Energy Changes - Topic 7

Question 1

Which changes do changes in heat energy accompany?

Answer 1

• Salt dissolving in water
• Neutralisation reactions
• Displacement reactions
• Precipitation reactions
  
  • When these reaction take place in solution, temperature solution can be measured to reflect the heat changes

Question 2

Exothermic reaction:

Answer 2

• a reaction where heat energy is given out to the surroundings so the temperature of the surroundings increases
• makes bonds
• temperature increases as chemical energy → thermal/heat energy

Question 3

Endothermic reaction:

Answer 3

• a reaction where heat energy is taken in from the surroundings so the temperature of the surroundings decreases
• breaks bonds
• temperature decreases as thermal/heat energy → chemical energy

Question 4

How do you know if the overall heat energy change for a reaction is exothermic?

Answer 4

• E is needed to break bonds and E is released when bonds are formed
• the overall heat energy change for a reaction is exothermic if more heat is released in forming bonds in the products than is required in breaking bonds in the reactants

Question 5

How do you know if the overall heat energy change for a reaction is endothermic?

Answer 5

• E is needed to break bonds and E is released when bonds are formed
• the overall heat energy change for a reaction is endothermic if less heat energy is released in forming bonds in the products than is required in breaking bonds in the reactants

Question 6

Activation Energy:

Answer 6

• the minimum amount of energy that particles must have for a reaction to take place
• it is the minimum amount of energy with which the particles need to collide with to make the collisions successful

Question 7

What happens if particles do not reach the activation energy?

Answer 7

The particles may still collide, but they will simply just bounce of each other

Question 8

Is the activation energy different for each experiment?

Answer 8

yes

Question 9

What do all substances have?

Answer 9

all substances have chemical energy - a form of stored or potential energy

Question 10

How does energy change in chemical reactions?

Answer 10

when chemical reactions occur and new substances are formed with different amounts of energy - energy can be exchanged with the surroundings and can be released or absorbed as heat

Question 11

Enthalpy change:

Answer 11

• The difference between the energy of the reactants and the energy of the products - approximately the amount of energy lost or gained during the reaction
• ΔH (“delta H”)

Question 12

What is the energy change for an exothermic reaction?

Answer 12

• energy change is negative
• as more heat energy has been released
• negative ΔH

Question 13

What is the energy change for an endothermic reaction?

Answer 13

• energy change is positive
• as heat energy has been absorbed/taken in
• positive ΔH

Question 14

How do you calculate the energy needed to break bonds and the energy released when bonds are formed?

Answer 14

the energy needed to break bonds and energy released when bonds are formed
can both be calculated from bond energies

Question 15

How do you calculate the overall energy change for a reaction?

Answer 15

overall energy change = sum of energy taken in to break bonds – sum of energy released to form bonds

Question 16

How can you tell from the energy change whether a reaction is exothermic or endothermic?

Answer 16

• if the energy out > energy in, the energy change will be negative showing an exothermic reaction
• if the energy out < energy in, the energy change will be positive showing an endothermic reaction

Question 17

Overall energy change of the reaction:

Answer 17

the difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed

Question 18

Reaction profile:

Answer 18

• graphs used to show the relative energies of reactants and products
• the activation energy and the overall energy change of a reaction

Question 19

Energy profile for exothermic reaction pathway:

Answer 19

Question 20

Energy profile for endothermic reaction pathway:

Answer 20

Question 21

Energy profile for reaction using catalyst:

Answer 21

Question 22

How do you compare the temperature changes of a neutralisation reaction when 2 different acids are used to neutralise an alkali solution?

Answer 22

• measure known volume of alkali solution
• add same volume of each of the acids
• stir the mixture
• record the initial and final temperatures/temperature change