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OCR CHEMISTRY A > x > Flashcards

x Flashcards

1
Q

enthalpy, H

A

measure of the heat energy in a chemical system

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2
Q

enthalpy change, ∆H

A

difference on enthalpy change

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3
Q

write the equation to find ∆H

A

∆H= H(products)- H(reactants)

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4
Q

law of conservation of energy

A

energy cannot be created or destroyed

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5
Q

where is the surrounding?

A

where the thermometer is

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6
Q

what is an exothermic change?

A

from the system to the surroundings

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7
Q

what is an endothermic change?

A

from the surroundings to the system

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8
Q

what is an activation energy?

A

minimum energy required for a reaction to take place

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9
Q

what is a standard pressure?

A

100kPa

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10
Q

what is the standard temperature?

A

25℃ and 298K

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11
Q

what is the standard concentration?

A

1 moldm⁻³

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12
Q

standard enthalpy change of formation definition

A

formation of 1 mol of a compound from its constituent elements, under standard conditions

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13
Q

standard enthalpy change of combustion definition

A

enthalpy change when 1 mole of substance is burnt in excess oxygen, under standard conditions

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14
Q

standard enthalpy change of neutralisation definition

A

formation of 1 mol of water from neutralisation, under standard conditions

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15
Q

how do you determine the temperature change of the surroundings?

A

∆T=T(final)- T(initial)

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16
Q

write the equation to calculate an energy change

A

q=mc∆T

17
Q

why are combustion reactions less exothermic that expected?

A
  • heat loss to the surroundings

- incomplete combustion of reactants

18
Q

how do you find the enthalpy change by using the energy change?

A

∆H=q/n

19
Q

what other factors could lead to an inaccurate ∆H result

A

non- standard conditions

20
Q

average bond enthalpy definition

A

breaking of 1 mol of bonds in gaseous molecules

21
Q

bond breaking

A

endothermic

22
Q

bond making

A

endothermic

23
Q

how to calculate the enthalpy change of reaction

A

∆H=Σ(bond enthalpies in reactions)- Σ(bond enthalpies in products)

24
Q

what is the limitation of average bond enthalpies?

A
  • actual energy involved in breaking and making individual bonds would ne slightly different
  • bonds may be in different environments
25
Q

how to work out the enthalpy change of formation

A

A=C-B

26
Q

how to work out the enthalpy change of combustion

A

A=B-C

OCR CHEMISTRY A (34 decks)

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