5.2 - Energetics Flashcards
What is an enthalpy change
The enthalpy change of a reaction is the heat energy it releases or absorbs per mole
What is the symbol used for enthalpy change
ΔH
What signs can enthalpy changes have
+ or -
What is the value in an enthalpy change
the number
What is the unit used for enthalpy change
kJ/mol
what tells us whether the reaction absorbs or releases energy
the sign
+ or -
If the sign is positive (+) has the reaction absorbed or released heat energy
absorbed
If the sign is negative (-) does the reaction absorbed or released heat energy
released
If the sign is positive (+) is the reaction endothermic or exothermic
endothermic
If the sign is negative (-) is the reaction endothermic or exothermic
exothermic
If the reaction has absorbed heat is the sign positive (+) or negative (-)
positive (+)
If the reaction has released heat is the sign positive (+) or negative (-)
negative (-)
If the reaction has absorbed heat is it endothermic or exothermic
endothermic
If the reaction has released heat is it endothermic or exothermic
exothermic
If the reaction is endothermic is its sign positive (+) or negative (-)
positive (+)
If the reaction is exothermic is its sign positive (+) or negative (-)
negative (-)
If the reaction is endothermic has it absorbed or released heat
absorbed
If the reaction is exothermic has it absorbed or released heat
released
What does the value of an enthalpy change show
how much heat energy is absorbed or released
What do the units of an enthalpy change always the same
they are always kJ/mol so that different reactions can be compared easily
what does how much energy a reaction absorbs or releases depend on
it depends on the balance between:
- breaking bonds, absorbs energy
- making bonds, releases energy
what does breaking bonds do
absorbs energy
what does making bonds do
releases energy
what does an energy level diagram look like for an endothermic
¦ ____products___
¦ ^
H¦ ¦
¦ ¦ΔH(+)
¦_____reactants_____¦
¦
In an endothermic reaction do the products end up with more or less energy than the reactants had
more
what does an energy level diagram look like for an exothermic reaction
¦
¦_____reactants_______
H¦ ¦
¦ ¦ΔH(-)
¦ ¦
¦ ˇ___products___
¦
¦
In an exothermic reaction do the products end up with more or less energy than the reactants had
less
in bond breaking is heat absorbed or released
absorbed
in bond breaking is the reaction endothermic or exothermic
endothermic
in bond making is heat absorbed or released
released
in bond breaking is the reaction endothermic or exothermic
exothermic
do stronger bonds require more or less energy to break
more
do stronger bonds release more or less energy when they are made
more
what is the equation for calculating bond energies in an enthalpy change reaction
ΔH = Bonds broken - Bonds made
when writing out a bonds energy calculation what should you do
write out the bonds broken and bonds made separately before you subtract them
how is a reaction profile diagram different to an energy level diagram
it shows the path the reaction actually takes from reactants to products
what is the inequality or energy absorbed and released in an exothermic reaction
energy absorbed < energy released
what is the inequality or energy absorbed and released in an endothermic reaction
energy absorbed > energy released
what is the energy needed to break the bonds called
activation energy
what is calorimetry
an experimental method used to measure an enthalpy change
in principle of calorimetry what is the reaction carried out near to
water
explain what happens for calorimetry for an exothermic reaction
- the reaction releases energy
- the heat energy is absorbed by the nearby water
- the temperature of the water increases
what are the two forumlae used in calorimetry to convert measurements into ΔH
Q=mcΔT
ΔH= -Q/moles
explain each component of Q=mcΔT equation
Q - heat energy (J)
m - mass of water (g)
c - specific heat capacity (degrees C)
ΔT - temperature change
explain each component of ΔH= -Q/moles
ΔH - enthalpy change (kJ/mol)
- Q - minus heat energy (degrees C)
moles - moles
what is the method for combustion calorimetry
- Weigh out 50 g of water and place in a can above the burner.
- Measure the initial mass of the burner and the initial
temperature of the water. - Ignite the burner, stir water for 30 seconds, then extinguish
the burner. - Measure the final mass of the burner and the final (maximum)
temperature of the water.
what are two sources of error in combustion calorimetry
heat loss - most of the heat energy dissipates into the air instead of the water
incomplete combustion - this releases less heat energy than expected
what is the method for solution calorimetry
- Pipette 25 cm3 of CuSO4(aq) into a polystyrene cup.
1 cm3
solution has a mass of 1 g - Measure the initial temperature of the solution.
- Add 1 g of Mg(s) and stir continuously.
- Measure the maximum temperature of the solution.
what are sources of error for solution calorimetry
heat loss - most of the heat energy dissipates into the air instead of the water
incomplete combustion - this releases less heat energy than expected
water being heated is not longer pure water - it has things dissolved in it so the specific heat capacity is no longer exactly 4.18 J/g/degrees C
