Energetics Flashcards
2 types of reaction
Endothermic
Exothermic
Enthalpy change
Heat energy change measured under conditions of constant pressure
standard enthalpy change symbol
ΔH°
What are standard conditions for standard enthalpy changes
100kPa
And a stated temp eg 298 Kelvin
Enthalpy of formation
ΔfH is the enthalpy change when one mole of compound is formed from its constituent elements in their standard states under standard conditions (298K and 100 kPa).
enthalpy of combustion
ΔcH is the enthalpy change when one mole of substance in its standard state is burnt in excess oxygen under standard conditions.
Exothermic definition
Heat energy given out
Reactants have more energy than products
🔼H = neg
Endothermic def
Heat given in
Reactants have less energy
🔼H = positive
Enthalpy change units
Kj/mol
Kjmol^-1
Axis of Enthalpy change diagrams
X axis = progress of reaction
Y axis = energy
Activation energy on a energy profile graph
From highest point to reactants
Making bonds is …
Exothermic
Releases energy
-ve 🔼H
Breaking bonds is …
Endothermic
Req energy
🔼H = positive
Whne is a reaction exothermic
When energy rel from bond making is greater than energy taken in from bond breaking
calorimetry equation
q = m x c x ΔT
what is calorimetry
Calorimetry is the measurement enthalpy changes in chemical reactions
q ?
heat change (j)
m?
mass (g)
of water whereby 1cm3 = 1g
c?
specifc heat capacity
4.18
ΔT
change in temp
K
After calc q you …
convert to kj
divide by number of mol
hess law states
enthalpy change is independent of the route taken
how to calc enthalpy change of combustion
arrows point down toward
co2 and h2o at bottom
enthalpy chage = reac - proud
State the meaning of the term average bond Enthalpy for the _____ bond
bond Enthalpy change to break 1 mol of _____ bonds
How to calc Enthalpy of formation
arrows point upwards
Elements at the bottom
Prod - reactants
define mean bond enthalpy
The average heat energy required to break one mol of covalent bonds over a range of compounds
How to calculate mean bond enthalpy
Change in H = sum bonds broken - sum bonds formed
This gives 1 mark
Why may Enthalpy change of an element thag you would expect to be 0 not be 0
Element not in its standard state
State why standard Enthalpy of formation of oxygen is 0
By definition 🔼Hf is formation from an element
describe how you would carry out reaction to find enthalpy of solution od mgcl2
(1a) Measures water with named appropriate apparatus
(1b) Suitable volume/mass / volume/mass in range 10 – 200 cm3
/g
(1c) Into insulated container / polystyrene cup (NOT just ‘lid’)
(1d) Add known mass of MgCl2(s)
(1e) Use of ‘before and after’ weighing method. NOT ‘added with washings’
Stage 2 Measurements (could mark from diagram)
(2a) Record initial temperature (min 2 measurements)
(2b) Record T at regular timed intervals for 5+ mins / until trend seen
(2c) Plot T vs time
Stage 3 Use of Results (3a and 3b could come from diagram)
(3a) Extrapolate lines to when solid added (to find initial and final T)
(3b) Tfinal – Tinitial = ΔT / idea of finding ΔT from graph at point of addition
