How Do I Calculate Energy Changes in Thermodynamics Problems?

[Roro312]

Tomorrow is my test and i understand partially everything... and than i stumbled upon these two questions and if i just know what's used for each formula .. i can do it

1) Calculate the amount of energy required to evaporate an ice cube having a mass of 45g.

I know that first it must go for solid to liquid which is E=mf and than from liquid to gas which is E=mLv .. can someone please show me how to do them... my test is tomorrow:(

Lf water= 3.3x10^5 J/Kg
water=4.2x10^3 J/Kg'C
ice= 2.1x 10^3 J/Kg'C

the other question i didnt get was:

2)An 80g mandarin orange is burned in bomb calorimeter. The 1.55 kg of water change in temperature from 21.0'C to 39.4'C. Calculate the energy per gram in the mandarin orange.


I would show you my attempt except its just ridiculous becuase i reallyyyy don't get it.


Your help is muchhhh appreciated.. i need help ASAP ...thank you

 

[rock.freak667]

Well for the first one, you will need energy to

1)convert the ice to liquid which is (solid to liquid)
2) Raise the temperature to the boiling point of water
3) convert all the liquid to vapor

So find the individual energy requirements and add them up.


For the second one, how much energy would it take to raise the temperature of water of mass 1.55kg from 21C ro 39.4C?.

 

[Roro312]

Thats the problem... i don't know how to convert solid to liquid.. what formula do i use?

 

[Gravitino22]

Q=mC C is a constant you can look up for water its different depending on which phase transition you are calculating.
Q=mc(Tf-Ti) for when your raising the temperature c i think its 1cal/gC for water.

Add all the Qs and viola