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A Level Chemistry-CIE > Chemical Energetics > Flashcards

Chemical Energetics Flashcards

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1
Q

Define Enthalpy

A

The total thermal energy stored inside a chemical system is called the enthalpy. Enthalpy cannot be measured but enthalpy change can be measured when energy is transferred between the system and the surroundings.

Enthalpy is also sometimes called heat content.

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2
Q

What is enthalpy Change? ΔH?

A

The heat energy transferred during a chemical reaction.

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3
Q

What are the units of enthalpy change ΔH?

A

Kilojoules per mole. KJ mol l-1

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4
Q

Define a chemical system.

A

System refers to the reaction mixture containing reactant and product.

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5
Q

Define the chemical surroundings.

A

The surroundings is everythings outside the chemicals.
The surrounding includes:
*The solvent (e.g. water).
*The container/test tube itself.
*Anything dipping inside the reaction mixture (e.g a thermometer).
*The air around the container.

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6
Q

Define exothermic reactions

A

A reaction is exothermic when the products have less energy than the reactants and heat energy is given off by the reaction to the surroundings.

This means that:
the tempretaure of the inviroment increases-this can be measured by a thermometer.
The temperature of the system decreases.

Exothermic reaction is a bond forming process.

Exothermic reactions have a ΔH=-ive

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7
Q

Define Endothermic Reactions.

A

A reaction is endothermic when the products have more energy than the reactants and heat energy is absorbed from the surroundings.

This means that:
the tempreature of the envirmoent decreases-this can be measured by a thermometer.
The temperature of the system increases.

Endothermic reaction is a bond breaking process.

Exothermic reactions have a ΔH=+ve

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8
Q

Give examples of exothermic reactions.

A

Combustion of fuels
Neutralisation reactions
Respiration
Nucleur Reactions
Change of states from gases to liquids and solids.

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9
Q

Give examples of endothermic reactions:

A

Thermal Decomposition
Photosynthesis
Electrolysis
Change in state from solid to liquid to gas.

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10
Q

Which reaction type is more feasible?

(b/w exo or endo)

A

Exothermic reactions tends to be more feasible as no energy is required except activation energy.

Endothermic more feasible at higher temperatures as heat energy needed

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11
Q

Define activation energy

A

Activation energy is the minimumum energy that colliding particles must posses to break the bonds to start a chemical reaction.

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12
Q

Describe the value of the enthalpy change of activation energy.

A

Activation energy always has a positive value of ΔH because enough energy has to be absorbed to increase kinetic energy of the reactant molecules so that they collide with enough force to break the bonds.

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13
Q

Important tip for drawing reaction pathway diagrams (aka enthalpy profile diagrams):

A

Remember that the activation energy arrow should start at the level of the reactants and end at the highest point on the energy hump . When drawing the arrow for ΔH make sure that the arrow points downwards for an exothermic reaction and upwards for an endothermic reaction.

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14
Q

What is the transition state in energy level diagrams?

A

*The transition state is a stage during the reaction at which chemical bonds are partially broken and formed
*The transition state is very unstable – it cannot be isolated and is higher in energy than the reactants and products
*The activation energy (Ea) is the energy needed to reach the transition state

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15
Q

Which type of reaction has a lower activation energy?

B/W exo and endo

A

In an exothermic reaction, the reactants are higher in energy than the products
The reactants are therefore closer in energy to the transition state
This means that exothermic reactions have a lower activation energy compared to endothermic reactions

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16
Q

What is meant by standard enthalpy change?

A

To fairly compare the changes in enthalpy between reactions, all reactions should be carried out under standard conditions. These are:
* A pressure of 101Kpa (1atm)
* A temperature of 298K (25°C)
* Each substance involved in the reactions in its normal physical state (solid, gas and liquid).

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17
Q

What is the symbol for standard enthalpy change?

A

To show that a reaction has been carried out under standard conditions, the symbol is ⦵ used.
* ΔH^⦵=standard enthalpy change.

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18
Q

Define Standard enthalpy change of formation.

A

The enthalpy change when one mole of a compound is formed from its consitutent elements under standard conditions is called standard enthalpy change of formation.

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19
Q

What type of reaction is standard enthalpy change of formation?

B/W Exo and Endo

A

Standard enthalpy change of formation can be exothermic or endothermic.
If it is exothermic than the compound is stable.
If it is endothermic then the compound is unstable.

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20
Q

How can we calculate standard enthalpy changes of formation?

A

We cannot determine the value of enthalpy change of formation directly so we use different enthalpy changes in order to get the values.

