Enthalpy Changes (OCR AS Chemistry A): Revision Note

Endothermic & Exothermic Enthalpy Changes

• The total chemical energy inside a substance is called the enthalpy (or heat content)

• When chemical reactions take place, changes in chemical energy take place and therefore the enthalpy changes

• An enthalpy change is represented by the symbol ΔH

  • Δ= change; H = enthalpy

• An enthalpy change can be positive or negative

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

• Exothermic reactions tend to be thermodynamically feasible (the enthalpy of the reactants is higher than that of the products), but feasibility also depends on the entropy change and the temperature, through ∆G = ∆H – T∆S

• However, if the rate is too slow, the reaction may not occur

  • In this case the reaction is kinetically controlled

The enthalpy changes during an exothermic reaction

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

The enthalpy changes during an endothermic reaction

Enthalpy Profile Diagrams

• An energy level diagram is a diagram that shows:

  • The energy level of the reactants

  • The transition state - the arrangement of atoms at the maximum of the energy profile, higher in energy than the reactants and products; it exists only for an instant and is not an intermediate (an intermediate sits in a dip in the profile)

  • The energy level of the products

  • The activation energy (E_a)

    • 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) 

    • This can be describes as the overall energy taken in from / given out to the surroundings OR the energy difference from reactants to products

The energy level diagram for the reaction of hydrogen with chlorine to form hydrogen chloride gas

Standard Enthalpy Definitions

• To be able to compare the changes in enthalpy between reactions, all thermodynamic measurements are carried out under standard conditions

• These standard conditions are:

  • A pressure of 100 kPa

    • OCR uses 100 kPa as the standard pressure (use this value in calculations)

  • 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

  • ΔH^Ꝋ = the standard enthalpy change

• There are a number of key definitions relating to enthalpy changes that you need to know

Definitions

• All standard enthalpies are performed under standard conditions

• Standard enthalpy change of reaction, ΔH^θ_r 

  • The enthalpy change when the reactants in the stoichiometric equation react to form the products, with all species in their standard states

  • Can be both exothermic and endothermic

• Standard enthalpy change of formation, ΔH^θ_f 

  • The enthalpy change when one mole of a compound is formed from its elements in their standard states

  • Can be both exothermic and endothermic

• Standard enthalpy change of combustion, ΔH^θ_c 

  • The enthalpy change when one mole of a substance, in its standard state, is burnt in excess oxygen

  • It is an exothermic change

• Standard enthalpy change of neutralisation, ΔH^θ_neut 

  • The enthalpy change when one mole of water is formed by reacting an acid and an alkali

  • It is an exothermic change