Chapter 9: Enthalpy (9.1-9.4) Flashcards

1
Q

exothermic reactions

A
  • products have less enthalpy than the reactants (negative ΔH)
  • chemicals lose energy, the surroundings gain this energy (hence increase in temp)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

endothermic reactions

A
  • products have more enthalpy than the reactants (positive ΔH)
  • the surroundings lose energy (hence decrease in temp) which goes to the chemicals
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

standard enthalpy changes are measured using the standard conditions.
what are those conditions?

A

1 atmosphere pressure / 100 kPa
room temperature of 25 degrees / 298K
solutions must have a concentration of 1mol/dm3
all substances in their standard states (physical state under standard conditions)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

equation for the heat energy change

A

q = m X c X ΔT

q: heat energy change in joules
m: mass of surroundings (the thing that you measure the temp change of) in grams
ΔT: temperature change

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

unit for ΔH

A

kJ/mol

make sure you work it out per mole!

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

common errors when determining enthalpy change of combustion (experimentally) and how to minimise them

A

errors: heat loss to surroundings, incomplete combustion of reactant in spirit burner, evaporation of reactant from wick, non-standard conditions being used (heated up during combustion)
solutions: adding a lid to water beaker, using draft shields around apparatus, insulating system

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

breaking bonds

A

endothermic (positive ΔH)

energy needed from the surroundings to break the bonds

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

making bonds

A

exothermic (negative ΔH)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

how to work out the enthalpy change of a reaction from average bond enthalpies - gaseous molecules of covalent substances

A

ΔrH = ∑(bond enthalpies of reactants) - ∑(bond enthalpies of products)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

how activation energy effects reaction rate

A

small activation energy: rapid (energy needed to break bonds is readily available from surroundings)
Higher activation energies: slow (large energy barrier that reactions happen very slowly or not at all)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

what is average bond enthalpy

A

the mean amount of energy required to break 1 mole of a specified type of covalent bond in a gaseous molecule

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

limitations of average bond enthalpies

A

The bond is in a different environment

Actual value for a bond in a certain molecule may be slightly higher/lower.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

state Hess’ law

A

Hess’ law states that, if a reaction can take place by more than one route, and the starting and finishing conditions are the same, the total enthalpy change is the same for each routes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Enthalpy change of neutralisation

A

The standard enthalpy change of neutralisation is the enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Enthalpy change of formation

A

the enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states under standard conditions

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Enthalpy change of combustion

A

the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard condition, all reactants and products being in their standard states.

17
Q

what is Hess’ law used for?

A

to calculate enthalpy changes that are not easy to measure directly in experiments

18
Q

Working out enthalpy change (e.g delta h in diagrams)

A

H(products) - H(reactants)