energetics Flashcards
what is an exothermic process
energy is released from the chemical to the surroundings during the process
the temperature of the surroundings increase
exo-energy exits
what is an endothermic process
energy is taken in by the chemical from the surroundings during the process
the temp of the surroundings decrease
en-energy enters
definitions for enthalpy change
enthalpy change is the change in heat energy at constant pressure
for exothermic processes the sign for deltaH is
negative
for endothermic processes the sign for deltaH is
positive
what are the standard conditions
100 KPa ( or 1 atmosphere, 1 atm) pressure
298 (25c) temp
1 mol dm-3 concentration for all solutions
bend and mex
breaking bonds - endothermic
making bonds - exothermic
definition of activation energy
the minimum energy needed to start a reaction
mean bond enthalpy
a mean bond enthalpy is the energy required to break 1 mol of a covalent bond into gaseous atoms averaged over a range of different compounds
mean bond enthalpies are ..
endothermic process and have a positive deltaH sign
standard enthalpy of formation
the enthalpy change when 1 mol of substance is formed
from its constituent elements under standard conditions
with all reactants and products being in their standard states
what does standard enthalpy of formation need to show
formation of 1 mol
the standard enthalpy of formation of all elements in their standard states is …
zero
standard enthalpy of combustion
the enthalpy change when 1 mol of a substance is burnt completely in excess oxygen
under standard conditions and all reactants and products in their standard states
substances that cant be combusted
have 0 enthalpy of combustion
why may the enthalpy of combustion be difficult to measure
incomplete combustion may occur
hess law
hess’ law states that the enthalpy change for a chemical reaction is the same whatever route is taken from reactants to products
bond enthalpy hess cycle
reactants -> products
both arrows face down to gaseous atoms
BERP-deltaH=sum of reactants - sum of products
formation enthalpies hess cycle
reactants -> products
both arrow face up from gaseous atoms
deltaH = sum of products - sum of reactants
combustion enthalpies hess cycles
reactants -> products
both arrows face down to gaseous atoms
CRP=deltaH = sum of reactants - sum of products
Q = mcdeltaT
Q=energy(J)
m=mass of water (surroundings) g
c = specific heat capacity ( usually 4.81)
delta t = change in temp Celsius
Q to enthalpy change ( deltaH)
deltaH=Q/mols
Q needs to be divided by 1000 from J to KJ
Assumption made in calorimetry
The energy transferred to the water from the reacting chemicals (or vice versa for endothermic reactions) is equal to the energy released (or taken in) by the reaction.
sources of error in combustion calorimetry
- Heat loss to the surroundings
- Incomplete combustion of the fuel
- Heat energy transferred to the metal calorimeter
- Some fuel evaporates
sources of error in solution calorimetry
heat loss to surroundings
improvements to minimise sources of error in combustion calorimetry
- Add a lid – reduces heat loss
- Insulate sides of calorimeter – reduces heat loss
- Reduce distance between flame and beaker – reduces heat loss
- Put sleeve around flame to protect it from draughts
improvements to minimise sources of error in solution calorimetry
- Add a lid – reduces heat loss
- Insulate calorimeter – reduces heat loss
Steps to Measure an Enthalpy Change Using a Cooling Curve
- Record the temperature for a suitable time (3 minutes) before adding reactants together
- To establish an accurate initial temperature
- Mix reactants then record temperature every minute until a trend is seen
- Plot a graph of temperature against time
- Extrapolate the cooling curve back to the point of addition
- To establish a theoretical temperature change accounting for heat loss
