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3 Energetics Flashcards

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1
Q

What is an exothermic reaction?

A

It is a reaction in which heat energy is given out to the surroundings.

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2
Q

What is an example of an exothermic reaction?

A

Adding water to calcium oxide. If you add water to solid calcium oxide, the heat produced is enough to boil the water and produce steam. Calcium hydroxide is produced.

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3
Q

What has more chemical energy in an exothermic reaction, the reactants or the products?

A

The reactants.

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4
Q

Why do the reactants have more chemical energy?

A

This is because the chemical energy in the reactants is converted to heat energy during the reaction, which is released to the surroundings. This means that the temperature of the mixture and its surroundings go up.

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5
Q

What is a diagram that shows how heat is lost during an exothermic reaction?

A
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6
Q

What are some examples of exothermic reactions?

A
  • Any reaction in which a flame is produced (combustion).
  • The reactions of metals with acids.
  • Neutralisation reactions.
  • Displacement reactions.
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7
Q

What is an example of combustion?

A

When hydrogen burns in oxygen - producing water and lots of heat.

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8
Q

What is an example of a reaction of metals with acids?

A

When magnesium reacts with dilute sulfuric acid - the mixture gets very warm.

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9
Q

What is an example of a neutralisation reaction?

A

When sodium hydroxide solution reacts with dilute hydrochloric acid - the temperature rises.

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10
Q

What is an example of displacement reactions?

A

The reactions between powdered aluminium and iron oxide, this reaction releases a large amount of heat.

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11
Q

How we measure the amount of heat energy taken in or released in a chemical reaction?

A

By the enthalpy change.

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12
Q

What is the enthalpy change?

A

It is the amount of heat energy taken in or given out in a chemical reaction. IT is the difference between the energy of the products and the energy of the reactants.

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13
Q

What symbol is given to the enthalpy change?

A

ΔH.

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14
Q

How do you show whether heat is being given out or absorbed by the reaction?

A

By using + and - signs following the answer found for ΔH.

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15
Q

How do we determine whether or not we put a plus or a minus sign?

A

You look at it from the point of the reactants.

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16
Q

When do we use a - sign?

A

When the reactants lose energy.

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17
Q

When do we use a + sign?

A

When the reactants gain energy.

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18
Q

What are the units for ΔH?

A

KJ/mol (kilojoules per mole).

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19
Q

What is an an endothermic reaction?

A

A reaction that absorbs heat from the surroundings is said to be endothermic.

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20
Q

What has more chemical energy in an endothermic reaction, the reactants or the products?

A

The products.

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21
Q

Why do the products have more chemical energy?

A

In order to supply the extra energy that is needed to convert the reactants (lower energy) to the products (higher energy) - heat energy needs to be absorbed from the surroundings. This heat energy is then converted to chemical energy which is then stored in the bonds of chemicals.

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22
Q

What happens to the temperature of the reaction mixture and surroundings?

A

They go down because heat energy has been converted into a different form of energy.

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23
Q

What is the specific heat capacity?

A

It is the amount of heat needed to raise the temperature of 1g of substance by 1*C.

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24
Q

What is the amount of heat energy required directly proportional to?

A

The mass and the temperature change.

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25
Q

How do you represent the temperature change?

A

ΔT.

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26
Q

What is the equation for energy change?

A

Q = m x c x ΔT

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27
Q

What is Q in this reaction?

A

The heat energy change.

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28
Q

What is m in this reaction?

A

The mass.

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29
Q

What is c in this reaction?

A

Specific heat capacity.

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30
Q

What is ΔT in this reaction?

A

The temperature change?

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31
Q

What is the value that we use for specific heat capacity?

A

4.18J/g/c. Which is the specific heat capacity of water.

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32
Q

How can we measure the amount of heat absorbed or given out in reactions or physical changes?

A

By using calorimetry.

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33
Q

What is calorimetry?

A

It is measuring the heat given out or taken in by a chemical reaction.

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34
Q

What is the idea of calorimetry based on?

A

That if we use the heat from a reaction to heat another substance, such as water, we can then use the equation for heat energy change to calculate the amount of heat released.

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35
Q

What is ΔH?

A

The molar enthalpy change.

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36
Q

What is the molar enthalpy change?

A

The change in enthalpy when 1 mole of a particular reactant reacts.

37
Q

How do you measure the enthalpy changed in combustion reactions?

A

It is to measure the amount of heat given off when a number of small alcohols are burned. You could use methanol, ethanol, propan-1-ol and butan-1-ol.

38
Q

Where are the alcohols burned?

A

In a small spirit burner.

39
Q

Where does the heat produced by these burning alcohols go?

