C11 Energetics
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38 Terms
What is an exothermic reaction in terms of thermal energy transfer
thermal energy released to surrounding gs
Do products in exothermic reaction have more or less energy than reactants
Less
How might energy be transferred to surroundings other than thermal
Sound/light
What polarity is ΔH in exothermic reaction
Negative
What is an endothermic reaction in terms of energy transfer
Thermal energy transferred from surroundings to reaction
Is making bonds exothermic or endothermic
Exothermic
Does making bonds release or remove heat from surroundings
Release
Is breaking bonds exothermic or endothermic
Endothermic
Does breaking bonds release or remove heat from surroundings
Remove
Do products have more or less energy than reactants in endothermic reaction
More
What unit is ΔH normally given in
KJ/mol
What is the polarity of ΔH for an endothermic reaction
+ve
For a reversible reaction, how does ΔH of the forward and backwards reactions compare
Equal magnitude, opposite polarity
What are bond energies
Average bond strength for certain covalent bond
If a reversible reaction is exothermic in one direction, the other direction will be
Endothermic
How to calculate enthalpy change using bond energies
Sum of bond energies of bonds broken - sum of bond energies of bonds made
What kind of reaction takes in energy
Endothermic
What kind of reaction releases energy
Exothermic
What is an endothermic reaction in terms of bonds broken/made
More bonds broken than formed
What is an exothermic reaction in terms of bonds broken/made
More bonds formed than broken
When can you use bond energies to approximate enthalpy change
Reaction that takes place entirely as gas
Q = mcΔΤ : what is q
Energy change (J)
Q = mcΔT what is m
Mass of water (g or kg)
Q = mcΔT what is c
Specific heat capacity of water (J/kg or J/g)
Relationship between ΔH (enthalpy change per mol) and q (energy change)
ΔH = q / number of moles
Specific heat capacity of water
4.2J/g degree C, or 4.2kJ/kg degree C
What is specific heat capacity
Energy required to heat up given mass of substance by 1 degree C
What is a more accurate way to determine enthalpy change
Calorimetry
What is a limitation of bond energies being average values
Not specific to molecules used; measured value can be over 10% dif from calculated
Why are polystyrene/styrofoam often used for calorimetry cup
Insulator
What is assumed about the density of solution for calorimetry
Same as density of water
What is assumed about shc in calorimetry
Same SHC as water
Calorimetry in combustion experiment setup
Fuel in burner heats water in copper calorimeter
Why is incomplete combustion a problem with calorimetry
Less energy is released
Why is incomplete combustion a bigger problem for larger molecules used as fuel
Ratio of fuel:oxygen is higher
Why is heat loss to surroundings more of a problem in combustion calorimetry
Open system - can be lost before reaching calorimeter / from calorimeter
Is enthalpy change affected by a catalyst
No - ΔH is independent of reaction route