4th Class Power Engineering chemistry, thermodynamics and steam practice questions (4A).

Five free practice questions on chemistry, thermodynamics and steam: unit B of paper A (4A) of the 4th Class Power Engineer certificate exam, written to the SOPEEC syllabus. They are in the exam's format: four options, one correct answer. Answer each one before opening the explanation. No account, no email.

01The questions

1. Recall · Describe the three main ways atoms bond together: covalent, ionic, and metallic bonding

   In a methane (CH₄) molecule, each hydrogen atom is joined to the carbon atom by

   1. a covalent bond, a pair of electrons shared by the two atoms
   2. an ionic bond, an electron passed from one atom to the other
   3. a metallic bond, outer electrons pooled among many atoms
   4. a hydrogen bond, a weak pull between nearby molecules
   Show the answer

   A. Correct. Carbon has four outer electrons and each hydrogen has one. Neither gives its electrons up; instead the carbon shares one pair with each hydrogen, so every atom ends with a full outer shell. Bonds formed by sharing between non-metals are covalent, as in water, methane and carbon dioxide.

   Why not the others

   B. Electron transfer forming charged ions is ionic bonding, typical of a metal with a non-metal, as in sodium chloride. Carbon and hydrogen are both non-metals; they share electrons, which is a covalent bond.

   C. A pool of free electrons shared by a whole lattice of atoms is the metallic bond, found only in metals. Inside a methane molecule each hydrogen shares one electron pair with the carbon: a covalent bond.

   D. Despite its name, a hydrogen bond is a weak attraction between separate molecules, such as between water molecules; it does not hold the atoms of one molecule together. Within methane the atoms are joined by covalent bonds.

2. Critical · Define heat and specific heat, and perform sensible heat calculations

   At a district energy plant, storage tank 1 holds 2 000 kg of water at 50 °C and tank 2 holds 200 kg of water at 90 °C. Both are cooled to 20 °C. Which statement is right?

   1. Tank 2 gives up more heat, since its water is hotter.
   2. Tank 1 gives up about 4.3 times as much heat as tank 2.
   3. Both give up the same heat, since both end at 20 °C.
   4. Tank 2 gives up about 2.3 times as much heat as tank 1.
   Show the answer

   B. Correct. Heat given up = m c ΔT, with the same c for both. Tank 1: 2 000 × 30 = 60 000 kg·K; tank 2: 200 × 70 = 14 000 kg·K. Ratio 60 000 ÷ 14 000 ≈ 4.3. Temperature says how hot the water is; the quantity of heat also depends on the mass.

   Why not the others

   A. This confuses temperature with heat. Tank 2 is hotter, but it holds a tenth of the water: 200 × 70 = 14 000 kg·K against 2 000 × 30 = 60 000 kg·K for tank 1.

   C. The final temperature is the same, but the heat released depends on mass × temperature drop: 60 000 kg·K for tank 1 against 14 000 kg·K for tank 2.

   D. This compares only the temperature drops, 70 ÷ 30 ≈ 2.3, and leaves out the mass. With mass included, tank 1 gives up 60 000 ÷ 14 000 ≈ 4.3 times as much as tank 2.

3. Procedural · Describe heat transfer fluids and how they affect the operation of a heat exchanger, including fouling, leakage and vapour locking

   A plant needs process heat at 300 °C and installs a heat-transfer oil system rather than a steam system. The main reason is that the oil

   1. gives up latent heat at a constant temperature as it condenses
   2. has a higher specific heat than water
   3. reaches 300 °C in a system at low pressure
   4. cannot foul the heater tubes the way hard water scales them
   Show the answer

   C. Correct. A heat-transfer oil chosen for the duty stays liquid at 300 °C at a low system pressure, so the heater and piping need only modest pressure ratings. Water's saturation pressure climbs steeply with temperature (to 22.064 MPa at its critical point, 373.95 °C), so steam at 300 °C would mean a high-pressure system throughout.

   Why not the others

   A. The oil is used as a liquid and gives up sensible heat only, so its temperature falls as it heats the process. Condensing at constant temperature is the advantage of steam, not of oil.

   B. The reverse: heat-transfer oils have a lower specific heat than water, so more mass must be circulated for the same duty. The reason to use them is high temperature at low pressure.

