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IGCSE Chemistry > Energetics - 5th > Flashcards

Energetics - 5th Flashcards

1
Q

what is the enthalpy change of a reaction?

A
  • the heat energy it releases or absorbs per mole
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2
Q

what is the symbol of enthalpy change?

A
  • DELTA H
  • has a sign, value and units
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3
Q

what does this tell us:
N2 + 3H2 -> 2NH3
DELTA H = -92 kJ/mol

A
  • this tells is that the reaction releases 92 kJ of heat energy every time 1 mole of N2 and 3 moles of H2 react to make 2 moles of NH3
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4
Q

what sign tells us whether the reaction absorbs or releases energy?

A

+ = POSITIVE = ABSORBED = ENDOTHERMIC
- = NEGATIVE = RELEASED = EXOTHERMIC

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5
Q

what tells us how much heat energy is absorbed or released?

A
  • the value tells us how much heat energy is absorbed or released
  • the units are always kJ / mol so that different reactions can be compared easily
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6
Q

what is the meaning of this reaction and enthalpy change:
BaCO3 + heat
DELTA H = +245 kJ/mol

A
  • absorbs a lot of heat energy
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7
Q

what is the meaning of this reaction and enthalpy change:
NaCl + H2O
DELTA H = +35 kJ/mol

A
  • absorbs a small amount of heat energy
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8
Q

what is the meaning of this reaction and enthalpy change:
CH4 + O2
DELTA H = -890 kJ/mol

A
  • releases a lot of heat energy
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9
Q

what is the meaning of this reaction and enthalpy change:
C8H18 + O2
DELTA H = -5330 kJ/mol

A
  • releases a huge amount of heat energy
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10
Q

what does how much heat energy a reaction absorbs or releases depend on?

A
  • the balance between breaking bonds (which absorbs energy) and making bonds (which releases energy)
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11
Q

how are enthalpy changes represented diagramically?

A
  • on an energy level diagram
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12
Q

what happens to heat energy and enthalpy change when the reaction is endothermic?

A
  • heat energy is absorbed
  • enthalpy change is positive
  • products end up with more energy than the reactants had
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13
Q

what happens to heat energy and enthalpy change when the reaction is exothermic?

A
  • heat energy is released
  • enthalpy change is negative
  • products end up with less energy than the reactants had
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14
Q

what contributes towards a reaction’s enthalpy change?

A
  • breaking bonds and making bonds
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15
Q

is bond breaking exothermic or endothermic and does bond breaking absorb or release heat energy?

A
  • absorbed
  • endothermic
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16
Q

is bond making exothermic or endothermic and does bond making absorb or release heat energy?

A
  • released
  • exothermic
17
Q

how can the enthalpy change for a reaction be calculated?

A
  • bonds broken - bonds made
18
Q

what is a reaction profile diagram?

A
  • like an energy level diagram
  • but it shows the path the reaction actually takes from reactants to products
19
Q

in an exothermic reaction, which is greater, energy absorbed or energy released?

A
  • energy released > energy absorbed
20
Q

in an endothermic reaction, which is greater, energy absorbed or energy released?

A
  • energy released < energy absorbed
21
Q

what is activation energy?

A
  • the energy needed to break the bonds
22
Q

what is calorimetry?

A
  • an experimental method used to measure an enthalpy change
23
Q

describe the experiment of calorimetry.

A
  • a reaction is carried out near some water
24
Q

what happens in an exothermic reaction when calorimetry is done?

A
  • reaction releases heat energy
  • heat energy is absorbed by the nearby water
  • the temperature of the water increases
25
Q

what are the two formulas needed to work out enthalpy change after doing calorimetry?

A
  • Q = mc deltaT
  • enthalpy change = -Q/moles
26
Q

what is c in Q=mc deltaT?

A
  • constant value
  • specific heat capacity of water
  • always has the value 4.18 J/g/degrees celcius
27
Q

describe the simplest type of calorimetry.

A
  • the simplest type of calorimetry involves combusting a fuel to heat up the water
  • to measure deltaH (enthalpy change) for the combustion of the liquid fuel methanol, you would combust some methanol using a burner under a can of water
28
Q

method for simplest type of calorimetry.

A
  1. weigh out 50g of water and place in a can above the burner
  2. measure the initial mass of the burner and the initial temp of the water
  3. ignite the burner, stir water for 30s, then extinguish the burner
  4. measure the final mass of the burner and the final (maximum) temperature of the water
29
Q

what are the two significant sources of error in this experiment?

A
  • heat loss - most of the heat energy dissipates into the air instead of the water
  • incomplete combustion - this releases less heat energy than expected
30
Q

what is solution calorimetry?

A
  • sometimes the reaction you are measuring happens dissolved in some water
  • in this case, the reaction still releases heat energy and the water still absorbs it- but it just happens that the reaction and the water are in the same place
31
Q

describe the method of solution calorimetry.

A
  • e.g. to measure enthalpy change for the displacement reaction between Mg(s) and CuSO4(aq)
  • pipette 25cm3 of CuSO4(aq) into a polystyrene cup
  • measure the initial temp of sol
  • add 1g of Mg(s) and stir continuously
  • measure the max temp of the solution
  • use equations to calculate
32
Q

what happens if you do a calorimetry experiment for endothermic reactions?

A
  • reaction absorbs heat energy from water, so the water cools down
  • this results in a negative temp change for the water
  • the minus sign gets passed through the equations and comes out at the end as a plus, because a minus times a minus is a plus

IGCSE Chemistry (16 decks)

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