enthalpy changes

• What is chemical energy?

  Chemical energy is a form of potential energy which is stored in chemical bonds
• What are chemical bonds?

  Chemical bonds are the attractive forces that bind atoms together.
• What changes occur to chemical bonds as a reaction occurs?

  • As a reaction takes place, bonds break in the reactants and new bonds are formed in the products.
  • The difference in energies of the bonds between the atoms of reactants and products is called chemical energy.
• What is enthalpy,H ?

  Enthalpy, H, is the heat content that is stored in a chemical system, as reactants or products.
• How can you measure the enthalpy content of a system?

  It is impossible to measure the enthalpy content of a system directly but we can measure the differences in enthalpy contents.

  • The difference in enthalpy contents is the energy either given out or absorbed.
• What is the first law of Thermodynamics?

  Energy cannot be created or destroyed just transferred between the system and the surroundings
• What is enthalpy change, DH?

  • This is the difference between the enthalpy contents of the products and the reactants (at constant pressure):

                  DH    =    Hproducts    -    Hreactants

  • It is rare for enthalpy contents of products and reactants to be exactly equal.
  • This means that there is almost always a transfer of energy between the system and the surroundings.
• Exothermic reactions?


  the enthalpy content of the products is smaller than the reactants.

  • The excess energy is transferred from the system to the surroundings - hot

                   Hproducts    <    Hreactants
• Exothermic reactions?

  DH=Negative value  

  Energy is released from the system to the surroundings
• Endothermic reactions?

  This means that the enthalpy content of the products is greater than the reactants.

  • The excess energy is transferred to system from the surroundings - cold
• Endothermic reactions?

  Hproducts    >    Hreactants

  =Positive value  

  Energy is transferred to the system from surroundings
• What are some exothermic reactions?

  Combustion, oxidation reactions and respiration.
• What are some endothermic reactions?

  Melting ice, dissolving ammonium nitrate in water, photosynthesis & thermal decomposition of limestone.
• What is activation energy?

  The minimum amount of energy required to begin breaking reactants bonds and start a chemical reaction.

  Once a reaction is started, the activation energy is regenerated and the reaction becomes self-sustaining.
• What is a system?

  The chemical reaction, in particular the heat stored within the chemical reaction.
• What is the surroundings?

  The rest of the Universe x
• What symbol is enthalpy change represented by?

  the symbol ΔH

  • Δ= change; H = enthalpy
• Exothermic reactions?

  • A reaction is exothermic when the products have less energy than the reactants
  • Heat energy is given off by the reaction to the surroundings
  • The temperature of the environment increases - this can be measured with a thermometer
  • The energy of the system decreases
  • There is an enthalpy decrease during the reaction so ΔH is negative
• How are exothermic reactions thermodynamically possible?

  because the enthalpy of the reactants is higher than that of the products

  • However, if the rate is too slow, the reaction may not occur
  • In this case the reaction is kinetically controlled
• Endothermic reactions?

  • A reaction is endothermic when the products have more energy than the reactants
  • Heat energy is absorbed by the reaction from the surroundings
  • The temperature of the environment decreases - this can be measured with a thermometer
  • The energy of the system increases
  • There is an enthalpy increase during the reaction so ΔH is positive
• What do enthalpy profile diagrams show?

  • The energy level of the reactants
  • The transition state(s) - an unstable intermediate in the reaction which cannot be isolated and is higher in energy than the reactants and products
  • The energy level of the products
  • The activation energy (Ea)
  • The minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction
  • The enthalpy change for the reaction (ΔH)
• What are the standard conditions?

  • A pressure of 100 kPa or 1 atm
  • A temperature of 298 K (25 oC)
  • Each substance involved in the reaction is in its standard physical state (solid, liquid or gas)
  • To show that a reaction has been carried out under standard conditions, the symbol Ꝋ is used
• What does a system mean in a chemical reaction?

  The atoms and bonds involved in the chemical reaction.
• What is the law of conservation?

  The amount of energy in an isolated system remains the same.

  Energy cannot be destroyed or created, it can only be transferred from one to another.
• What energy change is breaking bonds associated with?

  Energy is taken in to break bonds -> endothermic reaction
• What energy change is making bonds associated with?

  energy is released to make bonds -> exothermic reaction
• What is an endothermic reaction?

  A reaction with an overall positive enthalpy change.

  Enthalpy of the products is greater than the enthalpy of the reactants.
• What is an exothermic reaction?

  A reaction with an overall negative enthalpy change.

  Enthalpy of products is less than the enthalpy of the reactants.
• What is activation energy?

  The minimum energy required for a reaction to occur.
• Which way does the arrow for activation energy point on an enthalpy profile diagram?

  Always points upwards
• What does 'in standard state' mean?

  The state an element/compound exists at in standard conditions (100 kPa, 298K).
• What is the definition of enthalpy change of formation?

  The enthalpy change that takes place when 1 mole of a compound is formed from its elements in their standard states under standard conditions.
• What is an example of an equation which represents standard enthalpy of formation?

  H2 (g) + 1/2 O2 (g) -> H2O (l)
• What is the definition of enthalpy change of combustion?

  The enthalpy change that takes place when 1 mole of a substance is completely burned in oxygen, under standard conditions, with all reactants and products in their standard states.
• What is an example of an equation which represents standard enthalpy of combustion?


  C (s) + O2(g) -> CO2(G)
• What is the definition of enthalpy change of neutralisation?


  The energy change that takes place when 1 mole of water is formed from a neutralisation reaction (when solutions of an acid and an alkali react together under standard conditions)
• What is the definition of enthalpy change of reaction?

  The energy change associated with a given reaction.
• How can you calculate enthalpy change from experimental data?

  Use the equation Q=mc^T, where m is the mass of the substance being heated

  c is the specific heat capacity of that substance (water=4.18 j-1 k-1)

  ^T is the change in temperature