MODULE 3: Enthalpy

Enthalpy, H is the amount of energy that a substance contains (cannot be measure directly)

Enthalpy change (ΔH) = enthalpy of products (H) - enthalpy of reactants (H)

If a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change will be the same for each route

Heat energy is transferred from the reaction to the surroundings, enthalpy change is negative therefore energy is lost (bond making)

Heat energy is transferred from the surroundings into the reaction, enthalpy change is positive therefore energy is gained (bond breaking)

The minimum energy required to start a reaction

Standard pressure (100kPa), standard temperature (298K), standard concentration (1 mol dm-3), standard state (state in 101kPa and 298K)

The standard enthalpy change of a reaction is the enthalpy change which occurs when equation quantities of materials react under standard conditions, and with everything in its standard state

Enthalpy change that takes place when one mole of a compound is formed from its elements at standard conditions

The standard enthalpy change of combustion is the enthalpy change when 1 mol of a substance is burnt completely, in an excess of oxygen, under standard conditions of 298K and 101kPa

Using the equation ‘q = m x c x ΔT’, a Styrofoam cup (to prevent heat loss), and a temperature to record the temperature change

∆rHᶿ = ∑(bond enthalpies of reactants) - ∑(bond enthalpies of products)

Energy required to break one mole of bonds in a gaseous molecule

ΔrHᶿ = Bond enthalpies in reactants - bond enthalpies in products

The standard enthalpy change of neutralisation is the enthalpy change when an acid and alkali react together to form 1 mol of water, under standard conditions of 298K and 101kPa

q= mcΔT

Mass, grams

Specific heat capacity, g-1 K-1

Temperature change, kelvin

Heat loss to surroundings, incomplete combustion, evaporation of methanol from the wick, non-standard conditions

Thermometer, beaker full of water, spirit burner full of fuel

Positive if endothermic, negative is exothermic

Bond enthalpy can vary depending on the chemical environment of the bond

A + B = C