Thermochemistry

Define enthalpy change?

name given to amount of heat given out/absorbed in a reaction carried out at constant pressure

Exothermic

negative delta h values

Endothermic

positive delta h values

Standard Conditions for temperature

298 K

Standard conditions for pressure

1 atm

Define enthalpy change of formation

Enthalpy change when 1 mol of products is formed from its constiutent elements in their standard states under standard conditions

You must end up with 1 mol of the compound so

fractions on LHS

Enthalpy change of combustion

Enthalpy change when 1 mol of a substance is combusted in excess oxygen under standard conditions

You must end up with:

1 mol of what you’re burning

In __________ reactions, the heat transferred to _______________ can be measured by carrying out the reaction in an ___________ container: ______________

chemical
surroundings

insulated

calorimeter

Equation for q

mc∆t

equation for ∆H

-mc∆t / n

How can we go from J to KJ

x100

Define bond enthalpy

the enthalpy needed to break 1 mol if the bond to give separated atoms in a gaseous state

Define average bond enthalpy

average value of the enthalpy required to break a given type of covalent bond in the molecules of a gaseous species

∆H

Bonds Broken - Bonds Maked

Define Hess’ Law

Total enthalpy change for a reaction is independent of the route taken from the reactants to the products.

Combustion Hess Cycle

Formation Hess Cycle

How can we find total ∆H using hess cycles

Sum of anti clockwise arrows - Sum of clockwise arrows