Energetics - Unit 1, Section 4 Flashcards
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Enthalpy change (delta H)
the heat energy transferred in a reaction at constant pressure.
units = kJ mol -1
Standard Conditions
pressure = 100 kPa
temperature = 298 K
concentration = 1 mol dm3
Exothermic reactions
give out energy to the surrounding
temperature increases - feels hot
products have less energy than reactants
enthalpy change will be negative
oxidation is usually exothermic
examples = respiration and combustion
endothermic reactions
take in energy from their surroundings
temperature decreases - feels cold
products have more energy than the reactants
enthalpy change will be positive
examples = thermal decomposition, photosynthesis
what are bond enthalphies?
the energy needed to break a bond.
they have specific values that differ depending on which atoms are attached on either side of the bond.
breaking bonds
when a reaction happens, reactant bonds are broken. Energy is needed to break bonds so bond breaking is endothermic - positive.
forming bonds
when a reaction happens, product bonds are formed. Energy is released when bonds are formed so bond making is exothermic - negative.
what is a mean bond enthalpy?
the average energy needed to break a certain type of bond, over a range of compounds.
it is often given in data tables and used in calculations.
calculating enthalpy change
enthalpy change = total energy absorbed (reactants) - total energy released (products)
Standard enthalpy change of formation
the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.
Standard enthalpy change of combustion
the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions with all reactants and products in their standard states.
Standard enthalpy change of reaction
the enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation, under standard conditions with all reactants and products in their standard states.
q = mcΔ T
q = enthalpy change (J)
m = mass (g)
c = specific heat capacity (4.18 J g-1 K-1)
Δ T = temperature change (K)
calculating the standard enthalpy change of combustion
- use q = mcΔ T
- convert J to kJ
- calculate the number of moles of fuel - moles = mass/Mr
- calculate standard enthalpy change of combustion - Δ cH = q/n
what is Hess’s Law?
The total enthalpy change for a reaction is independent of the route taken.
it says that the total enthalpy change for route 1 is the same as for route 2.
