Chemical Energetics
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Define enthalpy change of reaction
Energy change in reaction when molar quantities of reactants specified in chemical equation react to form products
What are standard conditions
298K (25C) 1 bar / 10000Pa 1 mol dm^-3
Compare stability of products vs reactants for endothermic reaction
Products are energetically less stable than reactants in endothermic reaction
What are four factors that affect value of enthalpy change
Quantity of substances State of reactants and products Temperature Pressure
Define standard enthalpy change of formation
The energy change when one mole of substance is formed from constituent elements under standard conditions
Define standard enthalpy change of combustion
Energy evolved when one mole of substance is completely burnt in oxygen under standard conditions
Define Hess’ Law
The enthalpy change accompanying a chemical reaction is the same regardless of the route by which the chemical change occurs, provided the initial and final states are the same
Shortcuts for enthalpy change of reaction
By Hess’ Law, (state) Enthalpy = combustion(r-p) = formation (p-r) = B.E. (r-p)
Define standard enthalpy change of neutralisation
The energy evolved when one mole of water is formed from the neutralisation between an acid and base under standard conditions
Why is enthalpy change of neutralisation less than 57 kJ/mol for weak acids/bases
Some energy evolved from neutralisation process is used to further dissociate weak acid or weak base completely
How to get enthalpy change of reaction from calorimetric method
Hr = heat released or absorbed/n where heat = mcT
Purpose of beaker in simple calorimeter
To provide stability by preventing toppling of the set up
What are some assumptions when using simple calorimeter
Negligible heat lost/gained from surroundings Density of solution = 1.00g/cm Specific heat capacity = 4.18 J g^-1 K^-1
What is temperature correction method
Measure temperatures of reagent at 1-minute intervals for 3 mins. Mix reagents. From t=3.5 min record temp every 0.5 min. Extrapolate curve until t = 3 min.
What are components of copper calorimeter set up
Spirit burner containing fuel Draught shield Copper calorimeter Thermometer
Purpose of draught shield
Prevents heat loss and prevents flame from being burnt out by wind
Components of bomb calorimeter
Controlled temperature water jacket Steel bomb Fuse Stirrer
How to use bomb calorimeter
Determine heat capacity of bomb calorimeter by burning known mass of substance whose enthalpy change of combustion already known Water in calorimeter and outer bath adjust to same temp to ensure no heat loss
Define bond dissociation energy
The energy absorbed to break one mole of a particular covalent bond in a specific molecule in the gaseous state to form gaseous atoms
Define bond energy
The average energy absorbed to break one mole of covalent bonds in gaseous state to form gaseous atoms under standard conditions
Define standard enthalpy change of atomisation
Energy absorbed when one mole of gaseous atoms is formed from the element under standard conditions
Define first ionisation energy
The energy absorbed when one mole of gaseous atoms loses one mole of electrons to form one mole of singly charged gaseous cations
Define first electron affinity
Energy change when one mole of gaseous atoms acquires one mole of electrons to form one mole of singly charged gaseous atoms
Define lattice energy
The energy evolved when one mole of solid ionic compound is formed from constituent gaseous ions under standard conditions