Enthalpy changes

Question 1

Enthalpy H ;

Answer 1

heat content of system

@ constant pressure

Question 2

Enthalpy change ∆H°;

Answer 2

heat added to a system

@ constant pressure

Question 3

Enthalpy change ∆H EQ

Answer 3

(∆H = H products - H reactants)

Question 4

Exothermic energy profile

Answer 4

Reactants > Products

= -ve

Question 5

Endothermic energy profile

Answer 5

Products> Reactants

= +ve

Question 6

Conservation of energy;

Answer 6

energy cannot be

destroyed

created

only changed from one form to another

Question 7

Standard Conditions

Answer 7

• All substances = standard states
• 298k
• 1 atm
• conc - 1mol^-1dm³

Question 8

Standard enthalpy of formation;

Answer 8

One mole of substance is formed from constituent elements in standard states under standard conditions

Question 9

Standard enthalpy of combustion

Answer 9

One mole of substance is completely burned/ combusted in O₂ under standard conditions

Question 10

Standard state used should be………….

Answer 10

Elements that are most stable at 298k, 1 atm

Question 11

Enthalpy change of reaction

Answer 11

change in a reaction between number of moles of reactants shown in the equation form the reaction

Question 12

Hess’s Law

Answer 12

total enthalpy change for reaction

independent of route taken from reactants to products

Question 13

Hess’s law general EQ

Answer 13

ΔH₁= ΔH₂ + ΔH₃

Question 14

H.L Combustion

Answer 14

Arrows point down

Δ^cH = Reactants - Products

Question 15

H.L Formation

Answer 15

Arrows point up

Δ^fH = Products - Reactants

Question 16

Bond enthalpy;

Answer 16

Enthalpy required break covalent X-Y

into atoms X and Y

all in the gas phase

Question 17

Average bond enthalpy;

Answer 17

average value of enthalpy

required to break given type of covalent bond

gaseous species

Question 18

Breaking bonds = …………….

Answer 18

endothermic - req energy

Question 19

Making bonds = ……………..

Answer 19

exothermic - releases energy

Question 20

Calorimeter;

Answer 20

Vessel determination of heat/ temp changes over time

Question 21

Measuring enthalpy changes expression

Answer 21

q = mcΔT

mass refers to mass of water ; 100cm of water heated = 100g

c = 4.18Jg^-1k^-1

^{Always remember to convert answer to kJ by dividing by 1000}

Question 22

Calculating moles

Answer 22

n = m/M

n= c x v ^{(always remember to change volume into dm-3)}

Question 23

ΔH = ……….

Answer 23

ΔH = -q/n

Units = kJmol^-1

Question 24

Determining enthalpy of combustion - Prac

Possible improvements………. 1,2,3

Answer 24

1. Graph - extrapolating - reduces the error caused by heat loss to surroundings
2. Draught shield/ screen - maximise heat transfer from flame to water
3. Water - stirred continually - ensure even temp

Question 25

ΔH combustion = always lower than the book

Why?

Answer 25

• Energy transferred from burning alcohol = lost to surroundings + apparatus
• Alcohol not completely combusted = soot at bottom of calorimeter - indicator

Question 26

To determine more accurate enthalpy changes a ……….. can be used

Better - why?

Answer 26

bomb calorimeter

1. Stirrer - uniform temp
2. Insulation
3. Lid
4. ………………………

Question 27

Indirect determination of enthalpy change

Coffee cup calorimeter

What?

Answer 27

• Changes that take place in aq
• Sealed + insulated container
• Temperature noted at regular intervals over a period of time

Question 28

Why use polystyrene cup?

How could it be improved further?

Answer 28

• Insulates - heat lost or absorbed = negligible

Improved by

1. Placed inside another polystyrene cup
2. Beaker lagged with cotton wool

= Improved insulation

Question 29

ΔH = found using calorimetry = never completely accurate

Why?

Answer 29

• Energy easily lost through conduction or convection
• Inaccuracies measuring temperature

Question 30

Ex Calorimeter Experiment

Answer 30

Magnesium oxide + Acid (HCl)

Magnesium carbonate + Acid (HCl)