Question 3.
Answer:
They are equal in magnitude but opposite in sign.
Explanation:
Fusion is another word for melting 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)
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Question 4.
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)
Calculate q₁
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 = -5020 J
Calculate q₂
ΔH(cryst) = -334 J·g⁻¹
q₂ = 40.0 × (-334)
q₂ = -13 360 J
Calculate the total heat
q = -5 020 – 13 360
q = - 18 400 J
The negative sign shows that heat is removed, so you must remove
18 400 J.