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Chemisty A level > Enthalpy > Flashcards

Enthalpy Flashcards

1
Q

what is enthalpy?

A

a meaure of the energy stored in a chemical system

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2
Q

what is the conservation of energy?

A

energy cannot be created or destroyed only transferred

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3
Q

How do you calculate the enthalphy change?

A

product enthalpy - reactants enthalpy

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4
Q

does exothermic reactions have a positive or negative enthalpy change?

A

negative

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5
Q

does endothermic reactions have a positive or negative enthalpy change?

A

positive

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6
Q

What does exothermic mean?

A

the products have less energy than the reactants as heat is given out to the surroundings

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7
Q

what dpes endothermic mean?

A

the products have more enrgy than the reactants as energy has been taken in from the surroundings

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8
Q

what are examples of exothermic reactions?

A

condensation reaction
liquid solidifies
neutralisation
vapour condensing to liquid

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9
Q

what are examples of endothermic reactions?

A

decomposition reaction
liquid vapourising
solids melting

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10
Q

what is the standard state of a substance?

A

the substance’s physical state under standard conditions

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11
Q

What are the standard conditions?

A

1 atm / 100000 KPA
25 degrees / 298K

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12
Q

what is the standard enthalpy change of reaction?

A

the enthalpy change for a reaction in the molar quantities shown in a chemical equation, under standard conditions

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13
Q

what is the standard enthalpy change of combustion?

A

the enthalpy change for the complete combustion of 1 mole of a substance under standard conditions

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14
Q

What is the standard enthalpy chnage of formation?

A

the enthalpy change for the formation of 1 mole of a compound from its elements in their standard states, under standard conditions

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15
Q

what is the standard enthalpy change of neutralisation?

A

the enthalpy change for the formation of 1 mole of water from the neutralisation of an acid by a base under standard conditions

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16
Q

How do you calculate the heat evolved/ absored (q) in a reaction?

A

q = M x C x (T1 -T2)

m = mass of substance (g)
c = specific heat capacity of water = 4.18 Jg-1K-1
triangleT = temp change (degrees or kelvin)
q (J)

17
Q

How do you calculate the enthalpy change from q = MCtriangleT

A

use equation
covert to KJ (divide by 1000)
divide by moles reacting
decide if exo or endo
decide if + or -

18
Q

What measurements do you take for calorimetry?

A

weigh mass of spirit burner with and without fuel
mass of beaker with and without water

19
Q

what are reasons why the calorimetry enthalpy change is different to the calculated enthalpy change?

A

losing heat to the surroundings
not under standard conditions
incomplete combustion
evapouration of alcohol from wick

20
Q

What are the assumptions with enthalpy changes of solutions?

A

the density is the same as water = 1gcm-3
volume (cm3) = mass (g)
the specific heat capacity of the solution is the same as water = 4.18 Jg-1K-1

21
Q

What is a chemical system?

A

the atoms, molecules or ions making up the chemicals

22
Q

How could you minimise errors in calorimetry?

A

draught screens
input of oxygen gas

23
Q

Why is a polystyrene cup used in measuring the enthalpy change on reaction?

A

offers some insulation against heat loss to the surroundings

24
Q

What is average bond enthalpy?

A

the energy required to break one mole of a specified type of bond in a gaseous molecule

25
Q

Are bond ethalpies endothermic or exothermic?

A

require energy - endothermic = + value

26
Q

How do you calculate the enthalpy changes of reaction? (combustion)

A

tot bond enthalpies in reactants - tot bond enthalpies in products

27
Q

How do you calculate the average bond enthalpies of the enthalpy change of formation?

A

tot bond enthalpy in products - tot bond enthalpy in reactants

28
Q

What is Hess’ law?

A

if a reaction can take place by two routes, and starting and finishing conditions are the same, the total enthalpy change is the same for each route

29
Q

Which way to the arrows point in a reaction route with formation?

A

upwards

30
Q

Which way to the arrows point in a reaction route with combustion?

A

downwards

Chemisty A level (19 decks)

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