Topical Hub, IGCSE Chemistry 0620 exam-style questions: mark scheme
Mark scheme and worked solutions for 30 questions: 15 multiple choice, 15 structured.
Answer: A
Burning is a combustion reaction. The surrounding air warms up, which means thermal energy has moved from the reacting magnesium and oxygen out into the surroundings (the air). A reaction that transfers thermal energy to the surroundings is exothermic, so the answer is A.
Answer: B
As potassium nitrate dissolves, the temperature drops from 20°C to 12°C. A temperature drop in the surroundings means the process has absorbed thermal energy from those surroundings (the water). Taking in thermal energy from the surroundings, causing a temperature decrease, is the definition of endothermic, so the answer is B.
Answer: A
When the adhesive hardens, chemical bonds form between the two components. The mixture becoming warm shows that thermal energy is being released into the surroundings during this process. A reaction that releases thermal energy to the surroundings, raising its temperature, is exothermic, so the answer is A.
Answer: C
∆H is defined as the energy of the products minus the energy of the reactants. Here, ∆H = (−394) − (0) = −394 kJ/mol. The negative sign shows the products have less energy than the reactants, confirming this is an exothermic reaction (as expected for combustion), so the answer is C.
Answer: D
Burning petrol releases thermal energy as heat (and does useful work in the engine), so it is exothermic. By definition, ∆H is negative for exothermic reactions, so the answer is D.
Answer: B
Because the limestone only decomposes while it is being heated, thermal energy must constantly be absorbed from the Bunsen burner (or kiln burners) for the reaction to continue. This makes it endothermic, and endothermic reactions have ∆H positive, so the answer is B.
Answer: B
Activation energy, Ea, is defined as the minimum energy that colliding particles must have in order to react. Statement B matches this definition exactly, so the answer is B. The other options describe different energy quantities: overall energy released (A), total bond energy of reactants (C), and enthalpy change (D).
Answer: A
A catalyst speeds up a reaction by lowering the activation energy needed for particles to react — here, from 165 kJ/mol down to 55 kJ/mol. However, a catalyst does not change the energy of the reactants or the products, so the overall enthalpy change, ∆H, stays exactly the same. The answer is A.
Answer: C
Breaking the O=O double bond in oxygen requires an input of energy, since energy must be supplied to pull bonded atoms apart — this is always an endothermic process. Forming the new S=O bonds in sulfur dioxide releases energy, since forming bonds is always exothermic. This matches option C.
Answer: A
The Cl–Cl bond must be broken for one chlorine molecule to form two chlorine atoms. Breaking any bond requires energy to be absorbed, making this an endothermic process (the energy comes from the absorbed ultraviolet light). The answer is A.
Answer: C
For any reaction, ∆H = energy to break bonds in reactants − energy released forming bonds in products. If the reaction is exothermic overall, this value must be negative, meaning less energy was used breaking bonds than was released forming new ones. This matches option C.
Answer: C
First find the energy needed to break the bonds in the reactants: one H–H bond (436) plus one Cl–Cl bond (242), giving 678 kJ/mol. Next find the energy released forming the bonds in the products: two H–Cl bonds are formed (since there are 2 mol of HCl), so 2 × 431 = 862 kJ/mol. Then ∆H = energy to break bonds − energy released forming bonds = 678 − 862 = −184 kJ/mol. Checking by working backwards: −184 = 678 − 862 ✓. The negative sign confirms this matches option C.
Answer: D
The enzymes in yeast act as a biological catalyst, providing an alternative pathway with a lower activation energy so that more particles can react successfully, which speeds up the decomposition. Since the reactants and products themselves are unchanged, the enthalpy change, ∆H, of the reaction is not affected by the catalyst. This matches option D.
Answer: B
The temperature of the solution rises from 22°C to 34°C, showing that thermal energy is being transferred from the reacting mixture into the surroundings (the solution and the cup). A reaction that releases thermal energy to its surroundings, raising their temperature, is exothermic, so the answer is B.
