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1 1 1 aaaaaaa a a a alevel chemistry me > Chapter 9 > Flashcards

Chapter 9 Flashcards

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1
Q

What is enthalpy meaning

A

H, a measure of heat energy in a chemical System

  • can’t be measured but change can
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2
Q

What formula for enthalpy change

A

Enthalpy PRODUCTS - REACTANTS

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3
Q

what is conservation of energy and how used

A

Energy cannot be created or destroyed

Energy transferred or gained by reactions is gained or transferred by surroundings here

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4
Q

Exothermic = what, endothermic = what

A

Exothermic = drop, energy transferred from reaction to surroundings, increase temp, negative change

Endothermic = increase of graph , energy transferred from surrounding ti reaction , temp decrease, positive

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5
Q

What is activation energy

A

The minimum amount of energy required forma reaction to take place

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6
Q

What is standard conditions and why used!

A

Chemists use a standard of conditions so that reactions can be compared, snd they are close to typical working conditions .

Denoted by the circle with line at top, means enthalpy change under standard conditions

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7
Q

What are 4 standard Codntioms and their values

A

1) pressure = 100kpa
2) temperature = 25°, 298k
3) concentration (for solutions) = 1moldm^-3
4) state = whatever the state is at at 100kpa and 298k?

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8
Q

What is the standard enthalpy change of reaction and what about moles?

A

Just the enthalpy change of a reaction in standard conditions
Triangle r H °

  • given by KJ/ mol, of a reaction . If balancing moles are multiplied by 2, then so is enthalpy change
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9
Q

What is the standard enthalpy change of FORMATION

What to look out for?

A

triangle f H°

The enthalpy change takes place when one MOLE of a COMPOUND is formed from its elements , UNDER STANDARD CONDITIONS

As a result of you making MGO, it has to be ONE MOLE of MgO, so you need enthalpy change where it gives this even if balancing numbers are in 1/2 or something

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10
Q

What is the standard enthalpy change of formation for just an element?

A

Making one mole of an element from it’d element required no change= so enthalpy for all elements , have an enthalpy change of formation of just 0

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11
Q

Standard enthalpy change of combustion ?

What is it and where is the one

A

Is the enthalpy change that takes place when ONE MOLE of a substance reacts completely with oxygen (no matter how many mol) under STANDARD CONDITIONS!

  • so 1 mole c4h10 with like 7 mol oxygen is correct , gives different mole product calm
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12
Q

Standard enthalpy change of neutralisation?

What to look out for?

A

Is the enthalpy change that accompanied the reaction of an acid by a base to form ONE MOLE OF H20 UNDER STANDARD CONDITIONS

1 has to be the water

As a result it is H+ + OH- —> H2O, -57kj / mol
Here Esch reaction will involve one h + and one oh-, so the standard enthalpy change of neutralisation will be THE SAME FOR ALL REACTIONS

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13
Q

Where does the one go for everything

1) formation
2) combustion
3) neutralisation

A

1) product compound
2) reactant reacting with oxygen
3) water produced

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14
Q

Kelvin conversion

A

0k = absolute 0 = -273° so 0°= 273k

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15
Q

How do we technically measure the temperature change of surroundings ?

A
  • thermometer is part of surroundings co change in temp here = change in temp
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16
Q

What do you need to calculate an ENEGRY CHANGE (equation )

A

Heat energy = q

Q= mcAt,
Mass in grams (resemble the small values )
Specific heat capacity in J/ g K
Change in temp in Kelvin

17
Q

What is the specific heat capacity

A

The amount of energy needed to raise one gram of a substance by 1 Kelvin

18
Q

How are calculating enthalpy changes if combustion etc carried out

What to remember to do !!

A

Basically say you burning methanol, you will use something like water to measure the energy lost, as energy gained is that, and the water psrt of surroundings

So measure change in temp of water , mass used , specific heat capacity and find energy change

Then see how much methanol used as mass ti find out how many moles used, then scale to find out for one mole of methanol as that is DEFINITION OF THING

2) divide energy be 1000 to get kJ
- at the end at a symbol

19
Q

Called calorimetry

How to do experiment?