A

To heat some water in a copper can (the calorimeter).

40
Q

What is the procedure that we follow to measure the enthalpy change in combustion reactions?

A

1) Measure 100cm3 of cold water using a measuring cylinder and transfer the water to a copper can.
2) Take the initial temperature of water.
3) Weigh a spirit- burner containing ethanol with its lid on. The lid should be kept on when the wick is not lit to prevent the alcohol from evaporating.
4) Arrange the apparatus so that the spirit-burner can be used to heat the water in a copper can. The apparatus is shielded as far as possible to prevent draughts.
5) Light the wick to heat the water. Stop heating when you have a reasonable temperature rise of water. The flame can be extinguished by putting the lid back on the wick.
6) Stir the water thoroughly and measure the maximum temperature of the water.
7) Weigh the spirit burner again with its lid on.
8) The experiment can be repeated with the same alcohol to check for reliability, and then carried out again with whatever other alcohols are available.

41
Q

What are the steps used to calculate Q in this experiment.

A

1) You use the equation provided to find the Q.
2) The mass would be the volume of water being heated.
3) The specific heat capacity would be 4.18 - that of water.
4) ΔT would be the temperature change of water.
5) When you find Q, divide it by 1000 to ensure you find the answer in KJ rather than J.

42
Q

What equation do you use next after finding Q in this experiment?

A

n = m/mr.

43
Q

Why do we use the equation
n = m/mr?

A

To find out how many moles of ethanol are burned in your experiment.

44
Q

What would the m be?

A

The mass of ethanol burned.

45
Q

What would the Mr be?

A

The relative molecular mass of ethanol.

46
Q

What equation would you use after finding the number of moles?

A

The ΔH formula.

47
Q

What is the ΔH formula?

A

ΔH = Heat energy change (Q)
———————————-
Number of mols of ethanol burned (n)

48
Q

What do you do after finding ΔH?

A

You add the corresponding + or - sign to the value you have, depending on whether or not it is exothermic or endothermic.

49
Q

What are the sources of error in this experiment?

A
  • The warm water gives out heat to the air which means that the heat is lost from the flame which goes straight into the air rather than into the water and heat is the one thing we need as much of to raise the temperature of the water.
  • The incomplete combustion of alcohol. It occurs when there is not enough oxygen present. This kind of combustion releases less heat than complete combustion.
50
Q

How can we tell if incomplete combustion has occurred in this experiment?

A
  • The flame of the wick is often yellow/orange rather than blue.
  • We can see carbon (soot) build up at the bottom of the can.
51
Q

What should happen if complete combustion occured?

A
  • The flame of the wick should be blue
  • Carbon dioxide should be produced rather than carbon.
52
Q

What do can we use this experiment for if it isn’t accurate and reliable?

A

You can find out how the heat given out changes as the alcohol gets bigger. We have been able to observe that the combustion reaction gets more exothermic as the alcohol chain becomes longer.

53
Q

What does the heat given out by alcohols have in relation to their size?

A

Longer alcohols give out more heat energy per mole when they burn than shorter ones.

54
Q

Why do longer chain alcohols release more energy?

A

The greater amount of carbon atoms in the alcohol, accounts for the greater number of intramolecular bonds. Therefore, at combustion, more heat content can be released when there is a greater number of carbons due to an increase in intermolecular bonds.

55
Q

What is the procedure used to measure enthalpy changes for displacement reactions?

A

1) Place a polystyrene cup in a 250cm3 glass beaker.
2) Transfer 50cm3 of 0.200 mol/dm3 of copper sulfate solution into the polystyrene cup using a measuring cylinder.
3) Weigh 1.20g of zinc using a weighing boat on a balance.
4) Record the initial temperature of the copper sulfate solution.
5) Add the zinc.
6) Stir the solution as quickly as possible.
7) Record the maximum temperature reached.

56
Q

How do you calculate Q in this experiment?

A

Using the mass which is given.
The specific heat capacity of water (4.18).
ΔH can be found by minus-sing the maximum from the initial temperature of the copper sulfate solution.

57
Q

What do we assume in this experiment?

A
  • The density of the copper sulfate solution is the same as that of water, so 1cm3 of solution has a mass of 1g.
  • The specific heat capacity of the mixture is the same as that of water. This is a fairly reasonable assumption because the mixture is mostly water.
58
Q

What has happened to the zinc in this experment?

A

We have used excess zinc, meaning that more than enough zinc is present to ensure that all the copper sulfate reacts.

59
Q

After finding the Q in the experiment what do we do?

A

Find the number of moles of both things used in the experiment.

60
Q

What do we find the number of moles of using n = m/ar?

