- Energy can neither be created no destroyed. it is converted.
Forms of energy:
- work = when a force moves an object
- kinetic = energy due to motion of a body.
- gravitational potential energy = energy due to position of object relative to centre of the earth.
- chemical potential energy = energy stored in a molecule.
- radiant energy = energy in light and sound.
- electrical energy = in the form of electrical potential energy or energy due to an electric current
Heat is a form of kinetic energy. when an object is heated its particles gain more kinetic energy.
SI unit of energy is the joule (J) or kilojoule (kJ)
1kJ = 1000 J
When a reaction occurs at constant pressure and gives out or takes in energy there is an enthalpy change between the system(reactants and products) and the surroundings. (delta symbol H = Enthalpy Change)
- delta H = H products - H reactants
The enthalpy of a substance depends on:
- it's physical state
- it's amount (in moles)
- the temperature and pressure.
heat required = - (mass x specific heat capacity x the rise in temperature)
Specific heat capacity is the heat required to increase the temperature of 1g of water by 1 degree celcius.
Enthalpy Level Diagrams
Enthalpy level diagrams show the relative energy levels of reactants and products.
- Exothermic reactions get hot but the enthalpy change is negative. This is because the products are at a lower energy level than the reactants - the products have less energy than the reactants. In an exothermic reaction, chemical energy is converted into heat energy and the temperature of the system rises.
- Endothermic reactions get cold but the enthalpy change is positive. This is because the products are at a higher energy level than the reactants - the products have more energy than the reactants. In an endothermic reaction, heat energy is converted into chemical energy and the temperature of the system falls.
Most Enthalpy changes are called Standard Enthalpy changes. This means the heat produced is measured under standard conditions of:
- a pressure of 1 atm
- a stated temperature ( usually 298 K or 25 degrees celcius)
- solutions at a concentration of 1 mol dm ^ -3.
The symbol for this is a superscript and is shown as Delta H superscript.
It is expressed per mole of substance reacting.
the units are kJ mol ^-1
Definitions of Standard Enthalpies
- Standard Enthalpy of formation (Delta Hf superscript):
The enthalpy change when 1 mol of a substance is formed from its elements in their standard states at 1 atm pressure, 298 K.
- Standard Enthalpy of reaction (Delta Hr superscript):
The enthalpy change when the number of moles of the substance in the equation as written react under standard conditions of 1 atm pressure and 298 K.
- Standard Enthalpy of combustion (Delta Hc superscript):
The enthalpy change when 1 mol of a substance undergoes complete combustion with oxygen under standard conditions of 1 atm pressure and 298 K.
- Standard Enthalpy of neutralisation (Delta Hneut superscript):
The enthalpy change when 1 mol of water is produced by the neutralisation of a solution of an acid by excess base under standard conditions with all solutions of concentration 1 mol dm ^-3.
- Standard Enthalphy of atomisation (Delta Hat superscript):
The enthalpy change when 1 mol of gaseous atoms is formed from the element in it's standard state at 1 atm pressure and 298 K.
A number of assumptions are made when dealing with enthalpy of combustion changes:
- all the energy produced by the burning ehatnol is transferred to the water.
- no heat is absorbed by the calorimeter
- no heat loss to the room or from the surface of the water
- the fuel has completely combusted,
These assumptions lead to systematic errors - inaccuracies in calculated enthalpy change.
Random errors can also arise from measurement uncertainties e.g. the mass of water or recorded temperatures.
Hess' law states that:
- The total enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same.
In other words, for any chemical change, the enthalpy change is the same whatever reaction route is taken.
Hess' law is used to calculate the standard enthalpy change for the reactions that are difficult to measure or that cannot be measured directly. The requirements are that:
- there are two routes between the reactants and products to be able to draw the cycle.
- the data for the enthalpy changes for one route is given.
In a hess cycle, the equation must be balanced. You draw a Hess cycle by linking the formation of the fuel with the combustion of it and its constituent elements.
Bond dissociation enthalpy is the energy associated with 1 mole of a particular bond. It is often referred to as bond enthalpy. It is the energy:
- required to break 1 mole of bonds
- released on making 1 mole of bonds.
Mean Bond Enthalpy is the mean bond dissociation enthalpy for a particular bond in a range of compounds- it can be used to calculate the enthalpy change for reactions.
Enthalpy changes calculated from average bond enthalpy values are slightly different from experimental values as:
- the calculations use average bond enthalpies
- bond enthalpies only apply to substances in the gaseous states