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GCSE7 Energy

AQA GCSE Chapter 07 Energy

QuestionAnswer
Describe a SIMPLE energy diagram for an exothermic reaction (no need to discuss bond making or bond breaking) The levels of the reactants (on the left) are higher than the levels of the products (on the right); the arrow joining the two levels is going down
Describe a SIMPLE energy diagram for an endothermic reaction (no need to discuss bond making or bond breaking) The levels of the reactants (on the left) are lower than the levels of the products (on the right); the arrow joining the two levels is going down
Describe the FULL energy diagram for an exothermic reaction (including bond making or bond breaking) The levels of the reactants (on the left) are higher than the levels of the products (on the right); the arrow joining the two levels is going down; draw the "HUMP" to join the two levels
Describe the FULL energy diagram for an endothermic reaction (including bond making or bond breaking) The levels of the reactants (on the left) are lower than the levels of the products (on the right); the arrow joining the two levels is going down; draw the "HUMP" to join the two levels
For bond breaking process, is energy taken in or given out? Taken in ENTBB (Energy is Needed To Break Bonds)
When new bonds are made, is energy taken in or given out? Given out EGOMB (Energy is Given Out when Making Bonds)
Using a SIMPLE energy profile diagram, explain why a reaction is exothermic the energy LEVEL of the products is LOWER than the energy level of the reactants
Using a SIMPLE energy profile diagram, explain why a reaction is endothermic the energy LEVEL of the products is HIGHER than the energy level of the reactants
Explain why some reactions are exothermic, in terms of bond breaking and bond making Because there is more energy-given-out-when-new-bonds-are-formed than taken-in-when-bonds-are-broken ENTBB < EGOMB
Explain why some reactions are endothermic, in terms of bond breaking and bond making Because there is more energy-taken-in-when-bonds-are-broken than energy-given-out-when-new-bonds-are-formed than ENTBB > EGOMB
Name some chemical reactions that are endothermic Thermal decomposition, Photosynthesis, dissolving ammonium nitrate
What is the trick to remember for questions on energy compare ENTBB to EGOMB
what is a catalyst? a chemical that speeds up the reaction without getting used up (by lowering the activation energy and providing an alternative reaction pathway)
explain how a catalyst speeds up a reaction it lowers the activation energy and provides an alternative reaction pathway
what is the activation energy? the minimum of energy required for the particles to collide and bonds to break
How do manufacturers of cars ensure that the catalytic converters convert the maximum possible of harmful gases into Carbon Dioxide and Water? Increasing surface area (honeycomb structure); high temperature
In the catalytic converter, what are the harmful gases that need to be converted before coming out of the exhaust? Carbon monoxide and Unburnt Hydrocarbons
What is the difference between a full energy profile diagram for an uncatalysed and for a catalysed reaction? the uncatalysed reaction has one very tall “hump”; the catalysed reaction has a lower double hump
HIGHER Given bond energies, how do you work out the overall energy change 1)Draw structural formulae of all reactants; 2)count all the bonds on the reactant side and multiply by the relevant bond energy; 3)add it all up: this is ENTBB; 4) repeat steps 1 to 3 for the products side to work out EGOMB; 5)energy change=ENTBB-EGOMB
HIGHER Bond energies are in kJ.mol-1: C(triple)O=1077; C-O=360; C-H=410; H-H=436; O-H=460. What is the energy change for the reaction CO(g) +2H2(g) -> CH3OH(g) Draw energy 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
HIGHER AND CHALLENGE: 2X(g) + 6H(g)->X2H6(g) with deltaH=-2775kJ.mol-1. Bond energy 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
HIGHER Given SOME bond energies and the overall energy change, how do you work out the missing bond energy? 1) write overall=ENTBB-EGOMB 2) work out as much of ENTBB and EGOMB as possible, leaving the missing bond energy as "x" 3) rearrange the equation
Name some reactions that are exothermic combustion, neutralisation
if the temperature goes up, is the reaction endo or exothermic? Exothermic
how do you draw the arrow for the ACTIVATION energy on the full energy diagram? the arrow must start on the level of the reactant, goes up vertically to the top of the mountain
how do you draw the arrow for the OVERALL energy on the full energy diagram? the arrow must start on the level of the reactant, goes vertically to the level of the products; in an ENDO reaction, the arrow will go up; in an EXO, it will go down
Created by: ursulinechem2