DF - Enthalpy changes

Question 1

What is the chemistry word for energy?

Answer 1

Enthalpy.

Question 2

What does enthalpy mean?

Answer 2

Energy.

Question 3

What is the definition of enthalpy change?

Answer 3

The heat energy transferred in a reaction at constant pressure.

Question 4

What are the units for enthalpy change?

Answer 4

KJmol^-1

Question 5

What does it mean if enthalpy changes are under standard conditions?

Answer 5

The are done at:
25 degrees celsius/298K
1 atmosphere pressure/100 kPa
A concentration of 1moldm^-3

Question 6

How do you convert from Celsius to Kelvin?

Answer 6

Add 273 as the scale is the same but 0 degrees celsius is equal to 273 Kelvin.

Question 7

What happens to energy in an exothermic reaction?

Answer 7

It is given out.

Question 8

What happens to energy in an endothermic reaction?

Answer 8

It is absorbed.

Question 9

What happens to temperature in exothermic reactions?

Answer 9

Increases.

Question 10

What is the enthalpy change like in an exothermic reaction?

Answer 10

Negative.

Question 11

What type of reaction is exothermic?

Answer 11

Oxidation.

Question 12

What type of reaction is oxidation?

Answer 12

Exothermic.

Question 13

What are 2 examples of exothermic reactions?

Answer 13

Combustion and oxidation.

Question 14

What happens to the temperature (of the surroundings) in endothermic reactions?

Answer 14

Decreases.

Question 15

What are examples of endothermic reactions?

Answer 15

Thermal decomposition and photosynthesis.

Question 16

What type of reaction is bond breaking?

Answer 16

Endothermic

Question 17

Why is bond breaking endothermic?

Answer 17

It requires energy.

Question 18

What type of reaction is bond forming?

Answer 18

exothermic

Question 19

Why is bond forming exothermic?

Answer 19

It releases energy.

Question 20

How can enthalpy changes be calculated using average bond enthalpies?

Answer 20

Enthalpy change of reaction = total energy absorbed to break bonds - total energy released in making bonds.

Question 21

In terms of bond enthalpies, what is enthalpy change for a reaction?

Answer 21

The overall effect of the changes in energy when bonds are formed and broken.

Question 22

What is the enthalpy change like if you need more energy to break bonds than is released when bonds are made?

Answer 22

The enthalpy change is positive.

Question 23

What is the enthalpy change like if you need less energy to break bonds than is released when bonds are made?

Answer 23

The enthalpy change is negative.

Question 24

What is bond enthalpy?

Answer 24

The energy needed to break a bond, or the energy given out when a bond forms.

Question 25

What is the correlation between the strength of the bond and its bond enthalpy?

Answer 25

The stronger the bond, the higher the bond enthalpy.

Question 26

Are bond enthalpies exact?

Answer 26

No. For example, water has two O-H bonds but it takes different amounts of energy to break them both. The first bond takes more energy to break than the second one so the average bond enthalpy is the amount of energy needed to break both of the bonds divided by 2.

Question 27

What is average bond enthalpy?

Answer 27

The energy needed to break one mole of bonds in the gas phase, averaged over many different compounds.

Question 28

What is a bond enthalpy related to?

Answer 28

The length of the bond.

Question 29

What is bond length?

Answer 29

The distance between two nuclei is the distance where the attractive and repulsive forces balance each other. This distance is the bond length.

Question 30

What is the correlation between attraction between atoms and bond enthalpy and bond length?

Answer 30

The stronger the attraction between the atoms, the higher the bond enthalpy and the shorter the bond length. If there's more attraction, the nuclei are pulled closer together.

Question 31

Which bond has a greater bond enthalpy: | C=C or C-C?

Answer 31

C=C

Question 32

Which has a shorter bond length: | C=C or C-C?

Answer 32

C=C

Question 33

In which is the electron density greater: | C=C or C-C?

Answer 33

C=C as four electrons are shared rather than just 2 in C-C.

Question 34

As bond length increases what happens to average bond enthalpy?

Answer 34

Decreases.