Question 21M.2.SL.TZ1.3
| Date | May 2021 | Marks available | [Maximum mark: 12] | Reference code | 21M.2.SL.TZ1.3 |
| Level | SL | Paper | 2 | Time zone | TZ1 |
| Command term | Calculate, Determine, Estimate, Explain, Show that, State | Question number | 3 | Adapted from | N/A |
A mass of 1.0 kg of water is brought to its boiling point of 100 °C using an electric heater of power 1.6 kW.
The molar mass of water is 18 g mol−1. Estimate the average speed of the water molecules in the vapor produced. Assume the vapor behaves as an ideal gas.
[2]
Ek = « » = «J» ✓
v = «» = 720 «m s−1» ✓
State one assumption of the kinetic model of an ideal gas.
[1]
particles can be considered points «without dimensions» ✓
no intermolecular forces/no forces between particles «except during collisions»✓
the volume of a particle is negligible compared to volume of gas ✓
collisions between particles are elastic ✓
time between particle collisions are greater than time of collision ✓
no intermolecular PE/no PE between particles ✓
Accept reference to atoms/molecules for “particle”
A mass of 0.86 kg of water remains after it has boiled for 200 s.
Estimate the specific latent heat of vaporization of water. State an appropriate unit for your answer.
[2]
«mL = P t» so «» = 2.3 x 106 «J kg-1» ✓
J kg−1 ✓
Explain why the temperature of water remains at 100 °C during this time.
[1]
«all» of the energy added is used to increase the «intermolecular» potential energy of the particles/break «intermolecular» bonds/OWTTE ✓
Accept reference to atoms/molecules for “particle”
The heater is removed and a mass of 0.30 kg of pasta at −10 °C is added to the boiling water.
Determine the equilibrium temperature of the pasta and water after the pasta is added. Other heat transfers are negligible.
Specific heat capacity of pasta = 1.8 kJ kg−1 K−1
Specific heat capacity of water = 4.2 kJ kg−1 K−1
[3]
use of mcΔT ✓
0.86 × 4200 × (100 – T) = 0.3 × 1800 × (T +10) ✓
Teq = 85.69«°C» ≅ 86«°C» ✓
Accept Teq in Kelvin (359 K).
The electric heater has two identical resistors connected in parallel.
The circuit transfers 1.6 kW when switch A only is closed. The external voltage is 220 V.
Show that each resistor has a resistance of about 30 Ω.
[1]
«Ω» ✓
Must see either the substituted values OR a value for R to at least three s.f.
Calculate the power transferred by the heater when both switches are closed.
[2]
use of parallel resistors addition so Req = 15 «Ω» ✓
P = 3200 «W» ✓