Answer: D
The enthalpy change of the reverse reaction always has the same size but the opposite sign to the forward reaction, because the same amount of energy is involved, just transferred in the opposite direction. Since the forward reaction has ∆H = −92 kJ/mol, the reverse reaction (decomposition of ammonia) has ∆H = +92 kJ/mol. The answer is D.
An exothermic reaction is one that transfers thermal energy to the surroundings, causing the temperature of the surroundings to rise. In the hand warmer, iron reacts slowly with oxygen from the air to form iron(III) oxide. You can tell this reaction is exothermic because the sachet itself gets warm and stays warm for hours — this warmth is thermal energy that has been released by the chemical reaction into the surroundings (the sachet and the air around it, including the person holding it).
An endothermic reaction takes in thermal energy from the surroundings, cooling them down. Here, the temperature of the water falls from 21°C to 9°C, showing that dissolving ammonium nitrate absorbs thermal energy from the water. Since the process is endothermic, ∆H is positive. On a reaction pathway diagram, this means the products (the dissolved ions) are drawn above the reactants (the solid and water), because energy has flowed into the system to form them — the gap between the reactant and product energy levels represents the ∆H value, which is positive here.
The activation energy is the difference between the energy of the reactants and the highest point on the pathway: 160 − 100 = 60 kJ for the forward reaction. The enthalpy change is the difference between the energy of the products and the energy of the reactants: 20 − 100 = −80 kJ. Because this is negative, the products end up with less energy than the reactants, so the reaction is exothermic — the "extra" 80 kJ has been released to the surroundings. For the reverse reaction, the activation energy is measured from the products up to the same highest point: 160 − 20 = 140 kJ. Notice the reverse activation energy (140 kJ) is larger than the forward one (60 kJ) by exactly 80 kJ, which is the size of ∆H — this is always true for a reversible reaction.
Respiration breaks down glucose using oxygen and releases energy that the cell can use for its life processes, so it is exothermic. Photosynthesis does the opposite: it uses energy absorbed from sunlight to build glucose from carbon dioxide and water, so it is endothermic. The two equations are exact reverses of one another, and the energy released by respiration is ultimately the same energy that was stored in the glucose during photosynthesis — energy is being passed from sunlight, into glucose, and then released again for the cell to use.
Copper(II) carbonate only breaks down while it is being heated, showing that thermal energy must be continuously supplied for the reaction to keep going — this makes it endothermic. Activation energy is the minimum energy that colliding particles need in order to react. A catalyst works by offering an alternative reaction pathway with a lower activation energy, so more particles can react successfully at a given temperature. However, adding a catalyst never changes the energy of the reactants or products themselves, so it has no effect on the overall enthalpy change, ∆H, of the reaction.
In this reaction, the C=C double bond in ethene and the H–H bond in hydrogen are broken, while a new C–C single bond and two new C–H bonds are formed (the four C–H bonds already present in ethene are unchanged, so they are not counted). Breaking bonds needs 612 + 436 = 1048 kJ. Forming the new bonds releases 347 + (2 × 413) = 1173 kJ. So ∆H = 1048 − 1173 = −125 kJ/mol. Since more energy is released forming bonds than is needed to break them, the reaction is exothermic, which fits with hydrogenation reactions being used industrially without needing to add much extra energy.
The reactants contain two H–H bonds and one O=O bond, so the energy needed to break them all is (2 × 436) + 496 = 1368 kJ. The products are 2 mol of H2O, and each water molecule has two O–H bonds, so 4 O–H bonds form in total, releasing 4 × 463 = 1852 kJ. The enthalpy change is ∆H = 1368 − 1852 = −484 kJ/mol. This is negative, so the reaction is exothermic, which is why hydrogen fuel cells can be used to generate useful energy.
