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Chemistry OCR A A level > Enthalpy > Flashcards

Enthalpy Flashcards

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1
Q

What is enthalpy?

A

The measure of heat energy within a chemical system

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2
Q

How do you calculate enthalpy change?

A

Enthalpy of products - Enthalpy of reactants

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3
Q

Where is energy transferred in an exothermic change?

A

Energy is transferred from system to the surroundings

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4
Q

Where is energy transferred in an endothermic change?

A

Energy is transferred from the surroundings to the system

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5
Q

What are 4 key features of an exothermic reaction?

A

Enthalpy change is negative
Chemical system loses energy
Surroundings gain energy
Temperature of surroundings increases

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6
Q

What are 4 key features of an endothermic reaction?

A

Enthalpy change is positive
Chemical system gains energy
Surroundings lose energy
Temperature of surroundings decreases

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7
Q

What is activation energy?

A

The minimum energy required for a reaction to take place

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8
Q

What are standard conditions?

A

25 C or 298K
100kPa
1 mol dm^3

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9
Q

Define standard enthalpy change of formation

A

Enthalpy change when one mole of a substance is formed from its constituent elements with all substances in their standard states

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10
Q

What is the enthalpy change of formation for all elements?

A

0

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11
Q

Define standard enthalpy change of combustion

A

Enthalpy change when one mole of a substance undergoes complete combustion in oxygen with all substances in standard states

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12
Q

Define standard enthalpy change of neutralisation

A

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

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13
Q

What is 0 degrees C in K?

A

273K

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14
Q

Define specific heat capacity

A

The energy required to raise the temperature of 1g of a substance by 1K

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15
Q

What is the specific heat capacity of water?

A

4.18

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16
Q

What is the equation for calculating heat energy?

A

Q= mc delta T

17
Q

Give reasons for inaccuracy in heat energy of combustion practical

A

Heat energy lost to the surroundings
Incomplete combustion
Evaporation of alcohol from the wick
Non standard conditions

18
Q

How would you minimise effect of heat loss or incomplete combustion?

A

Draught screens

Input of oxygen gas

19
Q

What piece of equipment is used to measure enthalpy change of reaction between two solutions?

A

Polystyrene cups

20
Q

Define ‘Average Bond Enthalpy’

A

Energy required to break one mole of a specified type of bond

21
Q

Are bond enthalpies exothermic or endothermic?

A

Endothermic (always have a positive value)

22
Q

What is an average bond enthalpy calculated from?

A

Actual bond enthalpies in different environments

23
Q

Bond breaking- exothermic or endothermic?

A

Endothermic

24
Q

Bond making- exothermic or endothermic?

A

Exothermic

25
Q

What determines whether a reaction is overall exothermic or endothermic?

A

The difference between the value for making the bond and value for breaking bonds

26
Q

How is enthalpy change calculated from bond enthalpies?

A

Bond enthalpies in reactants - Bond enthalpies in products

27
Q

What are the limitations for using average bond enthalpies?

A

The bond enthalpies are slightly different in each environment, however not a major difference as an average bond enthalpy is used.
Need all molecules to be gaseous

28
Q

What does ‘Hess’ Law’ state?

A

If a reaction can take place by two routes, and starting and finishing conditions are the same, the total enthalpy change is the same for each route

Chemistry OCR A A level (20 decks)

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