C4 - Energetics Flashcards
What is an exothermic reaction?
Reaction that release energy
What is an endothermic reaction?
Reaction that absorbs energy
Is breaking bonds endothermic or exothermic?
Endothermic
Is forming bonds endothermic or exothermic?
Exothermic
What is enthalpy?
The enthalpy, H, of a system is a measure of the energy stored in (or heat content of) a system. It cannot be measured directly.
What is enthalpy change?
Enthalpy change (∆H) is the heat energy change measured under conditions of constant pressure
What is the units of enthalpy change?
kJ/mol
What are standard enthalpy change conditions?
100kPa and 298K
What is the standard enthalpy of formation ΔHf?
This is the enthalpy change when 1 mole of a substance is formed from its elements under standard conditions, with all being in their natural states
What is the standard enthalpy of combustion ΔHc?
This is the enthalpy change when 1 mole of a substance undergoes complete combustion under standard conditions, with all substances being in their standard states
What is the standard enthalpy of atomization ΔHa?
This is the enthalpy change when 1 mole of gaseous atoms of a substance are formed from the substance under standard condition
What is the standard enthalpy of neutralization ΔHn?
This is the enthalpy change when 1 mole of water is produced by the reaction of an acid and a base under standard conditions, with all substances being in their standard state
What does a negative enthalpy change represent?
Exothermic reaction
What does a positive enthalpy change represent?
Endothermic reaction
What is the rule when forming a standard enthalpy of formation equation?
The product has to have 1 mole
E.g. H2(g) + ½O2(g) → H2O(l)
What is the rule when forming a standard enthalpy of combustion equation?
The substance undergoing combustion has to be 1 mole
E.g. H2 (g) + ½O2 (g) → H2O(l)
What is the rule when forming a standard enthalpy of neutralisation equation?
Has to be 1 mole of water
E.g. H+(aq) + OH¯(aq) → H2O(l)
What is the specific heat capacity equation?
q = mc∆T
q = enthalpy change in joules
m = mass of substance being heated (often water) in grams
c = specific heat capacity in joules per Kelvin per gram (4.18 JK-1g-1 for water)
∆T = change of temperature in Kelvin
What is Hess’s law?
Hess’s law states that the overall enthalpy change for a reaction is independent of the route the reaction takes
When using Hess’s law to work out enthalpy of combustion what equation is used?
Sum of reactants - sum of products
When using Hess’s law to work out enthalpy of formation what equation is used?
Sum of products - Sum of reactants
What is the mean bond enthalpy?
The mean bond enthalpy is a measure of the energy required to break one mole of a covalent bond in the gaseous state averaged (mean) over a range of different compounds containing the bond
What do mean bond enthalpy’s relate to?
The strength of a covalent bond. A higher bond enthalpy = stronger covalent bond.
Typically the shorter the covalent bond, the stronger the bond. E.g a triple bond is shorter than a double bond
How do you use bond energies to calculate enthalpy change?
Write down equation
Work out how many of each bond type are broken
Use mean bond enthalpies
Substitute values into the equation to calculate enthalpy change when bonds are broken
Repeat for bonds formed
Value will be negative as energy is released
Add too values together for overall enthalpy change
