Higher Chemistry- Unit 3- energy

Question 1

what is the law of conservation of energy

Answer 1

energy cannot be created or destroyed but changed from one form to another

Question 2

if there is a temperature rise, is the reaction endo or exothermic

Answer 2

exothermic

Question 3

if there is a decrease in temperature, is the reaction exo or endothermic

Answer 3

endothermic

Question 4

what costs may be incurred if a reaction is endothermic

Answer 4

supplying heat energy in order to maintain the reaction rate

Question 5

what costs may be incurred if a reaction is exothermic

Answer 5

the heat produced may need to be removed to prevent the temperature rising

Question 6

what is the enthalpy change of an exothermic reaction

Answer 6

negative

Question 7

what is the enthalpy change of an endothermic reaction

Answer 7

positive

Question 8

what happens in an exothermic reaction

Answer 8

energy is given out as energy of the products is less than reactants
-it gets hotter

Question 9

what happens in an endothermic reaction

Answer 9

energy is taken in as the energy of the products is more than the energy of the reactants
- it gets colder

Question 10

what does an exothermic reaction graph look like

Answer 10

the products are lower than the reactants

Question 11

what does an endothermic reaction graph look like

Answer 11

the products are higher than the reactants

Question 12

what is enthalpy

Answer 12

chemical energy

Question 13

what is enthalpy change

Answer 13

the change in chemical energy associated with chemical reaction.

Question 14

how can you figure out the heat energy

Answer 14

Eh=cmΔT

mass of thing changing temperature

Question 15

how can you figure out the enthalpy change when you know the mass and heat energy

Answer 15

ΔH=Eh/n
ΔH= energy gained or lost by the water
Eh= energy gained or lost by the water
n= number of moles of fuel burned

Question 16

what must you remember when figuring out the enthalpy change

Answer 16

• if it is an endothermic reaction (decrease in temperature) it will be a positive enthalpy change (change at end
• if it is an exothermic reaction (increase in temperature) it will be a negative enthalpy change (change at end)

Question 17

what is Hess’s Law

Answer 17

the enthalpy change in converting reactants into products is the same regardless of the route by which the reaction takes place.

Question 18

how do you do a Hess’s Law question

Answer 18

1. write a balanced equation for what you are trying to calculate
2. write a balanced equation for the reactions which we know the enthalpy (of combustion) for
3. think about moles. make equations match moles of target equation
4. if reactants/ products are on the wrong side flip the equation (flip the symbol of the enthalpy change)
5. add reactions together to calculate ΔH for the reaction
6. to check write out equation as one, cancel out and should equal target equation

Question 19

define enthalpy of combustion

Answer 19

the heat energy given out (always exothermic) when one mole of a substance burns completely in oxygen

Question 20

define enthalpy of formation

Answer 20

the heat energy involved when 1 mole of a compound is formed from its elements in their standard states.

Question 21

define enthalpy of neutralisation

Answer 21

the heat energy released when 1 mole of water is formed by neutralisation of an acid with a base (exothermic)
- if more than one mole of water is produced the energy must be multiplied accordingly

Question 22

define enthalpy of solution

Answer 22

the enthalpy change when 1 mole of solute is completely dissolved (can be exothermic or endothermic)

Question 23

what is important to note in Hess’s laws calculations

Answer 23

remember the definitions

-use correct value in table (combustion or formation?)

Question 24

what is an evaluation for a Hess’s law investigation

Answer 24

• heat lost to the surroundings

- human errors in measuring

Question 25

what is required to break a bond

Answer 25

energy (endothermic)

Question 26

what can Hess’s law tell us about the energies involved in making and breaking bonds

Answer 26

the same amount of energy must be given out when a bond is broken, as when it is made

Question 27

what happens regarding energy when a bond is made

Answer 27

it releases energy (exothermic)

Question 28

define mean molar bond enthalpy

Answer 28

average energy required to break one mole of chemical bonds in the gaseous state to form gases atoms. (endothermic)

Question 29

what won’t combust

Answer 29

nitrogen or oxygen

Question 30

how do you do a mean molar bond enthalpy question

Answer 30

1. write out target equation
2. write down all the bonds which are being broken and made (broken on left, made on right) and the number of them
3. bond breaking is always endothermic and bond making is always exothermic
4. calculate the enthalpy for the reaction by adding them together. A + -B

Question 31

Calculate the mass of Vitamin C in a 1l carton of orange juice if 29.3ml of 0.002moll-1 I2 solution was required to oxidise 10ml of orange juice
C6H806 + I2 –> C6H606 + 2I-

Answer 31

1. moles of I2 for 10ml =cv
2. moles for 1000ml =cv x100=5.86x10-3
3. mole ratio- 1:1
4. mass of vitamin C for 1000ml =nxGFM = 1g

Question 32

why is the energy from burning ethanol less than predicted

Answer 32

think definition

• ethanol did not completely combust
• heat lost to the surroundings

Question 33

what is the molar bond enthalpy for a diatomic molecule XY

Answer 33

the energy required to break one mole of XY bonds

Question 34

how can the values for the enthalpy of combustion be measured

Answer 34

using a calorimeter