3.2 physical changes

Question 1

define enthalpy

Answer 1

the total energy stored in a chemical system

Question 2

when referring to a reaction, what is a system?

Answer 2

the actual chemical reaction and the bonds and atom involved

Question 3

when referring to a reaction, what are the surroundings?

Answer 3

they are everything that ISN’T the system - everything around the reaction - aqueous solutions, beakers, air around it etc

Question 4

define enthalpy change

Answer 4

the heat energy given out or taken in per mole as the reaction progresses

Question 5

what is meant by standard conditions of a reaction?

Answer 5

1 bar of pressure (1 atm) and 298 K

elements in their standard states which is there physical state in the given conditions above^

Question 6

define exothermic reaction

Answer 6

a reaction which gives out energy

energy is transferred from the system to the surroundings

Question 7

define endothermic reaction

Answer 7

energy is taken in

energy is transferred from surroundings to the system

Question 8

exothermic: increase or decrease in temp?

Answer 8

increase in temp

Question 9

endothermic: increase or decrease in temp?

Answer 9

decrease in temp

Question 10

is making bonds exothermic or endothermic?

Answer 10

exothermic because making bonds releases energy

(BEndo-MExo) mexo

Question 11

is breaking bonds exothermic or endothermic?

Answer 11

endothermic because breaking bonds requires energy (BEndo-MExo) bendo

Question 12

why do exothermic reactions give out energy?

Answer 12

they release more energy making bonds than they take in breaking bonds

so overall more energy is released

Question 13

why do endothermic reaction take in energy?

Answer 13

they take in more energy breaking bonds than they release making bonds

so overall more energy is taken in

Question 14

describe the energy level diagram for an exothermic reaction. is ∆H +ve or -ve?

Answer 14

reactants have higher energy than products - goes down

∆H = NEGATIVE

Question 15

describe the energy level diagram for an endothermic reaction. is ∆H +ve or -ve?

Answer 15

products have higher energy than reactants - goes up

∆H = POSITIVE

Question 16

give two equations used to find enthalpy change

Answer 16

enthalpy of reactants- enthalpy of products

energy taken IN - energy given OUT

bonds broken - bonds made

Question 17

define enthalpy change of neutralisation

give the symbol

Answer 17

enthalpy change when one mole of water is formed from a neutralisation reaction (under standard conditions)

symbol: ∆neutH⊖

Question 18

define enthalpy change of formation

give the symbol

Answer 18

enthalpy change when one mole of compound is formed from its elements in their standard states (under standard conditions)

symbol: ∆fH⊖

Question 19

define enthalpy change of combustion

give the symbol

Answer 19

enthalpy change when one mole of substance/fuel reacts/combusts completely with oxygen

symbol: ∆cH⊖

Question 20

give the equation for heat change

what are they measured in

Answer 20

heat change = mass x specific heat capacity x temp change
Q = mc∆T

Q = J
m - grams
t - ˚C

Question 21

how do you calculate enthalpy change from using heat change? (3 steps)

Answer 21

1. calculate heat change using Q=mc∆T
2. convert Q from J to kJ
3. find enthalpy change by doing -Q/n
   (n=moles)

Question 22

what is average bond enthalpy?

Answer 22

energy required to break 1 mole of bonds in gaseous molecules

Question 23

define activation energy

Answer 23

minimum energy required for a reaction to take place

Question 24

state hess’s law

Answer 24

total enthalpy change is not affected by the route the reaction takes, providing the start and end substances are the same in each route.

Question 25

what must particles do to react?

Answer 25

they must collide with: - the right orientation - and with enough energy these are successful collisions

