3.2.1 Enthalpy Changes

Question 1

what is the symbol for enthalpy?

Answer 1

H

Question 2

What is enthalpy a measure of?

Answer 2

the heat energy in a chemical system

thought of as the energy stored within bonds.

Question 3

How do you find enthalpy changes?

Answer 3

enthalpy of products - enthalpy of reactants

Question 4

law of conservation of energy

Answer 4

energy cannot be created or destroyed only transferred

Question 5

if energy is transferred from the system (chemicals) to the surroundings (thermometer, apparatus etc), the reaction is…

Answer 5

exothermic

Question 6

if energy is transferred from the surroundings to the system, the reaction is…

Answer 6

endothermic

Question 7

what is an enthalpy profile diagram?

Answer 7

a diagram for a reaction comparing enthalpy of the reactants with the products

Question 8

if the products have a higher enthalpy than reactants, the reaction is…

Answer 8

endothermic

Question 9

if reactants have a higher enthalpy than the products, the reaction is …

Answer 9

exothermic
the system releases energy into the surroundings and this causes a rise in temperature

Question 10

is enthalpy change of an exothermic reaction positive or negative?

Answer 10

negative because the system releases energy into the surroundings so the products have less energy than the reactants during the progress of the reaction.

Question 11

is enthalpy change of an endothermic reaction positive or negative and why?

Answer 11

positive because energy is transferred from the surroundings and gained by the chemical system during the progress of the reaction so the products have a higher enthalpy compared to the reactants and temperature decreases

Question 12

what happens to temperature during an endothermic reaction?

Answer 12

decreases because energy is lost by surroundings and gained by system. The thermometer is part of the surroundings.

Question 13

activation energy

Answer 13

the minimum energy required for a reaction to take place

Question 14

draw a full enthalpy profile diagram for an exothermic reaction including activation energy and enthalpy change labels

Question 15

draw a full enthalpy profile diagram for an endothermic reaction including activation energy and enthalpy change labels

Question 16

what is standard enthalpy change?

Answer 16

enthalpy change of a reaction under standard conditions

Question 17

What sign means standard conditions used?

Answer 17

(⦵)

Question 18

what are the units for standard enthalpy changes?

Answer 18

kJ/mol-1

Question 19

standard pressure

Answer 19

100kPa

Question 20

standard concentration

Answer 20

1 moldm-3 (relevant for solutions only)

Question 21

standard temperature

Answer 21

298K (25℃)

Question 22

standard state

Answer 22

the physical state of a substance under standard conditions

Question 23

Standard enthalpy change of reaction (∆rH⦵)

Answer 23

the enthalpy change of a reaction in the molar quantities shown in the chemical equation under standard conditions with all reactants and products under standard conditions

Question 24

why can the standard enthalpy change of a reaction change?

Answer 24

the enthalpy change refers to a stated equation, if the balancing numbers change, eg making the balancing numbers whole by multiplying by 2, the enthalpy change will also change and multiply by 2.

Question 25

standard enthalpy change of formation ΔfH⦵

Answer 25

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

Question 26

When would you not balance the equation using whole numbers when finding the standard enthalpy of formation?

Answer 26

when more than one mole of the compound is formed because the definition for the enthalpy would not match

Question 27

what is the standard enthalpy of formation of elements ?

Answer 27

0 kJ/mol-1 forming one mole of an element from its element has no change.

Question 28

Standard enthalpy change of combustion ΔcH⦵

Answer 28

enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions with all reactants and products in their standard states

Question 29

standard enthalpy change of neutralisation ΔneutH⦵

Answer 29

enthalpy change when an acid and base react to form one mole of water under standard conditions with all reactants and products in their standard states

Question 30

why is the standard enthalpy of neutralisation value the same for all neutralisation reactions?

Answer 30

all neutralisation reaction involve H+ and OH- ions reacting to form the same one mole of water

Question 31

what is the equation used to find energy/enthalpy change?

Answer 31

q = mcΔT

Question 32

what does q stand for in the energy change equation? what are the units?

Answer 32

heat energy Joules (J)

Question 33

what does ΔT stand for in energy change equation?

Answer 33

change in temperature measured in Kelvin (K)

Question 34

how do you convert from kelvin to degrees

Answer 34

- 273

Question 35

specific heat capacity

Answer 35

the amount of energy required to raise the temperature of 1g of a substance by 1k

Question 36

what is the c of water?

Answer 36

4.18 J g-1 K-1

Question 37

What mass do you measure when finding energy change?

Answer 37

the mass of the material thats changing temperature - where the thermometer is

Question 38

How do you convert from J to kJ?

Answer 38

divide by 1000

Question 39

How can you determine the enthalpy change of combustion experimentally?

Answer 39

burn a liquid fuel such as methanol with a spirit burner and use this to heat a beaker of water use this to find change in temperature of water and mass of fuel burnt. use energy change equation and find kJ/mol-1

Question 40

why might the data book values for enthalpy of combustion be different to the value obtained experimentally?

Answer 40

- heat loss to surrounding other than water - incomplete combustion of methanol - non standard conditions - evaporation of methanol from the wick

Question 41

How can you find enthalpy change of a reaction experimentally ?

Answer 41

use polystyrene cups to carry out reaction as they a re waterproof, offer some insulation and use a thermometer to record temperature change

Question 42

average bond enthalpy

Answer 42

energy required to break one mole of bonds in a gaseous molecule

Question 43

why are bond enthalpies always positive ?

Answer 43

Bendomex - breaking bonds requires energy so is an endothermic process

Question 44

BENDOMEX

Answer 44

breaking bonds is endothermic making bonds is exothermic

Question 45

when the energy needed to break bonds is higher than the energy releases to make bonds, the reaction is...

Answer 45

endothermic

Question 46

when the energy releases from making bonds is higher than the energy needed to break bonds, the reaction is...

Answer 46

exothermic

Question 47

equation to find enthalpy change of a reaction from bond enthalpies

Answer 47

total bond enthalpies of reactants - total bond enthalpies of products = enthalpy change of reaction

Question 48

what are the limitations to using average bond enthalpies?

Answer 48

- the enthalpy of a bond may be different in different conditions - not a standard enthalpy as average bond enthalpy needs all species in gaseous form

Question 49

Hess' Law

Answer 49

is a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route

Question 50

How does Hess' Law help to find enthalpy change?

Answer 50

can find enthalpy change indirectly through a different route

Question 51

enthalpy cycle

Answer 51

diagram showing different routes between reactants and products to indirectly find enthalpy change from other known enthalpy changes using Hess' Law

Question 52

draw an enthalpy cycle for enthalpy of formation, linking reactants and products with arrows

Question 53

draw an enthalpy cycle for enthalpy of combustion linking reactants and products with arrows