Question

400 g of wax at 10°C is heated to 80°C, when it starts melting. On complete melting wax is further heated so that temperature rises to 130°C. Calculate:

1. Heat energy required to bring the wax to its melting point.
2. Heat energy required to melt the wax.
3. Heat energy required to bring the molten wax to 130°C.

[sp. heat capacity of solid wax = 1.5 Jg⁻¹°C⁻¹; sp. heat capacity of liquid wax = 1.8 J⁻¹°C⁻¹; sp. latent heat of wax = 80 Jg⁻¹]

Answer 1

Given: m = 400 g, c_solid = 1.5 Jg⁻¹°C⁻¹, $$c_{\text{liquid}}$$ = 1.8 J⁻¹°C⁻¹, L = 80 Jg⁻¹, Melting point = 80°C

(a) Heat to raise solid wax from 10°C to 80°C:

Δθ = 80 − 10 = 70°C

H = mcΔθ

= 400 × 1.5 × 70

= 42000 J

(b) Heat to melt wax at 80°C:

H = mL

= 400 × 80

= 32000 J

(c) Heat to raise molten wax from 80°C to 130°C:

Δθ = 130 − 80 = 50°C

H = mcΔθ

= 400 × 1.8 × 50

= 36000 J