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21
Q

Define standard enthalpy change of combustion.

A

The enthalpy change when one mole of a susbtance is completely burnt in excess oxygen under standard conditions is called standard enthalpy change of combustion.

substance=element or compound

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22
Q

What type of reaction is standard enthalpy change of combustion?

A

It is always exothermic.

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23
Q

What products are always released when any organic substance is combusted?

A

Whenever any organic substance (something containing carbon and hydrogen) is combusted it always gives off carbon dioxide and water.

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24
Q

Define the standard enthalpy change of atomistation.

A

The enthalpy change when 1 mole of gaseous atoms is formed from its constituent elements in standard state under standard conditions is called standard enthalpy change of atomisation.

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25
Q

What type of reaction is standard enthalpy change of atomisation?

b/w exo and endo

A

It is always endothermic because it is a bond breaking process.

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26
Q

What is the enthalpy change of atomisation of a diatomic molecule?

A

Enthalpy change of atomisation of a diatomic molecule/gas is half of its bond energy.

For e.g.
1/2 Cl^2(g)→Cl(g)
1/2Br^2(l)→Br(g)
1/2I^2(s)→I(g)

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27
Q

In what physical state can electron transfer occur?

A

Electron transfer can only take place when the atom is in a gaseous state.

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28
Q

What is the symbol of standard enthalpy change of atomisation?

A

ΔHa^⦵

29
Q

Define standard enthalpy change of neutralisation.

A

The enthalpy change when one mole of water is formed by the reaction of an acid with an alkali under standard conditions.

30
Q

What type of reaction is standard enthalpy change of neutralisation?

b/w exo and endo

A

Neutralisation is exothermic because oppositely charged ions are involved in the formation of a covalent bond.

31
Q

What type of reaction is standard enthalpy change of neutralisation when a strong acid and strong base react together?

A

Whenever strong acid and strong base react together, neutralisation is always highly exothermic because of complete ionisation.

32
Q

What type of reaction is standard enthalpy change of neutralistation when a weak acid and weak base react together?

A

Whenever a weak acid and weak base react together, neutralisation is always less exothermic becuase of partial ionisation.

33
Q

What is the symbol of standard enthalpy change of neutralisation?

A

ΔHneut^⦵

34
Q

What is the symbol for standard enthalpy change of reaction?

A

ΔHr^Φ

Φ=standard conditions

35
Q

Define standard enthalpy change of reaction.

A

The enthalpy change when the amounts of reactants shown in the stochiometric equation react to give products under standard conditions.

36
Q

What type of reaction is standard enthalpy change of reaction?

b/w exo or endo

A

Enthalpy changes of reaction can be exothermic or endothermic.

37
Q

Why are bond forming reactions exothermic?

A

Energy is released from the reaction to the surroundings (in the form of heat) when new bonds are formed
Bond forming is therefore exothermic

38
Q

Why are bond breaking reactions endothermic?

A

Energy (in the form of heat) is needed to overcome attractive forces between atoms
Bond breaking is therefore endothermic

39
Q

What is bond disassociation energy?

A

The amount of energy required to break one mole of a SPECIFIC covalent bond in the gaseous state is called bond disassociation energy or exact bond energy or bond enthalpy/

40
Q

What is the symbol for bond energy?

A

The symbol for bond energy is E.

We put the type of bond broken in brackets after the symbol. For example E(C-H) refers to the bond energy of a mole of a single bonds between hydrogen and carbon atoms.

41
Q

Are the values of bond energies always positive or negative?

A

The values of bond energies are alway positive (endothermic) because they refer to bonds being broken.

42
Q

Describe the energy changes when bonds are formed and broken

A

When new bonds are formed the amount of energy released is always the same amount of energy absorbed when the same type of bond is broken.

For e.g

E(O=O)=+498 KJ Mol-1 O=O→2O(g)
E(O=O)=-498 KJ Mol-1 2O(g)→0^2

43
Q

What can we determine from bond energy?

A

Bond energy determines the strenght of a covalent bond. Greater the bond energy greater the strength of the covalent bond and vice versa.

44
Q

Define bond polarity.

A

Difference of electronegativity or unequal distrubution of e-.

45
Q

Define bond length.

A

The distance between the nuclei of two atoms.

46
Q

What is the relationshp between bond energy and size of the element.

A

As you go down the group atomic size increases.
The bond length increases.
Orbitals don’t overlap to the maximum extent.
Bond polarity Decreases.
Bond becomes weaker,
Bond energy value decreases.

47
Q

Which element is the exception when it comes to the relationship between bond enthalpy and atomic size.

