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enphalpy Flashcards

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1
Q

what is thermochemistry

A

thermochemistry is the study of heat changes during a chemical reaction

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2
Q

what happens when bonds are formed

A

energy is released when bonds are formed in the reactants.

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3
Q

what is the definition for an exothermic reaction

A

an exothermic reaction releases heat energy to the surroundings . the energy required to break the bonds in the reactants is less than the energy released when the bonds in the products are formed (temp will increase )(energy change is negative) eg combustion

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4
Q

what is the definition for an endothermic reaction

A

an endothermic reaction takes in heat energy from the surroundings . the energy required to break the bonds in the reactants is greater than the energy released when the bonds in the products are formed.(temp will decrease) (energy change is positive)

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5
Q

what are some examples of exo reactions

A

combustion , respiration , freezing , rusting , fission , neutralisation

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6
Q

what are some examples of endo reactions

A

evaporation , photosynthesis , ionisation , thermal composition

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7
Q

what is enthalpy change

A

enthalpy change is the heat energy change measured under conditions of constant pressure. it is usually measured in kilojoules per mol KJmol-1

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8
Q

what are the standard conditions of enphalpy changes

A

a pressure of 100kpa
a temperature of 298k
for chemicals in solution concentration of 1moldm-3
all chemicals must be in their standard states

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9
Q

how do you calculate the heat change

A

q = mc(delta t)
where q= the heat change measured in joules
m is the mass of the substance being heated (in grams)
c= the heat capactity of the substance being heated (JK-1g-1)
delta h= the temp change of the substance being heated ( temp change in kelvin )

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10
Q

how do you calculate the enphalpy change of a reaction

A

delta h = q/ n x 1000
where delta h is the enphalpy change in kilojoules per mol KJmol-1
n is the number of moles reacted of the limiting raegent in the reaction

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11
Q

what is the definition for the enphalpy change of combustion

A

the enphalpy change when one mol of substance is completely burnt in oxygen under standard conditions , all products and reactants being in their standard state

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12
Q

what is the definition for the enphalpy of formation

A

is the enphalpy change when one mol of compound is formed from its constituent elements under standard conditions , all products and reactants being in their standard state

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13
Q

what is the definition for the standard molar enphalpy of reaction

A

is the enphalpy change associated with a stated reaction under standard conditions , all products and reactants being in their standard state

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14
Q

what is the definition for the enphalpy of neutralisation

A

is the enphalpy change when 1 mol of water is formed from a neutralisation reaction under standard conditions , all products and reactants being in their standard state

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15
Q

what is the formula for chloroethane

A

c2h5cl

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16
Q

what is the formula for methyl propane

A

CH3CHCH3CH3

17
Q

what is an enphalpy change of formation that is also an enphalpy change of combustion

A

H2 + 1/2 O2—>H20

18
Q

what is the formula for propan - 2 - ol

A

CH3CHOHCH3

19
Q

what is the formula for ethanol

A

C2H5OH

20
Q
A
21
Q

Why do we use polystyrene cups for enthalpy changes

A

Cheap , waterproof , lightweight ,and offer some insulation against heat loss to the surroundings

22
Q

What state are average bond enpalpies in

A

Gaseous

23
Q

What is the definition for average bond enthalpy

A

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

24
Q

Is bond breaking endo or exothermic

A

Endothermic as energy is always put in to break a bond , energy change is positive

25
Q

Is bond forming endo or exothermic

A

Exothermic so energy change is negative

26
Q

Why is average bond enthalpy not a standard enthalpy change

A

Because they are determined for molecules in a gaseous state and under standard conditions not all molecules are gaseous

27
Q

How do you calculate enthalpy change from average bond enthalpies

A

Bonds broken - bonds formed ( sum of all energy stored in bonds in reactants - sum of all energy stored in products )

28
Q

What are the limitations of average bond enthalpies

A

As you are using average bond enthalpies , the actual energy involved in breaking and making individual bonds would be slightly different as the bonds may be in different environments

29
Q

What is the enthalpy change

A

The overall effect of these two changes ( bonds formed and broken ) if you need more energy to break bonds then it’s endothermic and if more energy is released it’s exothermic

30
Q

What is bond dissociation enthalpy

A

The energy needed to break the attraction between atoms and ions

31
Q

What is the enthalpy change of a reaction dependent on

A

The enthalpy change for a reaction is independent of the route taken , providing start and finish conditions are the same

32
Q

How do you do a Hess cycle for formation using formation data

A

Point arrrows up , bottom of Hess cycle is elements in standard states and enthalpy change = products - reactant

33
Q

What do you do with the arrows during combustion

A

Point them down , enthalpy change = reactants - products

34
Q

Is turning a liquid into a gas endo or exothermic

A

Endothermic

35
Q

What bonds are broken when a liquid turns into a gas

A

Intermolecular bonds ( London forces) between molecules are broken

chemistry (24 decks)

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