CHEM - Energetics
Chemistry, energetics, calorimetry, enthalpy change - IGCSE | IBDP | DSE | GCE | AP Chemistry
Exothermic vs. Endothermic
The energy level diagram for endothermic and exothermic reaction looks like this 👇
During a chemical reaction:
Determine whether the following reactions are endothermic or exothermic.
(a) H2 + Cl2 → 2HCl
Energy in = 436 + 243 = 679 kJ/mol
Energy out = 2 x 432 = 864 kJ/mol
Energy change = 679 - 864 = -185 kJ/mol
Since the energy change is negative, the reaction is exothermic.
(b) 2HBr → H2 + Br2
Energy in = 2 x 366 = 732 kJ/mol
Energy out = 436 + 193 = 629 kJ/mol
Energy change = 732 - 629 = 103 kJ/mol
Since the energy change is positive, the reaction is endothermic.
Calorimetry Experiment for Combustion
Energy released from combustion of alcohol = energy used to heat up the water
Energy used to heat up the water can be calculated by using the equation
Q = mcΔT
Q = energy transferred to water (J)
m = mass of water (kg)
c = specific heat capacity of water
= 4.2 J/g·°C
ΔT = change in temperature of water (°C)
The energy from burning 0.5 g of propane was transferred to 100g of water to raise its temperature by 20°C.
(a) Calculate the heat energy change (in kJ)
Q = 100 x 4.2 x 20 = 8400 J = 8.4 kJ
(b) Calculate the molar enthalpy change of propane.
Molar mass of propane = 44 g/mol
Moles of propane burnt = 0.5 / 44 = 0.01136 mol
Total enthalpy change = -8.4 kJ (negative since combustion is an exothermic reaction)
Molar enthalpy change = -8.4 / 0.01136 = -739 kJ/mol
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