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Chemistry > Energetics > Flashcards

Energetics Flashcards

1
Q

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

A

exothermic reaction

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2
Q

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

A

endothermic reaction

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3
Q

How would you define an exothermic reaction?

A

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

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4
Q

How would you define an endothermic reaction?

A

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

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5
Q

What is the meaning of enthalpy change for a process?

A

the TOTAL heat energy exchanged with the surroundings at constant pressure

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6
Q

What is the symbol for enthalpy change?

A

Q

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7
Q

What is the unit for enthalpy change?

A

kJ/mol^ -1

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8
Q

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

A

-ΔH

because heat is given out to its surroundings

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9
Q

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

A

+ΔH

because heat is absorbed from its surroundings

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10
Q

What kind of conditions are enthalpy changes usually measured in?

A

usually measured under standard conditions

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11
Q

What is the Law of Conservation of Energy?

A

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

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12
Q

Can energy be destroyed or created?

A

NO it can only be transferred

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13
Q

What happens to temperature in an exothermic reaction?

A

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

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14
Q

What kind of reactions are classes under exothermic?

A

neutralisation and combustion

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15
Q

What happens to temperature in an endothermic reaction?

A

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

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16
Q

Why can displacement reactions be exothermic OR endothermic?

A

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

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17
Q

What is common between combustion and neutralisation reactions?

A

both type of reactions always give out energy (exothermic)

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18
Q

What do we use simple calorimetry experiments for?

A

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

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19
Q

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

A

-enthalpy changes of reactions in solution

-enthalpy changes of combustion

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20
Q

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

A

measure the temperature change when the solutions are mixed together

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21
Q

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

A

to prevent heat loss

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22
Q

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

A

that it is ALWAYS 4.18 J/g/*C

(unless told otherwise)

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23
Q

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

A

-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

24
Q

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

A

energy given out by burning fuels

25
Q

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

A

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

26
Q

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

A

-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

27
Q

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

A

some heat is lost to the surroundings

some heat is absorbed by the calorimeter

28
Q

What happens in a reversible reaction?

A

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

29
Q

What is the symbol that shows a reaction is reversible?

A

⇌

30
Q

What can reversible reactions be seen in?

A

-dehydration of hydrated copper (II) sulfate

-thermal decomposition of ammonium chloride

31
Q

What can reversible reactions be classed as?

A

exothermic AND endothermic as they can do both

32
Q

What is the symbol for molar enthalpy?

A

ΔH

33
Q

What is the equation for molar enthalpy?

A

ΔH = -Q / n divided by 1000

34
Q

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

A

ΔH = molar enthalpy change

Q = the heat energy change

n = moles of the fuel burned

35
Q

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

A

because the reaction is exothermic meaning it releases energy

36
Q

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

A

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

37
Q

What are energy level diagrams?

A

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

38
Q

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

A

increasing enthalpy (represented as an arrow heading upwards)

39
Q

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

A

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

40
Q

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

A

it indicates whether the reaction is exothermic or endothermic

41
Q

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

A

the arrow points DOWNWARDS because energy is given out

42
Q

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

A

the arrow points UPWARDS because energy is absorbed

43
Q

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

A

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

44
Q

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

A

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

45
Q

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

A

-ΔH

46
Q

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

A

+ΔH

47
Q

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

A

NO the reactants are before the products

48
Q

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

A

YES

49
Q

What type of process is bond-breaking and why?

A

an ENDOTHERMIC process because energy is absorbed/taken in

50
Q

What type of process is bond-making and why?

A

an EXOTHERMIC process because energy is released

51
Q

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

A

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

52
Q

Why would a reaction be exothermic in terms of energy?

A

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

53
Q

Why would a reaction be endothermic in terms of energy?

A

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

54
Q

What is usually given in a bond energies equation?

A

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

55
Q

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

A

-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

56
Q

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

A

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

Chemistry (5 decks)

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