3.2 Physical chemistry

Question 1

enthalpy definition

Answer 1

measure of heat energy stored in a system (H)

Question 2

enthalpy change definition

Answer 2

(ΔH) difference between the enthalpy of reactants and enthalpy of products

Question 3

heat definition

Answer 3

process whereby thermal energy (J) is transferred from hotter object to a cooler object

Question 4

temperature

Answer 4

direction of energy transfer is determined by temperature of objects (measured in K, °C, °F)

Question 5

system definition

Answer 5

substances directly involved in a chemical reaction

Question 6

surroundings definition

Answer 6

apparatus, laboratory, anything not part of the system

Question 7

universe definition

Answer 7

system+surroundings

Question 8

law of conservation of energy

Answer 8

energy cannot be created or destroyed

Question 8

exothermic meaning

Answer 8

energy released to and gained by surroundings, temperature increased
ΔH is negative
more energy released from making bonds than energy absorbed from breaking bonds

Question 9

endothermic meaning

Answer 9

system takes in energy from surrounding, temperature decrease
ΔH is positive
more energy absorbed from breaking bonds than energy released from making bonds

Question 11

enthalpy of reaction definition

Answer 11

overall energy change in a reaction

Question 12

activation energy definition

Answer 12

minimum energy required for a reaction to take place

Question 13

standard enthalpy of reaction definition

Answer 13

overall energy change in a reaction under standard conditions
100 kPa, 298 K / 25°C, conc. of 1.0 mol dm^-3 (for reactions with aqueous solutions)

Question 14

standard conditions

Answer 14

100kPa,
298K/25°C
1 mol dm^-3 (for solution)

Question 15

standard state definition

Answer 15

physical state of substance under standard conditions

Question 16

standard enthalpy change of combustion definition

Answer 16

energy change when one mole of substance is combusted in excess oxygen

Question 17

standard enthalpy change of formation definition

Answer 17

overall energy change of 1 mol of a compound from its elements

Question 18

standard enthalpy change of neutralisation definition

Answer 18

overall energy change when 1 mole of water is produced in a reaction with an acid and base

Question 19

enthalpy change of reaction formula

Answer 19

ΔrH = sum of bond enthalpies of broken bonds - sum of bond enthalpies of bonds made

Question 20

average bond enthalpy definition

Answer 20

breaking of 1 mol of bonds in gaseous molecules

Question 21

why average bond enthalpy differs from actual bond enthalpy (and less accurate than formation enthalpies)

Answer 21

different bond enthalpies are measured in different conditions (no universal, unchanging standard saying which molecules used to determine each bond)

Question 22

Hess law definition

Answer 22

if reaction takes place by more than one route from initial to final conditions, total enthalpies change is the same for each route

Question 23

simple collision theory

Answer 23

reaction takes place when molecules collide with sufficient energy and correct orientation

Question 24

factors affecting rate of reaction

Answer 24

concentration/pressure increases rate of collision (particles closer together, more likely to collide)
temperature increases heat energy (more kinetic energy so faster molecules so higher rate of collision, also more molecules with heat energy equal or more than activation energy)
surface area increases likelihood of collision between molecules

Question 25

Boltzmann distribution curve features

Answer 25

area under graph = number of molecules shaded area = number of molecules able to react Ea line = activation energy peak of graph = most likely energy of a molecule

Question 26

how catalyst affects Boltzmann distribution curve

Answer 26

moves Ea line to the left (more shaded area)

Question 27

how temperature affects Boltzmann distribution curve

Answer 27

lowers peak of the graph then right

Question 28

how dynamic equilibrium exists

Answer 28

rate of forward and reverse reactions are equal | concentration of reactants and products do not change

Question 29

Le Chatelier’s principle

Answer 29

if a system at equilibrium exposed to a stress (e.g. concentration, pressure, temperature change), system will shift its equilibrium to relieve that stress

Question 30

how concentration affects equilibrium

Answer 30

more concentration of reactants, faster rate of forward reaction equilibrium shifted right more concentration of products, faster rate of reverse reaction

Question 31

how temperature affects equilibrium

Answer 31

when temperature increases if reaction is exothermic, rate of reverse reaction increases (equilibrium shifts left) if reaction endothermic, rate of forward reaction increases (equilibrium shifts left) and vice versa

Question 32

how changing volume/pressure affects equilibrium

Answer 32

``` lower pressure (more volume) = rate of reaction making more gas molecules increasing higher pressure (less volume) = rate of reaction making less gas molecules increasing ```

Question 33

how catalyst affects equilibrium

Answer 33

increases rate of forward and reverse reaction no effect on position of equilibrium reduces time taken to reach equilibrium

Question 34

how to determine position of equilibrium

Answer 34

find out equilibrium constant if Kc > 1, position of equilibrium closer to the right (products) if Kc < 1, position of equilibrium closer to the left (reactants)

Question 35

rate of chemical reaction definition

Answer 35

how fast a reaction is taking place how fast a reactant is used up how fast a product is made

Question 36

what concentration time graphs look like

Answer 36

``` steepest at start (rate of reaction fastest in the beginning) gradient lowers the longer the reaction is taking place (reactants concentration lower, less likely to collide and react) eventually plateaus (no more reaction taking place, one of reactants completely used up) ```

Question 37

how to measure rate of reaction if gas is produced

Answer 37

monitor volume of gas at regular intervals (with gas collection) monitor loss of mass of reactants with mass balance

Question 38

monitor production of gas with gas collection method

Answer 38

place measuring cylinder (held by clamp) submerged under trough of water measure initial volume in measuring cylinder place one reactant in conical flask place bung on top of conical flask quickly add other reagents to conical flask and replace bung and instantly start time record volume of gas at regular intervals until reaction is complete (no more gas produced) plot graph gradient (found using tangent) = rate of reaction at point of tangent

Question 39

monitor loss of mass of reactants using mass balance method

Answer 39

record mass of conical flask and reactants initially and regular intervals reaction complete when no more gas produced plot graph

Question 40

catalyst definition

Answer 40

substance that changes rate of chemical reaction without undergoing any permanent change itself

Question 41

how catalyst increases rate of reaction

Answer 41

provides alternate reaction pathway of lower activation energy

Question 42

catalyst facts

Answer 42

not used up in chemical reaction may react with reactant to form intermediate or provide surface where reaction can take place catalyst is regenerated at end of reaction

Question 43

homogenous catalyst definition

Answer 43

same physical state as reactants | reacts with reactant to form intermediate that breaks down to give products and catalyst

Question 44

heterogenous catalyst definition

Answer 44

different physical state from reactants usually solids in contact with gaseous reactants or reactants in solution reactant molecules adsorbed onto surface of catalyst, reaction takes place product molecules leave surface of catalyst via desorption

Question 45

adsorption definition

Answer 45

weakly bonding to surface

Question 46

desorption definition

Answer 46

molecules leaving surface of another substance

Question 47

autocatalysis definition

Answer 47

reaction products acts as catalyst for that reaction

Question 48

importance of catalysts

Answer 48

increases rate of industrial chemical reactions by lowering activation energy also reduces temperature needed for processes and energy requirements (less fossil fuels and electricity used) cuts cost more sustainable (higher atom economies, fewer pollutants, cuts CO emissions)