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A-Level Edexcel Chemistry > Chapter 8 - Chemical energies (L) > Flashcards

Chapter 8 - Chemical energies (L) Flashcards

1
Q

What two components make up chemical energy?

A

Kinetic energy

Potential energy

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2
Q

What is kinetic energy a measure of?

A

The motion of particles in a substance

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3
Q

What is potential energy a measure of?

A

How strongly particles in a substance interact with one another (attract and repel)

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4
Q

What is heat energy?

A

The portion of kinetic and potential energy of a substance that is responsible for the temperature of the substance

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5
Q

What is heating of a substance the result of?

A

The transfer of heat energy from one system to another, which in turn produces a change in temperature

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6
Q

What is the enthalpy change of a process?

A

The heat energy that is transferred between system and surroundings

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7
Q

What is an exothermic change?

A

When heat energy is transferred from the system to the surroundings

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8
Q

What is an endothermic change?

A

When heat energy is transferred from the surroundings to the system

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9
Q

In a chemical reaction, what does ‘system’ refer to?

A

The reaction mixture

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10
Q

In a chemical reaction, what does ‘surroundings’ refer to?

A

Everything outside of the system (which in practice is the air in the room of the reaction)

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11
Q

What is the definition of standard enthalpy change of reaction?

A

The enthalpy change of a reaction according to the molar quantities in the equation, under standard conditions

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12
Q

What are standard conditions?

A

100kPa pressure and a stated temperature (often 298K)

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13
Q

At the end of exothermic reactions, which has a greater enthalpy, reactants or products?

A

Reactants have a greater enthalpy, as during the reaction, heat energy has been transferred from the system to the surroundings

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14
Q

At the end of endothermic reactions, which has a greater enthalpy, reactants or products?

A

The products will have a greater enthalpy than the reactants, as heat energy has been transferred from the surroundings into the system

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15
Q

In exothermic reactions, is the enthalpy change positive or negative?

A

Negative

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16
Q

In endothermic reactions, is the enthalpy change positive or negative?

A

Positive

17
Q

How do you measure the change in enthalpy?

A

Hproducts - Hreactants

18
Q

What is the definition of standard enthalpy of neutralisation?

A

The enthalpy change when an acid and an alkali react to form 1 mole of water, under standard conditions

19
Q

What is the definition of standard enthalpy of combustion?

A

The enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions

20
Q

What is the definition of standard enthalpy change of formation?

A

The enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions

21
Q

What type of process is bond breaking?

A

An endothermic process, as energy needs to be absorbed for the bonds to break

22
Q

What type of process is bond forming?

A

Exothermic, as energy is released as a result of bonds being formed

23
Q

How do you work out enthalpy change from bond energies?

A

Energy needed to break the bonds - energy released from bond formation

24
Q

What is bond enthalpy?

A

The amount of energy required to break 1 mole of a bond type in a molecule (in gaseous state)

25
Q

Why do we take a mean bond enthalpy for the same type of bond (e.g. C-H bonds)?

A

Because different C-H bonds will have different individual bond enthalpies

26
Q

In calorimetry, what is the equation to work out heat energy change (Q)?

A

Q= m x c x ∆T

27
Q

What 3 things do you need to know to work out q (heat energy change)?

A

Mass of solution (m)
Specific heat capacity (c)
Change in temperature (∆T)

28
Q

What is the equation to work out enthalpy change after working out Q?

A

Enthalpy change = energy change / moles

29
Q

What are possible sources of error in working out the enthalpy of combustion?

A
  • Some heat energy produced would be transferred to the air and the copper can and not the water
  • Some incomplete combustion may occur, meaning the substance is not completely burned in oxygen
30
Q

How do you conduct an experiment to work out the enthalpy of combustion for ethanol?

A
  • Add known volume of water to copper can and measure temperature of water before and after
  • Weigh the spirit burner containing ethanol before and after
  • Work out temperature change and mass of ethanol burned
31
Q

How do you conduct an experiment to work out the enthalpy of neutralisation?

A
  • Add 25cm³ of 1 mole/dm³ acid into a polystyrene cup, and measure the temperature of the acid
  • Measure the temperature of the alkali, and then add 25cm³ of the alkali to the acid
  • Stir the solution until the maximum temperature is reached
32
Q

How do you work out temperature change in enthalpy of neutralisation experiments?

A

Take the average of the acid and alkali initial temperatures.
Subtract this value from the final solution temperature.

33
Q

What else must you work out in enthalpy of neutralisation calculations?

A

The moles, by multiplying the concentration and the volume (in dm)

34
Q

What uncertainties are there in enthalpy of neutralisation experiments?

A
  • Usual uncertainties of measurement involved with the use of the pipette and thermometer
  • Some heat energy would be transferred to the thermometer and cup
35
Q

Why are state symbols important in standard enthalpy of formation tests?

A

Because elements must be in their standard states; the value for enthalpy change of formation for gaseous ethanol is different to that of liquid ethanol

36
Q

Why is Hess’s law often used in standard enthalpy of formation calculations?

A

As most standard enthalpy of formation cannot be determined experimentally

37
Q

What does Hess’s law state?

A

The enthalpy change accompanying a chemical change is independent of the route by which the chemical change occurs.
I.e. going from reactants A -> products B and reactants A -> intermediates -> products B will have the same overall enthalpy change (much like vectors in maths)

A-Level Edexcel Chemistry (24 decks)

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