DP Physics (last assessment 2024)

Question 20N.2.SL.TZ0.3

Date	November 2020	Marks available	[Maximum mark: 8]	Reference code	20N.2.SL.TZ0.3
Level	SL	Paper	2	Time zone	TZ0
Command term	Calculate, Estimate, Explain	Question number	3	Adapted from	N/A

[Maximum mark: 8]

20N.2.SL.TZ0.3

A sample of vegetable oil, initially in the liquid state, is placed in a freezer that transfers thermal energy from the sample at a constant rate. The graph shows how temperature $T$ of the sample varies with time $t$ .

The following data are available.

Mass of the sample $= 0.32 kg$
Specific latent heat of fusion of the oil $= 130 kJ {kg}^{- 1}$
Rate of thermal energy transfer $= 15 W$

(a(i))

Calculate the thermal energy transferred from the sample during the first $30$ minutes.

[1]

Markscheme

$« 15 \times 30 \times 60 » = 27000 « J »$ ✓

(a(ii))

Estimate the specific heat capacity of the oil in its liquid phase. State an appropriate unit for your answer.

[2]

Markscheme

$27 \times 10^{3} = 0.32 \times c \times (290 - 250)$ OR $2100$ ✓

$J {kg}^{- 1} K^{- 1}$ OR $J {kg}^{- 1}^{0} C^{- 1}$ ✓

Allow any appropriate unit that is $\frac{e n e r g y}{m a s s \times t e r m p e r a t u r e}$

(b)

The sample begins to freeze during the thermal energy transfer. Explain, in terms of the molecular model of matter, why the temperature of the sample remains constant during freezing.

[3]

Markscheme

«intermolecular» bonds are formed during freezing ✓

bond-forming process releases energy
OR
«intermolecular» PE decreases «and the difference is transferred as heat» ✓

«average random» KE of the molecules does not decrease/change ✓

temperature is related to «average» KE of the molecules «hence unchanged» ✓

To award MP3 or MP4 molecules/particles/atoms must be mentioned.

(c)

Calculate the mass of the oil that remains unfrozen after $60$ minutes.

[2]

Markscheme

mass of frozen oil $« = \frac{27 \times 10^{3}}{130 \times 10^{3}} » = 0.21 « kg »$ ✓