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THERMOCHEMISTRY

From your syllabus:

Energetics of a reaction

Core

  • Describe the meaning of exothermic and endothermic reactions
  • Interpret energy level diagrams showing exothermic and endothermic reactions

Supplement

  • Describe bond breaking as an endothermic process and bond forming as an exothermic process
  • Draw and label energy level diagrams for exothermic and endothermic reactions using data provided
  • Calculate the energy of a reaction using bond energies

Energy transfer Core

  • Describe the release of heat energy by burning fuels
  • State the use of hydrogen as a fuel
  • Describe radioactive isotopes, such as 235U, as a source of energy Supplement
  • Describe the use of hydrogen as a fuel reacting with oxygen to generate electricity in a fuel cell
    (Details of the construction and operation of a fuel cell are not required.)

Energetics of a reaction

THERMOCHEMISTRY NOTES (MY HANDWRITTEN NOTES)

 Core

  • Describe the meaning of exothermic and endothermic reactions

EXOTHERMIC REACTION: 

A reaction which releases energy to the surroundings.

Examples:

  1. Adding water to anhydrous Copper (II) sulfate, releases energy.
  2. The reaction between Zinc metal and a solution of Copper (II) Sulfate, releases energy which can be measured using a thermometer. The solution gets hot and the temperature increases.
  3. Burning fuel releases energy in the form of light and heat. 

ENDOTHERMIC REACTION

A reaction which absorbs energy. 

Examples:

  1. AlkaSeltzer tablet added to water absorbs the energy from the water, which can be measured using a thermometer. The solution gets cold and the temperature drops. 
  • Interpret energy level diagrams showing exothermic and endothermic reactions

ENTHALPY, ENTHALPY CHANGE AND ITS RELATION WITH EXOTHERMIC AND ENDOTHERMIC REACTIONS:

The reactants and products in a reaction have a certain potential energy.

This energy measured at atmospheric pressure, has a special name: ENTHALPY.

Enthalpy is represented with the letter H, and the change in enthalpy is represented as ΔH

It is very difficult to know the exact value of the reactants and products enthalpy. In general, it is more useful to know the change in enthalpy for a certain reaction, since when the reactants change into products during the chemical reaction, we can calculate this change. 

This potential energy is also known as enthalpy (symbol, Delta symbolH). During a chemical reaction, the enthalpy of the reactants changes as the reactants form new products.

The enthalpy change (symbol, Delta symbolH) for a reaction can be calculated from a potential energy diagram.

Delta symbolH = H(products) - H(reactants)

Delta symbolH is measured in kilojoules per mole (kJ mol-1).

The following are profile diagrams for exothermic and endothermic reactions. 

In each graph, the enthalpy (energy at atmospheric pressure) is shown.

On the left the reactants have less enthalpy values than the products, so, the reactants need to absorb energy to react. 

The change in enthalpy (ΔH) is shown in blue. 

  • Describe bond breaking as an endothermic process and bond forming as an exothermic process

Bond energy is the amount of energy required to break a bond. Breaking a bond requires energy from the environment so it is an endothermic process. 

When two elements form a bond, they release energy. forming a bond is always an exothermic process. 

  • Draw and label energy level diagrams for exothermic and endothermic reactions using data provided

    (see the profile diagrams exercises)  Energy diagram exercises

  • Calculate the energy of a reaction using bond energies

 

  • Describe the release of heat energy by burning fuels

Fuels are compounds that release energy when they burn. we use that energy to 

  • State the use of hydrogen as a fuel

The reaction between of oxygen and hydrogen produces water and releases a great amount of energy. No by products are obtained. it is a clean fuel. besides it can be obtained by splitting the water molecule use some sun powered plants. It is a great source of energy and does not pollute. It is difficult though to store Hydrogen in safety containers since it can be highly explosive. 

  • Describe radioactive isotopes, such as 235-U, as a source of energy 

Uranium as any radioactive material releases energy while it decays. this energy can be used as fuel. 

  • Describe the use of hydrogen as a fuel reacting with oxygen to generate electricity in a fuel cell
    (Details of the construction and operation of a fuel cell are not required.)

 

   
© Analia Sanchez