10 questions🇬🇧English
🔥 Hess's Law & Enthalpy Changes – A-level
Chemistry · A-level — practise enthalpy definitions, calorimetry calculations, Hess cycles and mean bond enthalpies.
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Questions
Which statement correctly describes an exothermic reaction?
ΔH is positive and the surroundings cool down
ΔH is negative and the surroundings warm up
ΔH is negative and the surroundings cool down
ΔH is positive and the surroundings warm up
20s
Which conditions are used as standard conditions for standard enthalpy changes?
200 kPa and 298 K
100 kPa and 273 K
100 kPa and 298 K
10 kPa and 373 K
20s
Which equation represents the standard enthalpy of formation of water?
H₂(g) + ½O₂(g) → H₂O(l)
2H₂(g) + O₂(g) → 2H₂O(l)
2H(g) + O(g) → H₂O(l)
H₂(g) + ½O₂(g) → H₂O(g)
20s
The standard enthalpy of neutralisation is the enthalpy change when an acid reacts with an alkali to form 1 mole of what?
Hydrogen ions
Hydroxide ions
Water
Salt
20s
50.0 g of water (c = 4.18 J g⁻¹ K⁻¹) is heated and its temperature rises by 6.0 °C. How much energy did the water gain?
1.25 kJ
1254 kJ
0.21 kJ
2.51 kJ
30s
Bond enthalpies (kJ mol⁻¹): H–H 436, Cl–Cl 243, H–Cl 432. What is ΔH for H₂(g) + Cl₂(g) → 2HCl(g)?
+185 kJ mol⁻¹
−247 kJ mol⁻¹
−185 kJ mol⁻¹
+247 kJ mol⁻¹
30s
ΔfH (kJ mol⁻¹): CH₄(g) −75, CO₂(g) −394, H₂O(l) −286. What is ΔcH of CH₄(g)?
−1041 kJ mol⁻¹
−605 kJ mol⁻¹
−891 kJ mol⁻¹
+891 kJ mol⁻¹
60s
Burning 0.0200 mol of a fuel raises the temperature of 100 g of water by 10.0 K (c = 4.18 J g⁻¹ K⁻¹). What is ΔcH of the fuel?
+209 kJ mol⁻¹
−209 kJ mol⁻¹
−2090 kJ mol⁻¹
−20.9 kJ mol⁻¹
60s
ΔcH (kJ mol⁻¹): C(s) −394, H₂(g) −286, C₂H₅OH(l) −1367. What is ΔfH of ethanol?
−3013 kJ mol⁻¹
−279 kJ mol⁻¹
+279 kJ mol⁻¹
+687 kJ mol⁻¹
60s
Why does ΔH calculated from mean bond enthalpies often differ from the value obtained using ΔfH data?
Enthalpies of formation ignore the bonds broken
Bond breaking releases energy, so the signs differ
Mean values are averaged over many different compounds
Hess's law does not apply to gas-phase reactions
30s