Energetics
5 marks · EasyMagnesium reacts with dilute hydrochloric acid in a reaction described below:
The reaction is exothermic. An energy profile for this reaction is shown below, but the axes and key features have not been labelled.
On the diagram, clearly label: - the correct position of the reactants and products; - the activation energy; - the enthalpy change (ΔH), with an arrow showing its direction.
In a second experiment, a student dissolves ammonium chloride (NH₄Cl) in water. She uses 50.0 g of water at 24.0°C. After the salt dissolves, the final temperature is 19.7°C. The specific heat capacity, c, of water is 4.18 J/(g°C).
Calculate the heat energy change (Q) for the dissolving process. State if the reaction is exothermic or endothermic, and explain your answer.
Calculate the heat energy change, Q, for this process. Give the unit.
State if this process is exothermic or endothermic and explain your answer.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a(i)) Diagram correctly labelled: Reactants at higher energy, products at lower; activation energy upwards from reactants to peak; ΔH arrow downwards from reactants to products. · must show reactants at higher energy, products at lower energy, activation energy (Ea) as upward arrow/label, ΔH as downward arrow/label from reactants to products
(b(i)) Q = -893 J (to 3 significant figures) J
(b(ii)) Endothermic; temperature decreases, so heat is absorbed from the surroundings. · must show endothermic stated, reason: temperature goes down/heat absorbed
Common mistakes: ΔH arrow in wrong direction; identifying process as exothermic for temperature decrease; missing units in Q calculation
Show mark scheme and worked solution
Marking points
B1 (a(i)) · 1 mark · Labels the reactants at the higher energy side and products lower (relative positions).
B2 (a(i)) · 1 mark · Correctly marks both the activation energy (Ea, upwards arrow to peak) and ΔH (downwards arrow from reactants to products) on the diagram.
M1 (b(i)) · 1 mark · Uses Q = mcΔT correctly; Q = 50.0 × 4.18 × change in T.
A1 (b(i)) · 1 mark · Correctly calculates Q = -893 J (accept -892 to -895 J), unit required. (follow-through)
B3 (b(ii)) · 1 mark · States the process is endothermic and gives valid reason (temperature decrease or heat absorbed).
Worked solution
(a) On the diagram:
- The 'Reactants' label is at the higher energy level (point P). - The 'Products' label at the lower energy level (point Q). - Draw an upward arrow from the reactants to the peak (A): label this 'activation energy (Ea)'. - Draw a downwards arrow from reactants to products: label this 'ΔH'. - For an exothermic reaction, ΔH is negative (arrow points down).
(b) (i) Q = mcΔT; m = 50.0 g; c = 4.18 J/g°C; ΔT = (19.7 – 24.0)°C = –4.3°C
So Q = –899 J. (Accept values from –892 to –900 J with rounding, unit J is required.)
(ii) The process is endothermic because the temperature decreases—the system absorbs heat from the surroundings.