SPM Chemistry Form 5 Notes – Terminology and Concepts: Thermochemisty (Part 1)

by BerryBerryTeacher

in Berry Announcement,Berry Reference (Notes)

Thermochemistry is the science of energy and heat originating from chemical reactions or physical transformations. Berry Berry Readers should find this topic of SPM Form 5 Chemistry – Thermochemistry Part 1 relatively confusing at first due to the concepts of exothermic and endothermic reactions. Do not fret however, as it will become natural to you once you have completed a few calculation questions and understand the concepts behind it. So for this part, you’ll be introduced to the general concept of thermochemistry, exothermic reaction, endothermic reaction, heat of reaction and energy level diagram.

Tips to learn: Focus on exothermic reactions, then you only need to reverse most of the concepts (at least for this level) to understand endothermic reactions. Remember to identify the type of reactions by looking at the shape of the curve in the energy level diagram.

SPM Chemistry Form 5 – Terminology and Concepts: Thermochemistry (Part 1)

Thermochemistry:

  • The Law of Conservation of Energy – energy can neither be created nor destroyed but it can be changed from one form to the other form.
  • Exothermic reaction – a chemical reaction that gives out heat to the surrounding.
  • Endothermic reaction – a chemical reaction that absorbs heat from the surrounding.
  • Surroundings do not involve in the reactions. Example: water, container, the air, solvent and thermometer.
  • Heat of reaction – the heat change when the number of moles of reactants in the chemical equation reacts to form products in standard conditions.
  • Standard conditions: temperature (25˚C / 298 K), pressure (1 atm), concentration of solution (1.0 mol dm-3), reactants and products are at their normal physical states.
  • Heat of precipitation – the heat change when one mole of a precipitate is formed from their ions in aqueous solution.
  • Heat of displacement – the heat change when one mole of a metal is displaced from its salt solution by a more electropositive metal.
  • Heat of neutralisation – the heat change when one mole of water is formed from the reaction between an acid and an alkali.
  • Heat of combustion – the heat change when one mole of a substance is completely burnt in oxygen under standard conditions.

Exothermic reaction

  1. Chemical energy –> Heat energy
  2. The heat energy is transferred to the surrounding.
  3. Temperature of the surrounding increases.

Example of Chemical Reactions:

  • Respiration
  • Burning of metal
  • Reaction of an alkaline metals (Group 1) with water
  • Reaction of a reactive metal with acid
  • Neutralisation reaction between acid and alkali
  • Reaction of a carbonate with acid
  • Combustion of carbon compound
  • Displacement reaction of metals
  • Rusting of iron

Example of Physical Processes:

  • Freezing process
  • Condensation process
  • Dissolving an alkali in water
  • Dissolving an concentrated acid in water

Endothermic reaction

  1. Heat energy –> Chemical energy
  2. The heat is absorbed from the surrounding.
  3. Temperature of the surrounding decreases.

Example of Chemical Reactions:

  • Photosynthesis
  • Decomposition of nitrate salts
  • Decompositon of carbonates salts
  • Reaction between acid with hydrogen carbonates

Example of Physical Processes:

  • Melting process
  • Boiling process
  • Sublimation process
  • Dissolving of ammonium salts in water
  • Dissolving of potassium salts in water
  • Dissolving of thiosulphate  in water

Heat of Reaction

  1. Enthalpy (H) – absolute energy content of a substance.
  2. Change in energy content (ΔH) – absolute energy content cannot be determined, but ΔH can be determined when the reactants are converted to the products.
  3. 1 kJ (kilojoule) = 1000 J
  4. The unit ΔH  is kJ

Energy Level Diagram

  1. ΔH = H product – H reactant = negative value.
  2. Example: CH4(g) + 2O2(g) –> CO2(g) + 2H2O(l) ΔH = -890 kJ
  3. The value of ΔH is negative = exothermic reaction.
  4. ΔH = H product – H reactant = positive value.
  5. Example: N2(g) + 3H2(g) –> 2NH3(g) ΔH = +91.8kJ
  6. The value of ΔH is positive = endothermic reaction.

Try your best to understand the energy level diagram, it is the most crucial part for this subtopic. Try and try again until you understand it fully. You’ll learn about energy change during formation and breaking of bonds, application of exothermic and endothermic reactions in everyday life, heat of precipitation and heat of displacement in Part 2 of SPM Form 5 Chemistry Thermochemistry.

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