enthalpy changes

Question 1

what is the symbol for enthalpy change?

Answer 1

triangleH
triangle=change
H=enthapy

Question 2

what does enthalpy refer to?

Answer 2

all the heat energy stored in a chemical system

Question 3

what is a chemical system?

Answer 3

all chemicals present in the reaction

Question 4

how do you calculate enthalpy change?

Answer 4

enthalpy of reactant minus enthalpy of products

Question 5

how do exothermic reaction profiles look like?

Answer 5

enthalpy of products is lower than enthalpy of reactants as energy is lost to the surroundings.

Question 6

what are exothermic reactions?

Answer 6

reactions where heat energy was released from the chemical system to the surroundings e.g neutralisation.

Question 7

if the reactant chemicals are dissolved in water (aqueous) where will the heat energy go in a exothermic reaction?

Answer 7

to the surroundings which is the water, increasing the temperature

Question 8

how do you show activation energy on a reaction profile?

Answer 8

from the line of the products to the height of the bump of the curve with an arrow pointing up.

Question 9

how do you show enthalpy change on a reaction profile?

Answer 9

arrow pointing from line of products to line of reactants. Enthalpy change is negative in an exothermic reaction.

Question 10

what is the symbol of activation energy?

Answer 10

Ea

Question 11

what is activation energy?

Answer 11

minimum energy needed for a chemical reaction to take place.

Question 12

what happens to temperature of surroundings in endothermic and exothermic reactions?

Answer 12

endothermic, surroundings temperature decreases. Exothermic, surrounding temperature increases.

Question 13

Enthalpy change is always determined under standard conditions. What are they?

Answer 13

• temperature: 298K (25 degrees)
• pressure: 100kPa
• concentration: 1mol/dm cubed
• standard states

Question 14

what is the symbol of standard conditions?

Answer 14

circle with a line across it.

Question 15

what is the unit for standard enthalpy change?

Answer 15

kJ/mol

Question 16

what are the four standard enthalpy changes?

Answer 16

• standard enthalpy change of combustion
• standard enthalpy change of neutralisation
• standard enthalpy of formation
• standard enthalpy change of reaction

Question 17

what is the standard enthalpy change of combustion?

Answer 17

the enthalpy change when ONE mole of a substance completely combusts with excess oxygen under standard conditions and standard states.

Question 18

what is the standard enthalpy change of neutralisation?

Answer 18

the enthalpy change when ONE mole of water is formed a neutralisation reaction under standard conditions.

Question 19

why is the standard enthalpy change of neutralisation always -57kj/mol between any strong acid or alkali?

Answer 19

Because only the H+ ions and OH- ions are involved

Question 20

what is the standard enthalpy change of formation?

Answer 20

the enthalpy change when ONE mole of a compound is formed from its elements under standard conditions and all chemicals are in standard states.

Question 21

what is the standard enthalpy change of reaction?

Answer 21

enthalpy change when a reaction takes place in the molar quantities shown by the balanced chemical condition under standard conditions and all chemicals must be in standard state. This fits for any reaction that doesn’t fit to the other categories.

Question 22

During a chemical reaction when and how does an exothermic and endothermic process occur?

Answer 22

• chemical bonds are broken in the reactant molecules which requires energy therefore it is a endothermic process.
• then chemical bonds are formed from products, releasing energy which is a exothermic process.

Question 23

what is the definition of average bond enthalpy?

Answer 23

The average enthalpy change when one mole of a specific bond in a molecule is broken in its gaseous state

Question 24

why is average bond enthalpy called average?

Answer 24

because scientists take an average value for the bond enthalpies across a range of different molecules.

Question 25

why is it important to keep to lid of the spirit burner on when it is not lit?

Answer 25

fuel will evaporate making results less accurate

Question 26

how do you calculate thermal energy?

Answer 26

``` q=mc(triangle, delta)T q: energy change of water(J) m: mass of water (g) c: specific heat capacity (J/g/K) (triangle, delta)T: temperature change of the water (degrees Celsius) ```

Question 27

what is the specific heat capacity of water?

Answer 27

4.18(J/g/K)

Question 28

how do you calculate the standard enthalpy change of combustion?

Answer 28

thermal energy/ number of moles

Question 29

do exothermic reactions have a positive of negative sign?

Answer 29

negative

Question 30

when performing the spirit burner practical, why is the standard enthalpy of combustion we calculated usually lower then what it actually is?

Answer 30

- if the unlit spirit burner is uncapped, then the fuel will evaporate which will make it appear as if we burnt more fuel than we actually did, less exothermic. - not all heat energy is transferred to water, some is transferred to calorimeter (metal tin) and the air. - not all the fuel went under complete combustion. Incomplete combustion produces less thermal energy - experiment may not have been carried out in standard conditions

Question 31

how do you convert J into kJ?

Answer 31

divide by 1000

Question 32

how do you calculate standard enthalpy of reaction?

Answer 32

divide heat energy by number of moles. As the reaction is exothermic you should put a negative sign.

Question 33

why is the temperature change found in an experiment (on a graph) usually not accurate?

Answer 33

because heat is being lost throughout the reaction.

Question 34

How do you correct the graph to find the accurate value for the temperature increase?

Answer 34

draw a vertical line from the start of the equation (not zero, when curve appears) and draw a line back from the cooling part of the curve. The point where the lines meet is the point temperature would have reached if there was no cooling.

Question 35

what does Hess's law state?

Answer 35

if a reaction can be carried out by two different pathways then the total enthalpy change for these two pathways will be the same providing that the starting and final conditions are the same for both pathways

Question 36

why is the standard enthalpy change of formation of Cl2 zero?

Answer 36

because it is an element not a compound

Question 37

when you're working with Hess's law, what are the two most common type of data you will be provided with?

Answer 37

standard enthalpy change of formation data and standard enthalpy change of combustion data

Question 38

when using standard enthalpy change of formation data, which arrow changes.

Answer 38

the first arrow points down from up causing the signs of the reactants to change (positive). This is because the material is being formed, not broken down.

Question 39

where do the arrows point in enthalpy change of formation?

Answer 39

up

Question 40

where do the arrows point in the enthalpy changes of combustion?

Answer 40

down

Question 41

when using standard enthalpy of combustion data, which arrow changes?

Answer 41

second arrow points up instead of down causing the sign to change (positive)

Question 42

the bond enthalpy of a specific bond is dependant on what?

Answer 42

depend on the molecule that we find that bond in