Energetics

Question 1

What is enthalpy?

Answer 1

The word for energy in chemistry

Question 2

What are the units for enthalpies
What is the symbol for enthalpies

Answer 2

Kj/mol

The letter H

Question 3

What does the triangle symbol mean?
What does the circle with a line through it mean?

Answer 3

Triangle is a change in…
Circle is standard conditions

Question 4

What are the standard conditions?

Answer 4

298K

100 K pascals

Question 5

Define enthalpy change

Answer 5

The change in heat energy at constant pressure

Question 6

Define standard enthalpy change

Answer 6

The change in heat energy under standard conditions of temperature and pressure

Question 7

What is an endothermic reaction?
How is it measured?

Answer 7

A reaction that takes in heat energy from the surrounding
A decrease in the temperature of the surroundings

Question 8

What are 2 examples of endothermic reactions?

Answer 8

Photosynthesis
Thermal decomposition

Question 9

What is an exothermic reaction?
How is it measured?

Answer 9

A reaction that releases heat energy into the surroundings
It is measured by an increase in the temperature of the surroundings

Question 10

What are 3 examples of exothermic reactions?

Answer 10

Combustion
Respiration
Neutralisation

Question 11

Describe the enthalpy level diagram of an endothermic rewaction

Answer 11

The products are higher than the reactants
The activation energy goes from the reactants to peak of products
Enthalpy on y axis

Question 12

Describe the enthalpy level diagram of an exothermic reaction

Answer 12

The reactants are higher than the products
The activation energy still goes from the reactants to the top of the peak
Enthalpy on y axis

Question 13

What is a bond enthalpy?

Answer 13

The enthalpy change for breaking one mole of a particular covalent bond in the gas phase to form gaseous atoms

Question 14

What is the mean bond enthalpy

Answer 14

The average enthalpy change for breaking one mole of a particular covalent bond in a range of different compounds in the gas phase

Question 15

What is the difference in the processes of bond breaking and bond forming and why?

Answer 15

Bond breaking is endothermic because is put in to break the bonds

Bond forming is endothermic because energy is released when the bonds are formed

Question 16

What is the relation between energy to break a bond and energy to form a bond?

Answer 16

The energy released when a bond is formed is of the same magnitude as the energy needed to put in break it, the only difference is the sign

Question 17

What is the sign for an endothermic reaction?

Answer 17

It is positive ( + )

Question 18

What is the sign for an exothermic reaction?

Answer 18

It is negative negative ( - )

Question 19

What is the equation for calculating ^H (bond enthalpy change)

Answer 19

(The sum of the enthalpy change of all the reactants) - (The sum of the enthalpy changes of all the products)

Question 20

Why is bond breaking endothermic?

Answer 20

More energy is needed to break the bonds compared to the energy released when they are formed

Question 21

Why is bond forming exothermic?

Answer 21

More energy is released when the bonds are formed compared to thee energy needed to break the bonds

Question 22

Why are mean bond enthalpies different to data book values?

Answer 22

They are calculated from a range of compounds to provide us with the average instead of the exact value

Question 23

Define standard enthalpy change of combustion

Answer 23

When one mole of a substance undergoes complete combustion in excess oxygen where all the substances are under their standard conditions

Question 24

How do you write combustion equations?

Answer 24

Write the equation ( only products should be carbon dioxide and water )

Balance them in alphabetical order
Make sure you only have one mole of the reactant

Question 25

What 5 equations do you need to calculate enthalpy change

Answer 25

q = m x c x ^T

mass = volume x density

moles = mass / mr

^H = q / moles

^H /1000 = ^H in kj/mol

Question 26

What can the calorimetry experiment be used to calculate

Answer 26

Enthalpies of combustion

Enthalpies of reactions in solutions

Question 27

How can the enthalpies of combustion be found?

Answer 27

Using the heat energy released in a combustion reaction to heat a known mass of water

Question 28

What must you always include in your answers when calculate enthalpy change

Answer 28

The sign which shows whether it was endothermic or exothermic

+ = endothermic (dec temp)
- = exothermic (inc temp)

Question 29

What is specific heat capacity? (C)

Answer 29

The amount of energy required to heat up 1kg of a substance by 1 degree Celsius

Question 30

What are 2 rules when calculating enthalpy change if reactions in solutions?

Answer 30

The density and the specific heat capacity is taken to be the same as waters unless stated in the question

Question 31

What type of reactions are common that involves solutions

Answer 31

Neutralisation reactions

Question 32

What is the enthalpy of neutralisation

Answer 32

The enthalpy change per 1 mole of water formed

Question 33

When doing calculations of enthalpy change in solutions what do you always do first?

Answer 33

Write out the balances symbol equation

Question 34

How do you determine the limiting reagent when the ratio of reactants is 1 : 1

Answer 34

The one with the fewest moles is the limiting reagent

Question 35

How do you determine the limiting reagent if the ratio of reactants is n or 1 : 1

Answer 35

Calculate how many moles you have of each reactant and then use the ratio to calculate how many you need of each, the one that you have fewer moles of than you need is the limiting reagent

Question 36

Define standard enthalpy change of formation

Answer 36

Enthalpy change when one mole do a compound is formed from its elements under standard conditions, where all substances are in their standard states