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6. Thermochemistry

Nature of energy

thermodynamics: heat in chemistry

energy: the capacity to do work or produce heat

law of conservation of energy: energy cannot be created nor destroyed

first law of thermodynamics: the energy of the universe is always the same

heat: when energy is transferred from one thing to another

hot → cold

enthalpy (ΔH): measuring the transfer of energy/heat

endothermic: heat goes into the system (ΔH is positive)

exothermic: heat leaves the system (ΔH is negative)

know what is in the system and in the surroundings (affects answer)

thermal equilibrium: when you mix two things of different temperatures, heat transfers until they are the same temperature

phase change: changing state

solid → liquid (endothermic)

liquid → gas (endothermic)

gas → liquid (exothermic)

liquid → solid (exothermic)

Calorimetry

calorimetry: measuring heat in chemistry

calorie: amount of joules needed to heat 1 g 1℃

kilocalorie: calorie used in food (1000 cal)

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q: heat (J)

m: mass (g)

ΔT: change in temperature (℃ or K)

C: specific heat capacity (J/g℃)

e.g. H₂O: 4.184 - very high

lower: easier to heat up

constant pressure: q = ΔH

Enthalpy

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every compound has its own enthalpy value

all elements: H = 0

Formation

find reactants

balance for one mole of product (may need fractions)

find enthalpy

put heat on appropriate side of reaction based on sign of enthalpy

example: NO₂(g)

N₂(g) + O₂(g)

1 mol of product

1/2 N₂(g) + O₂(g)

ΔH = 34 kJ/mol

positive → endothermic → heat goes on left

34 kJ + 1/2 N₂(g) + O₂(g) → NO₂(g)

example: CH₃OH(l)

C(s) + 2 H₂(g) + 1/2 O₂(g)

ΔH = - 239 kJ/mol

negative → exothermic → heat goes on right

C(s) + 2 H₂(g) + 1/2 O₂(g) → CH₃OH(l) + 239 kJ

Reaction

use formula

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4NH₃(g) + 7 O₂(g) → 4 NO₂(g) + 6 H₂O(l)

example: NO₂: NH₃: - 46 kJ/mol, O₂: 0 kJ/mol, 32 kJ/mol, H₂O: - 286 kJ/mol; find ΔH for the reaction

ΔH = ( (4 mol)(34 kJ/mol) + (6 mol)(- 286 kJ/mol) ) - ( (4 mol)(- 46 kJ/mol) + (7 mol)(0 kJ/mol) )

ΔH = - 1396 kJ

example: 8 mol NH₃ and 21 mol O₂. what is the amount of energy released?

limiting reactant

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2792 kJ

Hess’s law

many chemical reactions occur in multiple steps

each step has its own enthalpy

Hess’s law: if you add up all the enthalpies of the steps, it will equal the total enthalpy

if you flip the products and reactants in an equation, the ΔH changes sign

if you multiply an equation by a number, the ΔH is multiplied by the same number

example

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example

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