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Chemistry A-Level > Enthalpy (Chapter 9) > Flashcards

Enthalpy (Chapter 9) Flashcards

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1
Q

What is the equation for energy change?

A

q = mcdeltaT

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2
Q

What is the mass in the equation for energy change?

A

The total mass of solutions

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3
Q

What is the c in the equation for energy change?

A

Specific heat capacity - 4.18 (4.2) J/g/K for water

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4
Q

What is the equation for the molar enthalpy change?

A

DeltaH = -q/n

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5
Q

Exothermic or endothermic with a positive molar enthalpy change?

A

Endothermic

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6
Q

Exothermic or endothermic with a negative molar enthalpy change?

A

Exothermic

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7
Q

What is the standard enthalpy change of reaction?

A

The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with reactants and products in their standard states

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8
Q

What is a standard state?

A

The physical state of a substance under standard conditions

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9
Q

What is the standard enthalpy change of formation?

A

The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states

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10
Q

What is the standard enthalpy change of combustion?

A

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions with all reactants and products in their standard states

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11
Q

What is the standard enthalpy change of neutralisation?

A

The enthalpy change that accompanies the reaction of an acid with a base to form one mole of H2O(l) under standard conditions with all reactants and products in their standard states

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12
Q

What are standard conditions?

A

A pressure of 100kPa
A stated temperature - usually 298K (25C)
Concentration of 1mol/dm3

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13
Q

What are two examples of an exothermic reaction?

A

Fuels burning, respiration

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14
Q

What are two examples of an endothermic reaction?

A

Thermal decomposition of limestone, photosynthesis

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15
Q

What do bond enthalpies tell us?

A

About the strength of the chemical bond

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16
Q

What is a (average) bond enthalpy?

A

The energy required to break one mole of a specified type of bond in a gaseous molecule

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17
Q

Is bond breaking exothermic or endothermic?

A

Endothermic

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18
Q

Why is bond breaking endothermic?

A

Because it requires energy

19
Q

Is bond making exothermic or endothermic?

A

Exothermic

20
Q

Why is bond making exothermic?

A

Because it releases energy

21
Q

What is first needed in a reaction?

A

Energy to break the bonds (even if bonds are also made after this)

22
Q

Why might the actual bond enthalpy differ from the average value?

A
  • The bond strength will vary across the different environments in which it is found hence data tables show average bond enthalpies
  • The experiment may also have been carried out under non-standard conditions
23
Q

In terms of bond enthalpies, how do you know if a reaction is exothermic?

A
  • The energy released from making bonds is greater than the energy absorbed breaking bonds
  • The bonds broken are weaker than the bonds made
24
Q

In terms of bond enthalpies, how do you know if a reaction is endothermic?

A

The energy absorbed breaking bonds is greater than the energy released making bonds
The bonds made are weaker than the bonds broken

25
Q

What is the equation for enthalpy change of reaction, in terms of bond enthalpies?

A

Sum(bond enthalpies of bonds broken(reactants) - Sum(bond enthalpies of bonds made(products))

26
Q

What is Hess’s Law?

A
  • If a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route
  • The enthalpy change that occurs is independent of the route taken
27
Q

What does Hess’s Law allow us to do?

A

Indirectly calculate enthalpy changes where we cannot do so directly

28
Q

Why can’t you always measure enthalpy change directly?

A

High activation energy
Slow reaction rate
More than one reaction taking place

29
Q

Whats the equation for Hess’s law when given formation enthalpies? (C to A and B)

A

DeltaHr = -DeltaH1 + DeltaH2

write out the elements of formation as C

30
Q

If A goes to C and C goes to B, what is the equation for Hess’s law?

A

DeltaHr = DeltaH1 + DeltaH2

31
Q

If A and B both go to C, what is the equation for Hess’s law?

A

DeltaHr = DeltaH1 - DeltaH2

32
Q

What is specific heat capacity?

A

The energy required to raise the temperature of 1g of a substance by 1K

33
Q

What is activation energy?

A

The minimum energy input required to break bonds and for a reaction to take place

34
Q

Why might experimental enthalpy change of combustion values be different from data book values?

A

Would lead to a less exothermic enthalpy change:
- Heat energy loss to the surroundings
- Incomplete combustion occurring
- Evaporation of water (mass not 100g) then mass is lower or alcohol if using wick
Could lead to a more or less exothermic enthalpy change
- Non standard conditions

35
Q

What is enthalpy change?

A

Enthalpy of products - enthalpy of reactants

36
Q

How to reduce heat loss?

A

Use a polystyrene cup and lid

37
Q

What is enthalpy?

A

A measure of the heat energy in a chemical system

- the energy stored within bonds

38
Q

What is standard pressure?

A

100kPa

39
Q

What is standard temperature (not stated)?

A

298K

40
Q

What is standard concentration?

A

1moldm3

41
Q

What is 0K equal to?

A

-273 degrees C

42
Q

What is different about a neutralisation reaction?

A

Two solutions react (so use the m of both solutions added together)

43
Q

When you work out energy change, what are the units of the answer?

A

KJ

Chemistry A-Level (27 decks)

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