A-LEVEL AQA ChEMISTRY NOTES
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- 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 & the standard state of the substance). Standard enthalpy changes are denoted by ∆Hϴ.
- Enthalpy change, ∆H = ∑H Products - ∑H Reactants
- ∑H Products Total enthalpy of products (kJmol^-1)
- ∑H Reactants Total enthalpy of reactants (kJ mol^-1)
- The standard enthalpy of formation (∆fHϴ) 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 (∆cHϴ) 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
Exothermic & Endothermic Reactions
- Exothermic reactions increase the temperature of the surroundings. ∆H is negative.
- Endothermic reactions decrease the temperature of the surroundings. ∆H is positive.
- Activation energy is the minimum energy required for a reaction to occur
- Calorimetry is the process of measuring the amount of heat given off or taken in during a chemical reaction
- Experimental values of enthalpy changes often differ from those found in data books because: the reaction may be incomplete, incomplete combustion of the fuel may occur, the heat capacities and densities of the solutions are only approximates, heat may be lost from the water to the surroundings, the experimental conditions may not be standard and some of the fuel or water may evaporate before reweighing.
- q = mc∆T
- q is the heat change (J)
m is the mass of the substance (g)
c is the specific heat capacity (Jg^-1 K^-1)
∆T 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 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.
- Bond enthalpies provide us with information about how much energy is needed to break a bond.
- Mean bond enthalpy is the enthalpy change when one mole of covalent bonds is broken to give the free atoms, averaged over a range of compounds
- Some bonds can only occur in one environment, such as Cl-Cl and H-Cl, as Cl and H can only form one bond.
- Some bonds can occur in multiple environments, such as C-H and C=O, as C can form multiple bonds with a wide range of elements – resulting in different environments
- The strength of these bonds will vary 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