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AS CHEMISTRY > 2.Energy changes and fuels > Flashcards

2.Energy changes and fuels Flashcards

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1
Q

What is enthalpy?

A

the heat content that is stored in a chemical system.

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2
Q

Endotheric-

A

+ve delta H

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3
Q

Exothermic-

A

-ve delta H

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4
Q

What are self heating cans?

A

a can which contains a chamber where an exothermic reaction can happen.

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5
Q

What are self cooling beer cans.

A

water evaporates to use heat energy.

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6
Q

Exothermic reaction example?

A

Combustion respiration.

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7
Q

Endotheric reaction example?

A

thermal decomposition of limestone ( CaCO3) photosynthisis

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8
Q

What is a enthalpy profile diagram?

A

A diagram for a reaction to compare the enthalph of the reactants with the enthalpy of the products. Shows Ea, and Delta H .

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9
Q

Define activation energy?

A

Ae is the minimum energy required to start a reaction by the breaking of bonds.

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10
Q

What are standard condions.

A

25 dec c(298K), 1atm, 1 moldm-3

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11
Q

Define the standard enthalpy change of reaction.

A

The enthalpy change that accompanies a reaction in the molar quantities(the number of moles of reactants as stated), expressed in a chemical equation under standard conditions, with reactants and products in their standard states.

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12
Q

Define the standard enthalpy change of combustion.

A

is the enthalpy change when one mole of a substance is completely reacted with oxygen under standard conditions, and all the products in their standard states

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13
Q

Define the standard enthalpy change of formation

A

The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.

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14
Q

Define 1st Ionisation energy

A

the required for 1 e- to be removed from every atom in 1 mole of gaseous atoms.

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15
Q

define mean bond enthalpy.

A

The average amount of energy needed to break 1 mole of a specific bond in the gaseous state.

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16
Q

Why might mean bond enthalpy be incorrect.

A

as its an average not for that specific molecule.

17
Q

what is specfic heat capacity?

A

the energy required to raise the temp of 1g of a substance by 1 deg C

18
Q

Q(heat exchange)=

A

m cp deltaT

19
Q

How is enthalphy change calculated?

A

-Q(in KJ)/mol

20
Q

What is the mass used in calorometry?

A

the mass of the thing being heated- usually water

21
Q

How do you find delta Hc

A

Burn a know mass of substance in air. measure the volume of water being heated. measure the waters temp change.

22
Q

Why might measure enthalpy of combustion be different?

A

Incomplete combustion? Heat loss to surroundings. heat lost to calorometer. mass of fuel lost as vapour

23
Q

Why might measure enthalpy of formation/reaction be different?

A

heat lost to surroundings heat lost to calorimeter heat lost to solid reactants.

24
Q

whats the relationship between carbon atoms in a alcohol and enthalph of combustion.

A

more c atoms = higher enthalpy of combustion.

25
Q

ENthalpy change using bond enthalpies=

A

Energy released when broken - energy requred to form product bonds.

26
Q

What is hess’ law?

A

if a reaction can take place by more then one route and the initial and final conditions are the same, the total enthalpy change is the same for each route.

27
Q

Why isnt it always possible to directly measure enthalphy change?

A

More then one reaction takes place a high Ae a very slow rate.

28
Q

Work our enthalpy change of reaction from their combustion energies.

A

construct a hess cycles/diagram. work out the reaction routes. (energy to combust left- energy to combust right = enthaply of reaction)

29
Q

Work our enthalpy change of reaction from their formation energies.

A

construct a hess cycles/diagram. work out the reaction routes. (energy to form right- energy to form left right = enthaply of reaction)

30
Q

Two hese cycles for formation and combustion (not andys way)

A
31
Q

Standarf for for anys hess cycle.

A

Elecments

fuel XCO2+yH2+O2

Xco2+yH20

32
Q

Why are some cyclic thigns more exothermic

A

the ring structure has been squashed from 109.5 to 60 in a tri, 90 in a but, etc, this requires energy which is released when its burnt.

33
Q

Why does enthalpy of combustionn increase linearly for homologeos series.

A

a regualr amour more of the same bond is being broken each increase= more energy released.

34
Q

What is a procedural error?

A

a fault with the method

eg incomplete combustion, non standard condtitions, cp of equipment ignored,head lost to environment…

AS CHEMISTRY (17 decks)

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