Topic 5 : Energetics/Thermochemistry

• Energy is a measure of the ability to do work. It is a form of energy • There are many different types of energy and heat is only one of them • During chemical reactions energy flows in and out of the reaction vessels • Inside the reaction vessel is known as the system • Outside the reaction vessel is known as the surroundings

• Open system • Closed system • Isolated

Isolated systems are very rare; most chemical reactions are open systems

Although energy can be exchanged between open and closed systems and the surroundings, the total energy of the process cannot change

• The total chemical energy inside a substance is called the enthalpy (or heat content) • When chemical reactions take place, changes in chemical energy take place and therefore the enthalpy changes • An enthalpy change is represented by the symbol ΔH (Δ= change; H = enthalpy) • An enthalpy change can be positive or negative

• A reaction is exothermic when the products have less enthalpy than the reactants (reactants are higher in energy than the products) • Heat energy is given off by the system to the surroundings in terms of the temperature of the surroundings increases or the temperature of the system decreases • There is an enthalpy decrease during the reaction so ΔH is negative • Exothermic reactions are thermodynamically possible (because the enthalpy of the reactants is higher than that of the products) • However, if the rate is too slow, the reaction may not occur. In this case the reaction is kinetically controlled

• A reaction is endothermic when the products have more enthalpy than the reactants • Heat energy is absorbed by the system from the surroundings so the temperature of the surroundings decreases or/and the temperature of the system increases • There is an enthalpy increase during the reaction so ΔH is positive

To compare the changes in enthalpy between reactions, all thermodynamic measurements are made under standard conditions

• A pressure of 100 kPa • A concentration of 1 mol dm-3 for all solutions • Each substance involved in the reaction is in its standard state (solid, gas or liquid) • Temperature is not part of the definition of standard state, but a temperature of 298 K (25 oC) is usually given as the specified temperature

• the enthalpy change when reactants in the stoichiometric equation react to give the products under standard conditions • Can be either endothermic or exothermic

• the enthalpy change when one mole of a compound is formed from its elements under standard condition • can be endothermic or exothermic

• the enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions • exothermic

• the enthalpy change when one mole of water is formed by reacting an acid and alkali under standard conditions • exothermic

• Calorimetry is a technique used to measure changes in enthalpy of chemical reactions • A calorimeter can be made up of a polystyrene drinking cup, a vacuum flask or metal can

• The energy needed to raise the temperature of 1 g of a substance by 1 K is called the specific heat capacity (c) of the liquid • The specific heat capacity of water is 4.18 J g-1 K-1

q = mc delta t q = heat transferred (J) m = mass c = specific heat capacity t = temperature (in kelvins)

• Enthalpy changes of reactions in solution • Enthalpy changes of combustion

• The principle of these calorimetry experiments is to carry out the reaction with an excess of one reagent and measure the temperature change over the course of a few minutes For the purposes of calculations certain assumptions are made: • That the specific heat capacity of the solution is the same as pure water, i.e. 4.18 J g-1 K-1 • That the density of the solution is the same as pure water, i.e. 1 g cm-3 • The specific heat capacity of the container is ignored • The reaction is complete • There are negligible heat losses

• For reactions which are not instantaneous there may be a delay before the maximum temperature is reached • During that delay the substances themselves may be losing heat to the surroundings, so that the true maximum temperature is never actually reached • To overcome this problem we can use graphical analysis to determine the maximum enthalpy change

• Take a temperature reading before adding the reactants for a few minutes to get a steady value • Add the second reactant and continue recording the temperature and time • Plot the graph and extrapolate the cooling part of the graph until you intersect the time at which the second reactant was added

• The principle here is to use the heat released by a combustion reaction to increase the heat content of water • A typical simple calorimeter is used to measure the temperature changes to the water • Not all the heat produced by the combustion reaction is transferred to the water: Some heat is lost to the surroundings and some heat is absorbed by the calorimeter • To minimise the heat losses the copper calorimeter should not be placed too far above the flame and a lid placed over the calorimeter • Shielding can be used to reduce draughts

• Heat losses • Incomplete combustion

• Hess’s law can be used to calculate the standard enthalpy change of a reaction from known standard enthalpy changes • Hess’s law states that: “The total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same.” • This means that whether the reaction takes place in one or two steps, the total enthalpy change of the reaction will still be the same • You can use cycles or equations to solve Hess’s Law

• When bonds are broken or made enthalpy changes take place • A chemical bond is a force of attraction between two atoms • The energy required to break a particular bond is called the bond dissociation enthalpy • This is usually just shortened to bond enthalpy or bond energy