Topic 8 - Energetics 1
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What is an endothermic reaction?
a reaction where energy is absorbed from surroundings -bonds broken +ve enthalpy change
What is an exothermic reaction?
a reaction where energy is given out to surroundings • bonds formed • ve enthalpy change
What is an enthalpy change?
the energy change at standard conditions
What are standard conditions?
temp 298K (25°C) pressure 1atm (100kPa)
What is standard enthalpy change of formation?
enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions -usually exothermic
What is standard enthalpy change of combustion?
enthalpy change when one mole of a substance (fuel) undergoes complete combustion under standard conditions -exothermic
What is standard enthalpy change of neutralisation?
enthalpy change when one mole of water is formed from the reaction of an acid and alkali under standard conditions -exothermic
Q =
mcΔT
What assumptions are made when using Q=mcΔT?
• all energy produced is exchanged with products and water in calorimeter • 1cm³ of dilute aqueous solution weighs 1g • 1g of water requires 4.18J to raise the temp by 1°C
What does Hess’ law state?
the overall enthalpy change of a reaction is independent of the route taken
What is bond enthalpy?
the energy required to break one mole of a covalent bond into gaseous atoms
What is the mean bond enthalpy?
the overall energy required to break one mole of a covalent bond into gaseous atoms across all molecules containing that bond