Unit 3.2.1 - Energetics

Question 1

Define what an exothermic reaction is?

Answer 1

A reaction that gives out energy to their surroundings, usually in the form of heat, causing the temperature to increase

Question 2

Is the ΔH negative or positive for an exothermic reaction and why?

Answer 2

Negative as the products have less energy than the reactants as energy is given out

Question 3

Define what an endothermic reaction is?

Answer 3

A reaction that takes in energy from its surroundings, usually in the form of heat, causing the temperature to decrease

Question 4

Is the ΔH negative or positive for an endothermic reaction and why?

Answer 4

Positive as the products have more energy than the reactants as they take in energy

Question 5

Give an example of an exothermic and endothermic reaction?

Answer 5

Exothermic - Oxidation

Endothermic - Thermal Decomposition

Question 6

What is the definition of enthalpy change for a reaction?

Answer 6

The heat energy change per mole at a constant pressure

Question 7

What is the definition of standard enthalpy change of a reaction?

Answer 7

The heat energy change per mole of a reaction under standard conditions of 100KPa and 298K with all the reactants and products in their standard states

Question 8

What is the definition of standard enthalpy change of ormation?

Answer 8

The enthalpy change when 1 mole of a compound is formed from its elements under standard conditions of 100KPa and 298K with all the reactants and products in their standard states

Question 9

What is the definition of standard enthalpy change of combustion?

Answer 9

The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions of 100KPa and 298K with all the reactants and products in their standard states

Question 10

What is the name of an experiment to measure enthalpy change ?

Answer 10

Calorimeter

Question 11

Name two calorimeter experiments you can do?

Answer 11

Coffee cup and copper calorimeter

Question 12

Describe what happens and what you measure during a coffee cup calorimeter?

Answer 12

You add one reactants to an insulated container and measure its temperature for three mins, at the forth minute you add the other reactant and stir and then measure the temperature again at the fifth minute up to 10 mins. Using this you can calculate the temperature change.

Question 13

Describe what happens and what you measure during a copper colorimeter?

Answer 13

You add a certain amount of water to a container, and burn a known mass of a reactant underneath it, using the heat to heat up the water. You then measure the temperature change throughout the experiment.

Question 14

During a coffee cup calorimeter why can’t you measure the temperature when you first mix the two reactants together?

Answer 14

Because this is when the reaction will first start to happen and so the temperature would be increasing at such a fast rate that it would be impossible to get an accurate reading

Question 15

What are the two main causes of uncertainty in any calorimeter experiment?

Answer 15

1. ) Heat loss

2. ) Incomplete reaction

Question 16

What do ΔH and ΔT stand for?

Answer 16

Enthalpy change and Temperature change

Question 17

How can you calculate the energy taken in or given out once you have the temperature change from a calorimeter experiment?

Answer 17

q=mcΔT

Question 18

Once you have calculated q from q=mcΔT how can you calculate ΔH?

Answer 18

As the q you get is in joules you first have to divide it by 1000 to get kj
Then you can use the equation: ΔH = kj/moles

Question 19

What does Hess’s Law state?

Answer 19

The total enthalpy change of a reaction is always the same and is independent of the route taken

Question 20

Draw a Hess cycle for the enthalpy change of combustion?

Answer 20

See flash card 1

Question 21

Draw a Hess cycle for the enthalpy change of formation

Answer 21

See flash card 2

Question 22

Is breaking bonds an exothermic or endothermic process?

Answer 22

Endothermic as energy is taken in

Question 23

Is forming bonds an exothermic or endothermic process?

Answer 23

Exothermic as energy is released

Question 24

In terms of bond strength describe an exothermic reaction?

Answer 24

Less energy is required to break the bonds in the reactants than is released when the bonds are formed in the products. So overall energy is released

Question 25

In terms of bond strength describe an endothermic reaction?

Answer 25

More energy is required to break the bonds in the reactants than is released when the bonds are formed in the products. So overall energy is taken in

Question 26

If you were given mean bond enthalpies how would you calculate the enthalpy change of a reaction?

Answer 26

ΔH = energy required to break the bonds in the reactants - the energy required to break bonds in the products

Question 27

Why do we have to use mean bond enthalpies?

Answer 27

The actual enthalpy of the bond varies depending on which condition the bond is under, so its hard to get the actual value

Question 28

What is mean bond enthalpy?

Answer 28

The average amount of energy needed to break one mole of a specific type of bond under different conditions to give separated atoms, with everything in the gaseous state

Question 29

Draw an energy level diagram for an exothermic reaction?

Answer 29

See flash card 3

Question 30

Draw an energy level diagram for an endothermic reaction?

Answer 30

See flash card 4

Question 31

Why are mean bond enthalpies not accurate?

Answer 31

They are only an average and include the bond when in many different compounds