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Chemistry > Chapter 9 > Flashcards

Chapter 9 Flashcards

1
Q

What is enthalpy?

A

Is the measure of heat energy within a chemical system. It is written as the letter H.

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2
Q

What does a chemical system refer to?

A

The atoms, molecules or ions which make up the chemicals.

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3
Q

What is enthalpy change?

A

It is the difference in enthalpies within the products and reactants. It is calculated by H(products)- H(reactants). It can be either positive or negative, it just depends on whether the products have more or less energy than the reactants.

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4
Q

What is the conservation of energy?

A

The law of conservation states that energy can’t be created or destroyed in the reaction. When a chemical reaction involves meant to be change heat energy is transferred between the system and the surroundings. The system is the chemicals – the reactants and products
The surroundings being apparatus – lab everything apart from the chemical system.

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5
Q

In terms of the conservation of energy what is the universe?

A

It is everything including the system and surroundings

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6
Q

What is an exothermic change?

A

It is when energy is transferred from the system to the surroundings increasing the temperature.

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7
Q

What is an endothermic change?

A

It is when energy is transferred from the surroundings to the system which causes a decrease in temperature.

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8
Q

What are the signs of exothermic and endothermic enthalpy changes.

A

Exothermic has a negative enthalpy change

Endothermic has a positive enthalpy change

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9
Q

Describe a complete reaction?

A

In a chemical reaction the bones need to break but input energy with new bonds being formed in the products completing the reaction.

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10
Q

What is the activation energy?

A

It’s the minimum amount of energy required for a reaction to take place (break the bonds). The energy input required to break the bonds acts as an energy barrier to the reaction.

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11
Q

Describe the speed of small activation energies?

A

It occurs rapidly as the energy needed to break the bonds is readily.

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12
Q

Describe the speed of large activation energies?

A

A large activation energy means that a very large energy barrier is present so direction will occur very slowly or not at all with a large amount of energy needed to break the bonds.

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13
Q

Describe an endothermic reaction?

A

It’s when energy is transferred from the surroundings to the system. The chemical system takes in heat energy from the surroundings. Any energy gained by the chemical system is balanced by energy lost by the surroundings. The enthalpy change is positive with the temperature of the surroundings decreasing as they lose energy.

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14
Q

Describe an exothermic reaction?

A

It’s when energy is transferred from the system to the surroundings. The chemical system releases energy into surroundings. Any energy loss by the chemical system is balanced by the same energy gain by their surroundings. The enthalpy change is negative with the temperature of the surroundings increasing as they gain energy.

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15
Q

What is the standard enthalpy change?

A

It refers to entropy change occurring under standard conditions the units are KJmol

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16
Q

What is the standard pressure?

A

100 KPa (close to 1 atm which is 101KPa)

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17
Q

What is the standard temperature?

A

298K (25 degrees Celsius )

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18
Q

What is the standard state?

A

Is the physical state of a substance under standard conditions the standard state is at 100 kPa and 298K

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19
Q

What is the standard concentration?

A

It is 1 mol/dm³.

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20
Q

What is the standard enthalpy change of reaction?

A

It is the enthalpy change that a common accompanies a reaction in the molar quantities. It is shown in a chemical equation under standard conditions with all reactants and products in their standard states. It refers to a state equation and its value depending on the balance numbers

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21
Q

What is the enthalpy change of formation?

A

It is the enthalpy change that occurs 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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22
Q

What is the enthalpy change of formation in elements?

A

It is zero as there is no chemical change taking place.

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23
Q

What is the enthalpy change of combustion?

A

The standard enthalpy change of combustion is the enthalpy change that occurs when one mole of a substance reacts completely with oxygen under standard conditions with all reactants and products in their standard states.

24
Q

What is the standard enthalpy change of neutralisation?

A

It’s the enthalpy change that accompanies the reaction of an acid and a base to form one mole of water understand the conditions with all reactants and products in their standard states.

25
Q

What is the standard enthalpy change of neutralisation value?

A

The value stays the same for all neutralisation reactions (a strong acid and a strong base will have the same value as they completely dissociate each other) the value is -57 kJ per mole.

26
Q

Is the thermometer part of the chemical system or the surroundings?

A

The surroundings

27
Q

Describe the Kelvin scale of temperature?

A

It starts at absolute zero (0K) which is equivalent to -273°C. On the Kelvin scale ice melts at 273K (0°C) and water boils at 373K (100°C). For 1K rise equals 1°C rise in temperature. The temperature change in degrees Celsius is the same as in Kelvin.

28
Q

What is the equation used to calculate the heat energy transferred, q ?

A

q= m x c x change in temperature

29
Q

Explain the mass of surroundings, m, used in the equation?

A

You need to identify the materials which are changing in temperature this is measured in grams.

30
Q

Explain the specific heat capacity, c, used in the equation?

A

Different materials require different quantities of energy to produce the same temperature change. The specific heat capacity of good conductors is low and insulated of heat have high specific heat capacities.

31
Q

What is specific heat capacity?

A

It’s the energy required to raise the temperature of 1 g by 1K

32
Q

What is the specific heat capacity of water?

A

4.18 KJg

33
Q

Explain the temperature change used in the equation?

