3.1.4 (energetics) endothermic and exothermic

Question 1

is bond breaking exothermic or endothermic?

Answer 1

endothermic

Question 2

is bond making exothermic or endothermic?

Answer 2

exothermic

Question 3

what is the enthalpy change of a system?

Answer 3

the heat energy change at constant pressure

Question 4

what is the symbol for enthalpy change?

Answer 4

ΔH

Question 5

what are the units of enthalpy change?

Answer 5

KJmol-1

Question 6

as enthalpy (energy) is lost by the system to the surroundings, the ΔH is what?

Answer 6

negative

Question 7

how do you know that a reaction is exothermic?

Answer 7

in a reaction, if more energy is released when new bonds are formed than was needed to break the bonds in the reactants, then the difference in energy is released to the surroundings and the reaction is dead to be exothermic

Question 8

what does the energy-level diagram for combustion look like?
(exothermic)

Answer 8

Question 9

what does the energy-level diagram for dissociation look like?

Answer 9

Question 10

as enthalpy (energy) is gained by the system from the surroundings, the sight on the enthalpy change of the system is what?

Answer 10

positive

Question 11

how do you know that a reaction is endothermic?

Answer 11

in a reaction if more energy is needed to break the bonds in the reactants that was released when new bonds formed, then the difference in energy is absorbed from the surroundings and the reaction is said to be endothermic

Question 12

different ……… have different bond enthalpies

Answer 12

bonds

Question 13

what does the overall enthalpy change for the reaction depend on?

Answer 13

the nature of the bonds in the reactants and products

Question 14

what do strong bonds need to break?

Answer 14

more energy

Question 15

what is bond enthalpy?
what are the values always?

Answer 15

the enthalpy change when a covalent bond is broken

always positive

Question 16

what does the energy needed to break a type of bond differ depending on?

Answer 16

whether it is in methane, ethane, ethanol etc

Question 17

what is the mean bond enthalpy?
what are they always?

Answer 17

the average enthalpy change when a specific bond is broken, in a range of compounds

always positive

Question 18

what are mean bond enthalpies used to calculate?

Answer 18

the enthalpy change in a redaction

Question 19

what is the bond dissociation energy? (definition)

Answer 19

the energy needed to break a particular covalent bond

Question 20

if the ΔH value is positive, is the reaction endothermic or exothermic?

Answer 20

endothermic

Question 21

if the ΔH value is negative, is the reaction endothermic or exothermic?

Answer 21

exothermic

Question 22

what’s the definition of: standard enthalpy change of formation?

Answer 22

the enthalpy change when 1 mole of a compound is formed from its elements with all reactants and products in the standard state and under standard conditions

Question 23

what’s the definition of: standard enthalpy change of combustion?

Answer 23

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

Question 24

is standard enthalpy change of combustion always endothermic or exothermic?

Answer 24

exothermic

Question 25

what’s the definition of: standard enthalpy change of reaction?

Answer 25

the enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation, under standard conditions with all reactants and products in their standard states

Question 26

the standard enthalpy change of formation of an element must be what?

all other enthalpies of formation are what?

Answer 26

zero

they can be endothermic or exothermic

Question 27

what does the first law of thermodynamics state?

Answer 27

energy cannot be made or destroyed, but it can be converted from one form to another

Question 28

what is the first law of thermodynamics also known as?

Answer 28

The principle of conservation of energy

Question 29

what does Hess’s law state?

Answer 29

the total enthalpy change for a reaction is independent of the route taken

Question 30

what is calorimetry used for?

Answer 30

to find the heat energy released in a reaction

Question 31

what is the principle of caliometry?

Answer 31

when water is heated, 4.18 J of energy required to increase the temperature of 1g of water by 1°C

this value is called the specific heat capacity of water

Question 32

what are the two methods of determining enthalpies of combustion by experiment?

Answer 32

using either:
simple calorimeter
or
bomb calorimeter

Question 33

how does the simple calorimeter work?

Answer 33

a spirit burner containing a liquid fuel is weighed.

the spirit burner is placed beneath a copper can (calorimeter) containing a known volume of water (at a known temperature) and then lit

the water is stirred at regular intervals

when the temperature of the water has risen by at least 10°C, the flame is out out and the spirit burner is re-weighed

using the mass of fuel used on the temperature rise of the water, the amount of heat energy released per gram of fuel can be calculated

if the relative molecular mass of the fuel is known, then the enthalpy of combustion can be calculated

Question 34

what are the steps to working out the standard enthalpy change of combustion?

Answer 34

1. calculate the amount of heat lost or gained during the combustion using q=mcΔT. change the units of q from J to KJ
2. calculate the number of moles of fuel that caused this enthalpy change, from the mass that reacted using n=mass/Mr
3. calculate the standard enthalpy change of combustion using the actual heat change for the reaction of the number of moles of fuel that burned using ΔC=q/n

Question 35

how is using a bomb calorimeter more accurate?

Answer 35

because the heat loss to the surroundings is reduced to 0