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A-level Chemistry Yr 12 Mine > 9 - Enthalpy > Flashcards

9 - Enthalpy Flashcards

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1
Q

What is Enthalpy?

A

Is a measure of the heat energy in a chemical system.

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2
Q

What does a chemical system refer to?

A

The atoms, molecules or ions making up the chemicals.

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3
Q

What is Enthalpy Change and the units for it?

A

Enthalpy change is the heat energy transferred in a reaction at constant pressure.

Units are kJ/mol

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4
Q

What is an Exothermic reaction?

A

Exothermic reaction give out energy to the surroundings from the system.

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5
Q

What is an Endothermic reaction?

A

Endothermic reactions absorb energy to the system from the surroundings.

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6
Q

How do you calculate Enthalpy change?

A

Heat of Products - Heat of Reactants

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7
Q

Is the enthalpy change negative or positive for Exothermic reactions and why?

A

The enthalpy change is negative.
This is because the chemical system loses energy and the surroundings gain energy p. So the temperature of the surrounding increase (reactants have higher temperature).
Remember you have measure the heat of the surrounding not the actual chemicals.

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8
Q

Is the enthalpy change negative or positive for Endothermic reactions and why?

A

The enthalpy change is positive.
This is because the chemical system gains energy and the surroundings lose energy. So the temperature of the surrounding decrease (products have higher temperature).
Remember you have measure the heat of the surrounding not the actual chemicals.

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9
Q

What are 3 examples of Exothermic reactions?

A

Combustions
Neutralisation
Respiration

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10
Q

What are 2 examples of endothermic reaction?

A

Photosynthesis
Thermal Decomposition

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11
Q

What are enthalpy profile diagrams?

A

They show how the enthalpy changes during a reaction.

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12
Q

What is activation energy?

A

The minimum amount of energy required for reaction to take place/start.

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13
Q

What are the standard conditions for pressure and temperature in Enthalpy Changes?

A

P- 100 kPa

T- 298 K or 25°c

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14
Q

Describe the enthalpy profile diagram for endothermic reaction?

A

The reactants start with a low enthalpy where the chemicals are more stable.
A large activation energy is required for the reaction to take place.
The enthalpy change is positive and acts upwards.
So the products have a high enthalpy (unlike reactants) where the chemicals are less stable.

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15
Q

Describe the enthalpy profile diagram for Exothermic reaction?

A

The reactants start with a high enthalpy where the chemicals are less stable.
A small activation energy (slight jump) is required for the reaction to take place.
The enthalpy change is negative and acts downwards.
So the products has a low enthalpy (unlike reactants) where the chemicals are more stable.

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16
Q

Does the enthalpy change diagram drop or rise for Exothermic reactions and endothermic reactions?

A

Rise for Endothermic reactions.

Drops for Exothermic reactions.

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17
Q

How do you calculate the activation energy on an enthalpy profile diagram?

A

From the line of reactants to the peak (highest) point in the graph.

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18
Q

How do you calculate the enthalpy change on a enthalpy profile diagram?

A

From the reactants line to the products line.

Positive for endothermic reaction and negative for Exothermic reactions.

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19
Q

What are the 4 different types of enthalpy change?

A
  1. Standard enthalpy change of reaction
  2. Standard enthalpy change of formation
  3. Standard enthalpy change of combustion
  4. Standard enthalpy change of neutralisation
20
Q

What is Standard Enthalpy change of reaction?

A

The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions (100 kPa, 298 K).

21
Q

What is Standard enthalpy change of formation?

A

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions.

22
Q

What is Standard enthalpy change of combustion?

A

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

23
Q

What is Standard enthalpy change of neutralisation?

A

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

24
Q

Why do you use standard conditions for enthalpy changes?

A

Enthalpy are affected by temperature and pressure. Using standard conditions means that everyone can know exactly what the enthalpy change is describing.

25
Q

What is standard state?

A

The physical state of a substance under standard conditions.

26
Q

What is the relationship between enthalpy and how stable a substance is?

A

The less enthalpy the more stable it is.

27
Q

What is the equation for heat loss / heat gain (q) and what are they and their units?

A

q = mc(change in T)

q = enthalpy change in Joules
m = Mass if water in grams
c = specific heat capacity of water which is 4.18 J g-1 K-1
Change in T = Change in temperature of solution (final - initial) in Kelvin

28
Q

In a laboratory experiment, 1.16 g of an organic liquid fuel was completely burned in oxygen. The heat formed during this combustion raised the temperature of 100g of water for, 295.3 K to 357.8 K. Calculate the standard enthalpy of combustion of the fuel. It’s Mr is 58.0.

