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Chemistry > Unit 1: Section 4: Energetics > Flashcards

Unit 1: Section 4: Energetics Flashcards

1
Q

What is enthalpy change

A

The heat energy transferred in a reaction at constant pressure

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2
Q

What is the notation for enthalpy change

A

ΔH

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3
Q

What are the units of enthalpy change

A

kj mol^-1

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4
Q

What are the standard conditions

A

100kPa or 1 atm of pressure

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5
Q

At what temperature are enthalpy changes measured at

A

298 K (25 degrees celcius)

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6
Q

What type of value will the enthalpy change be if the reaction is exothermic

A

Negative because the energy is given out to the surroudings

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7
Q

What type of value will the enthalpy change be if the reaction is endothermic

A

Positive because it’s taking in energy from the surroundings

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8
Q

What is bond enthalpy

A

The energy needed to break a bond

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9
Q

Is breaking bonds endothermic or exothermic

A

Need energy to break bonds so bond breaking is endothermic

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10
Q

What is the enthalpy change

A

The overall effect of these two changes

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11
Q

What is mean bond ethalpies

A

The average energy needed to break a cetain type of bond over a range of compounds

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12
Q

How do you calculate the enthalpy change

A

Enthalpy change = Totale energy absorbed - Total energy released

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13
Q

How do you find the total energy released by the reaction

A

Find the total energy needed to form all the new bonds

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14
Q

What are the three different types of entahlpy change

A

Standard enthalpy change of formation
Standard enthalpy change of combustion
Standard entahlpy change of reaction

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15
Q

Explain standard enthalpy change of formation

A

The enthalpy change when 1 mole of a compound is formed from its elements in their standard sates under standard conditions

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16
Q

Explain standard enthalpy change of combustion

A

The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions with all reactants and products in the ir standard states

17
Q

Explain standard entahlpy change of reaction

A

The enthalpy change when a reation occurs in molar quantities shown in the chemical equation, under standard conditions with all reactants and products in their standard states

18
Q

What are the three things you need to know when measuring the enthalpy change in a lab

A

Number of moles of stuff that is reacting
Change in temperature
How much stuff you’re heating

19
Q

What are experiments that measure heat given out by reaction called

A

Calorimetry experiments

20
Q

Describe the apparatus needed in when measuring enthalpy changes in a lab

A

Polystyrene beaker
Solution inside beaker
Thermometer inside solution

21
Q

Explain how you measure enthalpy change for reactions that happen in solution

A

Use the thermometer to measure the temperature at regular intervals
Use a polystryene beaker to insulate the reaction

22
Q

Describe the apparatus needed to measure the enthalpy change of combustion

A

Stirrer
Thermometer
Water
Combustion chamber
Fuel (reactan)
Air tube
Calorimeter

23
Q

Explain how you measure the enthalpy change of combustion

A

As the fuel burns it heats the water and you can measure the temperature change and find out how much heat the water has absorbed if you know the mass of the water and the specific capacity of the water

24
Q

Explain why the method for working out the enthalpy change of combustion may give inaccurate results

A

Some heat may be lost to surroundings and there might be some incomplete combustion

25
Q

Why is finding the starting value of the water and subtracting it from the final value not a good method of finding out the temperature change when measuring enthalpy change of combustion

A

Because some heat is lost fromp the calorimeter to the surroundings

26
Q

How do calculate the temperature change when measuring enthalpy change of combustion

A

Record temperature at regular intervals (begin a couple of minutes before you start the reaction)
Plot a graph (temp against time)
Plot two lines of best fit one for the results before the reaction and one for after
The distance between the two lines at the time the reaction started is the accurate temperature change for the reaction

27
Q

How do calculate the standard enthalpy change of combustion

A

Calculate the amount of heat lost
Calculate the number of moles of ffuel that caused thsi enthalpy change (number of moles = mass/Mr)
Use the equation - heat change/number of moles of fuel burned - to calculate

28
Q

What is standard enthalpies of combustion measured in

A

kj mol^-1

29
Q

What is the enthalpy of neutralisation

A

The energy change when 1 mole of water is formed by the reation of an acid and an alkali

30
Q

How do you find the enthalpy of neutralisation

A

Add a known volume of acid to an insulated conatainer
Then add known volume of alkali and record the temperature at regular intervals
Stir solution to make sure it’s evenly heated
Work out the heat energy given out by reaction using the heatc change/number of moles equation

31
Q

What is a dissolution reaction

A

When the reactants split up into it’s ions and makes it an aqueos solution

32
Q

How do you measure the enthalpy change of a dissolution reaction

A

Add known mass of a solid to a known mass of water in a polystyrene cup and stir
Record the temperature regularly

33
Q

How do you find out the molar enthalpy chanage of a dissolution reaction

A

Find the enthalpy change per mole of solute dissolved
Divide the heat energy change for the reactoin by the number of moles of solid that you started off with

34
Q

What is Hess’s Law

A

The total enthalpy change for a reaction is independent of the route taken

35
Q

What is the purpose of Hess’s Law

A

If you know the enthalpy change for one route of a reaction you can use that to find out the enthalpy change of the other route of the same reaction

36
Q

What are entahlpy changes of formation

A

The heat change of going from and element to a compound

37
Q

What does increasing the concentration of molecules in a reaction do to the Maxell-Boltzmann distribution curve

A

It increases the peak of the curve

Chemistry (9 decks)

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