Chapter 9 Enthalpy I

Measure of heat energy in a chemical system

The transfer of heat energy between the a chemical system and its surroundings at a constant pressure

The minimum amount of energy for a reaction to take place

X= Progress of Reaction Y= Enthalpy The activation energy goes from the reactants to the high point on the curve Endo= Reactants below products Exo= Reactants above products

100KPa 298 K/ 25 Degrees Celcius 1 mol/dm3 concentration All elements in standard state under these conditions

The enthalpy change that accompanies a reaction in its molar quantities shown in the chemical equation under standard conditions, with all elements in their standard states E.g 2Mg + O2 —> 2MgO give state symbols

The enthalpy change that accompanies the formation of one mole of a compound from its elements under standard conditions with all reactants and products in their standard states E.g 3 C (s) + 9/2 H2 (g) + 1/2 N2 (g) —> C3H9N (l) give state symbols

H-Br (g) —> H (g) + Br (g) must be gas

The enthalpy change that accompanies one mole of a substance completely reacting with oxygen (complete combustion) under standard conditions with all elements in their standard states E.g CH4 (g) + 2 O2 —> CO2 + 2 H20

The enthalpy change that accompanies the formation of 1 mol of water when an acid reacts with a base, under standard conditions with all reactants/products in their standard states

No, even if more reactants are used, as there is a bigger volume, the temperature change is spread over a larger area, so the same average temperature is the same

q= mass x change in temperature x SHC

4.18 j/k/g 1g/cm3

Calculate energy, converting units where necessary, and using measurements of the change in temperature of the water heated/ the surrounding N=m/mr of limited reactants Divide the energy in kj by the moles ADD A MINUS SIGN IF EXOTHERMIC

Heat loss to the surroundings Incomplete combustion (Evaporation from the wick)

The amount of energy required to break 1 mol of a specified bond in a gaseous molecule

Actual bond enthalpy values vary depending on the chemical environment of the bond The average bond enthalpy is only an average from multiple chemical environments

Bond breaking is endothermic Bond making is exothermic

Hess Cycle, to atoms so arrows downwards Or think about whether more energy is given out when bonds form or when broken , then add minus sign if exothermic

The overall enthalpy change of a reaction is independent of the route of the reaction, linked to the first law of thermodynamics- energy canot be destroyed or created

Point upwards

Point downwards

Reaction does not occur independently, multiple products will form Rate is too slow Activation energy is too high

Start with the largest equation Draw arrows in the correct direction at what can be substituted in Follow the cycle, reversing signs if necessary