The 75 kJ/mol figure is the activation energy without a catalyst — the minimum energy that colliding hydrogen peroxide molecules need in order to react. Manganese(IV) oxide works as a catalyst by providing a different reaction pathway that needs only 25 kJ/mol of activation energy. Since more particles will have this lower amount of energy at a given temperature, more successful collisions happen per second, so the reaction goes faster. However, the catalyst does not alter the reactants or products, so the enthalpy change stays at −98 kJ/mol whether or not the catalyst is used.
The highest recorded temperature is 27.5°C, so the temperature rise is 27.5 − 19.5 = 8.0°C. An exothermic reaction is one that transfers thermal energy to the surroundings, causing their temperature to rise. Because the temperature of the acid–alkali mixture rises as the two solutions react, thermal energy must be flowing out of the reacting particles and into the surroundings (the solution itself, which is being measured). This matches the definition of an exothermic reaction, confirming that neutralisation is exothermic.
The heat pack becomes warm because the reaction between iron and oxygen releases thermal energy into its surroundings — this release of energy, shown by the rise in temperature, is what makes a reaction exothermic. The cold spray becomes cold because the chemical dissolving inside it absorbs thermal energy from its surroundings (including your skin) rather than releasing it — this absorption of energy, shown by the fall in temperature, is what makes a process endothermic.
Breaking the reactant bonds needs 944 (N≡N) + (3 × 436) (H–H) = 2252 kJ. Each ammonia molecule has 3 N–H bonds, so 2 mol of NH3 contain 6 N–H bonds, releasing 6 × 391 = 2346 kJ when they form. So ∆H = 2252 − 2346 = −94 kJ/mol, showing the forward reaction is exothermic. Because the reaction is reversible, the reverse reaction (ammonia decomposing back into nitrogen and hydrogen) must have exactly the opposite enthalpy change: +94 kJ/mol, making it endothermic.
Since the forward reaction is exothermic with ∆H = −196 kJ/mol, the reverse reaction must be endothermic with ∆H = +196 kJ/mol — the same size, opposite sign. On a reaction pathway diagram, the activation energy for the reverse reaction is measured from the (lower) product energy level up to the same peak that the forward reaction is measured from, so Ea(reverse) = Ea(forward) + 196 = 250 + 196 = 446 kJ/mol. Because the products are much lower in energy than the reactants (a large exothermic ∆H), the reverse reaction has to climb a much bigger energy hill, which is why its activation energy is so much higher than the forward reaction's.
Sodium hydroxide's temperature rises by 32.5 − 20.0 = 12.5°C, while ammonium chloride's temperature falls by 20.0 − 14.5 = 5.5°C. A temperature rise means thermal energy has been released into the water, so dissolving sodium hydroxide is exothermic. A temperature fall means thermal energy has been absorbed from the water, so dissolving ammonium chloride is endothermic. This shows that dissolving is not always the same type of process — it depends on the particular substance being dissolved.
Breaking any chemical bond always requires an input of energy (endothermic), while forming any new bond always releases energy (exothermic). When ethanol burns, energy must first be absorbed to break the C–C, C–H, C–O, O–H and O=O bonds in the reactants. Then energy is released as new C=O and O–H bonds form in the carbon dioxide and water produced. Combustion reactions like this are exothermic overall because the amount of energy released forming the new, more stable bonds in the products is greater than the amount of energy absorbed breaking the bonds in the reactants — the surplus energy is released to the surroundings as heat.
Breaking the four C–H bonds in methane and the two O=O bonds in oxygen needs (4 × 413) + (2 × 496) = 2644 kJ. The products are one CO2 molecule (with 2 C=O bonds) and two H2O molecules (with 4 O–H bonds in total), so forming these releases (2 × 805) + (4 × 463) = 3462 kJ. This gives ∆H = 2644 − 3462 = −818 kJ/mol, confirming that methane combustion is strongly exothermic, as expected for a fuel used for heating. On a reaction pathway diagram, this means the products would be drawn at a lower energy level than the reactants, with the large drop in energy between them representing the ∆H value released to the surroundings.