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CheMisTrY - Developing Fuels > DF.1 > Flashcards

DF.1 Flashcards

Thermochemistry

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1
Q

What is thermochemistry?

A

Thermochemistry is the study of the energy and heat associated with chemical reactions.

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2
Q

What happens in an exothermic reaction?

A

Energy is released during the reaction and temperature of the surroundings increase.

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3
Q

What happens in an endothermic reaction?

A

Energy is taken in during the reaction from the surroundings, and therefore the temperature of the surroundings decrease.

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4
Q

How does the products’ energy store compare to the reactants’ store in an exothermic reaction?

A

The products has less energy than the reactants.

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5
Q

How does the products’ energy store compare to the reactants’ store in an endothermic reaction?

A

The product has more energy than the reactants.

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6
Q

What is enthalpy change the measure of?

A

The energy transferred to and from the surroundings. It is the difference between the reactants’ and products’ energy stores.

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7
Q

What is the symbol representation for enthalpy change?

A

ΔH

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8
Q

What sign would the enthalpy change have for an exothermic reaction?

A
  • negative, because energy is released to the surroundings.
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9
Q

What sign would the enthalpy change have for an endothermic reaction?

A

+ postive, because energy is taken in from the surroundings.

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10
Q

What is the equation that represents what enthalpy change is measuring?

A

ΔH = H(products) - H(reactants)

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11
Q

What does H represent?

A

Enthalpy

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12
Q

What is the typical unit for enthalpy change?

A

kJmol-1

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13
Q

What does system refer to in a chemical reaction?

A

The reactants and the products of the reaction.

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14
Q

What does the surroundings refer to in a chemical reaction?

A

The rest of the world, e.g. the test tube, you, the air.

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15
Q

What are some conditions that might affect the ΔH value of a reaction?

A
  • temperature
  • pressure
  • concentrations of solutions
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16
Q

Because ΔH varies with different conditions, it is important to keep a standard when measuring. What is the standard condition that has been set for measuring and comparing enthalpy changes?

A
  • Temperature = 25℃ / 298K
  • Pressure = 1 atmosphere
  • Concentration of solution = 1 moldm-3
17
Q

What is the symbol representation for an enthalpy change measured under standard conditions?

A

ΔH⦵

18
Q

How many Kelvin is 0℃?

A

273K

19
Q

What is the symbol representation for standard enthalpy change for a reaction?

A

ΔHr⦵, where r refers to reaction.

20
Q

What is the standard enthalpy change for a reaction?

A

The enthalpy change when molar quantities of reactants as stated in the equation react together under standard conditions.

21
Q

How can enthalpy changes of a reaction be measured in the lab?

A

By arranging for the energy involved in a reaction to be transferred to or from water surrounding the reaction vessel. If the reaction is exothermic, the water would get hotter and if the reaction is endothermic the water would get cooler. By measuring the temperature of the water, and knowing the mass and specific heat capacity of water, you can calculate the amount of energy that was transferred to or from the water during the reaction.

22
Q

What is the equation used to calculate the energy transferred to water?

A

Energy transferred (J) = specific heat capacity (of water 4.18Jg-1K-1) x mass (g) x temperature change

23
Q

What is the density of water?

A

1gcm-1

24
Q

How do you calculate the mass of the water from volume?

A

mass = density x volume.

Because density of water is 1, therefore its mass will be the same as its volume in cm3, numerically.

25
Q

What is the standard enthalpy change of combustion?

A

The enthalpy change that occurs when one mole of a substance is burnt completely in oxygen under standard conditions in standard states.

26
Q

What is the symbol representation for standard enthalpy change of combustion?

A

ΔHc⦵, where c refers to combustion.

27
Q

Is it possible to measure enthalpy change of combustion under standard conditions in practice?

A

It is impossible, therefore adjustments are made to allow for the non standard conditions.

28
Q

What sign would the enthalpy change of combustion always have?

A
  • negative, because all combustion reactions are exothermic.
29
Q

What is the standard enthalpy change of formation?

A

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

30
Q

What is the symbol representation of standard enthalpy change of formation?

A

ΔHf⦵, where f refers to formation.

31
Q

It is often impossible to measure enthalpy changes of formation directly, so how do we find out the values?

A

They can be found indirectly by making use of measurable quantities and incorporating them into enthalpy cycles.

32
Q

What is the standard enthalpy change of neutralization?

A

The enthalpy change when one mole of hydrogen ions react with one mole of hydroxide ions to form one mole of water under standard conditions.

33
Q

What is the symbol representation of standard enthalpy change of neutralization?

A

ΔHneut⦵, where neut refers to neutralization.

34
Q

When setting up a practical for measuring the enthalpy change of combustion of a fuel, why is a copper can used to hold the water?

A

Because the specific heat capacity of metal is very low therefore very little energy goes into heating up the can, so it affect the end result of the enthalpy change only by a little bit.

35
Q

Why is the real enthalpy change measured in practice (in lab practicals), never exactly the same as the theoretical/ proposed value?

A
  1. Incomplete combustion taking place.
  2. Energy lost to the surrounding and apparatus.
  3. Not done in standard conditions.

CheMisTrY - Developing Fuels (11 decks)

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