<em>Question 3.
</em>
Answer:
They are equal in magnitude but opposite in sign.
Explanation:
Fusion is another word for <em>melting</em> and crystallization is another word for freezing.
They are opposite processes, so the heat you must add (positive sign) to melt a substance is equal to the heat released but with a negative sign.
ΔH(fus) = -ΔH(cryst)
===============
<em>Question 4.
</em>
Answer:
18 400 J
Explanation:
There are two heat transfers in this question.
Total heat = heat to cool water + heat to freeze water
q = q₁ + q₂
q = mCΔT + mΔH(freeze)
<em>Calculate q₁
</em>
m = 40.0 g
C = 4.184 J·K⁻¹g⁻¹
ΔT = T_f – T_i = 0 °C - 30 °C = -30 °C
q₁ = 40.0 × 4.184× (-30)
q₁ = <em>-5020 J
</em>
<em>Calculate q₂
</em>
ΔH(cryst) = -334 J·g⁻¹
q₂ = 40.0 × (-334)
q₂ = <em>-13 360 J
</em>
<em>Calculate the total heat
</em>
q = -5 020 – 13 360
q = - 18 400 J
The negative sign shows that heat is removed, so you must remove
18 400 J.