A

It is determined by the thermometer readings. The final temperature - initial temperature.

34
Q

What is heat energy measured in, q?

A

Joules

35
Q

Describe the experiment used to determine the enthalpy change of combustion?

A
  1. Using a measuring cylinder to measure 150 cm³ of water. Pour the water in a beaker and record an initial temperature of the water to the nearest 0.5°C.
  2. Add methanol to the spirit burner, why the spirit burner containing the methanol.
  3. Place a spirit burner under the beaker and light it. The methanol needs to be burning while stirring the water with a thermometer.
  4. After about three minutes extinguish the flame and record the maximum temperature immediately.
  5. Reweigh the spirit burner containing methanol ensuring the wick is not burnt.
36
Q

How to calculate the enthalpy change of combustion?

A
  1. Calculate the energy change of water in KJ, using the equation (q=m x c x change in temp). Then divide by 1000
  2. Calculate the number of moles burnt e.g methanol (using n= m/M)
  3. Calculate the enthalpy change of combustion. (You get your q value and divide that by the amount/moles of the liquid burnt e.g methanol)
  4. Decide whether its an exothermic or endothermic reaction
37
Q

What is the density of water?

A

1 g/cm³ so 150cm³ is 150g

38
Q

Why are values of enthalpy change of combustion less accurate than the standard value?

A

Heat loss to the surroundings e.g the air surrounding the flame and the beaker
Incomplete combustion of the spirit/alcohol, which produces carbon monoxide and carbon (soot).
Evaporation of the spirit from the wick, it needs to be reweighed immediately after the flame is extinguished. However spirit burners do have a cover to reduce this error.
Non standard conditions, the data book is in standard conditions, with the experiment carried out less likely to be in standard conditions.

39
Q

How is accuracy improved with the enthalpy combustion value?

A

Draft screens are used with an input of oxygen gas to reduce errors from heat loss and incomplete combustion.

40
Q

Describe the determination of enthalpy change in reaction?

A

Many reactions occur between two solutions or a solid and a solution. This can be measured with polystyrene cups which are cheap, light, waterproof and provide some insulation. Carrying out this reaction between aqueous solutions the solution itself is the immediate surroundings. The particles may react when they collide and any energy transfer is between the chemical particles and the water molecules in the solution.

41
Q

Describe how you would calculate the enthalpy change of a reaction?

A
  1. Calculate the energy change of water in KJ, using the equation (q=m x c x change in temp). Then divide by 1000
  2. Calculate the number of moles reacted e.g copper sulfate (using n= m/M)
  3. Calculate the enthalpy change of reaction. (You get your q value and divide that by the amount/moles of the reactant e.g copper sulfate)
  4. Decide whether its an exothermic or endothermic reaction
42
Q

What do you do when calculating the enthalpy change of neutralisation.

A

Find the moles of water produced during the reaction.

43
Q

What do you do when calculating the enthalpy change of formation.

A

Find the number of moles formed.

44
Q

Why are cooling curves used?

A

A cooling curve extrapolation can be used to increase accuracy of the enthalppy change value as it corrects heat loss in the experiment.

45
Q

What is the average bond enthalpy?

A

It is the energy required to break one mole of a specified type of bond in a gaseous molecule.

46
Q

What type of reaction are bond enthalpies?

A

They are always endothermic so are positive. A larger bond enthalpy value will have a stronger bond. Energy is always required to break the bond.

47
Q

What are some limitations of average bond enthalpies?

A

An actual bond enthalpy can vary depending on the chemical environment of the bond.

48
Q

Is bond breaking exothermic or endothermic?

A

Energy required to break bonds is an endothermic reaction so the enthalpy change is positive.

49
Q

Is bond making exothermic or endothermic?

A

Energy released when bonds form/bond making is an exothermic reaction so the enthalpy change is negative.

50
Q

How is a reaction determined whether it’s an exothermic reaction or an endothermic reaction?

A

The difference between the energy required for bond breaking and the energy released by bond making determines the overall reaction.

51
Q

How is average bond enthalpies calculated?

A

sum of (bond enthalpies in reactants) - sum of (bond enthalpies in the products)

52
Q

What are the limitations of using average bond enthalpies?

A

Using an average instead of the actual energy involved in breaking and forming bonds will alter as some bonds could be in different environments. However the calculated enthalpy change of reaction should be a general agreement with the enthalpy change of reaction. All species must be gaseous when calculating the average bond enthalpies. Water has been found in the gaseous state which is not a standard enthalpy change but you do take into account the enthalpy change for condensing H2O (g) into H2O (l)

53
Q

What is Hess’ law?

A

He states that if a reaction can take place by two routes and the starting and finishing routes are the same the total enthalpy change is the same for each route.

54
Q

Give the equation to work out the enthalpy change of reaction using enthalpy change of formation?

A

Total enthalpy change of formation PRODUCTS - total enthalpy change of formation REACTANTS.

55
Q

Give the equation to work out the enthalpy change of reaction using enthalpy change of combustion?

A

Total enthalpy change of combustion REACTANTS - total enthalpy change of combustion PRODUCTS

Chemistry (23 decks)

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