Enthalpy changes

Enthalpy is the measure of the heat energy in a chemical system, which are atoms, molecules, or ions making up the chemicals.

Enthalpy cannot be measured but the changes can.

It is also sometimes thought of as the energy stored within bonds.

Enthalpy change:
The reactants and products often have different enthalpies in reactions.

Conservation of energy

In a chemical reaction which involves an enthalpy change heat energy is transferred between the system and the surroundings.

1.     The system is the chemicals- the reactants and products.

2.     The surroundings are the apparatus, the laboratory, and everything that is not the chemical system.

3.     The universe is everything and includes both system and surroundings.

, an enthalpy change can be determined experimentally by measuring the energy transfer between the system and surroundings.

Exothermic

Energy transferred from the system to energy transferred to the surroundings.

Any energy loss by the chemical system is balanced by the same energy gain by the surroundings.

Temperature of surrounding increases as they gain heat energy.

Endothermic:
Energy transferred from the surroundings to the energy transferred to the system. Chemical system takes in heat energy from the surroundings.

Any energy gain by the chemical system is balanced by the same energy loss by the surroundings.

The temperature of the surroundings decreases as they lose energy.

Activation Energy

During chemical reactions the bonds in the reactants need to be broken by an input of energy. New bonds in the products can then form to complete the reaction.

                  •The energy input required to break bonds acts as an energy barrier to the reaction, known as the activation energy.

                  •Activation energy is the minimum energy required for a reaction to take place.

                  •Small activation energies take place rapidly, because the energy needed to break bones is readily available from the surroundings.

                  •Very large activation energies may be present such a large energy barrier that a reaction may take place extremely slowly.

Standard enthalpy changes:

                  ⁃The enthalpy change for a reaction can vary slightly depending on the conditions used.

                  ⁃Chemists use standard conditions for physical measurements such as enthalpy changes, close to typical working conditions of temperature and pressure.

Standard conditions:

Shown by the sign Æ. A standard enthalpy change  Æ- enthalpy H change under standard conditions Æ

                  ⁃Standard pressure is 100 kPa. This is very close to a pressure of one atmosphere, 101kPa.

                  ⁃Standard temperature is a stated temperature, usually 298K.

                  ⁃Standard concentration is 1moldm-3 (Relevant for solutions only)

                  ⁃Standard state is the physical state of a substance under the standard conditions. 

Enthalpy change of reaction

The standard enthalpy changes of reaction ΔH_r^⊖ is the enthalpy change that accompanies a reaction in the molar quantities shown in the chemical equation under standard conditions, with all the products and reactants in their standard states.

ΔH_r^⊖ always refers to a stated equation and its value depends on the balancing numbers. If the equation is balanced with whole numbers, then the amounts are doubled and the enthalpy change is doubled.

   

Enthalpy change of formation

It’s the enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all the reactants and products at standard state. (ΔHf^⊖)     

Compounds:
The equation could be written with whole numbers but would not fit the definition as it requires the formation of one mole of the compound.

Elements:

From its definition it states the formation of one mole of a compound from its element. There is no change if an element is produced, so an enthalpy change of formation will be 0kJmol-1.

Enthalpy change of combustion- (ΔcH^⊖)     

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

Enthalpy change of neutralization

The standard enthalpy change of neutralization is the energy change that accompanies the reaction of an acid by a base to form one mole of water (l) under standard conditions, with the reactants and products in their standard states.

It involves the reaction of H+ and OH- to form water, therefore the value is the same for all neutralization reactions.