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Y4 Chem > Energy from chemicals > Flashcards

Energy from chemicals Flashcards

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1
Q

Energy law

A

Cannot be created or destroyed but it can be transformed from one form to another

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2
Q

Phase transition

A

change in state

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3
Q

Why steam burn worse than boiling water burn

A

The condensation of a vapour to a liquid is an exothermic process, and energy flows out of the system to the surroundings

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4
Q

What is the enthalpy change for endo and exo reactions (+/-)

A

Exo: < 0
Endo: > 0
change in H = H (products) - H (reactants)

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5
Q

Definition of exothermic change

A

A change in which heat is given out to the surroundings

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6
Q

Does temperature of reaction mixture for exo reactions increase/ decrease/ remain the same?

A

increase as heat is given out to the surroundings

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7
Q

what happens to the temperature of the reaction mixture after a while (exo)

A

it increases till the highest temperature is reached
when the reaction is complete, the temperature of the reaction mixture decreases till it reaches room temperature

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8
Q

What is a energy level diagram

A

👎

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9
Q

What are exothermic reactions

A
  1. condensation
  2. freezing
  3. dissolving an anhydrous salt in water
  4. dissolving an acid in water
  5. combustion of fuels (addition of O2)
  6. respiration
  7. neutralisation
  8. corrosion of metals (eg. rusting)
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10
Q

Definition of endothermic change

A

is a change in which heat is absorbed from the surroundings

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11
Q

Temperature of the reaction mixture for endothermic change

A

the temperature of the reaction mixture falls to the lowest temperature is reached before rising till it reaches room temperature

Temperature falls and container feels cooler

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12
Q

What is a endothermic change

A
  1. Evaporation
  2. Melting
  3. Dissolving some ionic compound and ammonium salts in water
  4. Photosynthesis
  5. Action of light on silver bromide in photographic film
  6. Thermal decomposition
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13
Q

Endo or exo?
1. Dissolving naoh in water
2. Precipitation
3. Harber process

A

all exo

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14
Q

Energy level diagram vs energy profile diagram

A

👎👎

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15
Q

Activation energy definition

A

the minimum energy that reacting particles must gain in order for a chemical reaction to occur (Ea)

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16
Q

Axis of energy profile diagram

A

Y axis is Energy/kJ
X axis is progress of reaction

17
Q

Enthalpy diagram vs energy profile diagram

A

they r the same heehee

18
Q

Bond breaking and bond forming
exo/endo?

A

bond breaking is endothermic
bond forming is exothermic

19
Q

Bonds in water

A

2O-H bonds

20
Q

Bonds in oxygen and hydrogen

A

O=O

and

H-H

21
Q

Phrasing for exo

A

The total energy released from forming the ( ) bonds in ( ) is greater than the total energy absorbed for the breaking of ( ) bonds in ( ) and ( ) bonds in ( ).

22
Q

Bond energies + SI Unit

A

energy required to break the covalent bonds in one mole of gaseous molecules into its atoms

SI Unit: kJ mol -1

23
Q

Why bond energy can vary

A

values used are AVERAGE bond energies

24
Q

how to calculate bond energies

A

total energy absorbed for bond breaking - total energy released for bond formation

25
Q

CO2 bonds

A

2 C=O

26
Q

Units when calculating enthalpy change

A

For bond breaking/forming, units is kJ

For enthalpy change (breaking-forming), units is kJ mol -1

27
Q

WHAT MUST ALWAYS BE WRITTEN FOR ENTHALPY CHANGE WHEN IT IS POSITIVE

A

THE POSITIVE SIGN

28
Q

Fuel cell

A

chemical cell in which the reactants are continuously supplied to produce electricity directly (chemical energy from the reaction is directly converted into electricity)

29
Q

Hydrogen Oxygen fuel cell
cathode and anode and overall reactions

A

O2 + 2H2O + 4e- –> 4OH-

2H2 + 4OH- –> 4H2O + 4e-

Overall: O2 + 2H2 –> 2H2O

30
Q

What is at the anode and cathode of a hydrogen oxygen fuel cell

A

Anode: Hydrogen (enter) and water (leave)
Cathode: Oxygen (enter) and water (leave)

31
Q

Hydrogen pros and cons

A

Pros:
- renewable source of energy (electrolysis of water)
- the only product is water (no pollutants)
- hydrogen is efficient (releases 2 times as much energy as compared to other fuels)

Cons:
- expensive to obtain through electrolysis so currently obtained from non-renewable sources (petroleum)
- flammable so need special transport arrangements
- need more storage space because it is very light

32
Q

Hydrogen and oxygen is exo or endo

A

EXTRA VERY exo

33
Q

How to obtain hydrogen

A
  1. Electrolysis of water
  2. Hydrocarbons
    - Methane mixed with steam to form CO and H2
    - CO further reacts with more steam to form CO2 and H2

Y4 Chem (10 decks)

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