Question

How much heat energy is needed to melt 36.0 of ice at zero Celsius (H of f=6.01 kJ/mol, H of v=40.7 kJ/mol)
a. 12.0 kJ b.216kJ c.81.4kJ d. 1470 kJ
I know it's A but I don't understand how to get that and I couldn't do subscripts so h of v and h of f is actually h with a subscript of f or v.

Answer 1

@abb0t please help me? :)

Answer 2

I'm going to assume it's 36.0 g because that gives A as the answer. Ice is frozen water so it's chemical formula is H2O with a molar mass of 18 g/mol. The equation you need for this question is:

\[q=\Delta Hn\]
Where q is the total heat, ΔH is the molar enthalpy (of fusion), and n is the moles. We know the enthalpy per mole, so if we know how many moles we have, we know how much heat we have too.

Moles of H2O = 36.0 g / 18 g/mol = 2 mol

q = (6 kj/mol)(2 mol) = 12 kJ

We use the heat of fusion here because fusion refers to state changes between solid and liquid. Heat of vaporization refers to state changes between liquid and gas and so isn't useful here. Hope that helps!

Answer 3

Ok got it does the 6 come from the heat of fusion value and is that a set value?

Answer 4

Yeah sorry should have written 6.01. Yes it is a set value, and it is different for every compound.

Answer 5

Ok so how do I find it for different compounds? I'm sorry to bother you with this I am just horrid at chemistry!

Answer 6

It's a characteristic of a compound like density or boiling point. It's a value you'd usually be given. If you were asked to calculate it, you would need more information. Using the equation above, you could calculate heat of fusion if you know total heat and moles.

Answer 7

Ok so the delta H value for the heat of fusion you looked up for that compound than, by finding the moles, you calculated it?

Answer 8

Well in this case the heat of fusion was given in the question. I used heat of fusion (ΔH) and moles (n) to find total heat energy (q). What I meant was if you're instead given n and q, you can use them to find ΔH using the same equation.

Answer 9

Ok so like algebra and isolating the variable I get it I think.

Answer 10

Yup exactly. Just solving for the variable you don't know.

Answer 11

Ok thank you sooo much do you think you have the time to help me with another problem?

Answer 12

Sure!

Answer 13

Ok cool :) Which isotope is produced when Ca-42 undergoes alpha decay?
a. Ti-46 b. Ti-38 c.Ar-46 d.-38

Answer 14

During alpha decay you lose a helium nucleus (an alpha particle). What this means is that you've actually lost 2 protons and 2 neutrons. Because you lose two protons, your atomic number goes from 20 (which is calcium) to 18 (which is argon). Because you've lost 4 atomic mass units, you mass goes down from 42 to 38.

The isotope produced is therefore Ar-38!

Answer 15

Awesome! I get it so its 4 no matter what the product is? Thank you so much by the way :)

Answer 16

Yes whenever you have alpha decay, you're always losing a helium nucleus off whatever your starting atom is.

Answer 17

Ok I actually get how you explain things I am struggling in chemistry and my final is a two day test so you are saving my life studying with me :)

Answer 18

No problem - happy to help :)

Answer 19

Well someday if you ever are struggling with writing or history I can return the favor haha just not in math or chemistry... Could I ask you if an equation's reactants are CH3NH2+H2O would CH3NH2 be the acid? The numbers are subscripts.

Answer 20

Never mind I got it :)

Answer 21

I would say it's the base because it accepts a proton from water.