8: Energetics 1

Question 1

Define enthalpy change (∆H)

Answer 1

Heat energy change in a reaction at constant pressure

kJ/mol

Question 2

What is ∆H^θ?

Answer 2

Enthalpy change under standard conditions

Also elements were in their standard states

Question 3

What are standard states?

Answer 3

Reacted in physical states under standard conditions

Question 4

What are standard conditions?

Answer 4

100 kPa of pressure

298K

Question 5

What is an exothermic reaction?

Answer 5

Reactions which give out heat energy

∆H is negative
Temp often increases

Question 6

What is an endothermic reaction?

Answer 6

Reactions that absorbs heat energy

∆H is positive

Temp often decreases

Question 7

What is an enthalpy level diagram?

Answer 7

Diagram which shows relative energies of reactants and products of a reaction

Does NOT show E_a or progress of reaction

Question 8

What is on the axis of an enthalpy level diagram?

Answer 8

y axis - enthalpy

no x axis

Question 9

What value is shown in an enthalpy level diagram?

Answer 9

∆H

This is the difference between the enthalpies of the two substances

Question 10

How does enthalpy relate to stability?

Answer 10

The less the enthalpy, the more stable

Question 11

How does the enthalpy of the products of an exothermic reaction compare to the reactants?

Answer 11

Products have a lower enthalpy than the reactants

Question 12

How does the enthalpy of the products of an endothermic reaction compare to the reactants?

Answer 12

Products have a higher enthalpy than the reactants

Question 13

What is a reaction profile diagram?

Answer 13

Diagram which shows how the enthalpy changes throughout the reaction

Question 14

What is shown in a reaction profile diagram?

Answer 14

Curve showing the enthalpy throughout the reaction

Activation energy

∆H

Question 15

What is the abbreviation for the activation energy?

Answer 15

E_a

Question 16

What are the relative positions of the products and reactants in a reaction profile diagram?

Answer 16

Products are higher than the reactants

Question 17

What on the axis in a reaction profile diagram?

Answer 17

x axis - progress of reaction

y axis - enthalpy (kJ/mol)

Question 18

How can enthalpy of a system be measured?

Answer 18

Cannot measure directly

This is irrelevant, enthalpy change is what matters

Question 19

How can you find enthalpy changes?

Answer 19

Experimentally or using data books

Question 20

What is to note about the enthalpy changes found in data books?

Answer 20

They are usually done under standard conditions

Question 21

Why is it important enthalpy changes are measured in standard conditions?

Answer 21

Changes in enthalpy are affected by temperature and pressure

Question 22

What is the standard enthalpy change of reaction ∆_rH^θ​?

Answer 22

Enthalpy change when the reaction occurs at the molar quantities shown in the chemical equation

Under standard conditions

Question 23

What is the standard enthalpy change of formation ∆_fH^θ?

Answer 23

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

Under standard conditions

Question 24

What is the standard enthalpy of combustion ∆_cH^θ?

Answer 24

Enthalpy change when 1 mole of a substance is completely burned in oxygen

Under standard conditions

Question 25

What is the standard enthalpy change of neutralisation ∆_neutH^θ?

Answer 25

Enthalpy change when an acid and alkali react together to form 1 mole of water Under standard conditions

Question 26

What is required when calculating enthalpy changes in experiments?

Answer 26

Number of moles of the reactants Change in temperature of the reaction

Question 27

When is the heat change the same as the enthalpy change?

Answer 27

Under constant pressure

Question 28

How can you experimentally measure temperature when you have solids and liquids?

Answer 28

Mix them and measure the temperature change using a thermometer

Question 29

What is the experimental procedure for determining enthalpy change when mixing two liquids?

Answer 29

Add set volume of liquid to a polysterene cup and put lid on with a hole for a thermometer Measure temp after 30s to stabilise Add known amount of solid/liquid Measure temp every 30s until reached max or min temp and has been returning to normal for a few minutes Draw graph of temperature against time Extrapolate line from where reaction is returning, back to original temp when added Read of temp when reaction starts, and calculate change in temp Then use to find enthalpy test

Question 30

What is done when the enthalpy of reactions involving gases is being experimentally calculated?

Answer 30

Calorimeter used to calcualte how much heat is given out by reaction Measures the temperature change of water around it

Question 31

Which reactions are usually involving gases for enthalpy calculations?

Answer 31

Combustion

Question 32

What is the set up for a calorimeter?

Answer 32

Spirit burner with known volume of fuel Underneath a container with a known volume of water present Container should be insulated and stirrer may be present Thermometer should be placed in the water

Question 33

How does combustion calorimtetry mean enthalpy can be measured?

Answer 33

Fuel burning heats the water Calculate heat absorbed by water USe to calculate enthalpy changes

Question 34

What is the equation for calculating the heat energy lost/gained?

Answer 34

q = mc∆T q - heat lost/gained (J) m - mass of water in calorimeter or solution in container (grams) c - specific heat capacity of water (4.18 J/g/K) ∆T - change in temperature of water (K)

Question 35

What is the specific heat capacity of water?

Answer 35

4.18 J g^-1 K^-1

Question 36

What is the specific heat capacity of water?

Answer 36

Amount of energy required to raise the temperature of 1 gram of water by 1 K

Question 37

Why do you not have to convert between ºC and K for ∆T?

Answer 37

The change is the same

Question 38

What can be done after calculating the heat energy gained/lost from q=mc∆T?

Answer 38

Find the moles of fuels which caused this change Divide heat energy (q) by the moles Value calculated is the enthalpy change of combustion

Question 39

What is Hess's Law?

Answer 39

Total enthalpy change of a reaction is always the same, no matter which route is taken

Question 40

When is Hess's law especially important?

Answer 40

Calculating enthalpy changes which cannot be measured directly by an experiment

Question 41

What is the ∆_fH^θ of elements?

Answer 41

Always 0

Question 42

Is breaking bonds endo or exothermic?

Answer 42

Endothermic as energy required to break bonds ∆H is +ve

Question 43

Is forming bonds endo or exothermic?

Answer 43

Exothermic as energy is released ∆H is -ve

Question 44

What is the ∆H of a reaction based on in terms of bonds?

Answer 44

Overall affect of energy required to break bonds and the energy released when new ones form

Question 45

What is bond enthalpy?

Answer 45

The energy required to break 1 mole of a type of bond in a molecule in the gas phase

Question 46

What keeps molecules together in ionic and covalent bonding?

Answer 46

Ionic - ions are electrostatically attracted to each other Covalent - nuclei are attracted to shared pair of electrons

Question 47

Are bond enthalpies always the same?

Answer 47

No Range of values depending which compound they are found in

Question 48

What is the mean bond enthalpy?

Answer 48

Energy needed to break 1 mole of bonds in the gas phase, averaged over many different compounds

Question 49

What values related to bond enthalpies are found in data books?

Answer 49

Mean bond enthalpies

Question 50

What is the ∆H of a reaction in terms of bonds?

Answer 50

Enthalpy change of reaction = Sum of bond enthalpies of reactants - Sum of bond enthalpies of products

Question 51

What occurs to the enthalpy change of combustion when you blow out the flame of the water?

Answer 51

Blows away unburnt fuel More evaporation so mass loss greater than true amount that has undergone combustion Calculated value for enthalpy of combistion will be less negative than true value