C3.2.1

• Enthalpy (H) is a measure of the heat energy in a chemical system and is sometimes thought of as the energy stored within bonds . Enthalpy cannot be measured but enthalpy changes can .
• The Enthalpy change (ΔH) is the difference in enthalpies of the reactants and products
• ΔH = H ( products ) - H ( reactants )
• A positive ΔH is an endothermic reaction where energy is transferred from the surroundings to the system
• A negative ΔH is an exothermic reaction where energy is transferred from the system to the surroundings
• The law of conservation of energy states that energy cannot be created or destroyed . During enthalpy changes heat energy is transferred between the system ( made of products and reactants ) and the surroundings .
• In an exothermic reaction:

  • ΔH is negative
  • energy is transferred from the chemical system to the surroundings
  • temperature of surroundings increases
• What type of reaction is shown in the picture?

  Exothermic

  
• What is a?

  Enthalpy ( H )

  
• What is B?

  reactants

  
• What is c?

  products

  
• What is d?

  progress of reaction

  
• What is e?

  negative enthalpy change

  
• Describe what is happening at F?

  The chemical system loses energy

  
• Describe what is happening at G?

  The surroundings gain energy and the temperature increases

  
• What type of reaction is the picture describing?

  Endothermic

  
• What is a?

  Enthalpy ( H )

  
• What is b?

  progress of reaction

  
• What is c?

  reactants

  
• What is D?

  products

  
• What is E?

  positive enthalpy change

  
• Describe what is happening at F?

  The chemical system gains energy

  
• Describe what is happening at G?

  The surroundings lose energy and the temperature falls

  
• In an endothermic reaction…

  • enthalpy change (ΔH) is positive
  • Energy is transferred from the surroundings to the chemical system
  • temperature of surroundings decreases
• The Activation energy (Eₐ) is the minimum amount of energy required to break bonds in the reactants and start a chemical reaction
• The Standard conditions (⦵) are…

  • Pressure of 100KPa
  • Temperature of 298K ( 25 degrees )
  • Concentration of 1 mol dm-3
• The standard state (⦵) of a substance is the physical state of a substance under standard conditions
• The enthalpy change (ΔH) for a reaction can vary slightly depending on the conditions used. To standardise results standard conditions are used when measuring enthalpy changes
• The symbol for standard conditions is ⦵ . For example standard enthalpy change is ΔH⦵
• What type of reaction is shown in the picture?

  Exothermic

  
• What is A?

  enthalpy change

  
• What is B?

  Activation Energy

  
• What reaction is shown in the picture?

  Endothermic

  
• What is A?

  Activation Energy

  
• What is B?

  enthalpy change

  
• Define the standard enthalpy change of reaction (ΔrH⦵)

  The enthalpy change that accompanies a reaction in the molar quantities expressed in a stated chemical equation under standard conditions , all reactants and products being in their standard states
• The enthalpy change of reaction always refers to a stated equation and it’s value depends on the balancing numbers
• Reactions can have the same reactants and products but different enthalpy changes of reaction as they have different amounts of moles . Enthalpy changes of reaction are specific to equations

  
• Define the standard enthalpy change of formation (ΔfH⦵)

  The enthalpy change that takes place when one mole of a compound is formed from it’s constituent elements in their standard states under standard conditions
• Fractional balancing numbers can be used to ensure that only one mole is formed in enthalpy change of formation equations