AQA GCSE Chapter 07 Energy
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| 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
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| 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
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| 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
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| 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
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| For bond breaking process, is energy taken in or given out? | Taken in ENTBB (Energy is Needed To Break Bonds)
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| When new bonds are made, is energy taken in or given out? | Given out EGOMB (Energy is Given Out when Making Bonds)
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| 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
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| 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
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| 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
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| 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
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| Name some chemical reactions that are endothermic | Thermal decomposition, Photosynthesis, dissolving ammonium nitrate
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| What is the trick to remember for questions on energy | compare ENTBB to EGOMB
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| 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)
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| explain how a catalyst speeds up a reaction | it lowers the activation energy and provides an alternative reaction pathway
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| what is the activation energy? | the minimum of energy required for the particles to collide and bonds to break
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| 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
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| 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
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| 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
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| 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
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| 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
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| 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
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| 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
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| Name some reactions that are exothermic | combustion, neutralisation
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| if the temperature goes up, is the reaction endo or exothermic? | Exothermic
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| 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
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| 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
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