Topic 8: Chemical Energetics 1️⃣
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What are the 2 components that make up chemical energy
Kinetic energy- measure of the motion of the particles in a substance Potential energy- measure of how strongly the particles interact (attract and repel)
What is heat energy
The portion of potential energy and kinetic energy of a substance that is responsible for the temperature of the substance
What is enthalpy
The measure of the total energy of a system
Define exothermic
Heat energy is transferred from the system to the surroundings
Define endothermic
Heat energy is transferred from the surroundings to the system
Define the standard enthalpy change of reaction
Enthalpy change measured at 100 kPa and a stated temp (usually 298K) when the number of moles of substances in the equation react 
Define the enthalpy change of combustion (ΔcH⦵)
Change that accompanies the complete combustion of one mole of a substance under standard conditions in their standard state
Explain how you would find the enthalpy change of combustion of a liquid
• The spirit burner containing the liquid and the test is weighed • A known vol of water is added to a copper can • The temperature of the water is measured • The burner is lit • The mixture is constantly stirred with the thermometer • When the temp of the water has reached approx 20° above its initial temp the flame is extinguished and the burner is immediately reweighed • The final temperature is measured
Give the equation for the calculation of enthalpy change
q = mCΔT q : heat energy (J) m: mass (g) c : specific heat capacity (4.18 unless told otherwise) ΔT: temp change
What are some of the possible sources of error in the reaction measuring the enthalpy change of combustion of a liquid
• Some of the heat energy produced in burning is transferred to the air and not the water • Some of the heat energy produced in burning is transferred to the copper cannot the water • The conditions are not standard e.g. water vapour not liquid water is produced
Define standard enthalpy change of neutralisation ΔneutHθ
The enthalpy change that accompanies the production of 1 mole of water produced by the neutralisation of an acid with an alkali under standard conditions in their standard states
Describe the steps in a reaction you would take to find out the enthalpy change of neutralisation
Pipette 25 cm³ of 1moldm3 acid into a polystyrene cup Measure the temperature of the acid Pipette 25 cm³ of the alkali of a concentration slightly greater than 1moldm3 (ensures all the acid is neutralised) into a beaker Measure the temperature of the alkali Add the alkali to the acid Stir with thermometer & measure the maximum temperature reached 
Define the standard enthalpy change of formation △fHθ
The enthalpy change that accompanies the formation of one mole of a substance from its elements in their standard states and conditions
Define Hess’ law
The enthalpy change for a chemical reaction that is independent of the route taken
Define bond enthalpy
Change when one mole of a bond in the gaseous state is broken 
Define mean bond enthalpy 
The enthalpy change when one mole of a bond, averaged out over many different molecules, is broken
What is the equation to calculate the enthalpy change of reaction using mean bond enthalpies
△rH = Σ(bonds broken) - Σ(bonds made) When calculating enthalpy change of combustion only use the fuel for bonds broken
Why does a calculation of enthalpy change for a reaction from mean bond enthalpies often not equal the same value as the data book value
Use of mean bond enthalpy values rather than actual values for molecules involved Substances are in the wrong state for bond enthalpy calculations
What type of Hess’ cycle should you draw and what does it look like if you are given combustion data
Use a combustion cycle : Reactants ———-> Products Arrows pointing downwards . CO2 + H2O
What type of Hess’ cycle should you draw and what does it look like if you are given formation data data
Use a formation cycle Reactants —————> Products . Arrows pointing upwards . Elements