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energetics Flashcards

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1
Q

define enthalpy and deduce its symbol

A

the heat energy stored within a chemical system
symbol= H

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2
Q

define enthalpy change and deduce its symbol

A

the change in heat energy under constant pressure
symbol = △H

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3
Q

what are the standard conditions of a reaction

A

100kPa (1atm)
298K (25°C)

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4
Q

draw an energy profile diagram for an endothermic reaction

A

see paper

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5
Q

draw an energy profile diagram for an exothermic reaction

A

see paper

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6
Q

does the temperature in an endothermic reaction increase or decrease?

A

decrease

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7
Q

does the temperature in an exothermic reaction increase or decrease?

A

increase

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8
Q

what value is the enthalpy change of an exothermic reaction?

A

negative

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9
Q

what value is the enthalpy change of an endothermic reaction?

A

positive

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10
Q

define the enthalpy of formation

A

enthalpy change when 1 mol of a compound is formed from its constituent elements in standard state under standard conditions

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11
Q

define the enthalpy of combustion

A

enthalpy change when 1 mole of a compound burns completely/ with an excess of oxygen in standard state under standard conditions

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12
Q

define enthalpy of neutralisation

A

enthalpy change when 1 mole of water is formed from a reaction between an acid and an alkali in standard state under standard conditions

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13
Q

how do you calculate the energy change/heat change of a reaction?

A

Q=mc△t
m= mass of h20/g (1cm^3=1g)
c= specific heat capacity
t= temperature

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14
Q

how do you calculate enthalpy change?

A

enthalpy change = q/moles of limiting reagent

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15
Q

what is enthalpy change measured in?

A

kJ mol^-1

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16
Q

4 reasons for uncertainties in calorimetry practical?

A

-heat loss to surroundings
-incomplete reaction
-fuel may have evaporated
-not in standard conditions

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17
Q

what is calorimetry

A

process that allows us to calculate the energy change of a reaction by measuring the temperature change of a solution/water

18
Q

what cups are used for the calorimetry practical and why?

A

-polystyrene cup for when you have a reaction taking place in a solution (polystyrene prevents heat loss to surroundings)
-copper (metal) can for when you are burning a fuel underneath to heat water

19
Q

what is the enthalpy of formation for elements in their standard states?

A

0 kJ mol^-1

20
Q

draw the Hess’s cycle diagram for the enthalpy of formation

A

see paper

21
Q

draw the Hess’s cycle diagram for the enthalpy of combustion

A

see paper

22
Q

what is the enthalpy of combustion of oxygen

A

0 kJ mol^-1

23
Q

what is specific heat capacity?

A

the energy required to heat 1g of a substance by 1 °C

24
Q

what kind of reaction is bond breaking? why?

A

endothermic
because energy is released from the reaction to the surroundings in the form of heat when bonds are made

25
Q

what kind of reaction is bond making? why?

A

exothermic
because energy in the form of heat required to overcome attractive forces between atoms

26
Q

define the enthalpy change of reaction

A

the enthalpy change when the reactants in the stoichiometric equation react to give the products under standard conditions

27
Q

3 examples of eothermic reactions

A

combustion of fuels
neutralisation
hydration

28
Q

example of endothermic reaction

A

thermal decomposition

29
Q

difference between heat and temperature

A

heat is the sum of all the particle’s energy and therefore affected by the amount of substance
temperature is related to the mean kinetic energy of the particles in a system so is independent of the particels present

30
Q

what is Hess’s Law?

A

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

31
Q

when is Hess’s Law used?

A

when the reactions cannot be measured directly (ie too fast, too slow, impossible to measure)

32
Q

what can you use to make experimental determination of enthalpy change more accurate

A

cooling curves

33
Q

hatwhat is a flame calorimeter; how does it differ from a simple calorimeter

A

reduces heat loss to surroundings to give more accurate results
has a spiral chimney made of copper
an enclosed flame, the fuel is burnt in pure oxygen, not air

34
Q

what is mean bond enthalpy?

A

the average energy required/ released when 1 mole of covalent bonds are broken/ formed in their gaseous statel

35
Q

how do you calculate enthalpy change using mean bond enthalpies?

A

enthalpy change = Σ reactant bond enthalpies - Σ products bond enthalpies

36
Q

do you use coefficients in bond enthalpy calculations?

A

yes
2CO(g)—> 2 x 107

37
Q

why is using bond enthalpies not as accurate as using standard enthalpy of combustion/formation?

A

bond enthalpies are an average of the bond enthalpies of different molecules whereas the enthalpies of combustion and formation apply just to that molecule, making it more accurate

38
Q

formula for mol when goven conc

A

mol=conc/vol

39
Q

(organic) how do you write a general formula for a homologous series

A

CnH2nX —> use this and the formula of a compound in the homologous series to work it out

40
Q
A

Chemistry (10 decks)

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