Enthalpy changes

Endothermic enthalpy profile diagram

Exothermic enthalpy profile diagram

Enthalpy change

The heat energy transferred during a reaction at constant temperature

What happens in an exothermic reaction?

Energy is transferred from the system to the surroundings

What happens in an endothermic reaction

Energy is transferred from the surroundings to the system

How can you tell an energy profile represents an exothermic reaction

The products are at a lower energy than the reactants

Examples of exothermic reactions

• Combustion

• Respiration

• Neutralisation - H=-57kJmol^-

How can you tell an energy profile represents an endothermic reaction?

The products are at a higher energy than the reactants

Examples of endothermic reactions

• Photosynthesis

• Thermal decomposition

• Melting of ice

Activation energy

The minimum energy required for a reaction to take place

Standard conditions

• Temperature - 298K

• Pressure - 100kPa

• All solutions at - 1 moldm^-3

Standard enthalpy change of a reaction

The enthalpy change for a given reaction in the molar quantities shown in the equation

• Can be either a endothermic or exothermic reaction

Standard enthalpy of combustion

The enthalpy change when 1 mol of a substance completely combusts at 298K and 100kPa

• Exothermic reaction

Standard enthalpy of formation

The enthalpy change when 1 mol of a compound is formed from its elements at 298K and 100kPa

• Can be either endothermic or exothermic

• Enthalpy of formation of an element is zero by definition

Standard enthalpy change of neutralisation

The enthalpy change when 1 mol of H₂O is formed from a reaction of H⁺ and OH^-

• Always an exothermic reaction

Calorimetry

q=mcΔt

• q = energy change - J

• m = mass of solution - g

• c = specific heat capacity - Jg^-1K^-1

• Δt = Change in temperature of solution ^.C or K

Calorimetry 2

ΔH = +-q/n

• ΔH = enthalpy change - kJmol^-1

• q = energy change - kJ

• n = moles of reactant

  If there are 2 reactants we would use the limiting reactant

Combustion Calorimetry

• The water acts as the surroundings

• The fuel burning acts as the chemical system

How come the calculated value from an experiment is different to the data book in combustion calorimetry?

• Heat loss to surroundings

• Incomplete combustion will occur

How to reduce sources of error in combustion calorimetry

• Ensure a ventilated room

• Move beaker closer to the flame

• Copper beaker instead of glass beaker

• Add a lid to the beaker

• Cover the wick

Calorimetry with a reaction in solution

• The solution acts as the surroundings

• The chemical reacting or dissolving acts as the chemical system

Sources of error in calorimetry reaction in solution

• Heat loss/gain to/from surroundings

• Water evaporates from the beaker

• Incomplete reaction

• C of solution is not the same as water

How to minimise these errors

• Use a polystyrene cup

• Use a lid to prevent heat loss/gain

• Ensure standard conditions

Predicting temperature changes 1

ΔH is the same, If n is doubled q is doubled, Volume is doubled, So Δt is doubled

Predicting temperature changes 2

ΔH is the same, If n is halved q is halved, volume is halved, Δt stays the same

Average bond enthalpy

The enthalpy change for the breaking of 1 mole of bonds in gaseous molecules

Bond enthalpies

• The bond enthalpy value indicates the strength of a covalent bond

• The more positive the bond the larger the amount of energy needed to break the bond so the stronger the bond

Why is a reaction exothermic?

The energy required to break the bond is less than the energy released when making bonds

Why is a reaction endothermic?

The energy required to break the bonds is more than the energy released when making the bonds

Mexican Ben

ΔrH

ΣΔBeH (bonds broken) - ΣΔBeH (bonds formed)

Why is it not always possible to measure enthalpy changes directly from experiments?

• A very high activation energy

• A very slow rate of reaction

• Occurrence of side reactions

Enthalpy of Combustion Hess’s Law

Enthalpy of Formation Hess’s Law