A

Fluorine has a smaller bond energy than chlorine and bromine. Due to small atomic size fluorine molecules experiences lone pair-lone pair replusion and so its bond length is greater than chlorine and bromine.

48
Q

What is the relationship between the presense of multiple bonds and bond energy.

A

The greater the number of bonds between atoms the stronger the bond and thus higher the bond energy.

49
Q

What is the relationship between the presense of more electronegative elements and bond energy?

A

Presence of more electronegative elements make the bond stronger and therefore bond energy is greater.

50
Q

What is average bond energy?

A

The average bond energy needed to break a specific covalent bond averaged from a variety of molecules in the gaseous state.

51
Q

Why do we need average bond energy?

A

Bond energies are affected by other atoms in the molecule. (The enviroment). Therefore an average number of the same type of bond but in different enviroments is calculated. This bond energy is known as average bond energy.

The O-H bond in water has a different bond energy value to that of O-H bond in ethanol.

52
Q

What is the formula for calculating ΔHr^Φ using bond energies?

A

ΔHr^Φ= Σ Bonds Broken - ΣH Bonds formed.
ΔH=ΣReactants- ΣProducts

53
Q

How can we measure enthalpy changes?

A

Calirometery is a technique used to measure enthalpy changes in some chemical reactions.
The Apparatus used is called a calirometer. A calirometer used can be made up of a polystyrene cup, a vaccume flask or a metal can.

54
Q

What is specific heat capacity?

A

The energy needed to increase the temperature of 1g of a substance by 1°c (or 1K) is called specific heat capacity.

55
Q

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~~~

What is the specific heat capacity of water?

A

4.18 J g^-10 C^-1

56
Q

What should the reading of a calirometer be exact to?

A

A calirometer should be accurate to 0.1 or 0.2 °C.

57
Q

What is the formula to calculate the energy transferred as heat for a number of moles of reactants in the calirometer?

A

q=mcΔT
where q is heat transferred in J
m is mass of water in g
C is specific heat capacity in J g-1 °C-1
ΔT is temperature change in °C.

In this case we dont need to change the temperature in Kelvin into celcius. We can put ΔT in C or K.

With solutions (acids, alkali’s and salts) we can make the assumption that:
1cm^3 of solution has mass of 1g.
The solution has the same specific heat capacity as water.

58
Q

How can we calculate the enthalpy change per mole of stated product or reactant?

A

ΔH=-mcΔT

59
Q

Define Hess’s law.

A

The total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same.

60
Q

Suggest why enthalpy change can sometimes not be measured using calirometery?

A

Rate of reaction is too slow
Ea is too high
Co2 and H20 can be formed instead when reacting with oxygen.

61
Q

How do you draw enthalpy cycle of enthalpy change of reaction from enthalpy changes of formation?

A

To calculate the enthalpy change of reaction using this type of enthalpy cycle, we use the following procedure:
Write balanced equation at the top
Draw cycle with elements at bottom
Draw all arrows ensure correct direction (away from element)
Apply Hess’s law, taking into account the number of moles of reactant and product.

62
Q

What is the standard enthalpy change of formation of an element in its standard state?

A

Remember that the standard enthalpy change of formation of an element in its standard state is zero.

63
Q

How do you draw enthalpy cycle of enthalpy change of formation from enthalpy changes of combustion.

A

To calculate enthalpy change of formation using this type of cycle:
Write the equation for enthalpy chnage of formation at the top; add oxygen on both sides of the equation to balance the combustion reactions.
Draw the cycle with combustion products at bottom.
Draw in all arrows ensuring correct directions (towards combustion products).
Apply hess’s law taking into account the number of moles of each reactant and product.

64
Q

Why do experimentally determines values for enthalpy not always match with values in the data booklet?

A

Standard conditions were not used
Incomplete combustion of substance
Heat Loss to surroundings

65
Q

What is the standard state of flourine,chlorine,bromine,sulfur and iodine.

A

Flourine and Chlorine are gases
Bromine is a liquid
Iodine is a solid
Sulfur is a solid

66
Q

What is the energy change of a reaction when chemical energy is coverted into heat energy?

A

Chemical energy being converted into heat energy is exothermic.

67
Q

What is standard enthalpy chnage of solution?

A

The energy hange when one mole of a compound is dissolved in large amounts of water to give an infinite dilute solution.

68
Q

What type of reaction is starndard enthalpy change of solution?

A

It can be exothermic or endothermic.
If exothermic then compound is soluble and if it is endothermic then compound is insoluble.

A Level Chemistry-CIE (13 decks)

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