3.2.1 Energetics - Enthalpy change (ΔH)
Students should:
|
Exothermic and endothermic reactions
If a reaction produces heat (increases the temperature of the surroundings) it is exothermic. If the temperature of the reaction mixture decreases (i.e. heat is absorbed) then the reaction is endothermic.
- Exothermic
a reaction which produces heat (ΔH has a negative
value by convention, -ve) - Endothermic a reaction which absorbs heat (ΔH has a positive
value by convention, +ve)
Enthalpy of reaction: The change in internal (chemical) energy (H) in a reaction = ΔH.
The most stable state is where all energy has been released. When going to a more stable state, energy will be released, and when going to a less stable state, energy will be gained (from the surroundings). On an enthalpy level diagram, higher positions will be less stable (with more internal energy) therefore, if the product is lower, heat is released (more stable, ΔH is -ve) but if it is higher, heat is gained (less stable, ΔH is +ve).
It is important to get the following facts clear in your mind:
- Formation of bonds causes an energy release (exothermic).
- Breaking of bonds requires energy (endothermic).
Calculation of enthalpy changes
When energy is used to cause a temperature change in a substance or body:
Change in energy = mass x specific heat capacity x change in temperature
E = m x c x ΔT
Energy is a extensive quantity
Enthalpy changes (ΔH) are related to the number of mols in the reaction...if all the coefficients are doubled, then the value of ΔH will be doubled.
When a reaction is carried out in aqueous solution, the water will gain or lose heat from (or to) the reactants. Therefore, the change in energy, and so the ΔH value, can be calculated from E = m x c x ΔT where m is the mass of water present (kilograms), and c = 4.18 kJ Kg-1 K-1. The ΔH value can then be calculated back to find the molar enthalpy change for the reaction.
Experimental
A known mass of solution should be placed in a container, as insulated as possible, to prevent as much heat as possible from escaping. The temperature is measured continuously, the value used in the equation is the maximum change in temperature from the initial reading.
The result will be a change in temperature. This can be converted into a change in heat (or energy) by using the above equation E = m x c x ΔT.
ΔH may then be calculated for the amount of reactants present, and then this can be used to calculate for a given number of mols.
Definitions
Standard state -- Pressure 100 kPa, temperature 298 K (or 1 atm, 25 degrees celsius). The standard state of an element or compound is the form in which it exists under standard conditions (not to be confused with STP)
Standard enthalpies of combustion (ΔHc )
The standard enthalpy of combustion is the energy released when 1 mole of a substance is burnt in excess air or oxygen, all quantities being measured under standard conditions.
This is particularly important historically, as combustion enthalpy is one of the easiest quantities to measure experimentally.
Standard enthalpies of formation (ΔHf )
The enthalpy change when 1 mole of a substance is made from its elements in their standard states. There MUST be 1 mole of the substance formed AND all of the elements forming it MUST be in their standard states.
|
Example: The standard enthalpy change of methane is shown by the following equation: C(graphite) + 2H2(g) |