Energetics
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- Created by: Hindleyc
- Created on: 16-06-18 23:06
What is enthalpy change ?
The amount of heat taken in or given out during any change in a system provide the pressure is constant
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If enthalpy change occurs then what is transferred between systems or surroundings?
Energy
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What is system? Which is surroundings?
Chemicals. everything outside chemicals
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What happens in an exothermic change? what is deltaH
Energy is transferred from system (chemicals) to surroundings . Products have less energy than reactants. Negative. Sum of bonds in reactant molecules are less than sum of bonds in product molecules
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What happens in an Endothermic change ?
energy is transferred from the surroundings to the system (chemicals )- require imput of heat energy. Products have more energy than reactants and deltaH is positive
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What is the standard enthalpy of formation of a compound?
Enthalpy change when 1 mole of the compound is formed from its elements under standard conditions with all reactants and products being in their standard states
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What is the enthalpy of formation of an element?
0
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What is the standard enthalpy of combustion of a substance?
Enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions with all reactants and products in their standard states
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What will incomplete combustion lead to?
Soot (carbon) carbon monoxide and water. will be less exothermic than complete combustion
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What are standard conditions?
100kPa, 298K, Solu at 1mol/dm^3 -all substances should've their normal state at 298k
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How do you measure enthalpy change for reaction experimentally? (EQN)
Energy change= mass of solution X heat capacity X temp change (Q=MCdeltaT)
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What are the units for each part
Q=J M=g C=J/G/K deltaT=K
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What does this give? what is normal done?
Gives energies for actual quantities used- normally converted into energy changer per mole of one of the reactants (enthalpy change of reactant)
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How do you calculate enthalpy change using formation?
p-r
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How do you calculate enthalpy change using combustion?
r-p
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What is the general method?
1. using q=mcAt cal energy change for quantities used 2.work out moles of reactant sued 3. divide q by number of moles of the reactant not in excess to give deltaH 4. Add sign and unit- divide by 1000 to kj/mol
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What happens if reactant dissolved in water?
Same heat capacity of pure water
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What do solutions have same density as?
water -1g/cm^3
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What is calorimetry?
Enthalpy of combustion can be calculated using this- fuel burnt and flame used to heat up water in metal cup
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What are the errors?
Energy losses from calorimeter, incomplete combustion of fuel, incomplete transfer of energy, evaporation of fuel after weighing, heat capacity of calorimeter not included, measurement not carried out under standard conditions as h2o Is a (g) not (l)
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if given formation
p-r
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If given combustion
r-p
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If given bond energies
r-p
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What is Hess's law
States that total enthalpy change for a reaction is independent of the route by which the chemical change takes place (energy is always conserved)
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Why are hess law cycles used?
To measure enthalpy change for a reaction cannot be measured directly by experiments
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What is done instead?
Alternative reactions are carried out that can be measured experimentally
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What is it used to work out?
Enthalpy change to form hydrated salt from an anhydrous salt
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Why can't this be done experimentally?
cannot be done experimentally because impossible to add the exact amount of water and it is not easy to measure temp change of a solid
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What is done instead?
Both salts dissolved in excess water to form a solution of copper sulphate and temp change can be measured for these reactions
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What are the mean bond energies ?
Enthalpy needed to break the covalent bond into gaseous atoms averaged over different molecules
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What is it always? why?
Positive as energy is required to break a bond- start and end in gaseous state
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If all substances are gases what is the calculation?
deltaH=Bonds broken-bonds formed
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Why are values calculated using this method less accurate than using formation/combustion data?
mean bond energies- not exact- averaged values of bond enthalpies from various compounds
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Why is experimental less calculated?
because significant heat loss- also incomplete combustion so less energy being released
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Other cards in this set
Card 2
Front
If enthalpy change occurs then what is transferred between systems or surroundings?
Back
Energy
Card 3
Front
What is system? Which is surroundings?
Back
Card 4
Front
What happens in an exothermic change? what is deltaH
Back
Card 5
Front
What happens in an Endothermic change ?
Back
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