Chemistry 1.4- Energetics

Bond breaking is…

Bond making is…

(Endo/exothermic)

Bond breaking is endothermic, it takes in energy

Bond making is exothermic, it releases energy

What is an enthalpy change? When is it positive and negative?

• Enthalpy change (ΔH) is the heat energy change measured under constant pressure, which reflects the change in chemical energy

• Exothermic reactions have a negative ΔH because energy is lost to the surroundings, so the products have less chemical energy stored

• Endothermic reactions have a positive ΔH because energy is taken in from the surroundings, so the products have more chemical energy stored

Exothermic energy level diagram

• Products have less energy stored than the reactants, because that energy has been lost to the surroundings, so the enthalpy change (∆H) is negative

• Requires less activation energy (E_a) than endothermic reactions

Endothermic energy level diagram

• Products have more energy stored than the reactants, because energy has been taken in from the surroundings, so the enthalpy change (∆H) is positive

• Requires more activation energy (E_a) than exothermic reactions

What are standard conditions for standard enthalpy changes?

• A pressure of 100 kPa

• A temperature of 298 K (25 ^oC)

• Each substance is in its standard physical state (eg. oxygen is a gas, water is a liquid)

What is the symbol for standard enthalpy change?

ΔH^Ꝋ

What is the standard enthalpy change of combustion?

• The enthalpy change when one mole of a substance is burnt in excess oxygen (complete combustion) in standard states

• ΔH_c^ꝋ

• Eg. C₂H₆ (g) + 3½O₂ (g) → 2CO₂ (g) + 3H₂O (I) (one mole of ethane is burned)

What is the standard enthalpy change of formation?

• The enthalpy change when one mole of a compound is formed from its elements under standard conditions

• ΔH_f^ꝋ

• Eg. H₂ (g) + ½O₂ (g) → H₂O (I) (one mole of water formed)

What is the standard enthalpy change of reaction?

• The enthalpy change when a reaction occurs in the stoichiometric (simplest) equation under standard conditions

• ΔH_r^ꝋ

• Eg. MgCO₃ (s) → MgO (s) + CO₂ (g) (all in simplest ratio)

What is the standard enthalpy change of neutralisation?

• The enthalpy change when one mole of water is formed in the reaction between an acid and alkali under standard conditions

• ΔH_neut^ꝋ

• Eg. ½H₂SO₄ (aq) + NaOH (aq) → ½Na₂SO₄ (aq) + H₂O (I) (one mole of water formed)

What is specific heat capacity?

The energy needed to increase the temperature of 1g of a substance by 1^oC

What is the formula for the enthalpy change of a reaction?

q=mcΔT

enthalpy change = mass x specific heat capacity x change in temperatre

What is Hess’s law?

The enthalpy change of a reactions is the same regardless of the route the reaction takes

ie. The enthalpy change for the reaction (A → B), ΔH_r is the same as the sum of the intermediate reaction enthalpies, ΔH₂ + ΔH₃ (A → C → B) or ΔH₄ + ΔH₅ + ΔH₆ (A → D → E → B)

How do you calculate the enthalpy of reaction using Hess’s law?

ΔH_r = ΔH₂ (formation of products) - ΔH₁ (formation of reactants)

How do you calculate the enthalpy of formation using Hess’s law?

ΔH_f = ΔH₁ (combustion of reactants) - ΔH₂ (combustion of products)

What is mean bond enthalpy?

The enthalpy required to break a covalent bond, averaged over the substances in which the bond is found

• These are always given as positive enthalpies, as energy must be put into a system to break a bond

How can we use bond enthalpies to calculate the enthalpy of a reaction?

Enthalpy to break the bonds in the reactants - enthalpy released when making the bonds in the products

Why are Hess’s law calculations more accurate than bond enthalpy calculations?

Bond enthalpy calculations use mean values for each bond, averaged across each of its compounds, so it may use values that are inaccurate for the bonds in the specific compounds used in a reaction