Chemistry - Chemical reactions and Energy

Question 1

What is an energy change?

Answer 1

-Almost every chemical reaction is accompanied by an energy change.
-When a chemical reaction happens, energy is transferred to or from the surroundings.
-Energy changes can be explained by examining the changes in chemical bonding during a reaction.
-Energy changes are used to classify reactions as exothermic or endothermic.

Question 2

What is an exothermic reaction? Where are they used?

Answer 2

-An exothermic reaction happens when energy is transferred to the surroundings.
-During an exothermic reaction, the temperature of the surroundings increases.
-Exothermic reactions are used in self-heating food cans and outdoor hand warmers.

Question 2

What is an endothermic reaction? Where are they used?

Answer 2

-An endothermic reaction happens when energy is taken in from the surroundings.
-During an endothermic reaction, the temperature of the surroundings decreases.
-Endothermic reactions are used in instant ice packs used to treat injuries.

Question 3

What does an exothermic reaction graph look like?

Answer 3

-The products are at a lower energy level than the reactants.
-Energy is given out to the surroundings.
-The downwards arrow shows that energy is given out.

Question 3

What does an endothermic reaction graph look like?

Answer 3

-The products are at a higher energy level than the reactants.
-Energy is taken in from the surroundings.
-The upwards arrow shows that energy is taken in.

Question 4

What is a reaction profile graph?

Answer 4

-A reaction profile shows how the energy of the chemicals changes during a reaction.
-A reaction profile also includes the activation energy - the minimum energy needed to start a reaction.
-The activation energy is shown as a ‘hump’ in the line.

Question 5

What is activation energy in a graph?

Answer 5

-Starts at the energy of the reactants
-Is equal to the difference in energy between the top of the ‘hump’ and the reactant.

Question 6

What is the overall change in energy in a reaction?

Answer 6

-The overall change in energy in a reaction is still the difference between the energy of the reactants and the energy of the products.

Question 7

How are bonds broken and made in a reaction?

Answer 7

-Energy is needed to break the bonds in the reactants
-Energy is released when the bonds in the products are formed
-The difference between the energy needed to break bonds and the energy released when forming bonds determines the type of reaction.

Question 7

How do breaking and making bonds influence an exothermic reaction?

Answer 7

-More energy is released when new bonds are made than is needed to break existing bonds.
-The overall energy change is negative – meaning that energy is given out to the surroundings.

Question 8

How do breaking and making bonds influence an endothermic reaction?

Answer 8

-More energy is taken in when the existing bonds are broken than is released in making new bonds
-The overall energy change is positive – meaning that energy is taken in from the surroundings

Question 9

How do you calculate bond energies?

Answer 9

1. Add together the bond energies for all the bonds in the reactants - this is the ‘energy in’.
2. Add together the bond energies for all the bonds in the products - this is the ‘energy out’.
3. Overall energy change = energy in – energy out.

Negative energy change = exothermic reaction
Positive energy change = endothermic reaction