Enthalpy
- Enthalpy, H, is the thermal energy that is stored in a system
- Enthalpy change is the heat energy change measured under conditions of constant pressure.
- Enthalpy change values are usually given under standard conditions (100 kPa, 298K, 1 mol dm^-3 & the standard state of the substance). Standard enthalpy changes are denoted by ΔH^ϴ.
- The standard enthalpy change of reaction (ΔHr^ϴ) is the enthalpy change that accompanies a reaction in molar quantities shown in a chemical equation under standard conditions
- The standard enthalpy of formation (ΔfH^ϴ) is the enthalpy change when one mole of a substance is formed from its constituent elements, with all reactants and products being in their standard states and under standard conditions
- The standard enthalpy of combustion (ΔcH^ϴ) is the enthalpy change when one mole of a substance is completely burnt in oxygen, with all reactants and products being in their standard states and under standard conditions
- The standard enthalpy change of neutralisation (ΔneutH^ϴ) is the energy change that accompanies the reaction of an acid by a base to form one mole of H2O (l) under standard conditions
Exothermic & Endothermic Reactions
Exothermic & Endothermic Reactions
- Exothermic reactions increase the temperature of the surroundings. ΔH is negative
- Endothermic reactions decrease the temperature of the surroundings. ΔH is positive.
- Activation energy is the minimum energy required for a reaction to occur
Bond Enthalpies
- Average bond enthalpy is the energy required to break one mole of a specified type of bond in a gaseous molecule
- The strength of a covalent bond varies according to the environment in which they are found, so an average value is taken
- The actual bond enthalpy is specific to each individual molecule
- Bond breaking is endothermic, while bond making is exothermic.
- Enthalpy change can be predicted using bond enthalpies
- The enthalpy change values calculated from bond enthalpies are approximate and not as accurate as those calculated from Hess’ Law cycles
Calorimetry
- Calorimetry is the process of measuring the amount of heat given off or taken in during a chemical reaction
q =mcΔT
q is the heat change (J)
m is the mass of the substance (g)
c is the specific heat capacity (J g^-1 K^-1)
ΔT is the temperature change (K or °C)
q is the heat change (J)
m is the mass of the substance (g)
c is the specific heat capacity (J g^-1 K^-1)
ΔT is the temperature change (K or °C)
Coffee Cup Calorimetry
- A coffee cup calorimeter can be used to calculate enthalpy changes of neutralisation
- The reaction mixture is placed in a Styrofoam cup with a lid to keep it insulated, along with a stirrer and a thermometer. The Styrofoam cup can be held within another Styrofoam cup in a beaker for maximum insulation
- A measured volume of the first reactant is added, and the temperature is recorded until stable. A measured amount of the second reactant is added, and the temperature is measured evert minute whilst constantly stirring.
- A graph of temperature against time is plotted, with a line of best fit
- q can be calculated using ∆T and converting the volumes of the solutions into masses using their densities (assumed to be 1 g cm^-3)
Spirit Burner Calorimetry
- A spirit burner calorimeter can be used to calculate enthalpy changes of combustion
- The substance being heated or cooled, usually water, is placed in a beaker with a thermometer. A spirit burner is placed underneath the beaker to heat it
- The spirit burner containing the fuel is weighed and a known volume of water is added to the beaker and its initial temperature measured
- The spirit burner is burnt, and the water continuously stirred
- After a few minutes, the flame is extinguished, and the spirit burger reweighed. The final temperature of the water is measured
- The measured values are used to calculate q
Hess’s Law
Hess’s Law
- Hess’s law is that the enthalpy change of a reaction is independent of the route taken
- Enthalpy changes of combustion can be used to find the enthalpy change of a reaction
- Enthalpy changes of formation can be used to find the enthalpy change of a reaction
- The enthalpy of formation for an element is 0 kJ mol-1.
