Chem Chapt 6: Chemical Energetics

Define standard enthalpy change of reaction and what is its value for elements?

It is the energy change when 1 mole of substance in its standard state is formed from its constituent elements in their standard states, at a specified temperature, usually 298K. The enthalpy change of reaction for elements is usually zero

Define the standard enthalpy change of combustion

It is the energy released when 1 mole of substance is completely burnt in excess oxygen with all reactants and products in their standard states, at a specified temperature, usually 298K

Define the standard enthalpy change of neutralisation

It is the energy released when 1 mole of water is formed in the neutralisation reaction between acid and a base, all in their standard states, at a specified temperature, usually 298K

State the usual value of enthalpy change of neutralisation for strong acids and bases and why it is less exothermic for weaker acids and bases

The value for strong acids and bases is usually 57kJ mol-1 as they all dissociate fully. The value for weaker acids and bases is less exothermic as they do not dissociate fully and energy is absorbed to help dissociate the undissociated weak acid or base

Define Bond energy

It is defined as the average energy required to break 1 mole of covalent bond between two atoms in the gaseous state

Explain why enthalpy changes calculated with bond energies in date booklets may deviate from actual enthalpy change values

Bond energy values from the data booklet are average value derived from a wide range of molecules with the same covalent bond. Furthermore, the value assumes that the chemical species is in its gaseous state(only for molecules in liquid state) and does not account for energy required to vaporise it into a gas. Hence the actual enthalpy change could be more/less exothermic

Define standard enthalpy change of atomisation of an element

It is energy required to form 1 mole of gaseous atoms from the element, all in standard states, at a specified temperature, usually at 298K

Define standard enthalpy change of atomisation for compounds

It is the energy required to form gaseous atoms from 1 mole of the compound, all in their standard states, at a specified temperature, usually 298K

How can enthalpy change of atomisation be derived from BE? What is the difference between enthalpy change of vaporisation and sublimation from atomisation?

Enthalpy change of atomisation is half of bond energy. Enthalpy change of vaporisation is for conversion of liquid to gas, sublimation is for diatomic solids to gas while atomisation is for monoatomic solids to gas

Define Lattice energy

Lattice energy of an ionic compound is the energy released when 1 mole of solid ionic compound is formed from its constituent gaseous ions

State the formula for magnitude of LE and determine whether MgO or NaCl would have a more exothermic LE

The formula is (charge of cation x charge of anion)/(cationic radius + anionic radius). Mg2+ has a higher charge than Na+ and O2- has a higher charge than Cl-. Mg2+ has a smaller ionic radius than Na+ and O2- has a smaller ionic radius than Cl-. Hence MgO will have a more exothermic lattice energy than NaCl

Why would there be a deviation between experimental and theoretical lattice energy

This is because theoretical LE is based on the assumption that the compound is completely ionic when there may be covalent character. The higher the charge and smaller the ionic radius of the cation, the higher its charge density and greater its polarising power to distort electron cloud to a greater extent. The larger the ionic radius of the anion, the more polarisable it is and the more distorted it would be by cation. This renders a covalent character where bonds are strengthened, making LE more exothermic than theoretical

Define 1st and 2nd ionisation energy

1st IE is defined as the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of singly charged gaseous cations. 2nd IE is defined as the energy required to remove 1 mole of electrons from 1 mole of singly charged gaseous cations to form 1 mole of doubly charged gaseous cations

Explain why IE increases after each removal

It increases after each removal as shielding effect decreases with lesser electrons while nuclear charge remains constant, hence effective nuclear charge increases and electrostatic forces of attraction between nucleus and valence electrons increases

Define 1st and 2nd electron affinty

1st EA is defined as the energy change when 1 mole of electrons is added to 1 mole of gaseous atoms to form 1 mole of singly charged gaseous anions. 2nd EA is defined as the energy change when 1 mole of electrons is added to 1 mole of singly charged gaseous anions to form 1 mole of doubly charged gaseous anions

Explain why 1st EA is usually exothermic while 2nd EA us usually endothermic

1st EA is usually exothermic as more energy is released from the attraction between nucleus and the electron added, than that required to overcome the repulsion between the electron added and those electrons that are already in the atom. 2nd EA is usually endothermic as more energy is required to overcome the repulsion between the electron added and the electrons that are already in the atom

What is the heat capacity of water and what does all experimental derivation of enthalpy change assume which leads to deviation from theoretical value?

