4.2- ENTHALPY

Question 1

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

Answer 1

conditions

Question 2

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

Answer 2

temp
pressure
concentration of solutions

Question 3

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

Answer 3

conditions under which measurements made

Question 4

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

Answer 4

enthalpy change

Question 5

What is the symbol for enthalpy?

Answer 5

H

Question 6

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

Answer 6

pressure of 100kPa

temperature of 298K

Question 7

What is the pressure of 100kPa approximatly?

Answer 7

approximately normal atmospheric pressure

Question 8

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

Answer 8

normal room temp, 25°C

Question 9

What is the standard state of an element?

Answer 9

state in which it exists at 298K and 100kPa

Question 10

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

Answer 10

ΔH∘298

298 usually left out

Question 11

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

Answer 11

until products have cooled back to 298K

Question 12

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

Answer 12

enthalpy change for reaction

Question 13

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

Answer 13

less heat energy than starting materials

Question 14

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

Answer 14

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

Question 15

What is the ΔH of exothermic reactions like?

Answer 15

negative

Question 16

What sign do exothermic reaction have?

Answer 16

negative sign

Question 17

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

Answer 17

absorb heat from water + cool it down

Question 18

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

Answer 18

dissolving ammonium nitrate in water

Question 19

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

Answer 19

products have warmed up to temp at which they started

Question 20

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

Answer 20

more energy than starting material

Question 21

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

Answer 21

positive

Question 22

What is the sign for an endothermic reaction?

Answer 22

positive

Question 23

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

Answer 23

reaction that involves gases

Question 24

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

Answer 24

some of the energy required to push away the atmosphere

Question 25

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

Answer 25

the more energy is used for this

Question 26

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

Answer 26

less energy remains to be given out as heat by reaction

Question 27

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?

Answer 27

importance of having a standard of pressure for measuring energy changes

Question 28

What else affects the enthalpy change of a reaction?

Answer 28

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

Question 29

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

Answer 29

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

Question 30

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

Answer 30

state symbols

Question 31

What are enthalpy level diagrams sometimes called?

Answer 31

energy level diagrams

Question 32

What are enthalpy level diagrams used to represent?

Answer 32

enthalpy changes

Question 33

What do enthalpy level diagrams show?

Answer 33

relative enthalpy levels of reactants (starting materials) + products

Question 34

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

Answer 34

enthalpy

Question 35

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

Answer 35

extent of reaction