Chemistry 30 – Formulas and Information
Thermochemistry:
Formula: Variables: When to use:
Q = mc∆t Q = total heat/enthalpy (J or kJ)
m = mass (g)
c = specific heat capacity (J/g◦C or kJ/g◦C)
∆t = temperature change (◦C)
If given the specific heat capacity, mass and temperature change of
one substance.
nH = mc∆t n = moles of substance reacting (n = cv or m/M) (mol)
H = molar heat/ enthalpy of substance reacting (J/mol or kJ/mol)
m = mass of water (1g = 1 mL)
c = specific heat capacity of water (J/g◦C or kJ/g◦C)
∆t = temp. change of water (◦C)
Calorimetry questions:
- if given information about a reacting substance and water
(remember water is present to determine the energy released or
absorbed by the reaction)
- usually asked to find the molar enthalpy (H)
∆H = ∑nfH(products)
- ∑nfH(reactants)
∆H = total heat/ enthalpy (J or kJ)
∑nfH (products) = sum of heats of formations of products
∑nfH (reactants) = sum of heats of formations of reactants
Hess’s Law:
- if only have a reaction with no other energy information
- remember:
- use data booklet for heats of formations of several compounds
- elements have a heat of formation of zero
% efficiency =
energy output x 100
energy input
% efficiency (%)
energy output (J or kJ released by a system)
energy input (J or kJ absorbed by a system)
If asked for % efficiency and given the energy released and used
by a system
Electrochemistry:
Formula: Variables: When to use:
E◦net = E◦red + E◦ox E◦net = net voltage of a cell (V)
E◦red = voltage of the reduction half reaction (V)
E◦ox = voltage of the oxidation half reaction (V)
If asked to find the voltage of a cell:
- the voltages come from the data booklet
- the oxidation half reaction comes from flipping the reduction half reaction in the
data booklet (remember to flip the voltage sign as well, but you don’t need to multiply
the voltage if reaction was multiplied for equal electron transfers)
q = It q = charge (C)
I = current (C/s or I)
t = time (s)
Electrical Stoichiometry:
- if given info about an electrolytic process such as running a current and the length
of time
ne = q or It
F F
ne= moles of electrons (mol)
q = charge (C)
I = current (C/s or I)
t = time (s)
F = 9.65 x 104 C/mol (Faraday’s Constant)
n = moles of reacting substance (mol)
Electrical Stoichiometry:
- if given info about an electrolytic process such as running a current and the length
of time used to produce a solid metal
- need to write the half reaction of the metal (substance) being electroplated or refined
- use the ne to calculate the n of the reacting substance (multiply n by the
coefficient ratios wanted/ given)
