Energy Calculations

Question 1

When fuels and foods react with oxygen, are the reactions endothermic or exothermic?

Answer 1

Exothermic

Question 2

What two unit is energy measured in?

Answer 2

Joules and calories

Question 3

What device can be used to measure the amount of energy released when substances burn?
What is the simplest version of this device and how can it be used?

Answer 3

A calorimeter
Water in a glass beaker or metal can, when a substance burns and heats the water, the temperature rise depends on the amount of energy released.
A thermometer is used to measure the temperature change which takes place during the experiment.

Question 4

Energy transferred can be calculated using which equation?

Answer 4

Q = mc∆T
Where:
Q is the amount of energy transferred to the water in joules, J
m is the mass of water in grams, g
c is the specific heat capacity of water in J/g/degrees celsius
∆T is the temperature change in degrees celsius

Question 5

What is the problem with simple calorimeters?

Answer 5

Simple calorimeters do not give accurate results for the energy released because much of it is used to heat the surroundings.
However they cane be used to compare the energy released by different fuels.

Question 6

How can energy released in KJ/mol be calculated?

Answer 6

It can be calculated by multiplying the energy change in KJ/g by the Mr of the substance.

Question 7

When a reaction takes place in a solution, what can be done to reduce the amount of energy transferred to the surroundings?

Answer 7

The reactions can be done in insulated containers.

Question 8

When a reaction takes place in a solution, how is energy transferred?

Answer 8

Energy is transferred to or form the solution

Question 9

In calculations using the temperature change of the solution to work out the energy transferred, it is assumed that the solutions behave like which substance?
What characteristics does this mean the solutions have?

Answer 9

Water

1cm cubed of the solution has a mass of 1g and the shc of the solution is 4.2J/g/degrees celsius

Question 10

When a solid is added to an aqueous solution or water what is assumed about the solution?

Answer 10

It is assumed that the volume of the solution does not change and that the solution still behaves like water.

Question 11

What do energy level diagrams show?

What is the difference between the energy levels of the reactants and product?

Answer 11

The energy changes for reactions

Energy change for the particular reaction

Question 12

How will a catalyst affect an energy level diagram?

Answer 12

A catalyst provides a different reaction pathway with a lower activation energy.

Catalyst increases the rate of reaction

Question 13

What is the activation energy for a reaction?

Answer 13

It is the minimum amount of energy required for the reaction to take place.

Question 14

What is the endothermic part of a reaction?

Answer 14

During a chemical reaction bonds in the reactants must be broken for the reaction to take place, breaking bonds is endothermic because energy is taken in.

Question 15

What is the exothermic part of a reaction?

Answer 15

Energy is released when bonds are formed in the products, this is exothermic.

Question 16

What makes the overall reaction exothermic or endothermic?

Answer 16

The difference between the energy taken in when bonds are broken in the reactants and the energy released when new bonds are formed in the products.

Question 17

What is bond energy?

Which unit is bond energy measured in?

Answer 17

Bond energy is the amount of energy required to break the bond between two atoms.
KJ/mol

Question 18

What is needed to calculate the energy change for a reaction?

Answer 18

The balanced symbol equation of the reaction.

Question 19

Give the method used to calculate the energy change for a reaction.
How de we know if the reaction is exothermic or endothermic?

Answer 19

1) Energy needed to break bonds in the reactants calculated using bond energies.
2) Energy needed to form bonds in the products calculate using bond energies.
3) Calculate the difference

If the difference is a positive number, the reaction is exothermic; if the difference is a negative number then the reaction is endothermic.

Question 20

Why is the need to develop alternative fuels to fossil fuels becoming more urgent?

Answer 20

Fossil fuels are non-renewable and they cause pollution.

Question 21

Give four advantages and three disadvantages of using hydrogen as a fuel.

Answer 21

Advantages:
Hydrogen burns easily and releases a large amount of energy per gram.
It produces only water when it burns, no carbon dioxide.
It can be used in combustion engines or fuel cells to power vehicles.
It can be produced from renewable sources.
Disadvantages:
It is hard to store
It is hard to source and supply
There are safety issues, e.g. it is highly flammable