Energetics

Question 1

Enthalpy Change

Answer 1

the change in heat energy at constant pressure

Question 2

enthalpy change units

Answer 2

kJ mol-1

Question 3

What sign is enthalpy change for endothermic processes?

Answer 3

positive

Question 4

What sign is enthalpy change for exothermic processes?

Answer 4

negative

Question 5

What are the standard conditions for enthalpy change measurements?

Answer 5

100kPa pressure
298k temperature
1.0 moldm-3 concentration for all solutions

Question 6

Is breaking bonds endothermic or exothermic?

Answer 6

endothermic

Question 7

is making bonds endothermic or exothermic?

Answer 7

exothermic

Question 8

What is activation energy?

Answer 8

the minimum energy needed to start a reaction

Question 9

Define mean bond enthalpy

Answer 9

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

Question 10

Are mean bond enthalpies endothermic or exothermic?

Answer 10

endothermic

Question 11

are mean bond enthalpies positive or negative?

Answer 11

positive

Question 12

the more positive the bond enthalpy the ___________ the amount of energy needed to break the bond and so the ___________ the bond

Answer 12

the more positive the bond enthalpy the LARGER the amount of energy needed to break the bond and so the STRONGER the bond

Question 13

define standard enthalpy of reaction

Answer 13

any chemical reaction under standard conditions

Question 14

define standard enthalpy of formation

Answer 14

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

Question 15

why is the enthalpy of Na(s) zero?

Answer 15

Na is an element in its standard state

Question 16

Why is the enthalpy of formation of liquid Na not zero?

Answer 16

Na(l) is not the standard state of Na

Question 17

calculation for enthalpy of formation

Answer 17

FPR
formation = products - reactants

Question 18

what is the standard enthalpy of formation of all elements in their standard state?

Answer 18

zero

Question 19

define standard enthalpy of combustion

Answer 19

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

Question 20

which substances cannot be combusted?

Answer 20

water, carbon dioxide, and most other oxides

Question 21

what is the enthalpy of combustion of water and carbon dioxide?

Answer 21

zero

Question 22

why may the enthalpy of combustion of a compound be difficult to measure?

Answer 22

incomplete combustion may occur

Question 23

calculation for enthalpy of combustion

Answer 23

CRP
combustion = reactants - products

Question 24

define Hess’ Law

Answer 24

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

Question 25

Give one reason why the bond enthalpy that you calculated above is different from the mean bond enthalpy quoted in a data book

Answer 25

The data book value is averaged over a range of different compounds

Question 26

what is the equation for calculating energy released or taken in by reactions during calorimetry?

Answer 26

Q = mc∆T

Q = Energy (J)
m = mass of water (surroundings) (g)
c = Specific Heat Capacity (usually 4.18 J K−1 g−1)
∆T = Change in temp (in Kelvin or ⁰C)

Question 27

How do you calculate enthalpy change from Q (energy released/taken in)?

Answer 27

∆H = Q / n

∆H = Enthalpy change (kJ mol−1)
Q = Energy taken in or released by the reaction (kJ)
n = number of moles reacted or formed

Question 28

how do you decide the sign of ∆H in calorimetry calculations?

Answer 28

if temp increases = exothermic = -∆H
if temp decreases - endothermic = +∆H

Question 29

what is the major assumption made in calorimetry experiments?

Answer 29

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

Question 30

sources of error for combustion calorimetry

Answer 30

heat loss to the surroundings
incomplete combustion of the fuel
heat energy transferred to metal calorimeter
some fuel evaporates

Question 31

sources of error for solution calorimetry

Answer 31

heat loss to the surroundings

Question 32

improvements to minimize sources of error for combustion calorimetry

Answer 32

add a lid - reduces heat loss
insulate sides of calorimeter - reduces heat loss
reduce distance between flame and beaker - reduces heat loss
put sleeve around flame to protect it from draughts

Question 33

improvements to minimize sources of error for solution calorimetry

Answer 33

add a lid - reduces heat loss
insulate calorimeter - reduces heat loss

Question 34

Steps to measure an enthalpy change using a cooling curve

Answer 34

record the temp for a suitable time (3 mins) before adding reactants together
to establish an accurate initial temperature
mix reactants the record temperature every minute until a trend is seen
plot a graph of temperature against time