the change in heat (q) when 30 grams of copper melt.

Question

Determine the change in heat (q) when 30 grams of copper melt.

 

( L_f = 205 J/g     L_v = 4807 J/g )

Answer 1

Here the temperature of copper is not given.

Hence I am assuming that the copper is at its melting point.

At the melting point ,when copper melts there is no change in temperature. The heat required to melt is the latent heat of fusion.

Given L_v= 205j/g

This means that to melt 1 g of copper the heat needed is 205 J .

Hence the heat required to melt 30 g of copper would be

mxL_v

_{= 30g x 205 j/g}

_=6150j

_{=6000j ( 1sig fig in the answer)}

Answer 2

We will need to know the original temperature of the piece of copper. We also need to know the melting point of copper and the specific heat (m.p. = 1085ºC; sp. heat = 24.44 J/molº; Wikipedia). I'm assuming the L_f and L_v are latent heat of fusion and latent heat of vaporization, respectively.

If you are at the melting point (1085ºC) for copper, the the heat required to melt the sample is...

q = (m)(L_f) = (30 g)(205 J/g) = 6150 J = 6000 J (1 sig. fig.)

If the original temperature of the copper is something other than 1085ºC you will use q = mC∆T and then q = mL_v and add the two values.

q = mC∆T

q = heat to go from starting temperature to melting point

m = mass of Cu

C = specific heat of Cu = 22.44 J/molº = 0.353 J/gº

∆T = 1085 - orig. temp.

Solve for q and then add this value to the above value (6000 J)