3- Energy Calculations

Question 1

What can you use to measure the amount of energy released when a substance is burnt?

Answer 1

A calorimeter- a simple one is water in a glass beaker or metal can
The burning substance heats the water, the temperature rise depends on the amount of energy released

Question 2

What equation is used to calculate the amount of energy transferred to the water from the substance/solution?

Answer 2

Q=m x c x T

Q=energy transferred (J) m=mass of water (g)
c=specific heat capacity (4.2J/g’C) T=temperature change (degrees C)

Question 3

What is the advantages and disadvantages to a simple calorimeter?

Answer 3

Ad: cheep and easy to make, good for comparing
Dis: do not give accurate results because much of the energy released heats the surroundings and not the water

Question 4

How can the energy change in kg/mol be found?

Answer 4

Energy change in kg/g x relative formula mass of the substance

Question 5

What happens when fuels and food react with oxygen?

Answer 5

The reaction is exothermic
Different amounts of energy are released by different fuels and foods (usually measured in Joules, but sometimes in calories)
1cal=4.2J

Question 6

How can you make energy transfer and released experiments more accurate?

Answer 6

Do the reactions in an insulated container to reduce energy transfer to the surroundings

Question 7

What do we assume when calculating energy transferred?

Answer 7

Solutions behave like water:
1cm3 of solution =1kg
The specific heat capacity is always is 4.2J/g’C

Question 8

Give two examples of reactions that can use the calorimeter technique?

Answer 8

Neutralisation and displacement reactions

Question 9

He do you find the energy changed (kJ/mol) from the energy transferred (J)?

Answer 9

Divide the energy transferred (J) by 1000 (kJ)
Then divide the energy transferred (kJ) by the mass (g) to get kJ/g
Then multiply that by the substances relative formula mass
to get (kJ/mol)

Question 10

What is the energy change and how is it shown?

Answer 10

It is the difference between the energy levels of the reactant and the product.
It is shown on an energy level diagram

Question 11

What must happen for a reaction to happen?

Answer 11

Bond must be broken (endothermic- taking in)

Question 12

What is the activation energy?

Answer 12

The minimum energy needed for the reaction to happen

Question 13

After the activation energy, why does the line on the graph start going downwards?

Answer 13

Energy is being released as products form (exothermic)

Question 14

How can you tell on a energy level diagram if the reaction is
1) endothermic 2) exothermic?

Answer 14

1) the products have more energy than the reactants

2) the reactants have more energy than the products

Question 15

How can a catalyst effect an energy level diagram?

Answer 15

It can lower the activation energy

Question 16

What is bond energy?

Answer 16

The energy needed (activation energy) to break the bond between two atoms - an equal amount of energy is needed when bonds are formed

Question 17

What needs to be worked out to find the energy change for a reaction?

Answer 17

A balanced equation
Total Energy needed to break all the bonds in the reactants
Total Energy needed to make all the bonds in the products
The difference between the two totals

Question 18

What are the advantages of using hydrogen and a fuel?

Answer 18

It burns easily
Releases a large amount of energy per gram
Produces no carbon dioxide, only water
Can be produced from renewable sources

Question 19

What are the disadvantages of using hydrogen as a fuel?

Answer 19

It is in short supply
Safety problems
Can reduce a vehicles performance

Question 20

What’s the problem with fossil fuels?

Answer 20

They are non renewable and cause pollution when extracting