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Chemistry > Energetics > Flashcards

Energetics Flashcards

1
Q

What is enthalpy change?

A

The heat energy change at a constant pressure

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2
Q

What are the units for enthalpy change?

A

KJ mol-1

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3
Q

What is the enthalpy of a reaction?

A

The heat energy stored in a chemical reaction

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4
Q

What is an exothermic reaction? Is ΔH +ve or -ve?

A

Heat energy is given out to the surroundings, causing the temp of surroundings to increase.
The reactions system looses energy so ΔH is -ve?

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5
Q

What is an endothermic reaction? Is ΔH +ve or -ve?

A

Heat energy is taken in from the surroundings, so temp of surroundings decreases.
The reactions system gains energy, so ΔH is +ve

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6
Q

Give two important examples of expthermic reactions

A

Combustion of fuels
Respiration

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7
Q

Give two important examples of endothermic reactions

A

Photosynthesis
Thermal decomposition of calcium carbonate

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8
Q

What key features should an enthalpy profile diagram show?

A

Products
Reactants
Activation energy
Enthalpy change

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9
Q

What are enthalpy profile diagrams?

A

Diagram with heat energy against time, within a system
Shows the enthalpy changes of a reaction
Box surrounding with thermometer to show surroundings

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10
Q

What is the activation energy?

A

The minmum energy required to start a reaction, by the breaking of bonds

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11
Q

What is bond enthalpy?

A

The heat energy required to break one mole of a covalent bond within a molecule in the gaseous state

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12
Q

Why are bond enthalpys always positive values?

A

Becasue heat energy is ALWAYS required to break a bond

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13
Q

What is mean bond enthalpy?

A

The heat energy required to break one mole of a covalent bond, averaged for that type of bond within a range of different compounds in the gaseous state.

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14
Q

Why might the mean bond enthalpy used rather the bond enthalpy?

A

Because the bond enthalpy for the same covalent bond may be different in different compounds

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15
Q

Give the equation for enthalpy change?

A

ΔH= energy requored to break bonds (energy in) - energy released when new bonds form (energy out)

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16
Q

What are the standard contitions considered in standard enthalpy changes?

A

298K, 100KPa

17
Q

What is a standard state?

A

The physical state of a compound when it is in standard conditions

18
Q

What is the standard enthalpy change of formation?

A

The enthalpy change when one mole of a compound is formed from its constituent elements, when all reactants and products are in the standard states

19
Q

What is the standard enthalpy change of formation of an element?

A

0

20
Q

How does the standard enthalpy change of formation of a compound tell you how stable it is?

A

The more negative the value, the stable the compound is

21
Q

What is the standard enthalpy change of combustion?

A

The enthalpy change when one mole of a compiund reacts completely in oxugen, with all reactants and products in the standard states

22
Q

State the first law of thermodynamics

A

Energy cannot be created or destroyed, but it can be changed from one form to another

23
Q

State Hess’s law

A

The enthalpy change of a chemical reaction is independent of the route taken.

24
Q

What is Hess’s law used for?

A

Used to measure the enthalpy change of a reaction that cannot be measured directly, using the enthalpy cycle

25
Q

Why might it not be easy to measure the enthalpy change of a reaction directly?

A
  • the rate of reaction may be too slow
  • there may be a very high activation energy
26
Q

When using Hess’s law, which way should the arrows piint when using combustion or formation data?

A

Formation- arrows point upwards
Combustion- arrows point downwards

27
Q

What is the formula for q (change in heat)?

A

q = mc∆T

28
Q

What is the firmula for enthalpy change?

A

∆H = q / n

29
Q

Why might the measured enthalpy change of combustion of a experiment not be accurate?

A

Due to heat loss or incomplete combustion

Chemistry (13 decks)

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