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Chem Everything > 8 Energetics 1 > Flashcards

8 Energetics 1 Flashcards

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1
Q

What does system mean in a chemical reaction?

A

The atoms and bonds involved in the
chemical reaction

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2
Q

Explain the law of conservation

A

The amount of energy in an isolated system remains the same. Energy cannot be destroyed or created, It can only be transferred from one form to another

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3
Q

What energy change is breaking bonds associated with?

A

Energy is taken in to break bonds → endothermic reaction

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4
Q

What energy change is making bonds associated with?

A

Energy is released to make bonds → exothermic reaction

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5
Q

What is an endothermic reaction?

A

A reaction with an overall positive enthalpy change (+ΔH) → enthalpy of products > enthalpy of reactants

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6
Q

What is an exothermic reaction?

A

A reaction with an overall negative enthalpy change (-ΔH) → enthalpy of products < enthalpy of reactants

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7
Q

Describe the diagram for an endothermic reaction

A

Reactants lower than products

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8
Q

Describe the diagram for an exothermic reaction

A

Reactants higher than products

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9
Q

What does activation energy mean?

A

The minimum energy required for a reaction to take place

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10
Q

Which way does the arrow for activation energy point on an enthalpy profile diagram?

A

Always points upwards

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11
Q

What are the standard conditions?

A

100 kPa
298 K

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12
Q

What does “in standard state” mean?

A

The state an element / compound exists at in standard conditions (100 kPa, 298 K)

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13
Q

Define enthalpy change of formation

A

The energy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard state under standard conditions

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14
Q

Give an example of an equation which represents standard enthalpy of formation

A

H 2 (g) + ½ O 2 (g) → H 2O (l)

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15
Q

Define enthalpy change of combustion

A

The energy change that takes place when 1 mole of a substance is burnt in excess oxygen under standard conditions.

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16
Q

Give an example of an equation which represents standard enthalpy of combustion

A

C (s) + O 2 (g) → CO 2 (g)

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17
Q

Define enthalpy change of neutralisation

A

The energy change that takes place when 1 mole of water is formed by reacting an acid and alkali under standard conditions.

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18
Q

What does enthalpy change of reaction mean?

A

The energy change when the amounts of reactants shown in the equation react under standard conditions to give the products in their standard states.

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19
Q

How can you calculate enthalpy change from experimental data?

A

Use the equation Q = mcΔT, where m is the mass of the substance being heated (usually water), c is the specific heat capacity of that substance (water’s SHC = 4.18gJ-1K-1) and ΔT is the change in temperature

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20
Q

Draw a simple calorimeter

A

A polystyrene cup, a vacuum flask or a metal can.
A plastic lid
A thermometer through the lid.

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21
Q

Why might experimental methods for enthalpy determination not be accurate?

A

Heat is lost to the surroundings
Not in standard conditions
Reaction may not go to completion

22
Q

What does average bond enthalpy mean?

A

The mean energy required to break 1 mole of bonds in gaseous molecules

23
Q

Why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation?

A

Bond enthalpies are a mean for the same bond across different molecules whereas standard enthalpy of combustion and formation apply just to that molecule, therefore they are more accurate

24
Q

How to calculate enthalpy change of reaction using average bond
enthalpies?

A

ΔH = Σ (bond enthalpies of reaction) - Σ (bond enthalpies of products)

25
Q

What is Hess’s Law?

A

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

26
Q

Is the enthalpy of reaction exothermic or endothermic?

A

Can be both

27
Q

Is the enthalpy of formation exothermic or endothermic?

A

Can be both

28
Q

Is the enthalpy of combustion exothermic or endothermic?

A

Exothermic

29
Q

Is the enthalpy of neutralisation exothermic or endothermic?

A

Exothermic

30
Q

What does enthalpy mean?

A

The total chemical energy inside a substance.

31
Q

What is enthalpy also known as?

A

Heat content

32
Q

What happens when there is enthalpy change?

A

A change in chemical energy due to a bond breaking or forming.

33
Q

When is activation energy shown on a diagram?

A

When it’s reaction profile diagram not enthalpy level diagram.

34
Q

What happens in an exothermic reaction?

A

Heat energy is given off to the surroundings. Hence the temperature of the surroundings increases and the energy of the system decreases.

35
Q

Why might an exothermic reaction not occur?

A

When the rate of reaction is too slow.

36
Q

When is a reaction kinetically controlled?

A

A reaction in which the product ratio is determined by the rate at which the products are formed.

37
Q

What happens in an endothermic reaction?

A

Heat energy is absorbed from the surroundings. Hence the temperature of the surroundings decreases and the energy of the system increases.

38
Q

What is calorimetry?

A

Calorimetry is the measurement of enthalpy changes in chemical reactions.

39
Q

What is the specific heat capacity of a liquid?

A

The energy needed to increase the temperature of 1g of substance by °C.

40
Q

What is the specific heat capacity of water?

A

4.18 J g^-1 K^-1

41
Q

What other substances can we use the SHC of water for and why?

A

Aqueous solutions of acids, alkalis, and salts.
They are assumed to be largely water.

42
Q

What is the formula for ΔH?

A

ΔH = (-Q)/n = (-(mcΔT))/n

43
Q

What is a closed system?

A

A closed system can exchange energy (as heat or work) but not matter, with its surroundings.

44
Q

What is the total energy present in a closed system?

A

Always constant

45
Q

What is bond dissociation enthalpy?

A

The energy required to break a particular bond.

46
Q

What is bond dissociation enthalpy also known as?

A

Bond enthalpy or bond energy.

47
Q

When are the products more stable than the reactants?

A

An exothermic reaction

48
Q

When are the products less stable than the reactants?

A

An endothermic reaction

49
Q

What is average bond enthalpy?

A

The energy needed to break one mole of bonds in a gaseous molecule averaged over similar compounds.

50
Q

What is the average bond enthalpy of C-H?

A

The bond dissociation enthalpy for the whole molecule and dividing it by the number of C-H bonds.

51
Q

Why would each bond enthalpy of C-H bonds in methane be different?

A

The first bond is easier to break than the second as the remaining hydrogens are pulled more closely to the carbon.

Chem Everything (7 decks)

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