Week 8

Question 1

System

Answer 1

the part of the universe we are focused on
* chemical reaction

Question 2

exothermic

Answer 2

Release heat
* Products have lower internal energy (U) than reactants
* Change is less than 0

Question 3

endothermic

Answer 3

Require heat to be supplied
* Products have greater internal energy than reactants
* Change is greater than 0

Question 4

Work (w)

Answer 4

Work done to a system
* Expansion and compression of gas
* Light
* Electrical

Question 5

First Law of Thermodynamics:

Answer 5

The change in internal energy of a chemical system is the sum of heat absorbed (q) by and the work (w) done on the system
* U(int) = q + w

Question 6

Work Arising from Expansion or Compression of V

Answer 6

One common form of work in systems in contact with the atmosphere arises when the system changes it volume V, either
* Expanding V > 0
* Compressing V < 0

volume change ∆V in contact with a pressure P is then given by:
* w= -P x V

Question 7

For a reaction involving a heat change and a change in the volume of gas:

Answer 7

ΔUint = q – PΔV

Question 8

Enthalpy of reaction (ΔH)

Answer 8

The heat of the reaction at constant pressure:
* ΔH = qP = ΔUint + PΔV
* Exo: ΔH is negative
* Endo: ΔH is positive

Question 9

Enthalpy of fusion (‘melting’)

Answer 9

H2O (s) (0°) → H2O (l) (0°)
* ΔfusH = +6 kJ mol-1

Question 10

Enthalpy of combustion

Answer 10

completely burning 1 mol of fuel:
* C4H10(g) + 6.5O2(g) → 4CO2(g) + 5H20(l)
* ΔcombH = -2659 kJ mol-1

Question 11

Enthalpy of atomisation

Answer 11

converting 1 mol of a compound into gaseous atoms
* C4H10(g) → 4C(g) +10H(g)
* ΔatomH = +5544 kJ mol-1
* Energy required to reduce a molecule to its atoms is the energy required to break all the bonds

Question 12

ΔatomH is always

Answer 12

greater than 0

Question 13

Hess’s Law

Answer 13

If you add up chemical equations to form a new (overall) equation, then the overall enthalpy is the sum of the enthalpies.
Total energy gained or lost depends only on the initial and final states, regardless of the number of steps or the path of the steps

Question 14

For molecules to react…

Answer 14

They must:
* Collide
* Be correctly orientated
* Have sufficient energy to react

Question 15

For molecules to react…

Answer 15

They must:
* Collide
* Be correctly orientated
* Have sufficient energy to react

Question 16

Activation energy (Ea)

Answer 16

The minimum energy that molecules must have to react

Question 17

Transition state

Answer 17

• Where the reaction occurs
• Rate determining step
• Collision

Question 18

Standard conditions

Answer 18

Pressure = 1 atm, temperature = 298 K, concentration = 1 mol L-1

Question 19

Enthalpy of formation

Answer 19

the enthalpy change when it is made from the stable form of its elements at standard conditions

Question 20

Rating fuels on volume

Answer 20

• Gases have low densities: harder to transport
• Liquids have quite high density and can be used in engines
• Solids have high densities but can’t be used in internal combustion engines

Question 21

Rating fuels on greenhouse gas production

Answer 21

• Unhydrogenated fuels lose out on energy from formation of H2O and so less energy per mole of CO2
• Natural gas is most efficient hydrocarbon per CO2 generated
• Ethanol is not cleaner than petrol but is renewable