Chem Energetics I

What is enthalpy change?

ΔH, Change in energy content of a chemical reaction at constant pressure in kJmol-1

What is the equation of enthalpy change?

ΔH = ΣH_products - ΣH_reactants

What is an endothermic reaction?

• A reaction in which energy is absorbed from the surroundings → Temperature of the surroundings decreases

• Products are energetically less stable than reactants

• ΣH_products > ΣH_reactants → ΔH>0 → More energy absorbed than released

What is an exothermic reaction?

• A reaction in which energy is released to the surroundings → Temperature of the surroundings increases

• Products are energetically more stable than reactions → More energetically feasible

• ΣH_reactants > ΣH_products → ΔH<0 → More energy released than absorbed

What is activation energy E_A

Minimum amount of energy reactant particles must possess before they can collide successfully to form products

What are the standard conditions for thermochemical measurements?

• Pressure → 1 bar (10⁵Pa)

• Temperature → 298K (25ºc)

• All substances are in standard states i.e. most stable form

What is standard enthalpy change of reaction?

ΔH_rº = Energy change in a chemical reaction when the molar quantities of reactants stated in the chemical equation react under standard conditions

What is the standard enthalpy change of formation of a substance?

ΔH_fº (substance) = Energy change when 1mol of pure substance in a specified state is formed from its constituent elements in their standard states under standard conditions

• More negative → More stable a compound is relative to constituent elements

What is the standard enthalpy change of formation of an element?

ΔH_fº (element) = Energy change when 1mol of an element in its standard state is formed under standard conditions = 0

What is the standard enthalpy change of combustion?

ΔH_cº = Energy released when 1mol of a substance is completely burnt in excess oxygen under standard conditions

• Formation of CO2 and H2O

What is the standard enthalpy change of neutralization?

ΔH_neutº = Energy change when the acid and base react to form 1mol of water under standard conditions

• Weak acid + Strong base → Less exothermic than strong acid + strong base

What is the standard enthalpy change of atomization of an element?

ΔH_atomº (element) = Energy absorbed when 1mol of gaseous atoms is formed from the element under standard conditions

What is the standard enthalpy change of atomization of a compound?

ΔH_atomº (compound) = Energy absorbed when 1mol of compound is converted to gaseous atoms under standard conditions

What is bond energy?

Average energy required to break 1mol of X-Y bonds in the gaseous state

What is the bond energy of a diatomic gas?

BE (X-X) = 2ΔH_atomº [X₂(g)]

What are the equations relating temperature change to heat change?

q = mcΔT or q = CΔT

• q → J

• m → g

• ΔT → ºc or K

• c = specific heat capacity → Jg^-1ºc^-1 / Jg^-1ºK^-1

• C = heat capacity Jºc^-1 / JK^-1

What is heat capacity and specific heat capacity?

• Heat capacity (C) = Quantity of heat required to raise the temperature of the substance by 1ºc/K

• Specific heat capacity (c) = Quantity of heat required to raise the temperature of 1g of the substance by 1ºc/K

• C = mc

What is the equation for experimentally determined heat change (q)

ΔH = (-q / n_A) x a

n_A = amount of substance A reacted or produced in the reaction

a = stoichiometric coefficient of substance A in the thermochemical equation

What does Hess’ Law state?

Enthalpy change of a reaction is determined by the initial and final states of the system, and is independent of pathways taken

What is the formula for calculating ΔH_rº from combustion data?

ΔH_rº = ΣnΔH_cº (reactants) - ΣmΔH_cº (products)

What is the formula for calculation ΔH_rº from formation data

ΔH_rº = ΣnΔH_fº (products) - ΣmΔH_fº (reactants)

What is the formula for calculating ΔH_rº from bond energy data?

ΔH_rº = ΣBE (reactants) - ΣBE (products)