Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

AS CHEMISTRY > ENERGETICS > Flashcards

ENERGETICS Flashcards

You may prefer our related Brainscape-certified flashcards:
Periodic Table flashcards Chemistry 101 flashcards AP® Chemistry flashcards Organic Chemistry 101 flashcards
1
Q

Define enthalpy change

A

The amount of heat energy taken in it given out during any change in a system provided the pressure is constant

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

Outline an exothermic reaction

A

Energy is transferred from system to surroundings

Products have less energy than reactants

Enthalpy change is negative

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

What are common exothermic processes

A

Combustion of fuels

Oxidation of carbohydrates
Eg glucose in respiration

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

Outline an endothermic reaction

A

Energy is transferred from surroundings to the system

Products have more energy than the reactants

Enthalpy change is positive

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

Define standard enthalpy change of formation

A

The enthalpy change when 1 mole of the compound
is formed from its elements under standard conditions
all reactants and products being in their standard states

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

What is the enthalpy of formation for an element?

A

0 kJ mol^-1

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

Define standard enthalpy change of combustion

A

The enthalpy change when 1 mole of a substance
Is combusted completely in oxygen under standard conditions
All reactants and products being in their standard states

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

How does incomplete combustion affect a reaction?

A

Soot (carbon)
Carbon monoxide
Water

Less exothermic than complete combustion

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

What are the standard conditions?

A

100 kPa pressure
298 K (25C)
Solutions at 1mol dm^-3
Standard states

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

What equation is used when calculating enthalpy change for a reaction in a solution?

A

Q = m x c x 🔼T

Energy change = mass of solution x heat capacity x temperature change

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

Outline the general calorimetric method

A
  • wash the equipment with the solutions and dry
  • place cup in beaker for insulation and support and clamp thermometer into place making sure bulb is immersed in liquid
  • measure initial temperatures
  • transfer reagents to the cup
  • stir mixture
  • measure final temperature
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What are the issues of calorimetry if the reaction is slow?

A

exact temperature rise is difficult to measure (cooling simultaneously)

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

How can issues with slow reactions be countered in calorimetry

A

Take readings at regular time intervals and extrapolate back to the time the reactants were added together

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

Name 3 errors that occur in calorimetry

A
  • Heat transfers to surroundings
  • Approximation in specific heat capacity of solution (assumes all solutions have heat capacity of water)
  • Neglects specific heat capacity of calorimeter
  • Reaction or dissolving may be incomplete or slow
  • Density of solution is taken to be the same as water
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Outline the general method for calculating enthalpy change from experimental data

A
  1. Q = MC🔼T
  2. Work out miles of reactants used
  3. Divide q by number of miles of the reactants bit in excess
  4. Add sign and unit (/1000 for Jmol-1 to kJmol-1)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Name three errors that occur when calculating enthalpy of combustion from experimental data

A
  • heat losses from calorimeter
  • incomplete combustion of fuel
  • incomplete transfer of heat
  • evaporation of fuel after weighing
  • heat capacity of calorimeter not included
  • measurements not carried out under standard conditions
17
Q

Outline hess’s law

A

The total enthalpy change for a reaction is independent of the route by which the chemical change takes place

18
Q

Outline the equation for calculating enthalpy change

A

🔼H = reactants - products

19
Q

Define mean bond energy

A

The enthalpy needed to break the covalent bond into gaseous atoms averaged over different molecules

20
Q

Is the value of mean bond energy negative or positive? Why?

A

Positive

Energy is required to break a bond

21
Q

When does the mean bond enthalpy definition no longer apply?

A

When either the products or/and reactants aren’t in a gaseous state

22
Q

How does mean bond energy differ between exothermic and endothermic reactions

A

The sum of bonds in reactants is less than that of products in an exothermic reaction

23
Q

Why aren’t bond energy calculations exact

A

The mean bond energies are taken from different molecules and averaged

So the mean bond energy is not exact

24
Q

What is the bond enthalpy calculation

A

🔼H = bond energies broken - bond energies made

25
Q

Explain the change in enthalpy of combustion in a homologous series

A

Constant rise in size of enthalpy

Number of carbon atoms increase

So there are more bonds broken and more bonds made

26
Q

How will experimental enthalpy of combustion results differ from calculated results in a homologous series?

A

Experimental will be much lower

Significant heat loss
Incomplete combustion (less energy released)

AS CHEMISTRY (17 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions