UNIT 1: SECTION 4 - ENERGETICS
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Define enthalpy.
The energy in a substance at constant Pressure.
What system measures enthalpy?
Calorimetry.
What is the 1st law of thermodynamics?
Energy cannot be created nor destroyed BUT can be converted from one to another.
True or false: In exothermic reactions, products have less energy than reactants.
TRUE
True or false: In endothermic reactions, Products have more energy than the reactants.
TRUE
If a reversible reaction is exothermic in the forward reaction, what is the reverse reaction?
Endothermic.
What are standard conditions.
298K 1Bar (100kPa) Standard states for elements. [if soln. used, conc of 1M]
Define enthalpy of combustion.
Heat given out when 1 mole is completely burned in Oxygen under standard conditions; all reactants and products in standard states. • always exothermic (-) • may have fractions.
Define standard enthalpy of formation.
Heat change when 1 mole of a compound is formed from its constituent elements under standard conditions. • can be exo/endo • formation of element = 0
Calorimetry equation?
q = mc^T q = energy transferred (J) m = mass of water/soln. (g) c = SHC (J K-1 g-1) ^T = change in temp (K)
What is specific heat capacity?
Hear required to raise 1g of a substance by 1°K.
Which 2 main types of reactions use calorimetry?
• Enthalpy of combustion of a fuel. - Enthalpy of neutralisation between acid and alkali.
Describe how we measure the enthalpy of combustion of a fuel. 2 sources of error in this experiment?
• Known mass of water in a copper calorimeter. • Spirit lamp burns ethanol (weighed before/after to find mass of ethanol burnt). • Initial temp of water and final temp measured to calculate rise. Sources of error: • Heat lost to surroundings, not to water. • Heat absorbed by calorimeter not considered.
Describe how we measure enthalpy of neutralisation. Describe how we plot this onto a graph and what we do with this graph. Are enthalpy of neutralisation endo or exo?
[Enthalpy change when 1 mole of H+ EXACTLY neutralised under S.C.] • Molar soln. of acid and alkali used. • Reaction carried out in polys. cup [GOOD INSULATION] supported in a beaker. • 50cm3 1M KOH pipetted into cup. • 50cm3 1M HNO3 pipetted into cup. • Temp of mixture measured every min. (stirred as-well to evenly heat) for an interval of time. • Graph plotted (temp y-axis: time x-axis). • extrapolation backwards along cooling curve to find max temp rise when acid added (at 4 mins). • Graph called a cooling curve. • Extrapolation used due to heat loss causing inaccuracies. • Enthalpies of neutralisation ALWAYS EXO.
State Hess’s Law.
Enthalpy of a reaction is always the same, regardless of the route taken from reactants to products.
What are the equations to work out: a) Standard enthalpy of combustion. b) Standard enthalpy of formation.
Formation = Products - Reactants. Combustion = Reactants - Products.
Which way do the arrows point in Formation Cycles?
Upwards.
Which way do the arrows point in Combustion cycles?
Downwards.
Is the standard enthalpy of combustion exothermic or endothermic?
EXOTHERMIC! NEGATIVE VALUEEE
Is the standard enthalpy of formation exothermic or endothermic?
Can be both…
Define Bond enthalpy.
Bond energy is • the energy needed to separate the constituent atoms of a covalent bond - - [to an infinite distance apart] • in the gaseous state under standard conditions. ALWAYS ENDOTHERMIC (+)
How does bond enthalpy decrease/increase?
• The shorter the bond is, the more energy is required to break it. • Multiple bonds stronger than single bonds.
Which bond is abnormal?
F-F: Abnormally short, substantial repulsion between lone pair of electrons on fluorine atoms. Repulsion weakens bond strength.
What is a disadvantage of mean bond enthalpy?
Doesn’t take into account that each individual bond dissociation enthalpy may be different. (Calculated from an average)