Enthalpy

Draw an exothermic reaction profile and describe what this means in terms of bonds

• The amount of energy given out is greater during bond making than the amount of energy absorbed when bonds are broken

• Increase in temp of surroundings

Draw an endothermic reaction profile and describe what this means in terms of bonds

• The amount of energy absorbed is greater during bond breaking than the amount of energy released when bonds are made

• Decrease in temp of surroundings

What are the standard conditions for enthalpy

• 100kPa

• 298K

  • this is recognised with: △Hθ

Define bond enthalpy

• average amount of energy required to break 1 mole of a covalent bond measured in the gaseous state in kJmol-1

• always a positive value: bond breaking requires energy, so energy is absorbed and is endothermic

What are rules of bond enthalpies

• The higher the bond enthalpy, the stronger the covalent bond

• Triple bonds: shortest but strongest

• Single bonds: longest but weakest

• Bond lengths increase down group 7

• Bond strength decrease down group 7

What are the formulas used to calculate enthalpy change

• q=mc△T

• J → kJ= ÷1000

• △H= q÷n

What is the definition for the enthalpy change of formation

Enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions

Define enthalpy change of combustion

Enthalpy change when 1 mole of a substance is completely burned in excess oxygen, under standard conditions to form CO2 + H2O

Definition of enthalpy change of neutralisation

Enthalpy change when an acid reacts with a base to form 1 mole of H2O under standard conditions

What does Hess’s law state

The enthalpy change for a chemical reaction is independent of the rate taken and depends only on the initial and final states

What does a combustion cycle look like

What does a formation cycle look like