Chap 5C - Enthalpy

Calculations

State the formulas between H and Hc and Hf and q

• In this reaction, the reaction between Zn and Ag+ heats up the solution it is in -> reaction between Zn and Ag+ is treated as the ‘system’

• The water or solution is treated as the ‘surrounding’

• When calculating mcT, mass m refers to the ‘surrounding’ which in this question is the 50 cm3 solution

State assumptions in calculations

1. No heat loss/gain from surrounding air 

2. Heat capacity of the calorimeter is omitted

• More convenient to measure the volume of the solution : To calculate the mass of the solution,  assume that the density of the solution is the same as that of water (1.00 g cm–3 ) since the solution is very dilute

• Specific heat capacity of the solution is the same as that of water (4.18 J g–1 K –1)

Describe Hess’s Law

• To make used of Hess’ Law, the enthalpy changes in a reaction can be represented either in the form of an energy cycle or an energy level diagram.

• Definition : Hess’ Law states that the enthalpy change of a chemical reaction is independent of the pathway which the reaction takes place, provided that the initial and final states of the system remains the same

• Simpler words : Total enthalpy change of a reaction is the same, regardless of the path taken, as long as the initial and final states are the same

• Sum of clockwise arrows = sum of anticlockwise arrows

State some impt stuff abt energy cycles

• Direction of the arrow DOES NOT indicate whether reaction is endothermic or exothermic

• Need to point arrows in the right direction to correctly indicate the enthalpy change that occurs

• Must include state symbols

State some impt stuff about energy level diagrams

• Direction of arrows = reaction is exothermic or endothermic

  • Upward arrows are used for endothermic reactions and downward arrows for exothermic reactions

• Length of arrows = magnitude of enthalpy change (so length matters!) 

• Always include the y-axis, which is labelled as the energy content of the substances

Describe the born-harder cycle

• The Born-Haber Cycle is a technique of applying Hess’ Law to the standard enthalpy changes during the formation of an ionic compound

• Lattice energy can be calculated from born-haber cycle 

• The cycle begins with the elements in their standard states, which is the 'zero reference line’

State formula relating LE, f, atom,