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Chemistry Module 3- Periodic table and energy > Enthalpy > Flashcards

Enthalpy Flashcards

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1
Q

What is enthalpy

A

The heat content stored in a chemical system. It is sometimes thought of as energy stored within bonds

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2
Q

What are the units of energy

A

Joules

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3
Q

What is the enthalpy change

A

Is a heat exchange with the surroundings during a chemical reaction. Heat exchange occurs because reactants and products have different enthalpies

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4
Q

What happens if a reaction is exothermic

A

Energy is transferred from the system to the surroundings. When a reaction is negative it is exothermic

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5
Q

What happens if a reaction is endothermic

A

Enthalpy of products is greater than reactants so enthalpy is positive as its transferred from the surroundings to the system

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6
Q

What is the activation energy

A

The minimum energy required to start a reaction

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7
Q

What are the units for standard conditions

A

Pressure is 100 kPa
Temperature 298 K
Concentration 1 mol dm-3

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8
Q

What is the enthalpy change of combustion

A

The enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions

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9
Q

What is the enthalpy change of formation

A

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

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10
Q

What is the enthalpy change of reaction

A

The enthalpy change that accompanies a reaction in their molar quantities expressed in a chemical equation under standard conditions

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11
Q

What is enthalpy change of neutralisation

A

The enthalpy change that accompanies the neutralisation of an aqueous acid by an aqueous base to form one molle of water under standard conditions

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12
Q

What measurements are taken for determination of enthalpy change for a polystyrene calorimeter

A

Mass of water
Temperature of water
Volume or concentration and mass of solutes reactants

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13
Q

What measurements are taken when you use a spirit burner ( for combustion)

A

Mass of water
Initial and final temp of water
Initial and final mass of fuel

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14
Q

What is the equation of heat exchange

A

q= mc delta T

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15
Q

What happens if temperature increases

A

The surrondings increase in heat energy system to surrondings so exothermic

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16
Q

What happens if temperature decreases

A

The surrondings decrease in heat energy so surrondings to system so its endothermic

17
Q

How do you evaluate if the volume of water was doubled

A

Firstly the mass would doulbe up however the temperature change would double down
This means q would be the same
No change in n as the system isnt affected
delta H wouldnt change

18
Q

What are the procedural errors for both calorimeters

A

Heat loss to all the surrondings
Not done under standard conditions

19
Q

What are the procedural errors for combustion experiments

A

Liquid fuel may evaporate
Heat capacity of the beaker isnt considered
Incomplete combustion may occur

20
Q

What evaluation do you make if the lid on the cup isnt used for exothermic reaction

A

The problem- Heat loss to surrondings
Delta T would decrease so q would decrease
So delta H would be less exothermic than it should be

21
Q

What evaluation do you make if the liquid fuel evaporates

A

There is no impact on q as the mass of water and temperature change are not affected
However n is bigger than it should be. As some fuel is evaporated as well as combusted
This means that delta H would be less exothermic than the actual value

22
Q

What evaluation do you make if incomplete combustion occurs

A

Delta T would be smaller as less energy is released for incomplete combustion so q would be smaller than it should be
There is no impact on n as all the fuel combusts
Since q is smaller and n is the same Delta H would be less exothermic than the actual value

23
Q

What happens if some methanol had evaporated from the wick before the final weighing

A

The calculated mass of methanol reacted would be higher than the actual value.
So calculated moles of methanol would be higher
Since Delta H is q over n Delta H would be less exothermic than the actual value

24
Q

What happens if the student used the Mr of ethanol instead of methanol

A

The Mr would be larger than methanol
Moles of methanol calculated is is smaller is n is mass divided by Mr
Since delta H is q divided by n delta H is more exothermic than the actual value

25
Q

What does Mexo Bendo mean

A

Bond making is exothermic
Bond breaking is endothermic

26
Q

What happens in exothermic reactions in terms of bond breaking and making

A

In exothermic reactions the energy released from bonds made is greater than the energy absorbed from bonds broken

27
Q

What happens in endothermic reactions in terms of bond breaking and making

A

In endothermic reactions the energy absorbed from bonds broken is greater than the energy released from bonds made

28
Q

What is the average bond enthalpy

A

The enthalply change when one mole of gaseous bonds are broken at 298 K

29
Q

Why do all bonds have a positive enthalpy value

A

Bonds are electrostatic attractions which require energy to break so are always endothermic

30
Q

What is the bond enthalpy equation

A

The sum of bond enthalpies of bonds broken - the sum of bond enthalpies of bonds made

31
Q

If the bond enthalpy of bonds broken are larger than those of bonds formed what will be the sign of delta H be

A

When the bond enthalpies of reactants are larger than products then delta H is larger than 0
When the bond enthalpies of products is larger than reactants then delta H is smaller than 0

32
Q

Why are the true values for energy change often slightly different from the values calculated using bond enthalpies

A

Reason 1- Bond enthalpies are averaged out over different molecules
Reason 2- Reactions may not be under standard conditions as it is only for gaseous bonds

33
Q

Why would actually measuring the enthalpy change in a reaction be difficult

A

1) A high activation energy
2) A slow reaction rate
3) More than one reaction is taking place

34
Q

What does Hess Law state

A

If a reaction can take place by more than one route and the initial and final conditions are the same the total enthalpy change is the same

35
Q

What happens if you use a Hess cycle with combustion data

A

If you follow the direction of an arrow you ADD
If it is opposite to the direction of an arrow you SUBTRACT

36
Q
A

Chemistry Module 3- Periodic table and energy (6 decks)

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