Learn about Senior Chemistry, Energetics 7, in this comprehensive video by bannanaiscool.
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Rob Lederer: Now these are the numbers necessarily to memorize, but you will probably have them memorized after you keep using them and using them. But big H in that NH formulae is the molar heat. So, it's not just written in joules that would just be heat. The little h would be joules, which isn't heat. but big H is joules or kilo joules per mole. This number here represents the amount of heat that needs to be absorbed when ice turns to liquid. One mole of it at zero degree Celsius. 6.01 kilo joules per mole. Now to take that one mole or 18.02 grams of water because, that's one mole right, that's the mole mass. 40.65 kilo joules per mole in order to vaporize that mole of water. Now, if you are going the other way and you are actually taking the waters as a gas and turning it into liquid, then the molar heat of condensation is negative because, when gas turns to liquid, water is released. We just remember because, this way because, if it goes from liquid to a gas, you know you have to add heat. So, now it would be endothermic, but if the water is going from gas to liquid, it releases heat, so that's exothermic, right? So, we put a negative for exothermic. Same if the water going from liquid to solid, we put a negative in front. By the way, you know that the heat capacity water is 4.19 and that's in joules per gram degree Celsius. But the heat capacity of water as a vapor is 2.02 joules per gram degree Celsius and for water as a solid its 2.00 joules per gram degree Celsius. Different heat capacities don't get tripped off. Because questions are coming now where, you have got to actually keep those under control. By the way, it can also be very convenient to represent this number here with slightly different units. If you multiply the numerator and denominator by 1000, then you can actually write Heat loss = Heat gain. What mass of ice at 100 C, can be vaporized by a 20.0kj of heat? There is a basic principle in order to calculate the answer to any of these questions that we'll be looking at and that is to recognize, that the amount of heat loss by something equals to amount of heat gain by something else. Now, we know that there is always heat that's lost in a reaction that we can't account for. Sure, okay but we are not going to really consider that. We try to minimize the amount of error there and I will show you that in an other calculation coming up too. But the point is, it's always heat loss equals heat gain. That's the first thing you always write down on paper. First thing I always do and I have been used for 20 years. You think I wouldn't have to do this anymore but I do because, it is a good example to you. Now look, heat loss equals heat gain. So, in this case what's losing the heat? Well, alright there must be the environment. Something in the environment we are not told, is actually giving heat to the water which is actually under going vaporization. So must be around 100 degree Celsius and ready to just take off as a vapor. So what's losing the heat? Well, the environment is losing that heat and what's gaining the heat? The system here is the water that's gaining the heat and because it's undergoing the phase change and a phase change only, you were told in the question. Then we write nH. Now we must have information for all of this. Well, two of those things anyway. One of them is unknown variable that we have to solve for and if we are looking for what mass of ice, what figure here is going to be able to help us to get mass. And which stands for the number of moles. So, n is number of moles and we find that we can then get the mass. So, what we have is the heat which is 20 kilojoules in this question. And we have a molar heat because, its water vaporizing that's 40.65. so, we rearrange what we do of course to solve for n is to divide each side by big H and so the formula actually then becomes, you always got a chemguy. The chemguy, he understands and when we manipulate this formula you get h over big H and the