topic 8 - energetics I

Question 1

**

8.1 what are the standard conditions?

Answer 1

100kPa, 298k and 1 mol dm-3

Question 2

define chemical and heat energy

Answer 2

chemical energy - made up of 2 components
1. kinetic energy - measure of the motion of particles in a molecule
2. potential energy - measures how strongly these particles interact with each other

heat energy - the total KE + PE of a substance responsible for the temperature of a substance

Question 3

what does system mean in a chemical reaction?
give the formula for enthalpy change when given product and reactant values

Answer 3

atoms and bonds involved in the reaction
H products - H reactants

Question 4

8.2 know what enthalpy and enthalpy change is

Answer 4

enthalpy - total measure of energy in a system
enthalpy change - heat energy that is transferred from the systems to the surroundings at constant pressure (kj mol -1)

Question 5

8.3 define an exothermic reaction and draw an enthalpy level diagram + give the sign of the enthalpy change
- give examples of exothermic reactions and how it affects temperature

Answer 5

heat energy is transferred from the system to the surroundings (releases energy to make bonds)
* diagram on physical flashcard
* increases temperature
* - enthalpy change
* examples - combustion, burning, combustion + respiration

Question 6

8.3 define an endothermic reaction and draw an enthalpy level diagram + give the sign of the enthalpy change
* give examples of endothermic reactions and how it affects temperature

Answer 6

heat energy is transferred from the surroundings to the system (energy is taken in to break bonds)
* diagram on physical flashcard
* + enthalpy change
* examples - photosynthesis, melting ice and thermal decomposition

Question 7

8.4 define standard enthalpy change of
i. reaction
ii. formation (give eg)
iii. combustion
iii. neutralisation

Answer 7

i. the enthalpy change when the number of moles in a substance in the equation react as written in their standard states under standard conditions
ii. the enthalpy change when 1 mole of a substance is formed from it’s constituent elements with all reactants and products in their standard states under standard conditions
eg - H2 (g) + 1/2O2 (g) -> H2O (l)
iii. the enthalpy change 1 mole of a substance is completely burnt in oxygen with all reactants and products in their standard states under standard conditions
eg - C(s) + O2 (g) -> CO2 (g)
iiii. the enthalpy change when 1 mole of water is formed from the neutralisation reaction between an acid + alkali under standard conditions

Question 8

8.5.i understand experiments to measure enthalpy changes in terms of: processing results using the expression:
energy transferred = mass x specific heat capacity × temperature change
(Q=mcΔT)

Answer 8

Q = energy transferred (J)
m = mass of substance being heated (g)
C = SHC (4.18 j-1k-1)
T =change in temp (K)

Question 9

8.5.ii evaluating sources of error and assumptions made in the experiments
* students will need to consider experiments where substances are mixed in an insulated container and the temperature change is measure * * enthalpy of combustion is measured, such as using a series of alcohols in a
spirit burner
* the enthalpy change cannot be measured directly.

Answer 9

• heat loss - energy is lost to the surroundings
• not in standard conditions, approximation of the SHC and density being the same as water
• neglecting the SHC of the calorimeter
• incomplete reaction
  combustion
• incomplete reaction - ethanol may not burn completely soot on bottom)
• conditions may not be standard - water vapour instead of water

Question 10

how do we improve accuracies from calorimeter experiements?

Answer 10

• using a bomb calorimeter to ensure that the sample is completely burnt
• insulating the outsides of a calorimeter
• using a lid on the calorimeter

Question 11

8.6. be able to calculate enthalpy changes in kJ mol-1 from given experimental results

Answer 11

Q = mc△T
△H = Q/moles

Question 12

define hess’s law

Answer 12

the enthalpy change is independent of the route by which the chemical reaction takes place
* given initial and final conditions are the same

Question 13

8.7 be able to construct enthalpy cycles using Hess’s Law

Answer 13

on physical flashcard

Question 14

8.8 be able to calculate enthalpy changes from data using Hess’s Law

Answer 14

on physical flashcard

Question 15

8.9 . know what is meant by the terms ‘bond enthalpy’ and ‘mean bond enthalpy’

Answer 15

• enthalpy change when 1 mole of a covalent bond in the gaseous state is broken
• enthalpy change when 1 mole of a covalent bond, averaged out between many similar molecules is broken

Question 16

8.10 be able to calculate an enthalpy change of reaction using mean bond enthalpies
and explain the limitations of this method of calculation

Answer 16

△rH = sum of bond energy broken (reactants) - sum of bond energy made (products)
limitations -
△H values are not accurate as bond energy values slightly in different compounds (affected by neighbouring atoms)
everything is in the gaseous state, if water was liquid as a product, more heat would be involved as it includes heat vaporisation of water

Question 17

why would the standard enthalpy change of formation of an element be zero?

Answer 17

the element is in it’s standard state as a solid