SME
Grade 10 Science · Chemistry
Thermochemistry: heat, enthalpy and calorimetry
Use this for “Read ΔH and the activation energy from an energy diagram. Is the reaction exothermic or endothermic?”
Diagram
The products are lower: the reaction gives off 90 kJ.
Exothermic.
Energy of the activated complex: 280 kJ
This is an example. Type your own values to start a new problem.
Hᵣ
Energy of the reactants
example · kJ
Hₚ
Energy of the products
example · kJ
Eₐ
Activation energy
example · kJ
ΔH
Enthalpy change
calculated · kJ
Eₚₑₐₖ
Energy of the activated complex
calculated · kJ
Eₐ′
Activation energy of the reverse reaction
calculated · kJ
Formulas
ΔH = Hₚ − Hᵣ
−90 = 110 − 200
Eₚₑₐₖ = Hᵣ + Eₐ
280 = 200 + 80
Eₐ′ = Eₐ − ΔH
170 = 80 − (−90)
Assumptions
•
Energy is stored in chemical bonds; breaking bonds takes energy, forming them releases it.
•
ΔH below zero is exothermic: the surroundings warm.
•
Only differences matter; the zero of enthalpy is chosen.
Step-by-step
WE KNOW
Hᵣ = 200 kJ, Hₚ = 110 kJ, Eₐ = 80 kJ
FIND
enthalpy change (ΔH), energy of the activated complex (Eₚₑₐₖ), activation energy of the reverse reaction (Eₐ′)
Step 1 · Find enthalpy change (ΔH)
Use ΔH = Hₚ − Hᵣ
ΔH is where the reaction ends minus where it starts: negative when energy is given off.
ΔH = 110 − 200
ΔH = −90 kJ
Step 2 · Find energy of the activated complex (Eₚₑₐₖ)
Use Eₚₑₐₖ = Hᵣ + Eₐ
The reactants must climb Eₐ to reach the top of the hump.
Eₚₑₐₖ = 200 + 80
Eₚₑₐₖ = 280 kJ
Step 3 · Find activation energy of the reverse reaction (Eₐ′)
Use Eₐ′ = Eₐ − ΔH
Going back, the climb starts at the products: the forward climb minus ΔH.
Eₐ′ = 80 − (−90)
Eₐ′ = 170 kJ
Check
Put the numbers back in. Both sides should match.
−90 = 110 − 200 ✓
280 = 200 + 80 ✓
170 = 80 − (−90) ✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
Refresh
More problem types
Calorimetry: q = mcΔT
Use this for “A reaction warms 100 g of water from 21 °C to 27.5 °C. How much heat did it give off?”, or a metal’s specific heat from its heat, mass and warming.
A cold pack: ΔH from a calorimeter
Use this for the ΔH per mole of a salt that cools the water as it dissolves.
Heat to turn ice into steam
Use this for the heat to take ice all the way to steam: warm it to 0 °C, melt it, warm the water to 100 °C and boil it away.
ΔH from heats of formation
Use this for ΔH of this reaction, CH₄ + 2O₂ → CO₂ + 2H₂O, from the heats of formation.
Hess’s law: adding steps
Use this for ΔH of this reaction, C + O₂ → CO₂, from C + ½O₂ → CO and CO₂ → CO + ½O₂.