3.1.4 Energetics Knowledge

Question 1

describe what happens during an exothermic process

Answer 1

energy is released from the chemical to the surroundings during the process. the temperature of the surroundings increases.

Question 2

describe what happens during an endothermic process

Answer 2

energy is taken in by the chemical from the surroundings during the process. the temperature of the surroundings decreases

Question 3

define the term ‘enthalpy change’

Answer 3

the change in heat energy at constant pressure. a measure of the energy change during a process, per mole of chemical - kJmol^-1

Question 4

what sign does delta H have in exothermic processes?

Answer 4

For exothermic processes, the sign for delta H is negative

Question 5

what sign does delta H have in endothermic processes?

Answer 5

For endothermic processes, the sign for delta H is positive.

Question 6

list the standard conditions for enthalpy change

Answer 6

• 100 kPa (or 1 atm) pressure
• 298K (25 degrees) temperature
• 1 mol dm^-3 concentration for all solutions

Question 7

why are standard conditions used when measuring enthalpy change?

Answer 7

the value of the enthalpy change for a reaction is affected by changes in temperature and pressure

Question 8

what is the state of a chemical under standard conditions called?

Answer 8

the state of a chemical under these conditions is called its standard state

Question 9

what is the symbol for standard enthalpy change?

Answer 9

ΔH°

Question 10

is breaking bonds endo or exothermic?

Answer 10

Breaking bonds requires energy and is an endothermic process

Question 11

is making bonds endo or exothermic?

Answer 11

Making bonds releases energy and is an exothermic process

Question 12

why do chemical reactions have an enthalpy change?

Answer 12

in order for reactions to happen, bonds in the reactants must be broken and bonds in the products must be formed.

Question 13

what is activation energy?

Answer 13

Chemical reactions require an initial input of energy to start - this is the activation energy (Ea) and is essentially the amount of energy required to break the reactant bonds

Question 14

define activation energy

Answer 14

the minimum needed energy to start a reaction

Question 15

why should you Always include state symbols in reaction equations for enthalpy changes?

Answer 15

as the states of reactants and products affect the enthalpy change of a reaction

Question 16

give an example of when the states of reactants and products affect the enthalpy change of a reaction

Answer 16

forming liquid water is a more exothermic process than forming gaseous water

Question 17

define mean bond enthalpy

Answer 17

the energy required to break one mole of a covalent bond into gaseous atoms, averaged over a range of different compounds

Question 18

why is bond enthalpy always an endothermic process?

Answer 18

Always endothermic process as energy is required to break bonds

Question 19

what does a more positive bond enthalpy indicate?

Answer 19

the more positive the bond enthalpy, the larger the amount of energy needed to break the bond and so the stronger the bond

Question 20

why are mean bond enthalpies averaged over a range of compounds ?

Answer 20

because the exact amount of energy required to break a covalent bond depends upon the molecule that bond is in

Question 21

what is the equation for Calculating enthalpy change using bond enthalpies

Answer 21

ΔH = ∑Δ(BE)H(Reactants) - ∑Δ(BE)H(Products)

Question 22

what is calorimetry?

Answer 22

An experiment is carried out & the change in temperature of the surroundings is measured; this is used to calculate the energy released or taken in by the reaction

Question 23

what is the density of water at 25 degrees?

Answer 23

1gcm^-3

Question 24

what is the equation to calculate heat energy change?

Answer 24

Q = mcΔT

Question 25

what does the Q in Q = mcΔT stand for?

Answer 25

heat energy released/taken in - Joules, J

Question 26

what is the units for Q in Q = mcΔT?

Answer 26

Joules

Question 27

what does the m in Q = mcΔT stand for?

Answer 27

mass of liquid that changes temperature - grams, g

Question 28

what is the units for m in Q = mcΔT?

Answer 28

grams

Question 29

what does the c in Q = mcΔT stand for?

Answer 29

specific heat capacity - Jk^-1g^-1

Question 30

what is the units for c in Q = mcΔT?

Answer 30

Jk^-1g^-1

Question 31

what does the ΔT in Q = mcΔT stand for?

Answer 31

change in temperature - K or degrees

Question 32

what is the units for ΔT in Q = mcΔT?

Answer 32

K or degrees celsius

Question 33

what is the equation for enthalpy change?

Answer 33

ΔH = Q / n

Question 34

what does the Q in ΔH = Q / n stand for?

Answer 34

energy taken in/released - kilojoules, kJ

Question 35

what is the unit for Q in ΔH = Q / n?

Answer 35

kilojoules, kJ

Question 36

what does the ΔH in ΔH = Q / n stand for?

Answer 36

enthalpy change - kJmol^-1

Question 37

what is the unit for ΔH in ΔH = Q / n?

Answer 37

kJmol^-1

Question 38

what does the n in ΔH = Q / n stand for?

Answer 38

moles of substance that causes temperature change - mol

Question 39

what is the unit for n in ΔH = Q / n?

Answer 39

moles

Question 40

how can a combustion reaction be used in calorimetry?

