Energetics Flashcards
Enthalpy Change (AH)
This is the change in heat energy in a reaction at constant pressure. The majority of reactions occur at atmospheric pressure in which the pressure remains constant throughout and so this is the enthalpy change.
Standard Enthalpy Change (AH)
This is the enthalpy change that occurs under standard conditions of 100 KPa pressure, 298K temperature and any solution at a concentration of 1 moldm-3. Reactants and products being in their standard physical state ( solid etc) under these conditions.
By measuring enthalpy changes under standard conditions- they can be compared fairly from one reaction to another.
Standard Enthalpy of Formation (AH® +)
Definition - The standard enthalpy of formation is the enthalpy change when I mole of a substance is formed from its constituent elements under standard conditions. All substances are in their standard States.
Standard Enthalpy of Combustion (AH° c)
Definition - standard enthalpy of combustion is the enthalpy change when 1 mole of a sustance undergoes compete combustion in excess oxygen under standard conditions. All substances are in their standard States.
for organic compounds like hydrocarbons and alcohols - the products of complete combustion are water and carbon dioxide.
Note that the enthalpy of combustion of oxygen is zero as it cannot be combusted. Likewise the enthalpy of combustion of oxides such as, CO2 is zero as they have already undergone complete combustion.
Exothermic reactions
Exothermic reactions are ones that get hot so they give out energy to the surroundings. The products have less energy than the reactants and AH values are negative.
If less energy is required to break old bonds in reactants than is released on forming new bonds in the products - then the reaction overall will be exothermic
Many reactions that occur spontaneously are exothermic - in these the products of the reaction are more Stable as they possess less energy.
Endothermic reactions
Endothermic reactions are ones that get cold so they take in energy from the Surroundings
The products have more energy than the reactants and AH
values are positive.
If more energy is required to break old bonds in reactants than is released on forming new bonds in the products - then the reaction overall will be
endothermic.
Hess’s law
This is a chemical representation of the first law of thermodynamics which states that energy can’t be created or destroyed.
Hess’s law states that the enthalpy change of a reaction is independent of the route taken.
Hess cycle formation example
Equation for formation
Mean Bond Enthalpy
Enthalpy needed to break 1 mole of gaseous covalent bonds averaged over breaking that bond in several different compounds. Bond enthalpies are positive as energy is required to break bonds.
Enthalpy Change in bond breaking
Energy has to be taken in, in order to break the
bond between two atoms ie overcome the attraction between them. This step is an endothermic step or process. AH for this must be + ve.
Enthalpy Change in bond forming
energy is released when bonds are formed. This step is an exomermic step or process. AH for this must be -ve.
Using Hess’s law and bond enthalpies to find an enthalpy change
Average Bond dissociation Energy
energy needed to break 1 mole of gaseous covalent bonds averaged over breaking that bond in several different compounds.
Harder bond enthalpy calculations - enthalpy of vaporisation
Bond enthalpy is defined as breaking bonds in the gaseous state. If the substance in the equation is not in the gaseous state it will need an additional enthalpy change in the cycle to turn it into a gas.
For a liquid this will mean using an enthalpy of vaporisation:
The enthalpy of vaporisation is the enthalpy change when one mole of liquid is turned into one mole of gas.
Eg for bromine AHvap is:
Br2(l) → Br2(g)
If the substance is a solid, the enthalpy of atomisation breaks all the bonds in the solid and turns it directly into gaseous atoms (definition - enthalpy of atomisation is the enthalpy change when one mole of gaseous atoms is formed from the element in its standard state).
Eg for carbon AHa is:
C(s) → C(g)
Experimental Measurement of Energy Changes in chemical reactions
This can be done using instruments called - CALORIMETERS.
The simplest type of calorimeter consists of an insulated container such as a polystyrene/styrafoam cup - this limits exchange of heat with the surroundings and allows the temperature change brought about by the chemical reaction.
The energy change of the reaction can then be calculated if it is known how much heat energy brings about a 1º C increase in temperature of a given mass of the substance being heated up.
Specific heat capacity
Is the energy (J) needed to raise 1 g of a substance by 1 Kelvin/celcius.
How to find delta H
Delta H = -Q/N
Q will be negtive if temp decreased and positive if temp increased.
Finding enthalpy of a molar enthalpy reaction - method
Finding enthalpy of a molar enthalpy reaction - sources of error
Heat loss to the surroundings
Some of the energy released going into heating up zinc / cup / the thermometer
The specific heat capacity of solution may not be the exact same as water
Methods of determining the enthalpy of combustion of an alcohol:
Set Up A:
Metal can with water (regular Calorimeter)
B- Bomb calorimeter
Benefits of using a Bomb Calorimeter
Better insulation - reduces heat loss to the surroundings
All heat energy goes to heating the water - has the sides of the metal can insulated and has a lid and uses heat shields which reflect heat towards the can.
Pure O2 ensures complete combustion of fuel
Constructing unfamiliar Hess cycles
