energetics (A1 physical chemistry) Flashcards
enthalpy change definition and calculation
the heat change in a reaction at constant pressure
ΔH, measured in kJmol⁻¹
enthalpy change = total energy to break bonds - total energy released forming bonds
endothermic reactions
absorb energy from surroundings (to break bonds), temperature of surroundings lowers
products higher in energy than reactants
ΔH is positive
process absorbs more energy when breaking bonds in reactants then it releases when forming new bonds in products
e.g. thermal decomposition of calcium carbonate (requires more heat energy to form products and products are less stable than reactants)
exothermic reactions
release energy to surroundings (when bonds are formed), temperature of surroundings increases
ΔH is negative
products lower in energy than reactants
process releases more energy when forming new bonds in products than it absorbs when breaking bonds in reactants
e.g. combustion of ethane
mean bond enthalpy definition
mean bond enthalpy is the energy required to break one mole of a specific type of bond in gaseous molecules, averaged over a range of compounds
bonds of the same type will have slightly different bond enthalpies, but we can find an average value
how to calculate energy and enthalpy change from a calorimetry experiment e.g. 100 g of water was heated from 23°C to 57°C by 1.8 g of ethanol, calculate the energy transferred and hence the enthalpy change of the fuel
q = mcΔT
q - heat energy lost or gained (J)
m - mass of water or solution (g)
c - specific heat capacity of water
ΔT - temperature change (K)
from the value calculated, we can use mass / Mr to find the moles of fuel and do q / moles to find the enthalpy change
q = 100g * 4.18 * 34 = 14212 J
moles of ethanol = 1.8 / 46 = 0.039 mol
enthalpy = -14.212kJ / 0.039 = -364.41 kJmol⁻¹
how to calculate energy from calorimetry experiments of solutions e.g. 25cm³ of 1 moldm⁻³ HCl had a temperature of 20°C, 25 cm³ of 1 moldm⁻³ of NaOH was added and this raised the temperature to maximum of 26°C, calculate the enthalpy of neutralisation for hydrochloric acid
q = mcΔT
q - heat energy lost or gained (J)
m - mass of water or solution (g)
c - specific heat capacity of water
ΔT - temperature change (K)
from the value calculated, we can use mass / Mr to find the moles of fuel and do q / moles to find the enthalpy change
q = 100g * 4.18 * 34 = 14212 J
moles of ethanol = 1.8 / 46 = 0.039 mol
enthalpy = -14.212kJ / 0.039 = -364.41 kJmol⁻¹
state Hess’s law
the total enthalpy change of a reaction is independent of the route taken
formation and combustion cycles
formation cycles are used when given ΔfH data, arrows face upwards
reactants ⮕ products
⬉ ⬈
elements
(in their standard states)
combustion cycles are used when given ΔcH data, arrows face downwards
reactants ⮕ products
⬊ ⬋
combustion products
(H₂O and CO₂)
ensure equation is balanced
enthalpy of elements that stay the same or are on both sides of the equation is 0 (will not be in data)
multiply value by number of moles
when we go against the arrow, we change the sign of the ΔH value on that side
define standard enthalpy of combustion and standard enthalpy of formation
ΔcH⦵ - the enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions, with all reactants and products in their standards states
ΔfH⦵ - the enthalpy change when one mole of a compound is formed from it’s elements in their standard states under standard conditions