A

Zinc.

61
Q

What do we find the number of moles of using n = v x c?

A

Copper sulfate.

62
Q

What do we have to do to the volume of copper sulfate?

A

Divide by 1000 to convert it to dm3, because concentration is given in mol/dm3.

63
Q

What equation do we use next?

A

ΔH = Heat energy change (Q)
//////////////////////////////////////////////////
Number of moles (n)

64
Q

What is the number of moles that we use in this equation?

A

The moles of copper sulfate reacted.

65
Q

For the ΔH of this reaction, what do we do?

A

We make it negative because we know that the experiment was exothermic since the temperature of the reaction mixture went up and heat was released.

66
Q

What is the procedure that we use to measure enthalpy changes when salts dissolve in water?

A

1) Place a polystyrene cup in a 250cm3 glass beaker.
2) Transfer 100cm3 of water into the polystyrene cup using a measuring cylinder.
3) Record the initial temperature of the water.
4) Weigh 5.20g of ammonium chloride using a weighing boat on a balance.
5) Add the ammonium chloride to water and stir the solution vigorously until all of the ammonium chloride has dissolved.
6) Record the minimum temperature.

67
Q

How do we know that this reaction is endothermic?

A

We know this because the temperature of the water decreases so the reaction is endothermic and heat is absorbed from the surroundings for the dissolving process to occur.

68
Q

How do you find the Q in this experiment?

A

Q = m x c x ΔT

69
Q

How do you find all those components that are used in this equation?

A

Everything can be found in the question. Use the mass as 100, the specific heat capacity as 4.18, and the temperature change as initial - minimum.

70
Q

What do we assume in this experiment?

A

1) The specific heat capacity of the diluted solution of the ammonium chloride is the same as that of water.
2) The mass of the solution is 100g. The mass of the ammonium chloride is ignored in the calculation.

71
Q

What are other sources of error in this experiment?

A

Heat is absorbed from surrounding air, which makes much more difference to the results.

72
Q

What do we do after we have found Q?

A

We find the number of moles.

73
Q

How do we find the number of moles?

A

Using the equation n = m/mr.

74
Q

Which substances number of moles are we finding?

A

Of ammonium chloride.

75
Q

What is going to happen to ΔH in this experiment?

A

It will have a positive sign in front of it because of the fact that it was endothermic.

76
Q

What is the procedure used to measure enthalpy changes of neutralisation between an alkali and an acid?

A

1) Place a polystyrene cup in a 250cm3 glass beaker.
2) Transfer 25cm3 of 2.00mol/dm3 of potassium hydroxide into the polystyrene cup using a measuring cylinder.
3) Record the initial temperature.
4) Fill a burette with 5cm3 of dilute hydrochloric acid.
5) Use the burette to add 5cm3 of dilute hydrochloric acid to the potassium hydroxide.
6) stir vigorously and record the maximum temperature reached.
7) Continue adding further 5cm3 portions of dilute hydrochloric acid to the cup, stirring and recording the maximum temperature each time, until a total volume of 50cm3 has been added.

77
Q

What happens in reference to the temperature in this reaction?

A

The temperature increases at first, but then decreases.

78
Q

What kind of reaction happens between the acid and the alkali?

A

An exothermic reaction as the temperature increases while there is a reaction going on.

79
Q

What causes the temperature to fluctuate throughout the experiment?

A

At the beginning, the temperature goes up because the acid reacts with the alkali, giving out heat. But when the alkali has been used up - we’re just adding cold acid to our warm solution; decreasing the temperature.

80
Q

What do we calculate now?

A

Q = m x c x ΔT

81
Q

What is the total volume of the solution?

A

53.

82
Q

Why is the total volume 53?

A

It is 53 because we have looked at the graph and added the volume that was present when the maximum temperature was reached.

83
Q

What do we assume in this experiment?

A
  • The density of the reaction mixture is the same as that of water, so 1cm3 of solution has a mass of 1g.
  • The specific heat capacity of the mixture is the same as that of water. This is a fairly reasonable assumption because the neutralised solution is mostly water.
84
Q

What do we do after we have found Q?

A

We find the number of moles.

85
Q

What formula do we use to find the number of moles?

A

n = c x v.

86
Q

What is the substance that we are trying to find the number of moles for?

A

KOH.

87
Q

In what equation do we then use the number of moles that we have found in?

A

ΔH = Q/n

88
Q

What is n in that reaction?

A

The number of moles of KOH.

89
Q

What happens to ΔH in this reaction?

A

The value that you found is expressed with a negative sign in front of it. This is because it is an exothermic reaction.

Chemistry (22 decks)

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