A calorimeter, heat capacity H = 42.7 J K , contains 0.80 kg of water at 15C. 0.40 kg of molten lead, specific heat capacity c = 157 J kg-1K-1and latent heat of fusion L = 2.31 x 10^4Jkg-1,is poured into the calorimeter. The final temperature of the system is 25.0 C. What was the initial temperature of the lead?
The specific heat capacity of water is 4182 J kg-1K-1
The melting temperature of lead is 327 C.
The specific heat capacity of solid lead is 136 J kg-1K-1.
This question is bit complicated. We need to understand this step by step.
- First we have a sample of water (0.8kg) at 15C in a calorimeter.
- Then molten lead at T temperature is added in to the calorimeter.
- T>melting point of lead. So initially lead drops it temperature while it comes to melting point.
- When lead comes to melting point it uses its latent heat of fusion and become lead solid.
- When it become solid it will further decrease its temperature and the system will come to equilibrium
- At all these time water and calorimeter will absorb all the energy released by lead.
Let us first consider energy released by Lead
Heat released to become lead at melting point = Q1
Heat released at latent heat of fusion = Q2
Heat released up to equilibrium = Q3
`Q = mCtheta`
`Q1 = 0.4xx157xx(327-T)`
`Q = mL`
`Q2 = -0.4xx2.31xx10^4 = -4000J = -9.24KJ`
`Q = mcCtheta`
`Q3 = -0.4xx136xx(25-327) = -16428.8 = -16.43KJ`
(-) sign is used because the energy is released.
For absorbing energy
Energy absorbed by water = Q4
Energy absorbed by calorimeter = Q5
`Q = mCtheta`
`Q4 = 0.8xx4182xx(25-15) = 33456J = 33.46KJ`
`Q5 = Ctheta`
`Q5 = 42.7xx10 = 427J = 0.427KJ`
Energy released = energy absorbed
`-0.4xx157xx(327-T)/1000+9.24+16.43 = 33.46+0.427`
`T = 457.84`
So the initial temperature of lead is 454.84C
There is no heat losses in the system other than the releasing and absorbing heat between water,lead and calorimeter.