3.1.4 Energetics

Question 1

Define Enthalpy

Answer 1

Heat content in a substance

Question 2

Define Enthalpy Change (ΔH)

Answer 2

Change in heat energy (of a reaction) at a constant pressure

Question 3

State the units for enthalpy change (ΔH)

Answer 3

kJ mol^-1

Question 4

What is meant by this symbol ⦵?

Answer 4

Means substances were in their standard states and measurement was under standard conditions

Question 5

Name the standard conditions

Answer 5

• 1 atm (100 kPa)
• 298 K

Question 6

Exothermic: ΔH is ______

Answer 6

negative

Question 7

Endothermic: ΔH is ______

Answer 7

positive

Question 8

Give 2 examples of exothermic reactions

Answer 8

• Neutralisation
• Combustion

Question 9

Give 2 examples of endothermic reactions

Answer 9

• Thermal decomposition
• Photosynthesis

Question 10

Sketch a graph to show an exothermic reaction

Answer 10

Question 11

Sketch a graph to show an endothermic reaction

Answer 11

Question 12

Why is bond breaking endothermic (ΔH = positive)?

Answer 12

Need energy to break bonds

Question 13

Why is bond making exothermic (ΔH = negative)?

Answer 13

Energy released when bonds formed

Question 14

If more energy is needed to break bonds than is released when bonds made, ΔH is _____

Answer 14

positive

(If it’s less = ΔH is negative)

Question 15

Define standard enthalpy of formation (Δ_fH^⦵)

Answer 15

Is the enthalpy change when 1 mole of a compound is formed from its constituent elements under standard states & standard condition

Question 16

Define standard enthalpy of combustion (Δ_cH^⦵)

Answer 16

Is the enthalpy change when 1 mole of a substance is burnt completely in oxygen, under standards states & standard conditions

Question 17

Standard enthalpy of formation equation

K₂Cr₂O_7(l)

Answer 17

2K_(s) + 2Cr_(s) + 7/2O_2(g) → K₂Cr₂O_7(l)

Question 18

Standard enthalpy of combustion equation

CH_4(g)

Answer 18

CH_4(g) + 2O_2(g) → CO_2(g) + 2H₂O_(l)

Question 19

How do we use calorimetry to find how much heat is given by reaction?

Answer 19

By measuring temperature change

Question 20

Describe how you can find the enthalpy of combustion of a flammable liquid using calorimetry

Answer 20

1. Burn flammable liquid inside apparatus (e.g. calorimeter)
2. Then can work out heat energy that’s been absorbed by water (heat given by fuel as it burns = be absorbed by water)

Question 21

Why is it hard to get an accurate result when using calorimetry? Name 3 reasons

Answer 21

• Heat is always lost to the surroundings
• Combustion may be incomplete (less energy given out)
• Flammable liquids often volatile = lose some fuel to evaporation

Question 22

Describe how you can use calorimetry to calculate the enthalpy change for a neutralisation reaction

Answer 22

1. Add known volume of acid to insulated container (e.g. polystyrene cup) & measure temperature
2. Add known volume of alkali and record temp. of mixture at regular intervals over period of time
   
   1. Stir solution to make sure it’s evenly heated
3. Find temp change of experiment
   
   1. Use it to calculate enthalpy change of reaction

Question 23

Calorimetry

Describe how you can use a graph to find an accurate temperature change

Answer 23

1. During experiment, record temp at regular intervals
   
   1. Start a couple mins before reaction
2. Plot graph of results
3. Draw 2 lines of best fit
   
   1. 1 through points before reaction started
   2. 1 through points after it started
4. Extend both lies so they both pass time when reaction started
5. Distance between 2 lines at time reaction (before any heat lost) = accurate temp change (ΔT) for reaction

Question 24

State what equation (& the units) you can use to calculate the enthaply change in calorimetry

Answer 24

Question 25

State the equation used to calculate heat energy given out per mole

Answer 25

ΔH = q / moles of reaction

Question 26

Define Hess's Law

Answer 26

Total enthalpy change of a reaction is independent of the route taken

Question 27

What is Hess's law used for?

Answer 27

Working out enthaply changes that you can't find directly by doing an experiment

Question 28

When working out enthalpies changes of formation, what data do you need?

Answer 28

Δ_fH^⦵ for all reactants and products that are compounds

Question 29

Value of Δ_fH^⦵ for all elements = \_\_\_\_

Answer 29

0 (∵ element's being 'formed' from element = no change)

Question 30

Enthalpies Changes of Formation

Answer 30

Question 31

Enthalpies Changes of Combustion 2C_(g) + 3H_2(g) + 1/2O_2(g) → C₂H₅OH_(I)

Answer 31

Question 32

What is bond enthalpy?

Answer 32

Energy required to break bonds

Question 33

Define mean bond enthalpy

Answer 33

Enthalpy heat (energy) change required to break a covalent bond on average across different compounds

Question 34

Draw the bonds between CO₂

Answer 34

Question 35

Draw the bonds between NH₃

Answer 35

Question 36

Draw the bonds between water

Answer 36

Question 37

Draw the bonds between oxygen

Answer 37

Question 38

Draw the bonds between nitrogen

Answer 38

Question 39

Mean Bond Enthalpies What formula do you use to calculate enthalpy changes?

Answer 39

Question 40

Why are enthaply change values calculated from mean bond enthalpies less accurate than using Hess's Law?

Answer 40

∵ use average values, enthalpy changes calculated using mean bond enthalpies aren't exact

Question 41

Answer 41

142 kJ mol^-1

Question 42

**Using calorimetry to find the enthalpy of combustion of a flammable liquid** Suggest 3 general improvements you can do to reduce errors due to heat loss

Answer 42

* _Put a draught shield around the apparatus_ * _Put a lid on the cup_ * Reduce the distance between the flame and the beaker * Use a copper calorimeter rather than a pyrex beaker

Question 43

Explain why the bond enthalpy of a Cl-Cl bond is greater than that of a Br-Br bond (2)

Answer 43

* Bond is shorter or bonding pair closer to nucleus * (Cl is a smaller atom) * So attraction (between nucleus and) (to) bond pair is stronger