Chemical Energy

Question 1

are combustion reactions exo- or endothermic

Answer 1

exothermic - fuels release energy

Question 2

problems with exothermic reactions

Answer 2

may incur costs in supplying heat to keep the reaction rate

Question 3

problems with endothermic reactions

Answer 3

may require heat to be removed to prevent the temperature from rising. could cause an explosion

Question 4

4 main ways to calculate chemical energy

Answer 4

1. proportion
2. Eh=cm delta T
3. bond breaking/ bond making
4. Hess’s law

Question 5

what is enthalpy

Answer 5

a measure of chemical energy in a substance

Question 6

what is the enthalpy of combustion

Answer 6

the energy released when one mole of substance burns completely in oxygen

Question 7

complete combustion

Answer 7

plentiful supply of oxygen

products - carbon dioxide and water (unless only carbon or hydrogen is being burned)

Question 8

incomplete combustion

Answer 8

not enough O2
when there is a poor supply of oxygen, the hydrocarbon cannot burn completely and so the products are carbon monoxide, carbon and water.

Question 9

steps for calculating the enthalpy of combustion from proportion

Answer 9

1. rewrite the information in the question as a proportion
2. now input the mass of one mole (gfm)
3. multiply diagonal numbers and divide by number in corner
   * negative sign to show energy is released*

Question 10

set up used to calculate the energy released by a fuel

Answer 10

a thermometer in glass beaker on top of alcohol in the spirit burner

Question 11

measurements taken in energy released by fuel experiment

Answer 11

1. mass of burner before + after burning

2. temperature of water before and after (temperature rise)

Question 12

specific heat capacity equation

Answer 12

Eh=cm delta T

Question 13

specific heat capacity definition

Answer 13

the energy required to raise the temperature of 1kg of a substance by 1-degree Celcius

Question 14

c of water

Answer 14

4.18 kJ per kg per degree

Question 15

general rule of thumb for decimal places

Answer 15

100s/1000s - no dp
10s - 1 dp
units - 2 dp

Question 16

calculating enthalpy of combustion

Answer 16

1. work out energy released (no need for negative sign)

2. use proportion to work out enthalpy of combustion

Question 17

why are experimental and theoretical values different

Answer 17

heat lost to the surroundings

Question 18

ways to improve enthalpy of combustion experiment

Answer 18

1. use a heat shield to prevent heat escaping

2. use a copper can around the glass beaker as copper is a good conductor of heat

Question 19

other sources of error in enthalpy of combustion experiment

Answer 19

• not enough oxygen for complete combustion
• some of the fuel evaporates (Mass decreases but fuel not burned to release energy)
• no lid on container
• if water has not been stirred
• some heat energy absorbed by beaker/ other apparatus

Question 20

the bomb calorimeter

Answer 20

can be used to improve experiment

• heat no longer lost to surroundings
• complete combustion occurs
• diagram in notes and camera roll*

Question 21

why is bond breaking positive

Answer 21

bonds form to allow atoms to achieve stability so to break chemical bonds, energy needs to be put in to destroy stability (endothermic process)

Question 22

unstable atoms

Answer 22

high in energy - reactive

Question 23

stable atoms

Answer 23

low in energy - less reactive

Question 24

why is bond making negative

Answer 24

when chemical bonds are formed, the new product is more stable and therefore lower in energy than the reactants
excess energy is released to surroundings as heat
(exothermic process)

Question 25

bond breaking...

Answer 25

energy required endothermic enthalpy change = positive

Question 26

bond making...

Answer 26

energy released exothermic enthalpy change = negative

Question 27

molar bond enthalpy

Answer 27

the energy required to break one mole of chemical bonds in the gaseous state and from two moles of gaseous atoms

Question 28

bond enthalpy table in data booklet

Answer 28

gives us bond enthalpies of common diatomic molecules | these bonds only exist in these diatomic molecules so values are very accurate

Question 29

why are values for triple bonds higher than single/double

Answer 29

it takes more energy to break a triple bond than a single or double bond as it is stronger

Question 30

mean bond enthalpy table in data booklet

Answer 30

these bonds can be found in more than one substance eg: C-H found in alkanes, alkenes, alcohols, aldehydes... the mean (average) value of the C-H bond in all compounds is calculated and displayed in the table

Question 31

bond breaking, bond making equation

Answer 31

enthalpy change = sum of all bonds broken + sum of all bonds made (break up AND make up) *bonds made will have a negative value*

Question 32

steps when using bond breaking, bond making

Answer 32

1. draw full structural formula to show all bonds 2. make a list of all bonds which are broken in reactants 3. fill in values using page 10 of db 4. repeat process for bond making 5. calculate total energy in and total energy given out. 6. calculate enthalpy change

Question 33

what is enthalpy of formation?

Answer 33

the energy change one mole of a compound from its elements in their standard states.

Question 34

what is Hess's law?

Answer 34

Hess's law states that the enthalpy change for a chemical reaction is independent of the route taken

Question 35

steps for basic Hess's law question

Answer 35

1. find starting point 2. follow alternative route and change signs if arrows are in wrong direction 3. enthalpy change is the sum of all steps

Question 36

applications of Hess's law

Answer 36

Hess's law can be used to calculate enthalpy changes, which are difficult to determine by experiment. Each of the substances shown in the equation can be burned and their enthalpies of combustion found by experiment.

Question 37

what is enthalpy of formation

Answer 37

quantity of energy taken in or given out when 1 mole of a substance is formed

Question 38

alternative method when it is difficult to determine by the direct route (enthalpy of formation)

Answer 38

use enthalpy of combustion to help work out the energies of the different steps (pg10)

Question 39

Hess's law calculations steps

Answer 39

1. write out target equation 2. write out *balanced* equations for the enthalpy of combustion for each substance 3. compare and rearrange the known equations to get to target equation 4. cancel out and add equations and enthalpy change values to get the target.

Question 40

enthalpy of combustion equations for C and H

Answer 40

C(s)+02-->CO2 | H2(g)+1/2O2(g)--> H20(g)