energetics

Question 2

What is enthalpy change?

Answer 2

change in heat energy at constant pressure.

Question 3

What is an exothermic reaction?

Answer 3

A reaction that releases heat, so external temp increases

Question 4

What is an endothermic reaction?

Answer 4

A reaction that absorbs heat

Question 5

What is the standard enthalpy change of reaction (ΔHᵣ⦵)?

Answer 5

Enthalpy change when a reaction occurs in molar quantities under standard conditions (100 kPa

Question 6

What is the standard enthalpy change of formation (ΔHf⦵)?

Answer 6

Enthalpy change when 1 mole of a compound is formed from its elements in standard states under standard conditions.

Question 7

What is the standard enthalpy change of combustion (ΔHc⦵)?

Answer 7

Enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions.

Question 8

What is the standard enthalpy change of neutralisation?

Answer 8

Enthalpy change when an acid and base form 1 mole of water under standard conditions.

Question 9

What is the equation used in calorimetry?

Answer 9

q = mcΔT (q = heat energy in J; m = mass in g; c = specific heat capacity (4.18); ΔT = temp change in °C).

Question 10

How do you calculate molar enthalpy change from q?

Answer 10

ΔH = −q / n (divide q in J by moles

Question 11

What are two sources of error in calorimetry?

Answer 11

Heat loss to surroundings; incomplete combustion.

Question 12

What is Hess’s Law?

Answer 12

The enthalpy change of a reaction is the same regardless of the route taken.

Question 13

What is the formula using formation enthalpies in Hess’s Law?

Answer 13

ΔH = ∑ΔHf(products) − ∑ΔHf(reactants).

Question 14

What is the formula using combustion enthalpies in Hess’s Law?

Answer 14

ΔH = ∑ΔHc(reactants) − ∑ΔHc(products).

Question 15

In a Hess cycle using formation data

Answer 15

where are the elements placed?

Question 16

In a Hess cycle using combustion data

Answer 16

where are the combustion products placed?

Question 17

What is bond enthalpy?

Answer 17

The energy needed to break 1 mole of bonds in gaseous molecules.

Question 18

Is breaking bonds endothermic or exothermic?

Answer 18

Endothermic — requires energy (positive ΔH).

Question 19

Is making bonds endothermic or exothermic?

Answer 19

Exothermic — releases energy (negative ΔH).

Question 20

What is the equation using mean bond enthalpies?

Answer 20

ΔH = ∑(bonds broken) − ∑(bonds formed).

Question 21

Why are bond enthalpy calculations approximate?

Answer 21

They use average bond enthalpies which vary by molecular environment.

Question 22

What are the units of enthalpy change?

Answer 22

kJ/mol.

Question 23

What is the specific heat capacity of water?

Answer 23

4.18 J/g°C.

Question 24

How do you convert J to kJ?

Answer 24

Divide by 1000.

Question 25

How do you convert cm³ to dm³?

Answer 25

Divide by 1000.

Question 26

What are standard conditions in energetics?

Answer 26

100 kPa

Question 27

Why is ΔH for combustion of alcohols often lower than expected?

Answer 27

Heat loss to surroundings and incomplete combustion.

Question 28

Why do experimental ΔH values differ from data book values?

Answer 28

Heat loss

Question 29

methanoic acid formula?

Answer 29

HCOOH

Question 29

define mean bond enthalpy (2)

Answer 29

the enthalpy change needed to break covalent bonds (1) averaged for that type of bond, over a range of compounds (1)

Question 30

define positional isomer (2)

Answer 30

compounds with the same molecular formula (1) but different structure due to different positions of the same functional group, on the same carbon skeleton/chain

Question 30

define enthalpy change (1)

Answer 30

change in heat energy under constant pressure

Question 31

why does Tollen’s/ Fehlings work on aldehydes but not ketones?

Question 31

bond enthalpy formula?

Answer 31

sum of bond breaking - sum of bond formation

Question 32

if external temp increases, what type of reaction is it?

Answer 32

exothermic

Question 33

define mean bond enthalpy (2)

Answer 33

the enthalpy change needed to break covalent bonds averaged for that type of bond over a range of compounds

Question 33

methanoic acid formula?

Answer 33

HCOOH

Question 34

why do tollens/ fehlings work on aldehydes but not ketones?

Answer 34

bc aldehydes are readily oxidised, but ketones aren’t

Question 35

define position isomer (2)

Answer 35

impounds with the same molecular formula but different structures due to the different positions of the same functional group on the same skeleton/chain