Energetics

Question 1

What would you describe a chemical reaction that of which heat energy is given out?

Answer 1

exothermic reaction

Question 2

What would you describe a chemical reaction that of which heat energy is taken in?

Answer 2

endothermic reaction

Question 3

How would you define an exothermic reaction?

Answer 3

a reaction that releases energy and gets hot so it loses energy to its surroundings

Question 4

How would you define an endothermic reaction?

Answer 4

a reaction that absorbs energy and gets cold so it takes energy in from surroundings

Question 5

What is the meaning of enthalpy change for a process?

Answer 5

the TOTAL heat energy exchanged with the surroundings at constant pressure

Question 6

What is the symbol for enthalpy change?

Answer 6

Q

Question 7

What is the unit for enthalpy change?

Answer 7

kJ/mol^ -1

Question 8

What enthalpy change symbol would an exothermic reaction have and why?

Answer 8

-ΔH

because heat is given out to its surroundings

Question 9

What enthalpy change symbol would an endothermic reaction have and why?

Answer 9

+ΔH

because heat is absorbed from its surroundings

Question 10

What kind of conditions are enthalpy changes usually measured in?

Answer 10

usually measured under standard conditions

Question 11

What is the Law of Conservation of Energy?

Answer 11

the overall amount of energy in a reaction does not change as energy is conserved in reactions

Question 12

Can energy be destroyed or created?

Answer 12

NO it can only be transferred

Question 13

What happens to temperature in an exothermic reaction?

Answer 13

there is an increase in temperature, the temperature of the surroundings increases and the heat content of the system falls

Question 14

What kind of reactions are classes under exothermic?

Answer 14

neutralisation and combustion

Question 15

What happens to temperature in an endothermic reaction?

Answer 15

there is a decrease in temperature, the temperature of the surroundings falls and the heat content of the system increases

Question 16

Why can displacement reactions be exothermic OR endothermic?

Answer 16

displacement reactions can either take in or give out (heat)energy

Question 17

What is common between combustion and neutralisation reactions?

Answer 17

both type of reactions always give out energy (exothermic)

Question 18

What do we use simple calorimetry experiments for?

Answer 18

to experimentally determine the relative amounts of energy released by a fuel

Question 19

What are the two types of simple calorimetry experiments we need to know?

Answer 19

-enthalpy changes of reactions in solution

-enthalpy changes of combustion

Question 20

How would you calculate the amount of energy produced by a chemical reaction in solution?

Answer 20

measure the temperature change when the solutions are mixed together

Question 21

Why would you use an insulated container when measuring the temperature change using a simple calorimeter?

Answer 21

to prevent heat loss

Question 22

What is always assumed about the specific heat capacity of the solution?

Answer 22

that it is ALWAYS 4.18 J/g/*C

(unless told otherwise)

Question 23

What is the method for calculating the enthalpy change in a chemical reaction using a simple calorimeter?

Answer 23

-a fixed volume of one reagent is added to the calorimeter and the initial temperature taken with a thermometer

-an excess amount of the second reagent is added and the solution is stirred continuously

-the maximum temperature is recorded and the temperature rise calculated

-the energy given out would be calculated using the equation
Q = m x c x ΔT

Question 24

What can be calculated using Q = m x c x ΔT ?

Answer 24

energy given out by burning fuels

Question 25

What does each letter stand for in the equation Q = m x c x ΔT ?

Answer 25

Q = the heat energy change, J m = the mass of the substance being heated, g c = the specific heat capacity, J/g/°C (always 4.18 unless told otherwise) ΔT = the temperature change, °C

Question 26

What is the method for the experiment that calculates the enthalpy change for the combustion of fuels?

Answer 26

-measure a fixed volume of water into an aluminium can -clamp the can so its base is approximately 10cm above the bench -weigh the spirit burner INCLUDING THE LID containing a fuel using a balance -place the burner underneath the can -measure the initial temperature of the water -remove the burner lid and immediately ignite the burner to prevent it drying out -burn the fuel and stir the water using a thermometer for 30 seconds without touching the sides -measure the maximum temperature of the water in the can -accurately reweigh the burner including its lid -calculate the enthalpy change of combustion using the equations

Question 27

Why is not all the heat produced by the combustion reaction is transferred to the water when using a simple calorimeter?

