Chemical Energetics 🔥

Describe endothermic reactions

1. Energy is absorbed to break bonds in reactants.

2. Products are thermally less stable than reactants.

3. (Draw the energy profile diagram).

Describe exothermic reactions

1. Energy is released when bonds are formed in products.

2. Products are thermally more stable than reactants.

3. (Draw the energy profile diagram).

Define standard enthalpy change of reaction

1. Standard enthalpy change of reaction is the enthalpy change when molar quantities of reactants as stated in the equation react together under standard conditions of 298K and 1 bar.

Define standard enthalpy change of formation

1. Standard enthalpy change of formation of a substance is the enthalpy change when one mole of the substance is formed from its constituent elements in their standard states at 298K and 1 bar.

2. Standard enthalpy change of formation of elements is 0.

3. Standard enthalpy change of reaction= standard enthalpy change of formation of products - standard enthalpy change of formation of reactants.

Define standard enthalpy change of combustion

1. Standard enthalpy change of combustion of a substance is the heat energy evolved when one mole of the substance is completely burnt in excess oxygen under standard conditions of 298 K and 1 bar.

2. Standard enthalpy change of combustion is always exothermic.

3. Standard enthalpy change of reaction= standard enthalpy change of combustion of reactants - standard enthalpy change of combustion of products.

Define standard enthalpy change of neutralisation

1. Standard enthalpy change of neutralisation is the enthalpy change when an acid and a base react under infinitely dilute conditions to form one mole of water under standard conditions of 298 K and 1 bar.

2. Standard enthalpy change of neutralisation is always exothermic.

Explain why standard enthalpy change of neutralisation is variable

1. Weak acids dissociate partially in solution to give H+(aq).

2. Part of the heat evolved from the neutralisation process is absorbed by the weak acid to further dissociate the weak acid to provide H+(aq) for neutralisation with the base.

3. Hence, weak acid-weak base reactions are less exothermic than strong acid-strong base reactions.

Define standard enthalpy change of atomisation

1. Standard enthalpy change of atomisation is the heat energy required to form one mole of free gaseous atoms from the element in its standard state at 298K and 1 bar.

2. Standard enthalpy change of atomisation is always endothermic.

State the common assumptions made in calculations of enthalpy change

1. The mass of any solid is ignored in the calculation of the mass of solution.

2. All heat energy were transferred from the reaction to solution with 100% efficiency.

3. The density and specific heat capacity of the aqueous solution, unless otherwise stated, are taken to be those of water.

4. Heat transfer to the calorimeter is negligible.

State the precautions taken when carrying out a calorimeter experiment

1. Use a wind-shield to minimise draught in the lab.

2. Use insulation.

3. Repeat the experiment and monitor temperature change at timed intervals, then determine maximum temperature change through graphical methods.

Define bond energy

1. Bond energy is the average energy required to break one mole of covalent bond in the gas phase into its constituent gaseous atoms under standard conditions of 298K and 1 bar.

2. Standard enthalpy change of reaction= bond energy of bonds broken in reactants - bond energy of bonds formed in products.

3. If standard enthalpy change of reaction is exothermic, bond energy absorbed to break bonds in reactants is less than energy released to form bonds in products.

4. If standard enthalpy change of reaction is endothermic, bond energy absorbed to break bonds in reactants is more than energy released to form bonds in products.

State the inaccuracies of using bond energies in calculations

1. Bond energies in the Data Booklet are average values and would not apply to the exact compounds in the reaction. Hence, the calculation is an approximation.

2. Standard enthalpy change of combustion calculated using bond energies applies for reactants and products in the gaseous phase, but standard enthalpy change of combustion is defined in terms of the formation of liquid water.

Describe the factors affecting average bond energies

1. Extent of orbital overlap. The greater the extent of orbital overlap, the more effective the orbital overlap, the greater the bond strength and hence the greater the bond energy.

2. Bond order. The greater the bond order, the greater the bond energy,

Define Hess’ Law

1. The enthalpy change for a chemical reaction remains the same regardless of whether the reaction takes place in one step or several steps, provided that the initial states of the reactants and the final states of the products are the same.

Define lattice energy

1. Lattice energy is the heat energy evolved when one mole of solid ionic compound is formed from its constituent gaseous ions under standard conditions of 298K and 1 bar.

Define first ionisation energy

1. First ionisation energy is the minimum energy required to completely remove one mole of valence electrons from one mole of ground state gaseous atoms to form one mole of gaseous singly charged cations.

2. X(g)—> X+(g) + e-.

Define first electron affinity

1. First electron affinity is the enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous singly charged anions.

2. X(g) + e—→ X-(g).

Explain why second electron affinities are always positive

1. Second electron affinities are always positive as energy needs to be supplied to overcome the repulsion between the two negatively charged species.

Define standard enthalpy change of solution

1. Standard enthalpy change of solution is the enthalpy change when one mole of solute is completely dissolved in a solvent to form an infinitely dilute solution under standard conditions of 298K and 1 bar.

2. Standard enthalpy change of solution = standard enthalpy change of hydration of cations + standard enthalpy change of hydration of anions - lattice energy.

Define standard enthalpy change of hydration

1. Standard enthalpy change of hydration of an ion is the heat energy evolved when one of mole of free gaseous ions is hydrated by water molecules under standard conditions of 298K and 1 bar.

2. Standard enthalpy change of hydration is always exothermic.

Define entropy

1. Entropy is a measure of the disorder of matter and energy of a system.

State the factors affecting entropy change of a system

1. Change in temperature.

2. Change in phase.

3. Change in the number of of gaseous particles.

4. Mixing of particles.

Describe how changes in temperature affects entropy

1. When temperature increases, kinetic energy of reactant particles increases and they move more vigorously.

2. There will be broadening of the Boltzmann energy distribution and there are more ways to distribute the energy of the reactant particles.

3. Number of ways to arrange the particles increases.

4. Degree of disorder of the system increases.

5. Hence, change in S>0.

Describe how changes in phase affects entropy

1. When H2O(s) melts to form H2O(l), the regular crystalline structure of ice is broken.

2. H2O molecules move freely in the liquid state.

3. Number of ways to arrange the H2O molecules increases.

4. Degree of disorder of the system increases.

5. Hence, change in S increases.

Describe how changes in number of gaseous particles affects entropy

1. When number of gaseous particles increases from 1 mol of Cl2(g) to 2 mol of Cl.(g).

2. Number of ways to arrange the particles in the system increases.

3. Degree of disorder of the system increases.

4. Hence, change in S increases.

Describe the significance of change in G

1. When change in G<0, reaction is energetically feasible and spontaneous in the forward reaction.

2. When change in G>0, reaction is not energetically feasible and spontaneous in the backward reaction.

3. When change in G=0, reaction is at equilibrium and there is no net change.

State the limitations in using change in G to predict spontaneity of a reaction

1. Change in G does not indicate the rate of a reaction.

2. Use of change in G to predict spontaneity is valid only under standard conditions.