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Chemistry - Y10 + Y11 > Energy Changes > Flashcards

Energy Changes Flashcards

1
Q

What do exothermic reactions do?

A

They transfer energy from the reacting molecules to the surroundings - surroundings get hotter

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2
Q

Examples of exothermic reactions

A
  • Neutralisation
  • Combustion
  • Oxidation
  • Freezing
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3
Q

What do energy profile diagrams show?

A

Scientists represent the energy changes that take place in reactions by drawing these

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4
Q

What are endothermic reactions?

A

They take in energy from their surroundings - surroundings get colder

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5
Q

Example of endothermic reactions

A
  • Thermal decomposition
  • Reaction between citric acid and sodium hydrogencarbonate
  • Melting
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6
Q

What is the activation energy?

A

Minimum amount of energy the reactants need to collide with each other and react

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7
Q

What does a greater activation energy mean?

A

The greater the ae. the more energy needed to start the reaction - this has to be supplied

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8
Q

What is the method to measure energy transfer?

A
  1. Use the 50cm3 measuring cyclinder to put 30cm3 dilute hydrochloric acid into the polystyrene cup
  2. Stand the cup in the beaker. This will make it more stable
  3. Use the thermometer to measure the temperature of the acid. Record it
  4. Pour5cm3 sodium hydroxide in the measuring cyclinder
  5. Pour sodium hydroxide into the cup - gently stir with thermometer in the hole of lid
  6. Repeat - pour more 5cm3 amounts to the cup - total of 40cm3 needs to be added
  7. Repeat previous steps for second trial
  8. Calculate mean of max temp
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9
Q

Exothermic reactions in everday life examples

A
  • Hand warmers - uses exothermic oxidation of iron in air to release energy
  • Self heating cans
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10
Q

Endothermic reactions in everday life examples

A
  • Sports injury packs
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11
Q

How can you reduce amount of energy lost to the surroundings?

A
  • Putting polystrene cup into a beaker of cotton wool to give more insulation
  • Putting a lid on the cup to reduce the energy lost by evaporation
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12
Q

What does height represent in reaction profiles?

A

Overall energy change in the reaction per mole

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13
Q

Is bonding breaking endothermic or exothermic, why?

A

Endo because energy must be supplied to break existing bonds

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14
Q

Is bonding forming endothermic or exothermic, why?

A

Exo because energy is released when new bonds are formed

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15
Q

How does the amount of energy related to bonds differ in exothermic reactions?

A

Energy released by forming bonds is greater than the energy used to break them

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16
Q

How does the amount of energy related to bonds differ in endothermic reactions?

A

Energy used to break bonds is greater than energy released by forming them

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17
Q

How do you work out the overall energy change?

A

Sum of energies needed to break bonds in the** reactants** - energy released when the new bonds are formed in the products

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18
Q

What is an electromechanical cell?

A

Basic system made up of 2 diff electrodes in contact with ann electrolyte

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19
Q

What causes the charge difference/ voltage of the cell?

A

Diff metals will react diff with the same electrolyte

20
Q

What is produced when there is a greater difference in reactivity of the metals?

A
  • greater pd produced by the cell
  • electrolyte also affects pd
21
Q

What else would affect the size of voltage in fuel cells?

A

Electrolyte used in a cell - diff ions in solution will react diff with the metal electrodes used

22
Q

How is a battery formed?

A

2 or more cells together in series = greater voltage

23
Q

Example of non-rechargable batteries

A

Alkaline batteries contain cells which use irreversible reactions - once 1 reactant is used up, they produce no more charge and replacement is needed

24
Q

How do rechargable cells work?

A
  • Can reverse the chemical reactions when we apply an electrical current
25
Q

Whats a fuel cell?

A
  1. supply fuel = hydrogen
  2. react with pure o2 or air
  3. chemical reaction = electric current
  4. waste product = water
26
Q

What happens to the fuel when it enters the cell?

