Exothermic vs. Endothermic
Endothermic reaction:
- Energy is absorbed from the surroundings during the reaction.
- Temperature of surroundings decreases.
- Enthalpy change is positive.
Exothermic reaction:
- Energy is released to the surroundings during the reaction.
- Temperature of surroundings increases.
- Enthalpy change (total heat energy change) is negative.
The energy level diagram for endothermic and exothermic reaction looks like this 👇
- For both endothermic and exothermic reactions, activation energy is required to start the reaction.
- For an endothermic reaction, energy level of products is higher than the energy level of reactants.
- For an exothermic reaction, energy level of products is lower than the energy level of reactants.
Bond Energy
During a chemical reaction:
- Energy is taken in to break bonds in the reactants.
- Energy is released to form bonds in the products.
If....
- energy required to break bonds > energy released to form bonds → the reaction is endothermic
- energy required to break bonds < energy released to form bonds → the reaction is exothermic
Bond energy: energy required to break one mole of the bond
It is possible to calculate the enthalpy change of a reaction with bond energy.
- Energy in = bond energy for reactant
- Energy out = bond energy for product
- Change in energy = energy in - energy out
If change in energy >0, the reaction is endothermic.
If change in energy <0, the reaction is exothermic.
Example
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
Experimental setup:
- Pour a known mass(volume) of water into the copper can.
- Measure the initial temperature of water.
- Fill the spirit burner with a known mass of test substance.
- Light the wick of the spirit burner.
- After some time, turn off the spirit burner.
- Measure the temperature of the water.
- Measure the mass of the substance in the spirit burner.
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)
Example
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