Energetics 1

Question 1

What is thermochemistry

Answer 1

The study of heat changes during a reaction.

Question 2

What type of process is bond breaking

Answer 2

Endothermic

Question 3

What type of process is bond formation

Answer 3

Exothermic

Question 4

What is an exothermic reaction

Answer 4

A reaction where energy is released into the surroundings.

Question 5

What is an endothermic reaction

Answer 5

A reaction that takes in energy from the surroundings.

Question 6

What is enthalpy change

Answer 6

When a heat change is measured at constant pressure

Question 7

What are the standard conditions for measuring enthalpy change

Answer 7

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

Question 8

What is standard state

Answer 8

The physical state of a substance under standard conditions.

Question 9

Define standard enthalpy change of reaction

Answer 9

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.

Question 10

Define standard enthalpy change of combustion

Answer 10

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.

Question 11

Define standard enthalpy change of formation

Answer 11

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.

Question 12

What will the overall enthalpy change of an exothermic reaction be

Answer 12

-Ve

Question 13

What will the overall enthalpy change of an endothermic reaction be

Answer 13

+ve

Question 14

What is temperature

Answer 14

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.

Question 15

What is heat

Answer 15

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.

Question 16

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

Answer 16

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

Question 17

What is specific heat capacity

Answer 17

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

Question 18

What is the equation used when calculating enthalpy change experimentally

Answer 18

Q=mc/\T

Question 19

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

Answer 19

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

Question 20

What is the specific heat capacity of water

Answer 20

4.18 Jg-1K-1

Question 21

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

Answer 21

• 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.

Question 22

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

Answer 22

• 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.

Question 23

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

Answer 23

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.

Question 24

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

Answer 24

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

Question 25

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

Answer 25

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.

Question 26

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

Answer 26

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.

Question 27

What is Hess’s law

Answer 27

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.

Question 28

What is the standard state of hydrogen

Answer 28

H2

Question 29

What is the standard state of carbon

Answer 29

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

Question 30

Describe the steps in constructing a thermochemical cycle/enthalpy diagram

Answer 30

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.

Question 31

Define bond dissociation enthalpy

Answer 31

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)

Question 32

Define mean bond enthalpy

Answer 32

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.

Question 33

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

Answer 33

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)

Question 34

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

Answer 34

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.

Question 35

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

Answer 35

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.

Question 36

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

Answer 36

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.

Question 37

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

Answer 37

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