Energetics
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What is enthalpy?
Measure of the amount of energy / energy content of a substance. It cannot be directly measured.
Define enthalpy change
A measure of heat energy given out or taken in when a chemical or physical change occurs at a constant pressure
What are the standard conditions?
25C or 298K 1atm or 101,000Pa
What is an exothermic reaction?
A reaction where energy has been given out to the surroundings as the reactants change to products
What is an endothermic reaction?
A reaction where energy has been taken in from the surroundings as the reactants change to products.
Define standard enthalpy of formation
The enthalpy change when one mole of a substance is formed from its constituent elements with all reactants and products in their standard states under standard conditions.
Define standard enthalpy of combustion
The enthalpy change when one mole of a substance is completely burned in oxygen with all reactants and products in their standard states under standard conditions.
Define standard enthalpy of neutralisation
Enthalpy change when one mole of water is formed in a reaction between an acid and an alkali under standard conditions.
What are the standard conditions?
100,000kPa of pressure (or 1 atm) 298K temperature (or 25 degrees, about room temperature)
Formula to find heat energy given out
q=mcT q is heat energy given out (J) m is mass of substance heated (g) T is temperature change c is specific heat capacity
Define specific heat capacity
The amount of heat needed to raise the temperature of 1g of substance by 1K Units: J/g/K
How to find the enthalpy change from the heat energy given out
Divide the heat energy given out (kJ) by the number of moles of the substance in the reaction (not the water).
What is the enthalpy of formation for elements?
0 kJ/mol
Calculation to work out enthalpy change of a reaction using enthalpy of formation values
Sum of enthalpy of formation of products - sum of enthalpy of formation of reactants (remember to times by all coefficients) (Hess’s cycle has arrows from constituent elements to reactants and products)
Hess’s law
The enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products.
Calculation to work out enthalpy change of a reaction using enthalpy of combustion values
Sum of the enthalpy of combustion of reactants - sum of enthalpy of combustion of products (remember to times by all coefficients) (Hess’s cycle has arrows from products and reactants to oxides)
Calculation to work out enthalpy change of a reaction using bond enthalpy values
Sum of the bond enthalpy of the bonds broken - sum of the bond enthalpy of the bonds formed (Remember the substances have to be gases so if they’re not you have to add the energy to make them a gas on to the bond enthalpy before the calculation) Always write out the displayed formula equation
Define bond dissociation enthalpy
The enthalpy change required to break a covalent bond with all species in the gaseous state. The mean bond enthalpy is the average bond dissociation enthalpy for a given type of bond.
What is important to remember when using a calorimeter
If you are using a spirit burner, the material containing the water should be a good conductor of heat. If the reaction is in the same container as the water, there should be as much insulation as possible to minimise heat loss to the surroundings. Record the temperature of the water a few minutes before and after the reactants are mixed so you can do an extrapolation curve to find the maximum temperature change.