   D. Heat-transfer oils do foul: overheated oil breaks down and leaves carbon deposits on heater tubes. The choice is made for high temperature at low pressure.

4. Procedural · Explain the various columns of the steam tables

   A boiler drum's steam space holds 6.0 m³ of dry saturated steam at 1 000 kPa absolute. Given: at that pressure vf = 0.001127 m³/kg and vg = 0.1944 m³/kg. What mass of steam is in the space?

   1. 0.0324 kg
   2. 1.17 kg
   3. 30.9 kg
   4. 5 320 kg
   Show the answer

   C. Correct. Specific volume is volume per kilogram, so mass = V ÷ vg = 6.0 m³ ÷ 0.1944 m³/kg = 30.9 kg. At this pressure steam takes about 170 times the volume of the same mass of saturated water (0.1944 ÷ 0.001127), so a steam space holds little mass.

   Why not the others

   A. The division is upside down: 0.1944 ÷ 6.0 = 0.0324. Mass = volume ÷ specific volume = 6.0 m³ ÷ 0.1944 m³/kg = 30.9 kg.

   B. Volume was multiplied by specific volume: 6.0 × 0.1944 = 1.17. Specific volume is cubic metres per kilogram, so divide: 6.0 ÷ 0.1944 = 30.9 kg.

   D. vf is the specific volume of saturated water, not of steam: 6.0 ÷ 0.001127 = 5 320 kg would be the mass if the space were full of water. For dry steam use vg: 6.0 ÷ 0.1944 = 30.9 kg.

5. Procedural · Describe the purpose and uses of the periodic table using the parts of an atom

   The periodic table gives oxygen an atomic number of 8, and its common atom has a mass number of 16. The nucleus of that atom holds

   1. eight protons and eight electrons
   2. eight protons and sixteen neutrons
   3. sixteen protons and eight neutrons
   4. eight protons and eight neutrons
   Show the answer

   D. Correct. The atomic number is the number of protons (8). The mass number counts protons plus neutrons (16), so there are 16 − 8 = 8 neutrons. The eight electrons that balance the charge are outside the nucleus, in shells.

   Why not the others

   A. Electrons do not sit in the nucleus; they occupy shells around it. The nucleus holds the protons and the neutrons: eight of each for this oxygen atom.

   B. The mass number already includes the protons. Neutrons = mass number − atomic number = 16 − 8 = 8, not 16.

   C. Atomic number and mass number are swapped here. The atomic number, 8, is the proton count and identifies the element; 16 is protons plus neutrons together.

02Answer key

03What unit B covers

Basic chemistry and heat for the plant: states of matter, chemical and physical change, atoms, bonding and balanced equations, simple stoichiometry, the first and second laws of thermodynamics, specific heat and sensible heat, expansion of solids and liquids, conduction, convection and radiation, heat exchanger design, fouling and vapour locking, and reading steam tables for saturated, wet and superheated steam.

The SOPEEC 4th Class syllabus names this unit “Elementary Chemistry and Thermodynamics” (ABSA numbers it A2) and lists four objectives under it. SOPEEC does not publish how many exam questions come from each unit, and neither do the ABSA, Technical Safety BC, TSASK or TSSA pages we checked. The right-hand column is how many questions our bank holds on each objective, Standard and Advanced together, not a count from the exam.

04What the questions turn on

Sensible heat is Q = mcΔT

The heat added or removed depends on the mass, the specific heat and the temperature change, not on the final temperature. A large tank of warm water can give up more heat than a small tank of hotter water.

Enter steam tables in absolute pressure

Steam tables are read with absolute pressure, so atmospheric pressure is added to a gauge reading first. Degrees of superheat are the steam temperature minus the saturation temperature at that absolute pressure.

Wet steam carries part of hfg

Dry saturated steam holds hg, which is hf plus hfg. In wet steam only the dry fraction has taken its latent heat, so h = hf + x hfg, while all of the water carries its sensible heat hf.

Exchanger leaks run high to low

A holed tube in a heat exchanger leaks from the higher-pressure side into the lower one. Oil heated at a higher pressure than its heating steam shows up as an oily film in the condensate, which is kept out of the boiler feed.

05The other units

Paper 4A has twelve units, A to L. Each has its own page of free questions:

The 4th Class Power Engineering practice exam (4A) page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.