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chemistry > Topic 8 - Energetics I > Flashcards

Topic 8 - Energetics I Flashcards

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1
Q

What is enthalpy change

A

the heat energy change in a reaction at constant pressure

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2
Q

What are standard conditions

A

100 kPa
298 K

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3
Q

What is enthalpy change if a reaction is exothermic

A

negative
(temp goes up)

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4
Q

What is enthalpy change if a reaction is enfothermic

A

positive
(temp goes down)

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5
Q

How are energy level diagrams different to reaction profile diagrams

A

energy level = overall change

reaction profile = how enthalpy changes throughout reaction

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6
Q

What does an enthalpy level diagram look like

A

one line drawn for reactants
one line drawn for products

vertical arrow in between

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7
Q

What does a reaction profile diagram look like

A

one line drawn for reactants
one line drawn for products

curved line connects the two
vertical arrow to top of curve = activation energy

vertical arrow between react/prod = energy change

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8
Q

What is standard enthalpy change of reaction

A

enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation under standard conditions

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9
Q

What is standard enthalpy change of formation

A

enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions

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10
Q

What is enthalpy change of combustion

A

enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions

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11
Q

What is standard enthalpy change of neutralisation

A

enthalpy change when an acid and an alkali react together to form 1 mole of water under standard conditions

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12
Q

How can you find enthalpy changes using an experiment

A

directly measuring temperature change of some reaction mixture

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13
Q

What is the problem with using an experiment to find enthalpy change

A

some heat lost to surroundings = temp change measured less than actual temp change

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14
Q

How can you solve the problem of energy loss when finding enthalpy change in an experiment

A

extrapolate the data

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15
Q

What is calorimetry used for

A

hard to measure temp change of gases = calorimeter to measure temp change of some water

to find enthalpy change of combustion of flammable liquids

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16
Q

How does calirometry work (include the set up)

A
  1. spirit burner containing fuel in combustion chamber with airflow
  2. closed lid, stirring rod and thermometer
  3. as fuel burns, water heats up
  4. work out heat absorbed by water if known mass of water, temp change of water and specific heat capacity
17
Q

What is the reality of using a calorimeter

A

some heat is always lost (to apparatus and surroundings)

18
Q

What is the formula to find heat lost or gained

A

q = m x c x ∆T

q = joules
enthalpy change = kJ (divide by 1000)

19
Q

Formula to find enthalpy change of combustion from finding heat loss (q)

A

∆H = q / moles

20
Q

What is Hess’s Law

A

totaly enthalpy change of a reaction is always the same , no matter which route is taken

21
Q

In enthalpy change of formation (using Hess’s cycle) what direction do the arrows point

A

away from elements to the reactants and products

22
Q

In enthalpy change of formation (using Hess’s cycle) what is the ‘elements’

A

what makes up each of the reactants/products

23
Q

What is the enthalpy change of formation for elements

A

zero

24
Q

In enthalpy change of combustion (using Hess’s cycle) what direction do the arrows point

A

towards combustion products (CO₂ + H₂O) and away from reactants/products

25
Q

What happens when as bond is broken

A

energy taken in
endothermic

26
Q

What happens when a bond is made

A

energy released
exothermic

27
Q

What is bond enthalpy

A

the amount of energy needed to break 1 mole of a type of bond in a molecule in the gas phase

28
Q

Why are mean bond enthalpies not exact

A

energy needed to break one mole of bonds in the gas phase, averaged over different compounds

29
Q

How can enthalpy changes be calculated from mean bond enthalpies

A

enthalpy change of reaction = (sum of bond enthalpies of reactants) - (sum of bond enthalpies of products)

chemistry (10 decks)

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