A

Methanol in burner, raised water heat it , and measure changes in temp mass etc

As long as you keep experiment same (same fuel burnt, or same water ) then comparison valid

20
Q

What are 3 reasons why an experimental value may be different showing LESS EXOTHERMIC
What one more reason could change the value (but no necessarily exo)

A

Basically anything that reduces heat loss, or changes mass (as this changes moles) will give you a different value

1) heat loss was to the surroundings OTHER THAN WATER. (So beaker, and air) (less heat = less value)
2) incomplete combustion of methanol = gives less heat output than expected
3) evaporation of methanol from wick
- here this would show more mass so moles would be wrong. This happens because as soon done needs to be measured, but some may evaporate before cap put on (more mass = more moles = less value )

4) NOT STANDARD CONDTIONS , book value is standard but unlikely to be exactly standard here, this can give both HIGHER OR LOWER!!!

21
Q

How can you reduce errors in combustion experiment? (Stopping two things from happening)

A

1) reduce heat loss = cover with lid and hole, insulate as much as you can + draught screens like trays to stop breezes
2) reduce incomplete combustion = use oxygen gas

carry out reaction in polystyrene cup, temperature hole, lid , insulated gel and cover

22
Q

What’s going on when doing reaction with two solutions?

A

The actual solution is the immediate surroundings, so measure temperature her, with polystyrene cup etc, so energy transfer is between chemicals in water in solution

You using the amount of solution in mass based on density to find hest change and moles of same solution if they give conc

23
Q

How to account for heat loss?

A

1) measure known volume with Pipette of standard conc in a cup, and wxcess powder
2) measure just volume every 30s until constant
3) then add zinc and continue to measure temp until it drops for a few kinures
4’ draw gradient and extrapolate to find estimated final temperature

24
Q

What is the downside to extrapolation method (2) , why do we use it anyways?

A

1) in real life we can’t assume that the reaction started straight away
2) also can’t assume rate of cooling is linear

  • however still good as it inaccuracy to heat loss to other surroundings could just be worse , without this we could never achieve a value close to the actual heat produced straight away
25
Q

What to use for mass when both are solutions in neutralisation?

A

The entire both of the solutions added, as the entire solution raised by a temp not just one!

26
Q

What is the AVERAGE bond enthalpy

Why is it always endothermic?

A

The energy required to break ONE MOLE of a specified type of bond in gaseous state

Bond breaking is endothermic, as energy is supplied to it

27
Q

What is the limitations of bond enthalpies?

A

1) a value for C-H bond in one molecule will be slightly different in another and another.
Thus an AVERAGE IS TAKEN . However this means value never truly accurate unless real used

2) can only be used for gas stated, not physical or liquid
- this means if you calculate for gas of water it is not standard condition and thus not standard enthalpy, so need to do more working

28
Q

Bond making breaking

A

Energy needed to break bonds = endothermic

Energy released when bonds are formed = exothermic

29
Q

WHAT IS ENTHALPY CHANGE DIFFERENCE IN BONDS

A

Energy in REACTANTS - PRODUCTS !

30
Q

One more time enthalpy change equation and bond enthalpy one

LEARN

A

Enthalpy change = products- reactants (that’s energy inside the )
Bind is total energy gained from bonds in REACTANTS - total energy released by PRODUCT

31
Q

What is meant by average bond enthalpy

A

Average bond enthalpy when one mol of gaseous COVALENT bonds are broken!

32
Q

HOW TO CALCULATE REVERSE ACTIVTAITON ENERGY

A

Add the enthalpy change too! , or subtract

33
Q

Outline stages for a catalyst to work for co and no (3)

A

Co and no are both gas so ABSORBED onto catalyst surface
Chemical Reaction takes place such that products are made
These DESORB from surface

34
Q

Why is it hard to measure the enthalpy change of something directly? (3)

A

Because

  • activation energy too high
  • rate of reaction to slow
  • or some random oxides compounds etc are made !
35
Q

Why doesn’t a reaction go to equilibrium ?

A

Because it not in closed system!

36
Q

Thermal decomposition of calcium carbiante?

A

CaO and co2 fool

37
Q

What type of reaction is one that makes a solid

A

Precipitation

38
Q

Why would heating for longer lesd to a more or less accurate result

A

More = temperature change more, so less uncertainty, same for mass of ethanol burnt

Less = more heat energy loss to surroundings

1 1 1 aaaaaaa a a a alevel chemistry me (43 decks)

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