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CHEM - Energetics

Chemistry, energetics, calorimetry, enthalpy change - IGCSE | IBDP | DSE | GCE | AP Chemistry

· chemistry,energetics,calorimetry,enthalpy

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 👇

broken image
  • 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

broken image

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

broken image

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

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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

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