Endothermic reactions
3 marks · NormalA reaction takes place in which 2.5 g of calcium chloride, , dissolves in water and releases 8.6 kJ of heat to the surroundings.
Show how you would calculate the molar enthalpy change, , for this process. Give your answer in kJ/mol and include the correct sign. Explain each step in your calculation.
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.
Show answer
-294 kJ/mol (rounded), with each calculation step explained: find moles, divide heat released by moles, show sign, reasoning for sign
Also accept: –294 kJ/mol (accept –296 for different rounding at intermediate steps if fully shown); +294 kJ/mol, if sign is justified and described as energy released
Common mistakes: Incorrect calculation of moles (wrong molar mass for CaCl2, should use 111 g/mol); Omission of sign or unit; Failing to explain method
Show mark scheme and worked solution
Marking points
M1 · 1 mark · Correctly calculates number of moles from mass and correct molar mass (111 g/mol for CaCl2).
M2 · 1 mark · Correctly divides heat released (8.6 kJ) by number of moles to find ΔH per mole (shows correct process, allows follow-through from wrong moles). (follow-through)
A1 · 1 mark · Final answer correct with sign (–294 kJ/mol for exothermic), and explanation of sign (energy released = negative ΔH). Unit required for full accuracy mark. (follow-through)
Worked solution
Step 1: Calculate the number of moles of calcium chloride dissolved.
Step 2: Calculate the molar enthalpy change, ΔH.
However, using significant figures based on original data (2 s.f. in 8.6 and 2.5), report as –380 kJ/mol. If 0.0225 mol is rounded to 0.023 mol, ΔH = –374 kJ/mol.
Step 3: The negative sign shows that the process is exothermic—the reaction releases energy to the surroundings.