2 - Energetics

Enthalpy Change (DELTA H) - is the overall energy exchanged with the surroundings when a change happens at constant pressure and the final temperature is the same as the starting temperature.

• The units for (DELTA H) are KJ/mol-1

Exothermic and Endothermic changes

Exothermic - Energy is given out to the surroundings. e.g - Burning, Respiration, hot packs and self warming drinks. 

Endothermic - Energy is taken in from the surroundings. e.g - Melting and vaporising. Photosynthesis can also be classed as endothermic as it takes in energy from the sun. 

Measuring enthalpy changes:

Heat taken in or given out during a chemical reaction can be measured constantly to determine enthalpy changes. 

SPECIFIC HEAT CAPACITY OF WATER = 4.2 J g-1 K-1

Energy transferred/j = Mass(g) * SHC(J g-1 K-1) * Change in temperature (Delta T[K])

Bomb Calorimeter - ensures that the sample has burned completely and that heat losses are avoided. this will give a more accurate value for the enthalpy changes of combustion. they can be used to determine the energy values of foods and fuels. 

Enthalpy Changes in Solution:

Enthalpy changes for reactions in solutions can be measured using insulated plastic containers such as polystyrene cups. 

If the reaction is exothermic, the energy released cannot escape to the surroundings so it heats up the solution.

If the reaction is endothermic, no energy can enter from the surroundings so the solution cools.  

Standard Enthalpy Changes:

Standard conditions are:

• 1 atmosphere of pressure
• 25`c temperature
• Substances in their standard state
• Solutions with a concentration of 1 mol/dm-3

Standard Enthalpy Change of a reaction - is the energy transferred when the molar quantities of reactants as stated in the equation react under standard conditions. 

Standard Enthalpy Change of neutralisation - is the enthalpy change when the amounts of acid and alkali in the reaction neutralise each other under standard conditions. 

Standard Enthalpy Change of Combustion -  is the enthalpy change when one mole of the substance burns completely in oxygen under standard conditions. 

Standard Enthalpy Change of Formation - is the enthalpy change when one mole of the compound forms from its elements under standard conditions with the elements in their standard states. 

Standard Enthalpy of Atomisation - is the enthalpy change when one mole of gaseous atoms is formed from the elements under standard conditions. 

HESS'S LAW AND THE INDIRECT DETERMINATION OF ENTHALPY CHANGES 

Hess's Law - the enthalpy change in converting reactants to products is the same regardless of the route taken, provided the initial and final conditions are the same. 

Enthalpies of Formation from Enthalpies of Combustion:-

Enthalpies of Reaction from Enthalpies of Formation:- 

Enthalpy Changes and bonding 

Bond Breaking - Endothermic

Bond Making - Exothermic

The bond enthalpy of a particular bond is the energy required to break one mole of the bonds in a substance in the gaseous state.