Chapter 9: enthalpy

What happens in terms of enthalpy and temperature to exothermic reactions?

enthalpy of products less than enthalpy of reactants

Heat loss from chemical system to surroundings

Surroundings temperature increases

Negative enthalpy change

activation energy

the minimum energy required to start a reaction through the breaking of bonds

standard conditions

100,000 Pa

298K

1 moldm^-3 concentration for reactions with aqueous solutions

standard state

the physical state of a substance under standard conditions of 100,000Pa and 298K

Standard enthalpy change of reaction

the enthalpy change that accompanies a reaction in the molar quantities expressed in the chemical equation

under standard conditions of 100,000Pa and 298K, all reactants and products being in their standard states

Standard enthalpy change of formation

the enthalpy change that takes place when 1 mol of a compound is formed from its constituent elements

under standard conditions of 100,000Pa and 298K, all reactants and products being in their standard states

Standard enthalpy change of combustion

the enthalpy change that takes place when 1 mol of a substance reacts completely with oxygen

under standard conditions of 100,000Pa and 298K, all reactants and products being in their standard states

Standard enthalpy change of neutralisation

the enthalpy change that accompanies the reaction of an acid by a base to form 1 mol of H₂O_(l)

under standard conditions of 100,000Pa and 298K, all reactants and products being in their standard states

value of standard enthalpy change of neutralisation for all neutralisation reactions

-57 kJmol^-1

calorimetry heat change equation

q=mcT

Where q= heat energy of surroundings

m= mass of surroundings

c= specific heat capacity

T= temperature change (K)

Why might experimental values of enthalpy change of combustion differ from actual values?

heat loss to surroundings

Incomplete combustion may occur

Evaporation of fuel/water

Not standard conditions

Equation linking heat energy and enthalpy change

Enthalpy change = -q/mol

Hess’ law

if a reaction can take place by more than one route, and the initial and final conditions are the same, the total enthalpy change is the same for each route

bond enthalpies in exothermic reactions

More energy is released when new bonds are formed than is taken in to break bonds in the reactants

average bond enthalpy

the average enthalpy change that takes place when breaking 1 mol of a given type of bond in the molecules of a gaseous species

why would average bond enthalpy differ from an experimental value?

bond enthalpies may not be the same as the average bond enthalpy

Bonds have different strengths in different environments

Experiment wasn’t performed under standard conditions

Energy could be lost in an experiment