Answer 40

energy released by the fuel is transferred to the water, and the temperature rise of the water is measured

Question 41

how do you calculate energy transfer from a combustion reaction/

Answer 41

- Calculate the energy transferred to the water using Q = mcΔT where 'm' is the mass of water heated and 1g of water has a volume of 1cm^3 - Calculate the enthalpy change using ΔH = Q / n (remember to divide the previous Q value by 1000 to convert to kJ) - Add negative sign to show change is exothermic

Question 42

what is the volume of 1g of water?

Answer 42

1cm^3

Question 43

how do you calculate energy transfer in a reaction of two solutions?

Answer 43

- Calculate the energy transferred to the solution using Q = mcΔT where 'm' is the mass of solution heated (use the total value e.g solution 1 + solution 2) - Calculate the enthalpy change using ΔH = Q / n (remember to divide the previous Q value by 1000 to convert to kJ) - only use one set of moles here; when moles are different, always use the limiting reagent - Add negative sign to show change is exo/endothermic

Question 44

how do you calculate energy transfer in a reaction of a solid and solution?

Answer 44

- Calculate the energy transferred to the solution using Q = mcΔT - only use the volume of the solution for m, not the solid - Calculate the enthalpy change using ΔH = Q / n (remember to divide the previous Q value by 1000 to convert to kJ) - only use one set of moles here; when moles are different, always use the limiting reagent - Add negative sign to show change is exo/endothermic

Question 45

what major assumption is made in calorimetry?

Answer 45

the energy transferred to the water from the reacting chemicals (or vice versa for endothermic reactions) is equal to the energy released (or taken in) by the reaction

Question 46

what are possible sources of error in combustion calorimetry?

Answer 46

- heat loss to the surroundings - incomplete combustion of the fuel - heat energy transferred to the metal calorimeter - some fuels are volatile and so evaporate

Question 47

what are possible sources of error in solution calorimetry?

Answer 47

- heat loss to the surroundings - incomplete reaction

Question 48

what improvements can be made to combustion calorimetry to reduce sources of error?

Answer 48

- add a lid - insulate sides of a calorimeter - reduce distance between flame and beaker - put a sleeve around the flame to protect it from drafts

Question 49

what improvements can be made to solution calorimetry to reduce sources of error?

Answer 49

- add a lid - insulate calorimeter/use a polystyrene cup

Question 50

how can you account for heat loss in calorimetry?

Answer 50

- record the temperature for a suitable time (3 mins) before adding reactants together to establish an accurate initial temperature - mix reactants then record temperature every min until a trend is seen (~10 mins) - plot a graph of temp against time - extrapolate the cooling curve back to the point of addition to establish a maximum theoretical temp change accounting for heat loss

Question 51

define the Standard Enthalpy of Formation

Answer 51

the enthalpy change when one mole of a substance is formed from its constituent elements under standard conditions with all reactants and products being in their standard states

Question 52

give the equation for standard enthalpy of formation of water

Answer 52

H2(g) + 1/2O2(g) ⇒ H2O(l)

Question 53

what is The standard enthalpy of formation of all elements in their standard states

Answer 53

0

Question 54

why is the enthalpy of formation of Na(s) 0?

Answer 54

Na is an element in its standard state

Question 55

why is the enthalpy of formation of Na(l) not 0?

Answer 55

Na(l) is not the standard state of Na

Question 56

define the Standard Enthalpy of Combustion

Answer 56

the enthalpy change when one mole of a substance is completely burnt in excess oxygen under standard conditions, all reactants and products being in their standard states

Question 57

give the equation for standard enthalpy of combustion of methan

Answer 57

CH4(g) +2O2(g) ⇒ CO2(g) + 2H2O(l)

Question 58

what is the enthalpy of combustion for water and co2?

Answer 58

Substances that cannot be combusted, like H2O, CO2, and most other oxides have 0 enthalpy of combustion

Question 59

why can the enthalpy of combustion of a compound can be difficult?

Answer 59

as incomplete combustion may occur, so the energy is less than expected (less exothermic)

Question 60

why do we use hess cycles?

Answer 60

Some types of enthalpy change are difficult to measure experimentally (i.e. through calorimetry)

Question 61

why are Some types of enthalpy change are difficult to measure experimentally (i.e. through calorimetry)?

Answer 61

due to high activation energy, slow reaction rate, or possibility of competing reactions

Question 62

define Hess' Law

Answer 62

Hess' Law states that the enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products

Question 63

give the layout of a hess cycle if the majority of the data given is bond enthalpies

Answer 63

Question 64

why will The bond enthalpy calculated using a Hess cycle be different from the mean bond enthalpy in a data book?

Answer 64

the data book value is averaged over a range of different compounds (not just a specific molecule)

Question 65

give the layout of a hess cycle if the majority of the data given is standard formation enthalpies

Answer 65

Question 66

give the layout of a hess cycle if the majority of the data given is standard combustion enthalpies

Answer 66

Question 67

why is The oxygen required to combust the reactants not included in a combustion Hess cycle?

Answer 67

we just assume these reactions are done in air where there is plenty of oxygen