Enthalpy- The basic stuff you must know!

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The rule of thermodynamics.

Energy may be exchanged between a chemical system and the surroundings but the total energy remains constant. 

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Exothermic and endothermic reactions.

If the reacting chemicals lose energy then the reaction is exothermic(gives off heat) this energy loss is balenced by the energy gain to the surroundings which causes a rise in temperature. The enthalpy change for these reactions are negative

However if the reacting chemicals gain energy then the reaction isendothermic (takes in heat). The energy gain is ballenced by the energy lost from the surroundings thus why there is a fall in temperature. The enthalpy change for these reactions are positive.

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The standard conditions.

  • A pressure of 100kPa
  • A temperature of 298K (or 25°C)
  • A concentration of 1 mole dm-3 (for aqueous solutions). 
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Standard enthalpy of combustion

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states. 

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Standard enthalpy of reaction.

The energy change that accompanies a reaction in the molar quantities that are expressed in a chemical reaction under standard conditions, all reactants and products being in their standard states.

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Standard enthalpy of formation.

 The enthalpy change that takes place when one mole of a compound in its standard state is formed from its constituent elements in their standard states under standard conditions. 

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Calculating the enthalpy change.

The heat energy change (Q) is something that can be calculated in the equation below:

 Q=mc∆T 

Where:

  • m is the mass of the surroundings.
  • c is the specific heat capacity. 
  • T is the temperature change (final temperature - initial temperature)

100g of ethanol heated 250cmof water from 20C to 55C. Find the enthalpy change for this reaction. 250cmhas a mass of 250g. Water has a specific heat capacity of 4.18 (unless it says otherwise). T= 55-20=35

  250 4.18 x 35= 36575J. It is an exothermic reaction so it becomes -36575J.

So now we find out the number of moles that reacted. One mole of ethanol has a mass of 46. 100/46= 2.17 moles. So for one mole of Ethanol we have to divide 36575 by 2.17 to get 16854.84j which in Kj is 16.85Kj. 

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