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Energetics
4.1 Endothermic and exothermic reactions: The results gained from this are inaccurate, as you assume
all the fuel burned went into heating the water, you can
Exothermic: when energy is given out in a reaction, reduce heat loss by using insulation, making sure the flame
products have a lower energy level than the reactants is enclosed, getting a spiral chimney made of copper and
eg. Neutralisation, forming bonds, burning it in pure oxygen not air
Endothermic: when energy is taken in during a
reaction, products have a higher energy level than the
reactants. Eg. Breaking bonds 4.4 Hess’s law
Hess’s law: the enthalpy change for a reaction is the same
4.2 Enthalpy:
whatever route is taken from reactants to products.
You can represent this on a thermodynamic cycle.
Enthalpy change: the heat change at a constant Eg.
pressure under standard conditions. you can produce ethene from
When measuring you must always use a balanced Converting ethyne -> ethene and
equation with state symbols so you can see necessary
· then ethene -> ethane.
changes in state for the reaction. You then follow the arrows and
Enthalpy changes can be shown in an Enthalpy add the enthalpy changes.
diagram If you follow direction of the arrow you add the number,
against the arrow you subtract.
You could also manipulate the enthalpy change of
formation as shown.
Ethyne is broken into
elements = negative enthalpy
Of formation. Then reform to
form ethane = positive
enthalpy of formation.
4.3 Measuring Enthalpy changes:
Standard molar Enthalpy of formation: the Enthalpy 4.5 Enthalpy change of combustion:
change when 1 mole of a substance is formed from it’s
elects under standard conditions and all reactants/ You can similarly use the enthalpy change of combustion
products are in their standard states as all substances readily burn in oxygen and so it can
Standard molar Enthalpy of combustion: the easily be measured.
Enthalpy change when 1 mole of a substance is 1. Write balanced equation
completely burnt in oxygen under standard 2. All arrows point towards elements
conditions, in standard states. 3. Combine the enthalpy changes for the products and
Temperature: the average kinetic energy of the particles then the enthalpy changes for the reactants, follow
in a system, the more the particles vibrate, the higher arrows and calculate.
the temperature.
Heat: measure of the total energy of all particles in a
system, moves from a high to low temperature.
Specific heat capacity: the amount of heat needed to
raise the temperature of 1g of a substance by 1K.
Enthalpy change = mass x specific heat x temperature Enthalpy of formation = products - reactants
Capacity Change Enthalpy of combustion = reactants - products
Calorimetry-
1. Set up apparatus as shown 4.6 Representing enthalpy changes
2. Burn your desired fuel to heat a
known mass of water You can represent the enthalpy
3. Record the initial temperature and the changes in a thermochemical cycle
final temperature as above or in an enthalpy diagram.
4. Calculate the enthalpy change Compare the compounds to 0 and
then show the different energy levels
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