3.2 Physical Chemistry

Question 1

What is the law of conservation of energy?

Answer 1

Energy can not be created or destroyed only transferred, stored or dissipated

Question 2

What’s an exothermic reaction?

Answer 2

Energy lost to surroundings,

reactants more energy than products (-ΔH )

Question 3

What’s an endothermic reaction?

Answer 3

Energy taken in from surroundings,

products more energy than reactants (+ΔH)

Question 4

What is activation energy?

Answer 4

The energy required to break bonds in the reactants

Question 5

What are standard conditions?

Answer 5

100 Kpa (100,000 pa) ot 1 atm
298k(k=c°+ 273)
1 mol dm-3

Question 6

What is enthalpy change of formation?

Answer 6

Energy change associated with formation of 1mol of compound from a substituent elements

Question 7

What is enthalpy change of combustion?

Answer 7

Energy change that accompanies the COMPLETE combustion of 1 mol of a substance

Question 8

What is enthalpy change of neutralisation?

Answer 8

Energy change associated with formation of 1mol of water from a neutralisation reaction

Question 9

What equation links energy in joules, mass of substance& specific heat capacity?

Answer 9

q=mcΔT
• q- energy in joules
• m- mass of substance heated or cooled
• c- specific heat capacity
• ΔT- temperature change

Question 10

What is average bond enthalpy?

Answer 10

The Mean energy needed for 1 mol of gaseous bonds to undergo homolytic fission

Question 11

What is Hess law?

Answer 11

States regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes

Question 12

Hess cycle for formation

formation up

Answer 12

reactants → products
⇖ ⇗
sub elements

Question 13

Hess cycle for combustion

combustion down

Answer 13

reactants → products
⇘ ⇙
combustion products

Question 14

What is the equation for rate of reaction?

Answer 14

rate=change in conc/ time

units =mol dm3 s-1

Question 15

What factors effect rate of reaction?

Answer 15

• Temperature
• surface area
• Concentration
• pressure (gaseous reactions only)
• catalyst (not for reversible)

Question 16

What’s collision theory?

Answer 16

For a chemical reaction to take place molecules must collide with sufficient energy in the correct orientation

Question 17

Effect of temp on exothermic reaction @ equilibrium?

Answer 17

• decrease temp favours forward (exo) reaction

* increase temp favours reverse reaction

Question 18

Effect of temp on endothermic reaction @ equilibrium?

Answer 18

• increase temp favours forward (endo)reaction

* decrease temp favours reverse reaction

Question 19

How does pressure effect a gaseous reaction @ equilibrium?

Answer 19

• increased pressure favours side with less gas mol

* decreased pressure favours side with more gas mol

Question 20

How can we measure reaction rates?

Answer 20

• collect gas in syringe formed during reaction
• monitor reactions mass
• record time taken for certain change to occur
• calorimeter

Question 21

What is a catalyst?

Answer 21

substances that increases the rate of reaction without being used up

Question 22

What’s a Homogenous catalyst

Answer 22

Catalyst in same phase(state of matter) as reactants

Question 23

What’s a Heterogenous catalyst

Answer 23

Catalyst in different phase(state of matter) as reactants

Question 24

What’s the economic reasons for catalyst?

Answer 24

• lower energy demands (activation energy) for reactions
• less fuel used so less emissions
• speed up reactions

Question 25

What’s the principle around Boltzmann distribution graph

Answer 25

• some molecules have lots of energy
• some molecules have little energy
• most molecules have average energy

Question 26

How does temp affect a Boltzmann distribution graph?

Answer 26

Lowers the peak and heightens the area of molecules in above activation energy.
**Does NOT change activation energy

Question 27

How do catalyst affect a Boltzmann distribution graph?

Answer 27

Decrease activation energy so moves the energy needed line closer to 0

Question 28

What is Le chateliers principle?

Answer 28

When a system in equilibrium is subject to change the position of equilibrium will shift to minimise the change

Question 29

What is the effect of concentration on equilibrium?

Answer 29

Whichever side has conc changed causes equilibrium to shift to counteract

Question 30

Equilibrium vs yield arguments

Answer 30

• favourable condition tend to be expensive/hard to maintain/dangerous
• some favourable conditions for yield are slow so compromise needed

Question 31

What is the Haber process?

Answer 31

Process to make ammonia

N2+3H2 ⇌ 2NH3

Question 32

What conditions would favour the Haber process?

N2+3H2 ⇌ 2NH3 ΔH= -92KJ

Answer 32

• high pressure (favour side with less Gaseous mol)

* low temp (exothermic)

Question 33

What compromises might a company make to have a sustainable Haber process?

Answer 33

• using a higher pressure so its safe to work
• temp high enough for reaction to happen at realistic rate
• catalyst

Question 34

What does a Kc value greater than 1 show?

Answer 34

Reaction favour products

Question 35

What does a Kc value less than 1 show?

Answer 35

Reaction favours reactants

Question 36

Uncertainty Eq?

Answer 36

(absolute uncertainty *2)/ value measured