Stoichiometry and Thermochemical Reactions

• the energy of a reaction is directly related to the quantity of products

• if 28.4 kJ of energy is required to melt 1 mol of NaCl at its melting point then it would require twice that amount (56.8 kJ) to melt 2 mol of NaCl

FORMULA: Q = nΔH

Ex) Given: The molar heat of fusion (Δ Hfusion) for sodium = 2.63 kJ/mol

a) How many kJ are required to melt 3 moles of sodium at its melting point?Q = n ΔH Q = (3 mol)(2.63kJ/mol)

= 7.89 kJ

b) How many kJ are required to melt 11.5 g of sodium at its melting point?Q = n ΔH n = m/M

= (11.5 g)/(22.99 g/mol)= 0.5 mol

Q = n ΔH = (0.5 mol)(2.63 kJ/mol)= 1.32 kJ

Ex. Aluminum reacts readily with chlorine gas to produce aluminum chloride. The molar enthalpy of this

reaction is ΔHºrxn = -1408 kJ/mol. What is the

enthalpy change when 1.0 kg of Al reacts completely with excess Cl2?

Therefore the enthalpy change is -2.6 x 104 kJ.

2Al(s) + 3Cl2(g) 2AlCl3 (s) + 1408 kJ

n

m

M

Q

37.06 mol 18.53 mol

1000 g

26.98 g/mol

2.6 x 104 kJQ = nΔH

HOMEWORKp226 #1-4

WS “Thermochemical Equations and Stoichiometry”