A
  1. Calculate the amount of heat given out.
    q = 100 x 4.18 x (357.8 - 295.3)
    = 26,125 J
    = 26.125 kJ
  2. How many moles of fuel produced.
    n = 1.16 / 58 = 0.0200 mol
  3. Find the heat produced by one mole.
    -26.125 / 0.0200 = -1310 kJ mol-1
29
Q

Why is there always activation energy in a reaction?

A

During chemical reactions, the bonds in the reactants need to be broken by an input of energy, the new bonds in the products can then form to complete the reaction.
Reactants bonds are broken, product bonds are formed.

30
Q

Is endothermic reaction bond breaking or bond making and why?

A

Bond breaking. You need energy to break bond.

31
Q

Is Exothermic reaction bond breaking or bond making and why?

A

Bond making. Energy is released when bonds are formed.

32
Q

What does the enthalpy change show for bond making and breaking of a reaction?

A

The overall effect of the two changes of energy being taken in and released.
If there is more energy being taken in (bond breaking) than released (bond forming) the change in enthalpy with be positive. Opposite way round the EC with be negative.

33
Q

What is average bond enthalpy?

A

The energy needed to break one mole of bonds in the gaseous molecules.

34
Q

Why are bond enthalpies given as an average?

A

As bonds can different bond enthalpies, due to electron repulsion, so they give an average of all the bond enthalpies.

35
Q

Calculate the overall enthalpy change for this reaction N2 (g) + 3H2 —> 2NH3 (g)?

N triple bond N = 945 kJ mol-1
H-H = 436 kJ mol-1
N-H = 391 kJ mol-1

A

Bonds broken:
1 x 945 = 945
3 x 436 = 1340

Bonds formed :
6 x 391 = 2253

Enthalpy change of reaction = 2253 - 2346 = -93 kJ mol-1

36
Q

How do you find the enthalpy change of reaction? (With bonds)

A

Total energy absorbed (bonds broken) - total energy released (bonds made)

37
Q

Explain the practical to find the combustion of a flammable liquid?

A
  1. Measure 150 cm3 of water and record the initial temperature to the nearest 0.5°C.
  2. Weight the methanol spirit burner.
  3. Place spirit burner under the beaker and light the spirit burner stirring the water with the thermometer.
  4. After about 3 minutes extinguish the flame and immediately record the maximum temperature of the water.
  5. Re-weigh the spirit burner.
38
Q

What are the two things you need to know to calculate the enthalpy change of a reaction?

A

Number of moles
Change in temperature

39
Q

How accurate is the experiment for the combustion of a flammable liquid?

A
  • Heat loss to the surroundings, other than the water, beaker but mainly the air surrounding the flame.
  • Incomplete combustion of methanol, carbon/carbon monoxide being produced instead of carbon dioxide resulting in black soot on the beaker.
  • Evaporation of methanol from the wick after the flame has been extinguished. A cover/cap reduce this.
  • Non-standard conditions, the data value book uses this.
40
Q

How do you calculate the enthalpy change?

A

q / mol

q = heat enthalpy
mol = mol of a substance

41
Q

What is the standard concentration?

A

1 mol dm-3

42
Q

Find the standard enthalpy change of formation for C2H5OH (l)?

A

2 C (s) + 3 H2 (g) + 1/2 O2 (g) ——> C2H5OH (l)

The element given will never have a number infront as it is one mole of this substance always in enthalpy change of formation.

43
Q

Find the standard enthalpy change of combustion for C2H5OH (l)?

A

C2H5OH (l) + 3 O2 (g) ——> 2 CO2 (g) + 3 H2O (g)

44
Q

Find the standard enthalpy change of neutralisation for HNO3 (aq) + Ba(OH)2 (aq)?

A

HNO3 (aq) + 1/2 Ba(OH)2 (aq) —> 1/2 Ba(NO3)2 (l) + H2O (l)

Remember there can be no number in front of water as is is for the one mole of water.
And the charge on NO3 is -1.

45
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 energy is the same for each route.

The total enthalpy change is always the same, no matter which route is tacken.

A-level Chemistry Yr 12 Mine (15 decks)

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