Themochemistry

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  • Created by: zoolouise
  • Created on: 27-04-16 14:22

Thermochemistry

Temperature Changes

  • An exothermic reaction is one that releases energy to the surroundings, there is a temperature rise and the ΔH is negative
  • An endothermic reaction is one that takes in energy from the surroundings, there's a temperature drop and ΔH is positive

The kinetic energy of a matter is the energy of motion at molecular level. Bond-breaking and bond-making involve changes in potential energy. The sum of the kinetic cenergy of all the particles in a system and their potential energy is the internal energy of a system.

Within a chemical reaction, the bonds are broken and new ones are made. This changes chemical energy of atoms and energy is exchanged between the chemical system and also the surroundings This leads to heat being given out or taken in from the surroundings. 

Most chemical reactions release energy to their surroundings. This is detected by a rise in temperature of the reaction mixture and surroundings. These are `reactions and some examples are known as:

  • Acids with metals
  • In hand warmers (oxidation of iron)
  • Thermite reaction (aluminium and iron(III) oxide)

However, in some reactions the chemical system will absorb energy from the surroundings in the form and heat and these reactions are endothermic. Examples are:

  • Melting ice
  • In cold packs
  • Thermal decomposition of group 2 carbonates

Enthalpy changes

  • Enthalpy, H, is the heat content of a system at constant
  • presusre.
  • Enthalpy change, ΔH, is the heat added to a system at a constant temperature.

The amount of heat which is transferred in a chemical reaction depends on the conditions in which the reaction occurs under. The total energy content of a system held at a constant pressure is defined as its enthalpy, H. 

Just like internal energy, enthalpy cannot be measured directly. However, an enthalpy change, ΔH, can be easily measured. Its units are joules, J, or kilojoules, kJ.

ΔH = Hproducts - Hreactants

For exothermic changes heat is given out to the surroundings so Hproducts < Hreactants and ΔH is negative

For endothermic changes heat is taken in from the surroundings so Hproducts > Hreactants and ΔH is positive

 

Conservation of energy

  • The principle of conservation of energy states that energy cannot be created or destroyed only changed from one form to another

Standard coniditions

As enthalpy change for reactions depends on the conditions, for values to be compared standard enthalpy cchange is measured when fixed conditions are used. The conditions are:

  • All substances in their standard states
  • A temperature of

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