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Physical Chemistry > Energetics 1 > Flashcards

Energetics 1 Flashcards

1
Q

What is thermochemistry

A

The study of heat changes during a reaction.

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2
Q

What type of process is bond breaking

A

Endothermic

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3
Q

What type of process is bond formation

A

Exothermic

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4
Q

What is an exothermic reaction

A

A reaction where energy is released into the surroundings.

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5
Q

What is an endothermic reaction

A

A reaction that takes in energy from the surroundings.

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6
Q

What is enthalpy change

A

When a heat change is measured at constant pressure

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7
Q

What are the standard conditions for measuring enthalpy change

A

Pressure: 100kPa
Temperature: 25 degrees Celsius or 298K
Concentration: 1.0M

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8
Q

What is standard state

A

The physical state of a substance under standard conditions.

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9
Q

Define standard enthalpy change of reaction

A

The standard enthalpy change of reaction is the enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states.

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10
Q

Define standard enthalpy change of combustion

A

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

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11
Q

Define standard enthalpy change of formation

A

The standard enthalpy change of formation is the enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states under standard conditions.

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12
Q

What will the overall enthalpy change of an exothermic reaction be

A

-Ve

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13
Q

What will the overall enthalpy change of an endothermic reaction be

A

+ve

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14
Q

What is temperature

A

Temperature is related to the average kinetic energy of the particles in a system. As the particles move faster, the average kinetic energy increases and the temperature goes up. It doesn’t matter how many particles there are- temperature is independent of the number of particles present.

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15
Q

What is heat

A

Heat is a measure of the total energy of all the particles present in a given amount of substance. I does depend on the amount of the substance present as the energy of every particle is included. Heat always flows from high to low temperature.

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16
Q

What three things do you need to know to measure the enthalpy change of a reaction experimentally

A

1) the number of moles of reactants
2) the change in temperature of the reaction
3) the mass of the liquid being heated

17
Q

What is specific heat capacity

A

The amount of heat needed to raise 1g of a substance by 1 kelvin. It is measured in Joules per gram per kelvin Jg-1K-1

18
Q

What is the equation used when calculating enthalpy change experimentally

A

Q=mc/\T

19
Q

What does each variable stand for in the equation q=mc/\T

A

Q= heat energy lost or gained (joules) which is the same as enthalpy change required the pressure stays constant.
M= mass of the solution in grams
C = specific heat capacity of the solution
/\T= change in temperature

20
Q

What is the specific heat capacity of water

A

4.18 Jg-1K-1

21
Q

Describe what the apparatus used to measure the approximate enthalpy change when a fuel burns consists of

A
  • It is called a calorimeter
  • A spirit burner heats a metal can filled with water. This metal can is surrounded by a polystyrene cup to try and conserve heat.
  • You burn the fuel to heat a known mass of water and then measure the temperature rise of the water. You assume all of the heat goes into the water.
22
Q

Describe the problems with the experimental calorimetry method for calculating enthalpy change.

A
  • not all of the heat energy will go towards heating the solution-some will heat the container and some will be lost to the surroundings.
  • some experiments may not always go to completion- there may be incomplete combustion so less energy would be given out.
  • fuels and flammable liquids are usually volatile (react easily) so may escape by evaporation before the reaction takes place.
23
Q

In what ways is the flame calorimeter an improved version of the simple calorimeters used to measure enthalpy change

A

It has the following features which aim to reduce heat loss:
1) there is a spiral chimney made of copper
2) the flame is enclosed
3) the fuel burns in pure oxygen,rather than air.

24
Q

How do you calculate the enthalpy change from the value of q from q=mc/\T

A

Divide by 1000 and then divide by the number of moles of the reactant.

25
Q

What problem is there with using an expanded polystyrene cup for calorimetry and how do we solve it

A

Although expanded polystyrene cups are good insulators, some heat will still be lost from the sides and top which leads to low values for enthalpy changes measured by this method- this is especially important in very exothermic reactions. We can allow for this by plotting a cooling curve.

26
Q

Describe how to use a cooling curve to allow for heat loss when measuring enthalpy change

A

1) Before the experiment all apparatus and solutions are left to stand in the laboratory for some time. This ensures that they all reach the same temperature (that of the laboratory )
2) Place a set volume of each solution in two separate polystyrene cups
3) Using an accurate thermometer, take the temperature of each solution every 30 seconds for four mins to confirm that both solutions remain at the same temperature, that of the laboratory.
4) Plot on your graph and draw a line of best fit through these points.
5) pour one solution into the other and continue to record the temperature every thirty seconds for a further six minutes.
6) Plot these values on the graph. (An electronic temp sensor and data logging software can be used to plot the graph directly and increases accuracy)
7) draw a line of best fit through the graph points and use this to extrapolate back to the temperature at the point of mixing.

27
Q

What is Hess’s law

A

Hess’s law states that the enthalpy change for a chemical reaction is the same, whatever route is taken from the reactants to the products.

28
Q

What is the standard state of hydrogen

A

H2

29
Q

What is the standard state of carbon

A

Graphite.
Shown by the notation C (s,graphite)

30
Q

Describe the steps in constructing a thermochemical cycle/enthalpy diagram

A

1) Write an equation for the reaction.
2) Write down the elements involved in the alternative reactions that are being used (products of combustion, elements from formation) and balance.
3) Draw arrows showing the direction of each reaction and add the enthalpy changes.
4) go round the cycle and add/subtract that enthalpy values depending on your direction.

31
Q

Define bond dissociation enthalpy

A

The enthalpy change required to break a covalent bond with all species in the gaseous state. (The same amount of energy is given out when the bond is formed)

32
Q

Define mean bond enthalpy

A

The mean bond enthalpy is the enthalpy change needed to break a covalent bond in gaseous atoms, averaged over different molecules.This is because a particular bond may have a slightly different enthalpy in different molecules so an average is taken.

33
Q

What is the issue with calculating enthalpy change using mean bond enthalpies

A

As mean bond enthalpies are averages, calculations using them for specific compounds will only give approximate answers. (however they are useful,quick and easy to use)

34
Q

Describe how to calculate the enthalpy change of a reaction using mean bond enthalpies

A

1) Write a balanced equation for the reaction
2) Draw out the displayed formulas for all of the reactants and products.
3) Count how many of each bond exist either side of the reaction.
4) Calculate the total energy of the reactants by adding up all of the bond enthalpies.
5) Do the same thing for the products.
6) Calculate the difference between these total enthalpies.
7) Work out the sign of the enthalpy change. If the reaction is endothermic (and the products have a smaller total enthalpy than the reactants), the enthalpy change is positive. If the reaction is exothermic (the total enthalpy of the products is higher than the reactants), the enthalpy change is negative.

35
Q

Why is the enthalpy change of an exothermic reaction negative event though the total bond enthalpies of the products when added together are higher than that of the reactants

A

Because when adding the bond enthalpies of the products, this shows the energy given out when the bonds form (an exothermic process). So if the total enthalpy of the products is higher than the reactants, more energy was given out than taken in.

36
Q

Why are thermochemical cycles better for calculating enthalpy change than mean bond enthalpies

A

Mean bond enthalpies are averages so only give an approximate value. However, a thermochemical uses accurate values for enthalpy of formation/combustion and so gives an approximate value.

37
Q

When are mean bond enthalpies a useful way of calculating enthalpy change

A

When we cannot obtain values for a compounds enthalpy of formation/combustion due to being unable to carry out the experiments.

Physical Chemistry (10 decks)

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