Energetics Flashcards
1
Q
- Define enthalpy change
A
- Enthalpy change is the amount of heat energy taken in or given out during any change in a system provided the pressure is constant
2
Q
- What is an exothermic change?
A
- Energy is transferred from the system (chemicals) to the surroundings
3
Q
- What is an endothermic change?
A
- energy is transferred from the surroundings to the system (chemicals).
4
Q
- Define the standard enthalpy change of formation
A
- enthalpy change when 1 mole of the compound is formed from its elements under standard conditions (298K and 100kpa), all reactants and products being in their standard states
5
Q
- Define the standard enthalpy change of combustion
A
- enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions. (298K and 100kPa), all reactants and products being in their standard states.
6
Q
- State 4 factors which constitute standard conditions
A
- 100 kPa pressure; 298 K; Solutions at 1mol dm-3 ; all substances should have their normal state at 298
7
Q
- What does incomplete combustion produce?
A
- Soot (carbon), carbon monoxide and water.
8
Q
- What is ∆H for exothermic reactions?
A
- Negative
9
Q
- What is ∆H for endothermic reactions?
A
- Positive
10
Q
- What’s the equation for working out enthalpy change experimentally?
A
- energy change = mass of solution x heat capacity x temperature change
11
Q
- What’s the unit of specific heat capacity?
A
- J g-1K-1
12
Q
- What’s the units of temperature change?
A
- K
13
Q
- With calorimetry, why is a polystyrene cup placed into a beaker?
A
- Insulation and support
14
Q
- What do we do in calorimetry if it is difficult to obtain the final temperature?
A
- Take readings at regular time intervals and extrapolate the temperature curve/line back to the time the reactants were added together
15
Q
- What are 4 errors in using the polystyrene cup method of calorimetry to measure the enthalpy change?
A
- Heat transfer from surroundings (usually loss); The method assumes all solutions have the heat capacity of water; neglecting the specific heat capacity of the calorimeter- we ignore any heat absorbed by the apparatus; reaction or dissolving may be incomplete or slow; Density of solution is taken to be the same as water
16
Q
- What’s a simple method using calorimetry to measure the enthalpy of combustion?
A
- fuel is burnt and the flame is used to heat up water in a metal cup. H……..(NOT FINISHED)
17
Q
- How do you convert from J mol-1 to KJ mol-1?
A
- Divide by 1000
18
Q
- What does Hess’ Law state?
A
- The total enthalpy change for a reaction is independent of the route by which the chemical change takes place
19
Q
- What is the first law of thermodynamics?
A
- Energy is always conserved
20
Q
- What is the equation using Hess’ law to determine the enthalpy of combustion?
A
- ∑∆H c reactants- ∑∆Hc products
21
Q
- What is the equation using Hess’ law to determine the enthalpy of formation?
A
- ∑∆Hf products- ∑∆Hf reactants
22
Q
- Define the mean bond enthalpy
A
- enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules
23
Q
- What are the units of mean bond enthalpy?
A
- kJ mol-1
24
Q
- What’s the equation for working out the mean bond enthalpy?
A
- ∆H = Σ bond energies broken - Σ bond energies made
25
25. Draw the bonds which make up carbon dioxide i.e. as a display formula
25. O = C = O
26
26. Draw the bonds which make up water i.e. as a display formula
O
/ \
H H
27
27. Draw the bonds which make up oxygen i.e. as a display formula
O = O
28
28. How many bonds make up ethanol?
8
29
29. How many bonds make up propan-1-ol?
11
30
30. How many bonds which make up butan-1-ol?
14
