1.4 - Energetics

Enthalpy Change

Enthalpy Change

What is a system?

The reactants and products of a reaction

What are the surroundings in a reaction?

Equipment + naturally occurring elements in the room

Define Exothermic (is it +ve or -ve enthalpy)

A reaction that releases energy to the surroundings, decreasing the thermal energy in the system:

• -ve

Define Endothermic (is it +ve or -ve enthalpy)

A reaction that absorbs energy from the surroundings, increasing the thermal energy in the system:

• +ve

Define the term Enthalpy Change

The heat energy change during a reaction under a constant pressure (kj/mol)

Calorimetry

Calorimetry

What is the relationship between thermal energy + mass?

Thermal energy is inversely proportional to mass

Define Specific Heat Capacity

The thermal energy needed to increase the temperature of a substance per gram per kelvin

Give the equation for Thermal energy (+ units)

q=mcΔT

• q = energy = Joules

• m = mass = Kg

• c = Specific Heat Capacity = JK^-1g^-1

• T = Temperature = Kelvin

What are standard conditions for reactions? (2)

• 100KPa

• 298K

Define the Standard Enthalpy of Formation

• (when is it = 0 and why)

When 1 mole of a substance is formed from 1 mole of its constituent elements under standard conditions + constant pressure:

• The formation of elements - Since it will be formed in its standard state

Define the Standard Enthalpy of Combustion

When 1 mole of a substance is completely burnt under standard conditions in their standard state

Under standard conditions what elements are:

• gaseous

• liquid

• solid

• GAS - Group 8, Fluorine, Chlorine, Oxygen, Hydrogen and Nitrogen

• LIQUID - Bromine, Mercury

• SOLID - Rest

What is a Combustion Reaction? (+ 2 types)

When a substance reacts with oxygen to release thermal energy:

• Complete = Forms water + carbon dioxide ONLY

• Incomplete = Forms water + carbon dioxide, with at least Carbon or Carbon Monoxide

Hess’ Law

Hess’ Law

Define Hess’ Law

Enthalpy change at a constant pressure is independent of the route taken

Use CHEM NOTES 1 (HESS’ LAW - MULTISTEP ENTHALPY CHANGE DIAGRAMS)

Use CHEM NOTES 1 (HESS’ LAW - MULTISTEP ENTHALPY CHANGE DIAGRAMS)

What is the Bond energy-making/breaking for:

• Endothermic

• Exothermic

B(endo)M(ex):

• Bond Breaking = Endothermic

• Bond Making = Exothermic

Define Bond Dissociation Enthalpy

The enthalpy change when 1 mole of a type of bond is broken to give 1 mole of gaseous dissociated atoms

Define Mean Bond Enthalpy

The enthalpy change when 1 mole of bonds are broken, taken as an average from a range of different compounds

What is the main limitation from Mean Bond Enthalpy? (why)

Mean Bond enthalpies are calculated from a range of different compounds:

Experimentally these enthalpies are in the sample - making values different from the ones we’re calculating