3.2.1 Enthalpy changes Flashcards
Energy profile diagram for exothermic reaction
Energy transferred from system to surroundings
Products have less energy than reactants
🔼H is negative
Endothermic reaction energy profile
Energy is transferred from surroundings to system
Require input of heat energy
Products have more energy than reactants
🔼H is positive
Activation energy
Defined as the minimum energy which particles need to collide for a reaction to proceed
What are standard conditions
100 kPa
298K
Solutions at 1 mol dm^-3
All substances in their normal state
Enthalpy change of reaction
Enthalpy change When the number of moles of reactants as specified in the balanced equation react together
Enthalpy change of formation
Enthalpy change when 1 mole of the compound is formed from its elements under standard conditions all reactants and products being in their standard states
Standard Enthalpy change of combustion
Enthalpy change that occurs when one mole of a substance is completely combusted in oxygen under standard conditions
Enthalpy change of neutralisation
Enthalpy change when solitiond of an acid and an alkali react together under standard conditions to produce one mole of water
How to calculate energy change
Q= mc🔼t
Q= energy change, J m= mass of solution, g c= specific heat capacity, J/g/K 🔼t= temperature change, K
Calorimetric method
Wash equipment with soltuions to be used
Dry cup after washing
Put polystyrene cup in beaker for insulation and support
Measure desired volumes of solutions with volumetric pipettes and transfer to insulated cup
Clamp thermometer into place
Measure initial temp of solution, measure temp every minute for 2-3 minutes
Transfer 2nd reagent into cup, if solid is used then add solution to cup then add solid weighed on balance
Use before and after weighing method
Stir mixture
Record temp every minute after addition for several minutes
Errors in the calorimetric method
Energy transfer from surroundings
Approximation in specific heat capacity, method assumes same as water
Neglecting specific heat capacity of calorimeter
Reaction or dissolving may be incomplete or slow
Density of solution is taken to be same as water
calculating the enthalpy change of reaction from experimental data
use q=mc🔼t to calculate energy change
work out moles of reactant used
divide q by moles of reactant
divide by 1000 to convert j to kj
measuring enthalpies of combustion using calorimetry
fuel is burnt and flame is used to heat up water in a metal cup
then use energy change equation and divide by moles then by 1000
errors in the calorimetry for combustion experiment
energy losses from calorimeter
incomplete combustion of fuel
incomplete energy transfer
evaporation of fuel after weighing
heat capacity of calorimeter not included
measurements not carried out in standard conditions because water is used as a gas rather than liquid
mean bond enthalpy
enthalpy change when one mole of bonds of (gaseous covalent bonds) is broken (averaged over different molecules)
ONLY APPLIES WHEN SUSTANCES START AND END IN THE GASEOUS STATE