The heat capacity of water is 4.18. They all assume negligible heat loss to surrounding air and to the calorimeter, and that the density of the reaction mixture is the same as that of water. Hence, theoretical value will be more exothermic

Explain why extrapolation is needed in temperature correction graphs (Temp-time graph)

Some reactions may proceed too slowly and maximum temperature would not be reached immediately due to heat loss to the surroundings and to the calorimeter in the reaction process. Hence, extrapolation is done to the point of mixing to obtain an accurate maximum temperature for calculations

What is the spirit burner set up used to calculate and what measurements must be taken

It is used to calculate enthalpy change of combustion, whereby the fuel that will undergo combustion will be placed into the spirit burner and its initial mass will be measured, along with the initial temperature of water, after the combustion reaction, the final temperature of water and the final mass of the spirit burner is measured again

Why is the bomb calorimeter more accurate in calculating enthalpy changes

There is less heat loss to the surroundings and the heat capacity of the bomb calorimeter and the heat evolved in burning can all be taken into account, making the deviation from theoretical value much smaller

What does Hess’ Law state

It states that the enthalpy change of reaction is determined only by the initial and final states and is independent of the reaction pathway taken

Define standard enthalpy change of hydration

It is the energy released when 1 mole of free gaseous ions in its standard state is dissolved in water to give a solution of infinite dilution, at a specified temperature, usually 298K

Explain why standard enthalpy change of hydration is always exothermic and what the formula for calculating its magnitude

It is always exothermic as it requires formation of ion-dipole interactions between ions and polar water molecules. Its formula is ionic charge over ionic radius. Hence the higher the charge density, the stronger the ion-dipole interactions formed and the more exothermic the standard enthalpy change of hydration

Define standard enthalpy change of solution

It is the energy change when 1 mole of substance in its standard state is completely dissolved in a solvent to give a solution of infinite dilution, at a specified temperature, usually at 298K

Explain the values of enthalpy change of solution with respect to the substances’ solubility. Explain whether there can be exceptions too

If the value is highly positive, then the substance is highly insoluble in water. If the value is highly negative, then the substance is highly soluble in water. However, some salts with highly positive values can still be soluble in water due to positive entropy values

Deduce the relationship between lattice energy, enthalpy change of solution and enthalpy change of hydration

If lattice energy is larger than enthalpy change of hydration, then then the enthalpy change of solution is endothermic, but if lattice energy is smaller than enthalpy change of hydration, then enthalpy change of solution is exothermic

Define entropy and what its values entail

It is the measure of randomness or disorder in a system, which is reflected in the number of ways that the energy of a system can be distributed through the motion of its particles. Hence, if entropy is positive, it means the reaction resulted in more ways to disperse energy, vice versa

Define spontaneous processes

They are processes that takes place naturally in the direction stated, and the change occurs without a need for continuous input of energy from outside the system

Explain how temperature changes affect entropy, eg. 1 mole of gas is heated to 373K

When temperature increases, entropy will increase as the average kinetic energy of particles increases and the range of energies increases, resulting in more ways for the energy to be distributed among gas particles

Explain how phase changes affect entropy. Eg solid to gas (sublimation)

In solids, particles vibrate about their fixed positions but in gases, particles are randomly arranged far apart and can move freely. Hence when a substance sublimes, the particles in their fixed positions move more freely and are more dispersed and disordered. This increases the number of ways for the particles and energy to be distributed

Explain how change in number of particles affect entropy. Eg Cl2(g) is photolysed at 100K to form 2 Cl free radicals

Entropy will increase as the number of gas particles increases, hence the system becomes more disordered and there are more ways in which the particles and energy can be distributed

Describe how mixing of particles will affect entropy (for mixing of gases and mixing of liquids)

When gases are mixed at constant pressure, entropy will increase as the volume available for each gas increases to keep total pressure constant, hence the number of ways in which particles and energy can be distributed increases. When liquids are mixed, entropy increases as the number of ways in which particles and energy can be distributed increases

Describe how the dissolution of ionic solids affect entropy (Mixing of solid and water)

entropy increases as the ions in the solid become free to move in solution, at the same time, the entropy of water decreases as its molecules were originally free to move but are now more restricted in motion with more ions.

Explain what the values of gibbs free energy entails

If the value is positive, then the reaction is thermodynamically feasible and spontaneous. If the value is equal to zero, then the system is at equilibrium. If the value is negative, then the reaction is thermodynamically feasible and spontaneous

State the limitations of gibbs free energy

They are only valid to predict spontaneity when reactions occur in standard states, values of gibbs free energy changes with temperatures and the measure of spontaneity may not mean the reaction is instantaneous, hence the rate of reaction is not accounted for

What does a negative H (enthalpy) and positive S (entropy) result for gibbs free energy value and the reaction?

Gibbs free energy value will be negative at all temperatures and reaction is spontaneous at all temperatures

What does a negative H (enthalpy) and negative S (entropy) result for gibbs free energy value and the reaction?

Gibbs free energy value is negative only at low temperatures and reaction is only spontaneous at low temperatures

What does a positive H (enthalpy) and positive S (entropy) result for gibbs free energy value and the reaction?

Gibbs free energy value is only negative at high temperatures and reaction is only spontaneous at high temperatures

What does a positive H (enthalpy) and negative S (entropy) result for gibbs free energy value and the reaction?

Gibbs free energy value is positive at all temperatures and reaction is non-spontaneous at all temperatures