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# AQA A1 energetics

### AQA energetics year12

2X(g) + 6H(g)->X2H6(g) with deltaH=-2775kJ.mol-1. Bond enthalpy of H-X is 395kJ.mol-1. H-H is 436. What is the bond energy of X-X? drawX2H6 to convince yourself that in this reaction 1X-X bond and 6 X-H bonds are made; so deltaH=-(X-X + 6H-X) negative because EGOMB; replace with values -2775=-(X-X+6*395); X-X=+405kJ.mol-1
deltaHf(TiO2(s))=-940kJ.mol-1 and deltaHf(CO(s))=-110kJ.mol-1. What is the enthalpy change for the reaction TiO2(s) +2C(s) -> Ti(s) + 2CO(g)? Construct Hess cycle with Ti, O2 under MAIN;deltaH=-(-940) + 2(-110)= +720 kJ.mol-1
Write the equation for the reaction that has an enthalpy change equal to the standard enthalpy change of formation of propane 3C(s)+4H2(g)->C3H8(g). Remember the state symbols!
The standard enthalpy changes of combustion are in kJ.mol-1. C=-394, H2=-286, C4H10=-2877. Work out the standard enthalpy change of formation of butane the MAIN equation is 3C(s) + 4H2(g) -> C4H10(g); Construct Hess cycle with 4 CO2 +5 H2O under MAIN;check moles and direction of arrows;deltaHf=+4(-394) +5(-286) –(-2877)=-129 kJ.mol-1
CHALLENGE: In an experiment, 1.60g of a fuel is burnt. 45% of the energy is absorbed by 200g of water, whose temperature rises from 18C to 66C. What is the total energy, inkJ.mol-1, released per gram of fuel burnt? Q=m.c.deltaT=200*4.2*48=40320J; but this is only 45%, so to find total energy given out 40320 *100/45=89600J for 1.60g of fuel; so 89600/1.60=56000J.mol-1 ie 56kJ.mol-1
Bond enthalpies are in kJ.mol-1: C(triple)O=1077; C-O=360; C-H=410; H-H=436; O-H=460. What is the enthalpy change for the reaction CO(g) +2H2(g) -> CH3OH(g) Draw enthalpy profile; draw structural formula of methanol; ENTBB=C(triple)O + 2H-H =1077+2*436=1949; EGOMB=3C-H + C-O + O-H=3*410+360+460=2050; Overall=ENTBB-EGOMB=1949-2050=-101kJ.mol-1
CHALLENGE: The enthalpy change for the following reactions are in kJ.mol-1: I2(g) + 3Cl2(g)->2ICl3(s) =-214; I2(s)->I2(g) =+38. What is the enthalpy change for the formation of Iodine Trichloride ICl3(s)? Equation for Formation is 0.5 I2(s) + 1.5 Cl2(g)-> ICl3(s); note that I2 is (s) and ONE mole is formed; so two steps half of I2(s)->I2(g) then half of the I2(g) + 3Cl2(g)->2ICl3(s);deltaH=0.5(+38)+0.5*(-214)=-176kJ.mol-1
Define bond enthalpy The enthalpy change needed to break ONE MOLE of a bond in a GASEOUS molecule
Define enthalpy change of combustion The enthalpy change when one mole of the substance is burnt in excess oxygen under standard conditions of 1 atm pressure at 298K.
Define enthalpy change of formation The enthalpy change when one mole of a substance is formed from its elements in their standard states at 1 atm pressure at 298K
When working out enthalpy change using Hess Law, what are the pitfalls? Tha arrows are the wrong way; you did not check the moles
How do you work out the enthalpy change, given some enthalpy change of formation and/or combustion Next to the data given, write the equations for the reaction; for each equation,identify in the MAIN equation either the reactants or products; the rest of the equation goes under the MAIN equation; check direction of arrows and number of moles
How do you work out the enthalpy change, given some bond enthalpies Draw the enthalpy profile for the reaction and work out ENTBB and EGOMB; remember that EGOMB is a negative value; add up ENTBB and EGOMB
Explain why reactions are endothermic in terms of bond enthalpies ENTBB>EGOMB
Define the term 'average bond enthalpy' The enthalpy change needed to break ONE MOLE of a bond in a GASEOUS molecule averaged when taken from many different compounds
What is the relationship between bond enthalpy and bond length? the stronger the bond, the shorter the bond (ie C-C has alower bond enthalpy than C=C and C-C is longer than C=C)
Why is the energy calculated from bond enthalpies different than in the data book? 1. Bond enthalpies are averaged values. 2. Bond enthalpy values apply to molecules in the GASEOUS state (for example the equation for the combustion of methane will produce H2O(l) and the bond enthalpies are given for H2O in a GASEOUS state)
Why is the energy calculated from experiment lower than in the data book? 1) heat loss to the surroundings 2) incomplete combustion takes place (definition deltaHc says COMPLETE); 3) heat absorbed by calorimeter
How do you work out the enthalpy change of combustion in an experiment given mass of water, mass of fuel and temp difference? work out Q=m.c.deltaT where m is the mass of WATER; work out the moles of fuel burnt n=m/RMM (where m is the mass of FUEL); then work out deltaHc=-Q/n; double check that the reaction is exothermic and the sign of deltaHc is negative; remember to convert J
How do you answer a question with delta Hf and deltaHc (ie a Hess Law)? 1) next to data, write the corresponding equation 2) spot in each equation which of reactant or product is in main equation 3) place the other part of equation under the main to form triangle 4) check direction arrow 5) check moles 6) write data ON CYCLE
What is the procedure for working out the enthalpy change of neutralisation or of solution? Record temperature of one chemical for 3mins in insulated cup; add the second chemical; carry on recording the temp; plot graph temp vs time; EXTRAPOLATE the line to work out temp change
Created by: UrsulineChem