energetics Flashcards

1
Q

what is an exothermic process

A

energy is released from the chemical to the surroundings during the process
the temperature of the surroundings increase
exo-energy exits

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2
Q

what is an endothermic process

A

energy is taken in by the chemical from the surroundings during the process
the temp of the surroundings decrease
en-energy enters

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3
Q

definitions for enthalpy change

A

enthalpy change is the change in heat energy at constant pressure

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4
Q

for exothermic processes the sign for deltaH is

A

negative

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5
Q

for endothermic processes the sign for deltaH is

A

positive

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6
Q

what are the standard conditions

A

100 KPa ( or 1 atmosphere, 1 atm) pressure
298 (25c) temp
1 mol dm-3 concentration for all solutions

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7
Q

bend and mex

A

breaking bonds - endothermic
making bonds - exothermic

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8
Q

definition of activation energy

A

the minimum energy needed to start a reaction

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9
Q

mean bond enthalpy

A

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

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10
Q

mean bond enthalpies are ..

A

endothermic process and have a positive deltaH sign

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11
Q

standard enthalpy of formation

A

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

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12
Q

what does standard enthalpy of formation need to show

A

formation of 1 mol

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13
Q

the standard enthalpy of formation of all elements in their standard states is …

A

zero

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14
Q

standard enthalpy of combustion

A

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

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15
Q

substances that cant be combusted

A

have 0 enthalpy of combustion

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16
Q

why may the enthalpy of combustion be difficult to measure

A

incomplete combustion may occur

17
Q

hess law

A

hess’ law states that the enthalpy change for a chemical reaction is the same whatever route is taken from reactants to products

18
Q

bond enthalpy hess cycle

A

reactants -> products
both arrows face down to gaseous atoms

BERP-deltaH=sum of reactants - sum of products

19
Q

formation enthalpies hess cycle

A

reactants -> products
both arrow face up from gaseous atoms

deltaH = sum of products - sum of reactants

20
Q

combustion enthalpies hess cycles

A

reactants -> products
both arrows face down to gaseous atoms

CRP=deltaH = sum of reactants - sum of products

21
Q

Q = mcdeltaT

A

Q=energy(J)
m=mass of water (surroundings) g
c = specific heat capacity ( usually 4.81)
delta t = change in temp Celsius

22
Q

Q to enthalpy change ( deltaH)

A

deltaH=Q/mols
Q needs to be divided by 1000 from J to KJ

23
Q

Assumption made in calorimetry

A

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.

24
Q

sources of error in combustion calorimetry

A
  • Heat loss to the surroundings
  • Incomplete combustion of the fuel
  • Heat energy transferred to the metal calorimeter
  • Some fuel evaporates
25
Q

sources of error in solution calorimetry

A

heat loss to surroundings

26
Q

improvements to minimise sources of error in combustion calorimetry

A
  • 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
27
Q

improvements to minimise sources of error in solution calorimetry

A
  • Add a lid – reduces heat loss
  • Insulate calorimeter – reduces heat loss
28
Q

Steps to Measure an Enthalpy Change Using a Cooling Curve

A
  • 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