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C4- ENERGETICS > 4.2- ENTHALPY > Flashcards

4.2- ENTHALPY Flashcards

1
Q

What does the amount of heat given out or taken in by a reaction vary with?

A

conditions

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2
Q

Examples of conditions that amount of energy given out or taken in by a reaction varies with? (3)

A

temp
pressure
concentration of solutions

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3
Q

As amount of heat given out or taken in be a reaction vary with conditions, what must be stated?

A

conditions under which measurements made

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4
Q

What is it called when you measure a heat change at constant pressure?

A

enthalpy change

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5
Q

What is the symbol for enthalpy?

A

H

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6
Q

What are the standard conditions for measuring enthalpy changes? (2)

A

pressure of 100kPa

temperature of 298K

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7
Q

What is the pressure of 100kPa approximatly?

A

approximately normal atmospheric pressure

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8
Q

What is the temperature of 298K around the same as?

A

normal room temp, 25°C

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9
Q

What is the standard state of an element?

A

state in which it exists at 298K and 100kPa

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10
Q

What is it written as when an enthalpy change is measured under standard conditions?

A

ΔH∘298

298 usually left out

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11
Q

When is a reaction thought to be over? (exothermic)

A

until products have cooled back to 298K

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12
Q

What is the heat given out to surroundings while reaction mixture cools?

A

enthalpy change for reaction

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13
Q

In an exothermic reaction what do the products end up with less of?

A

less heat energy than starting materials

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14
Q

Why do the products end up with less heat energy than the starting materials in an exothermic reaction?

A

as they have lost heat energy when they heated up their surroundings

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15
Q

What is the ΔH of exothermic reactions like?

A

negative

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16
Q

What sign do exothermic reaction have?

A

negative sign

17
Q

What do some endothermic reactions that take place in aqueous solution do?

A

absorb heat from water + cool it down

18
Q

What is an example of an endothermic reaction that happens in an aqueous solution and absorbs heat from water and cools it down?

A

dissolving ammonium nitrate in water

19
Q

When is a reaction thought to be over? (endothermic)

A

products have warmed up to temp at which they started

20
Q

In endothermic reactions what do the products end up with more of?

A

more energy than starting material

21
Q

What is the ΔH of endothermic reactions like as the products end up with more energy than the starting material?

A

positive

22
Q

What is the sign for an endothermic reaction?

A

positive

23
Q

When does pressure affect the amount of heat energy given out by a reaction?

A

reaction that involves gases

24
Q

If a gas is given out, what is required by some of the energy?

A

some of the energy required to push away the atmosphere

25
Q

What happens the greater the atmospheric pressure, when a gas is given out?

A

the more energy is used for this

26
Q

As more energy is used when there is greater atmospheric pressure and gas is given out, what does this mean?

A

less energy remains to be given out as heat by reaction

27
Q

What does increasing the atmospheric pressure, leading to more energy used when a gas is given out and less energy remaining to give out as heat by reaction show the importance of?

A

importance of having a standard of pressure for measuring energy changes

28
Q

What else affects the enthalpy change of a reaction?

A

physical states (gas, liquid or solid) of reactants + products

29
Q

Example of how physical states of reactants + products affect enthalpy change of reaction?

A

heat must be put in to change liquid to gas + is given out when gas changed to liquid

30
Q

As physical states affect enthalpy change of reaction, what must be included in the equations?

A

state symbols

31
Q

What are enthalpy level diagrams sometimes called?

A

energy level diagrams

32
Q

What are enthalpy level diagrams used to represent?

A

enthalpy changes

33
Q

What do enthalpy level diagrams show?

A

relative enthalpy levels of reactants (starting materials) + products

34
Q

What does the vertical axis of an enthalpy level diagram represent?

A

enthalpy

35
Q

What does the horizontal axis of an enthalpy level diagram represent?

A

extent of reaction

C4- ENERGETICS (7 decks)

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