CH9 - Enthalpy

• What is enthalpy

  A measure of the heat energy in a chemical system
• what is a chemical system

  the atoms, molecules, and ions that make up the chemicals in a reaction
• equation for enthalpy change

  enthalpy of products- enthalpy of reactants
• what is the law of conservation of energy

  a rule of science that states that energy cannot be created or destroyed
• what happens when a chemical reaction involving enthalpy change occurs

  heat energy is transferred between the chemical system and the surroundings
• what happens in an exothermic enthalpy change

  energy is transferred from the system to the surroundings, any energy loss by the chemical system is made up for by the gain in energy by the surroundings

  therefore enthalpy change is negative and the temp of the surroundings increases
• what happens in an endothermic enthalpy change

  energy is transferred from the surroundings to the system, any energy gain by the chemical system is balanced by the same energy loss from the surroundings

  therefore enthalpy change is positive and temp of environment descreases
• What does a negative ΔH indicate in a reaction?

  Exothermic reaction
• What does enthalpy (H) represent in a chemical reaction?

  Heat content of the system
• Draw an enthalpy profile diagram for an exothermic reaction


  :)

  
• Draw an enthalpy profile diagram for an endothermic reaction


  :)

  
• define activation energy

  the minimum energy required to start a reaction by the breaking of bonds
• define standard conditions

  a pressure of 100kPa, temp of 298K or 25 degrees Celsius and a concentration of 1 mol dm^-3
• define standard state

  the physical state of a substance under standard conditions
• define standard enthalpy change of combustion

  The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactions and products being in their standard state
• define standard enthalpy change of formation

  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
• define standard enthalpy change of neutralisation

  The enthalpy change that accompanies the reaction of an acid by a base to form one mole of H2O under standard conditions with all reactants and products in their standard states
• define standard enthalpy change of reaction

  the enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products in their standard states
• what is activation energy

  the minimum energy required for a reaction to take place
• what is calorimetry

  an experimental method for finding enthalpy change
• how does enthalpy change work

  it works by measure temperature change over a period of time