system). If you were using foot pounds, etc, then you would find energy measured in foot-pounds or BTUs. So, I would say yes, the joules work the same as the metric system since joules is part of the metric system.
• At about , he says that 1000 has 4 significant digits. But Doesn't 1000 have just 1 significant digit?
Your supposed to count the zeros only if they have a decimal point at the end right ( 1000.) ?
• In chemistry lab, we have some special rules about that. Specifically, if you actually measure the trailing zero, then it is significant.

How many significant digits there are depends on the device you used to measure it. So, you really cannot make a blanket statement about how many significant digits a measurement with trailing zeros has.

A lab balance (depending on its quality) will have between 3 and 5 significant digits. So, we would normally assume that 1000 g has at least 3 significant digits.

To avoid this ambiguity, we would typically write the measurement in scientific notation and include only significant digits or we would specifically state what the precision is.
• I asked my chemistry teacher why doesn't the temperature change during phase change, she showed me a diagram and said that it is because the energy is used in breaking the bonds. Now when I asked how are the bonds being broken she said that heat is kinetic energy and more kinetic energy means the bonds are broken.
I opened chemistry stack exchange it told me that the heat during phase change doesn't change the kinetic energy, but literally breaks the bond.
https://chemistry.stackexchange.com/questions/15852/during-a-phase-change-in-matter-why-doesnt-the-temperature-change
What I wanna know is how does this heat energy affects the strength of the bonds without affecting kinetic energy of the particles.
• We are doing enthalpy in AP Chemistry right now, and we learned that when a reaction is exothermic, because the system is giving off heat, the energy would be negative. Since the temperature is being raised in this reaction, isn't it exothermic, and shouldn't the resulting number of joules for part one (the temperature change) be negative? Basically, how do I know when to make it negative or positive?