Question 26

name 5 factors affecting rate of reaction

Answer 26

``` temp concentration pressure - gases surface area catalysts ```

Question 27

define rate of reaction

Answer 27

the change in concentration of reactant or product per unit of time

Question 28

how can you calculate ROR using a progress curve?

Answer 28

at a specific time: draw a tangent and find the gradient overall: change in products/reactants conc/time

Question 29

give 3 equations for calculating ROR

Answer 29

reactant lost/t product made/t 1/t where t = time

Question 30

state and describe the effect of increasing concentration on ROR

Answer 30

increased conc = faster ROR 1. increasing conc gives more particles in the same volume so they are closer together 2. this means there's a greater chance of them colliding for frequently and successfully 3. more chance they'll react and overcome activation energy

Question 31

state and describe the effect of increasing pressure on ROR

Answer 31

increased pressure = faster ROR 1. increased pressure means gas particles are pushed closer together 2. this means there will be chance for frequent and successful collisions

Question 32

what is the purpose of a catalyst? | what do they allow a reaction to do?

Answer 32

- increase ROR without being used up in the reaction | - allows reaction to take a different route with a lower activation energy

Question 33

define heterogeneous catalyst

Answer 33

a catalyst that is in a different phase from the reactants

Question 34

define homogeneous catalyst

Answer 34

a catalyst that is in the same phase as the reactants

Question 35

what is meant by the phase of a substance?

Answer 35

the physical state

Question 36

how can catalysts be more economical?

Answer 36

- by lowering energy demands for industrial processed | - so there lowering cost of producing such energy

Question 37

how can catalysts help the environment?

Answer 37

- catalytic converters in cars reduce the pollution released by cars by speeding up reactions in the engines - also reduce waste and improve atom economy by allowing reactions to take alternative routes

Question 38

describe the general shape of a Boltzmann distribution curve (BDC) what is the y and x axis?

Answer 38

bell shape y axis: no. of particles with given energy x axis: energy

Question 39

what does the peak on BDC show?

Answer 39

average energy (most particles have this amount of energy

Question 40

what is the area under a BDC?

Answer 40

the number of particles

Question 41

where is the activation energy on a BDC?

Answer 41

further along when the line goes down

Question 42

what does changing temp do to: 1. the overall shape of the BDC 2. the average energy (position of peak) 3. the area under graph

Answer 42

1. the curve is flatter 2. the average energy is higher (peak is further right) 3. area under graph stays - same no. of particles

Question 43

what does changing temp do to: 1. the activation energy 2. the number of particles exceeding Ea 3. rate of reaction

Answer 43

1. activation stays the same 2. more particles exceed activation energy 3. increases ROR - more particles with Ea - more successful collisions

Question 44

what does using a catalyst do to: 1. the overall shape of BDC 2. activation energy 3. number of particles exceeding Ea 4. ROR

Answer 44

1. shape does not change 2. Ea decreases 3. more can exceed activation energy 4. ROR increases - more particles can exceed Ea = more successful collisions

Question 45

describe the term dynamic equilibrium

Answer 45

occurs when, in a closed system, the rate of the forwards and backwards reactions are equal and the concentrations of the products and reactants remain constant

Question 46

give 3 factors affecting the position of equilibrium

Answer 46

- concentration - pressure - temperature

Question 47

what happens to the position of equilibrium when you: 1. increase concentration of products? 2. increases concentration of reactants? 3. why do these shifts occur?

Answer 47

1. equilibrium shifts to the reactants (left) 2. shifts to the products (right) 3. opposes change in concentration by producing more of the opposite substance

Question 48

what happens to the position of equilibrium when you: 1. increase the pressure? 2. decrease the pressure? 3. why do these shifts occur?

Answer 48

1. shifts to the side with fewer gas molecules/less moles 2. shifts to the side with more gas molecules/more moles 3. counteract changes in pressure - less molecules = lower pressure and more molecules = higher pressure

Question 49

what happens to the position of equilibrium when you: 1. increase temp? 2. decrease temp? 3. explain why this shifts occur

Answer 49

1. shifts in endothermic direction to try and decrease temp 2. shifts in the exothermic direction to try to increase the temp 3. equilibrium tries to oppose the change in temp

Question 50

how does a catalyst affect the position equilibrium? explain your answer

Answer 50

catalyst do not position of equilibrium - catalysts speed up the forward and backwards reactions by the same amount - so there is no need for the equilibrium to shift as the reaction is still balanced

Question 51

what is the equilibrium constant and why do we use it?

Answer 51

Kc is a constant that tells us where equilibrium lies using the ratio of reactants to products

Question 52

what do square brackets around an element mean?

Answer 52

concentration of ______. eg. [Fe] = conc of iron

Question 53

how do you calculate Kc?

Answer 53

eg. aA + bB ⇋ cC + dD Kc = [C]^c[D]^d / [A]^a[B]^b (conc of products / conc of reactants)

Question 54

what can you deduce about the position of equilibrium when Kc = 1?

Answer 54

equilibrium lies halfway between reactants and products

Question 55

what can you deduce about the position of equilibrium when Kc << 1?

Answer 55

lies to the left - towards the reactants | equilibrium favours reactants

Question 56

what can you deduce about the position of equilibrium when Kc >> 1?

Answer 56

lies to the right - towards the products | equilibrium favours products

Question 57

why is it important to compromise on a temperature in a reaction?

Answer 57

if temp is too low - ROR is slow | if temp is too high for an exothermic reaction - too much product would be formed = low yield

Question 58

why is it important to compromise on a pressure in a reaction?

Answer 58

if pressure is too low - reaction will be too slow - low yield (if products have less moles of gas) if pressure is too high - expensive, puts stress on equipment a d side reactions may occur