Energetics SL

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Exothermic reaction
a reaction that releases heat to the surroundings as a result of forming products with stronger bonds than the reactants
-∆H
Endothermic reaction
a reaction that absorbs heat from the surroundings as a result of forming products with weaker bonds than the reactants
+∆H
Standard enthalpy change of reaction (∆Hº)
the heat transferred during a reaction carried out under standard conditions:
-pressure 100kPa
-temperature 298K
-all substances pure and in their standard state
-1 atm pressure
Average bond enthalpy (kJ mol^-1)
the energy required to breaks a mole of covalent bonds in the reactant, all reactants and products being in the gaseous state
EX: X2(g) --> 2X (g)
It is an average value because it takes account of the different energies in a bond between the same atoms and different molecules.
average bond enthalpy can also be defined as the energy released on forming a moles of covalent bonds in the products, all reactants and products in the gaseous state.
Standard state*
(compound state) most stable state under the specified conditions:
-pressure 100kPa
-temperature 298K
-all substances pure and in their standard state
-1 atm pressure
Standard enthalpy change of formation (∆Hfº)
enthalpy change on the formation of one mole under standard conditions*
Standard enthalpy change of combustion (∆Hcº)
enthalpy change on the complete combustion of one mole of the compound in its standard state in excess oxygen under standard conditions*
Energy
the ability to do work; move a force through a distance. (measured in Joules / force x distance / J = N x m)
Enthalpy
internal energy stored in the reactants (H)
absolute value cannot be measured in products/reactants but what can be is the distance between them (∆H)
enthalpy change can be measured using ¡calorimeter!
Heat
measure of total energy in a given amount of substance and therefore depends on the amount of substance present
Tempurature
measure of 'hotness' of a substance.
represents average kinetic energy of substance but independent of substance present
Heat energy change
q = mc∆T
m: mass
c: specific heat capacity
∆T: change in tempurature
Calorimeter
EXTRAPOLATION GRAPH:
x axis = time
y axis = temperature
compensating for heat loss:
T0- initial temp of reactants
T1- highest temp actually reached
T2- temp that would have been reached if no heat lost to surroundings
∆T for reaction = T2 - T0
Hess' law
enthalpy change for a reaction depends only on the difference between the enthalpy of the products and reactants.
independent of reaction pathway.
used to determine enthalpy changes which cannot be measured directly.
represented by an energy cycle.
Average bond enthalpy
the enthalpy change for the process.
if all bond enthalpies are known for reactants/products then overall enthalpy change can be calculated.
limitations:
•can only be used if all react/prod are in gaseous state.
•averages are used considering a number of similar compounds - energy of a particular bond will vary slightly in different compounds as it will be affected by neighboring atoms - so ∆H values obtained will not be very accurate.
System
an area of interest
Surroundings
everything else in the universe
When 100 cm3 of 1.0 mol dm-3 HCl is mixed with 100 cm3 of 1.0 mol dm-3 NaOH, the temperature of the resulting solution increases by 5.0 °C. What will be the temperature change, in °C, when 50 cm3 of these two solutions are mixed?
5.0
Which statement about bonding is correct?
A. Bond breaking is endothermic and requires energy.
B. Bond breaking is endothermic and releases energy.
C. Bond making is exothermic and requires energy.
D. Bond making is endothermic and releases energy.
Bond breaking is endothermic and requires energy.
Consider the following reactions.
Cu2O(s) + O2(g) → 2CuO(s)
∆Hº = -144 kJ
Cu2O(s) → Cu(s) + CuO(s)
∆Hº = +11 kJ

What is the value of ∆Hº, in kJ, for this reaction?
Cu(s) + O2(g) → CuO(s)
-144 - 11
Which processes have a negative enthalpy change?
I. 2CH3OH(l) + 3O2(g) →
2CO2(g) + 4H2O(l)
II. HCl(aq) + NaOH(aq) →
NaCl(aq) + H2O(l)
III. H2O(g) → H2O(l)
All three.
Which process represents the C-Cl bond enthalpy in tetrachloromethane?
A. CCl4(g) → C(g) + 4Cl(g)
B. CCl4(g) → CCl3(g) + Cl(g)
C. CCl4(l) → C(g) + 4Cl(g)
D. CCl4(l) → C(s) + 2Cl2(g)
B. CCl4(g) → CCl3(g) + Cl(g)
What is the energy, in kJ, released when 1.00 mol of carbon monoxide is burned according to the following equation?

2CO(g) + O2(g) → 2CO2(g)

∆Hº = -564 kJ
282KJ
The specific heat of iron is 0.450 J g-1 K-1. What is the energy, in J, needed to increase the temperature of 50.0 g of iron by 20.0 K?
450
Application of Hess's LAW
Describe the bond strength in ozone in its importance to the atmosphere
#Bond Order
O2 bond : 2
Ozone: 1.5

Ozone weaker - broken by radiation of higher energy
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