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chemistry ocr a a-level yr 12 > enthalpy changes > Flashcards

enthalpy changes Flashcards

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1
Q

what is the symbol for enthalpy change?

A

triangleH
triangle=change
H=enthapy

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2
Q

what does enthalpy refer to?

A

all the heat energy stored in a chemical system

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3
Q

what is a chemical system?

A

all chemicals present in the reaction

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4
Q

how do you calculate enthalpy change?

A

enthalpy of reactant minus enthalpy of products

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5
Q

how do exothermic reaction profiles look like?

A

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

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6
Q

what are exothermic reactions?

A

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

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7
Q

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

A

to the surroundings which is the water, increasing the temperature

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8
Q

how do you show activation energy on a reaction profile?

A

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

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9
Q

how do you show enthalpy change on a reaction profile?

A

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

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10
Q

what is the symbol of activation energy?

A

Ea

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11
Q

what is activation energy?

A

minimum energy needed for a chemical reaction to take place.

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12
Q

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

A

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

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13
Q

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

A
  • temperature: 298K (25 degrees)
  • pressure: 100kPa
  • concentration: 1mol/dm cubed
  • standard states
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14
Q

what is the symbol of standard conditions?

A

circle with a line across it.

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15
Q

what is the unit for standard enthalpy change?

A

kJ/mol

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16
Q

what are the four standard enthalpy changes?

A
  • standard enthalpy change of combustion
  • standard enthalpy change of neutralisation
  • standard enthalpy of formation
  • standard enthalpy change of reaction
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17
Q

what is the standard enthalpy change of combustion?

A

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

18
Q

what is the standard enthalpy change of neutralisation?

A

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

19
Q

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

A

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

20
Q

what is the standard enthalpy change of formation?

A

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

21
Q

what is the standard enthalpy change of reaction?

A

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.

22
Q

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

A
  • 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.
23
Q

what is the definition of average bond enthalpy?

A

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

24
Q

why is average bond enthalpy called average?

A

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

25
Q

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

A

fuel will evaporate making results less accurate

26
Q

how do you calculate thermal energy?

A 
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)
27
Q

what is the specific heat capacity of water?

A

4.18(J/g/K)

28
Q

how do you calculate the standard enthalpy change of combustion?

A

thermal energy/ number of moles

29
Q

do exothermic reactions have a positive of negative sign?

A

negative

30
Q

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

A
  • 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
31
Q

how do you convert J into kJ?

A

divide by 1000

32
Q

how do you calculate standard enthalpy of reaction?

A

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

33
Q

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

A

because heat is being lost throughout the reaction.

34
Q

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

A

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.

35
Q

what does Hess’s law state?

A

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

36
Q

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

A

because it is an element not a compound

37
Q

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

A

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

38
Q

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

A

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.

39
Q

where do the arrows point in enthalpy change of formation?

A

up

40
Q

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

A

down

41
Q

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

A

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

42
Q

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

A

depend on the molecule that we find that bond in

chemistry ocr a a-level yr 12 (21 decks)

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