3.1.4 energetics

Question 1

change in enthalpy is

Answer 1

the change in heat energy measured at constant pressure, temperature and conc of 1M for solutions

those are the standard conditions needed for measuring enthalpy change

Question 2

ΔH∘r

Answer 2

standard enthalpy of reaction

Question 3

ΔH∘neut

Answer 3

standard enthalpy change of neutralisation

enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions with all reactants and products in their standard states

Question 4

how do you write an equation for the standard enthalpy of combustion

Answer 4

moles formed of water must be 1. use fractions for the other species if necessary

use standard state symbols

Question 5

standard states

Answer 5

state you find a substance in at 298K and 100kPa

Question 6

remember enthalpy change value is for the

Answer 6

specific molar quantities in that reaction. so its different for every reaction

Question 7

ΔH∘c

Answer 7

standard enthalpy change of combustion

enthalpy change when 1 mole of substance is burned completely in oxygen under standard conditions with all the P/R in their standard states

(always referring to complete combustion)

Question 8

ΔH∘f

Answer 8

standard enthalpy change of formation

enthalpy change when 1 mole of substance is formed from its constituent elements under standard conditions with all products and reactants in their standard states

Question 9

what is the ΔH∘f of an element

Answer 9

0

by definition
there is no ΔH because no energy is required to change an element into an element

Question 10

calorimetry

Answer 10

the process of measuring the heat of chemical reactions or physical changes as well as heat capacity.

is used to work out the ΔH of a reaction experimentally, using a calorimeter

Question 11

units of ΔH

Answer 11

kJmol^-1

Question 12

process of a calorimeter

Answer 12

1. Use measuring cylinder to add known mass of water to a copper cup. that is the calorimeter
2. take and record initial temp of water
3. use stirrer to evenly distribute heat throughout the water
4. clamp cup in the air. have screens on the sides to reduce heat loss to surroundings so more heat is transferred to water
5. record mass of burner w lid on to prevent evaporation
6. light it and begin. stir water to evenly distribute heat. ensure water is never brought to boil
7. record final temp of water and final mass of burner with lid on
8. calculate how much fuel is used up by doing initial mass burner - final mass

Question 13

equation to find energy released or taken in from calorimeter results

Answer 13

q = mc∆t

where q is the energy released/taken in
m is the mass water heated
c is the shc of water (always given)
∆t is the temp change

Question 14

how do you find the ∆H from calorimeter results

Answer 14

get q (q=mc∆t)

÷ by moles
for neutralisation you divide by moles water produced

for displacement you divide by moles used up in the reaction eg limiting reactant moles

Question 15

why is calorimetry useful for comparing fuels

Answer 15

for combustion reactions this works out ΔH∘c for that recation/substance

so who ever has higher kJmol^-1 has higher energy output

Question 16

for neutralisation the change in temp is so sudden. why

Answer 16

they are often v exothermic so more heat is lost at the moment of reaction than at any other point

Question 17

for neutralisation the change in temp is so sudden. how can we get a somewhat accurate value for it

Answer 17

plot graph of time on X and temp on y

record temp of reaction before it happens (so j the acid/alk in a beaker basc) for a few minutes

record it after neutralisation for a few minutes (so after the other was added)

extrapolate both data plots w a line of best fit to when the substance is added

find difference

record temp at regular intervals
temp against time
2 lines of b.f
establish theoretical max temp by extrapolation

Question 18

for the extrapolation of neutralisation temp graphs, how do you do it properly

Answer 18

1. extrapolate no further than the point of addition
2. the gap in time data will indicate when substance is added
3. miss the point of inflection, will help you decide what way you extrapolation should go
4. may be a straight line sometimes

Question 19

what are energy cycles and Hess cycles used for

Answer 19

to calculate ∆H that are difficult to achieve in reality.

ex the enthalpy of formation of methane; H2 is v flammable

Question 20

Hess law states that

Answer 20

∆H for any chemical reaction is independent of the intermediate stages, provided the initial and final conditions are the same for each route

basc to say that enthalpy change from reactant to intermediate products to products, all that added up, is the same as the enthalpy change from reactant to product

Question 21

In a hess cycle if you travel opposite of the arrow then

Answer 21

you invert the sign of that ∆H

Question 22

checklist for energy profiles

Answer 22

dotted line to extrapolate inherent energy in R/P to where the arrows come up/down from

yeah lol

Question 23

Bond enthalpy??

Question 24

In an exothermic change…

Answer 24

energy is transferred from the system to the surroundings

the products have less energy than the reactants

enthalpy change is negative

Question 25

in an endothermic change…

Answer 25

energy is transferred for the surroundings to the system

the require an input of heat energy

the products have more energy than the reactants

Question 26

Mean bond enthalpy

Answer 26

Average energy needed to break one mole of a certain type of bond in a gaseous state under standard conditions. this def only applies when all subatcnes start and end in a gaseous state

It is a mean of data from a range of different compounds containing that bond type

Always +ve

Question 27

Why is values from mean bond enthalpy calcs different from those determined by hess law

Answer 27

Mean bond enthalpies are not exact; they are averaged values of the bond enthalpies from various compounds