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Chemistry > Chapter 8 > Flashcards

Chapter 8 Flashcards

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1
Q

When bonds are broken and formed, what is often caused?

A

A change in energy

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2
Q

What does enthalpy change mean?

A

A change in energy

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3
Q

What is the scientific definition of enthalpy change?

A

The heat energy change in a reaction at constant pressure

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4
Q

What is the units of enthalpy change?

A

kJ / mol

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5
Q

When describing enthalpy change, why do you write delta H?

A

To show that the measurements were made under standard conditions and that the elements were in there standard states.

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6
Q

What are standard conditions?

A

100kPa pressure, and 298k

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7
Q

What do exothermic reactions do?

A

They give out energy

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8
Q

For exothermic reactions, what is the value of delta H?

A

It is a negative value

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9
Q

For endothermic reactions, what is the value of delta H?

A

It is a positive value

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10
Q

What are endothermic reactions?

A

Reactions that take in energy from the surroundings

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11
Q

What is the relationship between stability and the enthalpy of a substance?

A

The less enthalpy a substance, the more stable it is

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12
Q

What is the activation energy?

A

The minimum amount of energy needed to begin breaking bonds and start a chemical reaction

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13
Q

In endothermic reactions, are the reactants or products more stable?

A

The reactants are more stable

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14
Q

In exothermic reactions, are the reactants or products more stable?

A

The products are more stable

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15
Q

What is standard enthalpy change of reaction?

A

The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions

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16
Q

What is the standard enthalpy change of formation?

A

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

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17
Q

What is the standard enthalpy change of combustion?

A

The enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions.

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18
Q

What is the standard enthalpy change of neutralisation?

A

The enthalpy change when an acid and an alkali react together, under standard conditions, to form 1 mole of water.

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19
Q

When writing an equation for the standard enthalpy change of combustion, what must the amount of moles for the compound reactant be?

A

1 mole always, as the standard enthalpy change of combustion is when 1 MOLE of a substance is completely burned in oxygen.

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20
Q

What 3 reactants are required for the formation of methanol (CH3OH)?

A

Hydrogen, oxygen and carbon.

21
Q

What is the equation for the standard enthapy change of combustion of carbon?

A

C + O2 —> CO2

22
Q

When trying to find the enthalpy change for a reaction, what 2 things do you need to know?

A
  • The number of moles of the reactants

- The change in temperature of a reaction

23
Q

For reactions carried out using a constant pressure, what is the heat change the same as?

A

The enthalpy change

24
Q

What is the main problem with mixing reactants together, using a thermometer and measuring the overall temperature change?

A

Some heat will be lost to the surroundings, so the temperature change you measure will be less than the actual temperature change

25
Q

Describe an experiment that could be used to find the enthalpy change of the endothermic reaction between citric acid and sodium bicarbonate:

A

1) Add a set volume of citric acid to a polystyrene cup
2) Put a lid on the beaker and measure the temperature until stable
3) Add a set mass of sodium bicarbonate to the beaker, and stirred the mixture
4) Measure the temperature of the reaction every 30s until it reaches a minimum value (exothermic) for a couple of minutes.
5) Draw a graph of temperature against time
6) Find the extrapolated minimum temperature and find the total temperature change.

26
Q

How do you find the extrapolated minimum tempreature?

A

Draw a tangent on the time-temp graph at the point where the temperature is at its lowest value. Find the point where this tangent intersects the time where the reaction started

27
Q

To find the enthalpy changes of combustion, what do you need?

A

A calorimeter

28
Q

Describe an experiment to find out the enthalpy changes of comustion?

A

1) Burn a flammable liquid
2) As the fuel burns, it heats the water. If you know the mass of the water you can work out the heat absorbed by the water

29
Q

What is the main equation used to calculate enthalpy changes?

A

q = mcT

30
Q

In the equation q = mcT, what is q?

A

q=heat lost or gained

31
Q

If the pressure is constant, what is the heat lost or gained equal to?

A

The enthalpy change

32
Q

In the equation q = mcT, what is m?

A

The mass of the water in the calorimeter, or solution in the insulated container ( in grams).

33
Q

In the equation q = mcT, what is c?

A

The specific heat capacity of water

34
Q

What is the specific heat capacity of water?

A

4.18 J g-1 K-1

35
Q

In the equation q = mcT, what is T?

A

The change in temperature

36
Q

What does ‘calorimetry’ mean?

A

Measuring heat change

37
Q

What is located at the bottom of a calormeter?

A

A spirit burner containing the fuel

38
Q

What does Hess’s Law state?

A

The total enthalpy change of a reaction is always the same, no matter of what route is taken.

39
Q

By using Hess’s law, what does the enthalpy change of route 1 equal?

A

The enthalpy change of route 2

40
Q

Is bond breaking exothermic or endothermic?

A

Bond breaking is endothermic, so enthalpy change is positive.

41
Q

Why is bond breaking enothermic?

A

You need to supply energy to break the bonds

42
Q

Is bond forming exothermic or endothermic?

A

Bond formic is exothermic, so the enthalpy change is negative

43
Q

Why is bond forming exothermic?

A

Because energy is released when bonds are formed

44
Q

If you need more energy to break bonds than is released when bonds are made, what is the value of delta H?

A

Positive

45
Q

What is required to break bonds in ionic bonding etc?

A

Energy is required to break bonds

46
Q

What is Bond Enthalpy?

A

The amount of energy required to break 1 mole of a type of bond in a molecule in the gas phase.

47
Q

Why are mean bond enthalpies not exact?

A

Its the average for a bigger range of molecules. For example the O-H bond in water is in alcohols and carboxylic acids as well.

48
Q

What is the mean bond enthalpy?

A

The energy needed to break 1 mole of bonds in the gas phase, averaged over many different compounds.

49
Q

What is the equation for enthalpy change of reaction?

A

Enthalpy change of reaction = Bond enthalpies of reactants - Bond enthalpies of products

Chemistry (25 decks)

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