A

Becomes oxidised and sets up a pd within the cell

27
Q

What does a hydrogen-oxygen fuel cell do, type of reaction?

A

Combines O and H to produce water and release energy through a redox reaction

28
Q

What does a H-O fuel cell consist of?

A
  • Electrolyte - usually an acid (phosphuric aicd(
  • Electrodes - porous carbon with a catalyst
  • Negative anode and positive cathode
29
Q

Process of H-O fuel cell

A
  1. H goes into anode compartment
  2. O goes into cathode
  3. At anode, H loses electrons to produce H+ ions - oxidation
  4. H+ ions in the electrolyte move to cathode
  5. At cathode, O gains electrons and reacts with H+ ions from acidic eletrolyte to make water - reduction
  6. Electrons flow through an external circuit from the anode to cathode - electric current
30
Q

Pros on fuel cells

A
  • Not as many pollutants compared to other fuels - only by products are water and heat
  • do not get less efficient the longer they run
  • source of drinkable water
  • will produce electricity for as long as you provide hydrogen
31
Q

Cons on fuel cells

A
  • H is a gas, takes up more space to store than a rechargable battery
  • H is explosive when mixed with air - hard to store safely
  • H fuel made from hydrocarbons (from fossil fuels) or electrolysis of water (electricity from ff)
  • produce low pd so several are needed together
32
Q

How do you know whether a reaction is mainly endo or exo thermic by using bond energies?

A

If the answer is negative, its exothermic as it has given that amount of energy to the surroundings
If its positive, its endothermic, has gained that muh energy from surroundings

33
Q

What does bond energy mean?

A

Amount of energy required to break one mole of a particular covalent bond.

34
Q

Fuel cell equation at negative electrode

A

2H₂ –> 4H⁺ + 4e⁻

35
Q

Fuel cell equation at positive electrode

A

O₂+ 4H⁺ + 4e⁻ –> 2H₂0

36
Q

Overall equation for fuel cells

A

2H₂ + O₂ –>2H₂0

hydorgen is oxidised

37
Q

Practical Method: Temp changes

A
  1. MC for 30cm3 dilute HCL
  2. Transfer into polyesterne cup
  3. put into beaker - stops it falling over
  4. measure tem p of acid
  5. MC for 5cm3 sodium hydroxide and transfer into cup
  6. put a lid + thermometer into lid
  7. stir solution
  8. record temp rise + record highest temp reached
  9. Repeat, rinse and use 10cm3 sodium hydroxide solution
  10. increase vol of sodium hydroxide by 5cm3 until 40cm3
  11. repeat + mean value for max temp reached for each vol.
38
Q

Describe what happens as you increase the volume of sodium hydroxide solution in HCL

A
  • As you increase vol, max temp increases
  • because they react with HCL - more energy released as its exo
  • at a certain vol, max temp starts to decrease becaue there isnt enough HCl to react
38
Q

Why do we use a polysterene cup and a lid?

A

reduce heat losses
poly = good thermal insulator
lid = reduces heat loss to air

39
Q

How is electricity produced using metals?

A

2 diff metals, place them into an electrolyte

40
Q

How long can a cell produce electricity for?

A

a certain period of time, chemicals in the cell will run out and reaction stops

41
Q

When will cells only produce electricity?

A

if metals with diff reactivities are used

42
Q

Cons of rechargable batteries

A
  • run out and need to be recharged
  • store less electricity the more charging cycles they go - need to be replaced
43
Q

Pros of rechargeable batteries

A
  • no dangerous fuels are required - some can catch fire if not manufactured correctly
  • produce greater pd
44
Q

In a fuel cell, what charges is the anode and cathode?

A

anode = negative
cathode = positive

45
Q

What is used to make the electrodes and why?

A

Graphite (carbon) has delocalised elctrons - carry charge

Chemistry - Y10 + Y11 (10 decks)

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