C8 - Energetics I

Enthalpy change

• When chemical bonds break or form, energy is often released or absorbed as heat

• At constant pressure the energy change is enthalpy change

Exothermic

• Release heat energy

• Have a negative enthalpy change

• Temperature of surroundings increase

Endothermic

• Absorb heat energy from surroundings

• Have a positive ΔH

• Temperature of surroundings decreases

Energy profiles: Exothermic

Endothermic

Standard conditions

• Enthalpy changes are measured under standard conditions

  • Stated temperature (298 K)

  • 100 kPa

Standard enthalpy of reaction

Enthalpy change when a reaction occurs in the molar quantities shown in the chemical reaction under standard conditions

Standard enthalpy change of formation

Enthalpy change when 1 mole of a compound forms from elements under standard conditions

Standard enthalpy change of combustion

Enthalpy change when 1 mole of a substance combusts fully in oxygen under standard conditions

Standard enthalpy change of neutralisation

Enthalpy change when an acid and an alkali neutralise under standard conditions to form 1 mole of water

Combustion calorimetry

Used to find the enthalpy change when a fuel is burned

Combustion calorimetry: method

• A weighed fuel sample is burnt underneath a calorimeter containing a known mass of water

• As the fuel combusts, heat is transferred to the water

• Thermometer measures the temperature change

• Temperature change along with the mass and heat capacity of water are used to calculate heat energy

Sources of error in calorimetry

• Heat loss to surroundings - means less absorbed by the water so lower enthalpy change

• Incomplete combustions - less heat produced so lower enthalpy change

• Fuel evaporation - reduces the amount of heat generated, so decreases the measured enthalpy change

Solution calorimetry

• Used to find the enthalpy change for reactions occurring in aqueous solutions such as the enthalpy change of neutralisation

  • Used to find ΔH of

    • Neutralisation

    • Dissolution

    • Displacement

Method of solution calorimetry

• Add measured volume of one reactant to a polystyrene cup calorimetre and record initial temp

  • Are better insulators, lightweight, waterproof and has a low specific heat capacity

  • Reduces heat loss

• Add measured volume of second reactant and put on lid

• Carefully stir and record temps at intervals

Temperature time graphs

• During calorimetry, temperature fluctuates meaning that it would start to decrease again after reaching peak

• Means subtracting final and initial values would mean we underestimate the temperature change

• To accurately determine temperature change

  • Plot the data

  • Establish a baseline

  • Post reaction line

  • Extrapolation

  • True temperature change

    • Difference between two extrapolated lines of best fit

Calculating enthalpy change

• Q = mcΔT

  • mass of solution

• ΔH = q/n

Hess law

Route doesn’t affect ΔH

Hess cycles of combustion

Hess cycles of formation

• Can be used to calculate unknown enthalpy change of reaction

  • Calculated with sum of  ΔHf (products) - sum of  ΔHf(reactants)

Breaking and forming bonds

• BENDO - MEXO

• Overall enthalpy change of a reaction depends on the difference between energy needed to break and form bonds

  • If more energy is needed to break bonds rather than forming new ones, ΔH is positive

  • Vice verse means exothermic as ΔH is negative

Bond enthalpy

• Amount of energy needed to break one mole of a type of bond in the gas phase

  • Requires energy output so it will always be positive

• Depends on:

  • Strong electrostatic forces of attraction between oppositely charged ions (IONIC COMPOUNDS)

  • Nuclear attraction to the negatively charged electron pair that forms the covalent bond (COVALENT MOLECULES)

Mean bond enthalpies

• Average values rather than the actual value

• Are the mean whereas in reality they can vary slightly

• e.g. OH bonds

  • Breaking 1st OH bond requires more energy than second

  • Happens since after the first OH bond is broken, the remaining OH bond experiences greater electron repulsion

• Mean bond enthalpies provided in data tables are averages taken from main different molecules with that bond

Calculating enthalpy changes using mean bond enthalpies

ΔH_reaction = sum of bond enthalpies of bonds broken - sum of bond enthalpies of bonds formed