Answer 27

some heat is lost to the surroundings some heat is absorbed by the calorimeter

Question 28

What happens in a reversible reaction?

Answer 28

the product molecules can react with each other or decompose and form the reactant molecules again

Question 29

What is the symbol that shows a reaction is reversible?

Answer 29

⇌

Question 30

What can reversible reactions be seen in?

Answer 30

-dehydration of hydrated copper (II) sulfate -thermal decomposition of ammonium chloride

Question 31

What can reversible reactions be classed as?

Answer 31

exothermic AND endothermic as they can do both

Question 32

What is the symbol for molar enthalpy?

Answer 32

ΔH

Question 33

What is the equation for molar enthalpy?

Answer 33

ΔH = -Q / n divided by 1000

Question 34

What do the letter in the equation for molar enthalpy represent ΔH = -Q / n divided by 1000 ?

Answer 34

ΔH = molar enthalpy change Q = the heat energy change n = moles of the fuel burned

Question 35

When calculating the molar enthalpy change, why does the heat energy change (Q) have to be made negative?

Answer 35

because the reaction is exothermic meaning it releases energy

Question 36

How would you calculate the moles of the fuel burned (n) when calculating the molar enthalpy change?

Answer 36

change in mass of the fuel / Mr of the fuel = moles of the fuel burned (n)

Question 37

What are energy level diagrams?

Answer 37

energy level diagrams are graphical representations of the relative energies of the reactants and products in chemical reactions

Question 38

What is on the y-axis of an energy level diagram and what does it look like?

Answer 38

increasing enthalpy (represented as an arrow heading upwards)

Question 39

What is on the x-axis of an energy level diagram and what does it look like?

Answer 39

progress of reaction (represented as an arrow heading to the right)

Question 40

What do the arrow in the middle of a energy level diagram indicate?

Answer 40

it indicates whether the reaction is exothermic or endothermic

Question 41

What way the does arrow in the middle of the energy level diagram point for an exothermic reaction and why?

Answer 41

the arrow points DOWNWARDS because energy is given out

Question 42

What way the does arrow in the middle of the energy level diagram point for an endothermic reaction and why?

Answer 42

the arrow points UPWARDS because energy is absorbed

Question 43

On an energy level diagram for an exothermic reaction, are the reactants higher than the products?

Answer 43

YES because the reaction starts with more energy than it ends with

Question 44

On an energy level diagram for an endothermic reaction, are the reactants higher than the products?

Answer 44

NO because the reaction ends with more energy than it starts with

Question 45

What is drawn next to the downwards arrow on an exothermic energy level diagram?

Answer 45

-ΔH

Question 46

What is drawn next to the upwards arrow on an endothermic energy level diagram?

Answer 46

+ΔH

Question 47

Are the products before the reactants on an energy level diagram?

Answer 47

NO the reactants are before the products

Question 48

Should you write the actual chemical formula on the reactants and products line on an energy level diagram?

Answer 48

YES

Question 49

What type of process is bond-breaking and why?

Answer 49

an ENDOTHERMIC process because energy is absorbed/taken in

Question 50

What type of process is bond-making and why?

Answer 50

an EXOTHERMIC process because energy is released

Question 51

What does whether a reaction is endothermic or exothermic overall depend on?

Answer 51

the difference between the energy needed to break existing bonds and the energy released when the new bonds are formed

Question 52

Why would a reaction be exothermic in terms of energy?

Answer 52

energy needed to break the bonds is less than the energy released in making new bonds

Question 53

Why would a reaction be endothermic in terms of energy?

Answer 53

energy needed to break the bonds is more than the energy released in making new bonds

Question 54

What is usually given in a bond energies equation?

Answer 54

a chart with the average bond energy (kJ) for each bond to work out the difference between reactants and products

Question 55

How would you perform a calorimetry study of the reaction between HCl and NaOH to investigate temperature changes?

Answer 55

-using a measuring cylinder, place 25cm^3 of the NaOH solution into the calorimeter -measure and record the temperature of the solution -add 5cm^3 of the dilute HCl and stir -measure and record the highest temperature reached by the mixture -repeat these steps increasing the amount of acid added by 5cm^3 each time

Question 56

In a calorimeter study that investigates temperature changes what does a larger difference in temperature show?

Answer 56

the larger the difference in the temperature the more energy is absorbed or released