Quantitive chemistry and chemical changes

Question 1

how do you calculate the percentage mass of an element in a compound

Answer 1

atomic mass x number of atoms of the element / relative formula mass of the compound

Question 2

how do you calculate the number of moles

Answer 2

mass in g / relative formula mass

Question 3

in an exothermic reaction is heat given in or out

Answer 3

given out

Question 4

in an endothermic reaction is heat given in or out

Answer 4

taken in

Question 5

does an exothermic reaction transfer energy to or away from the surroundings

Answer 5

transfers energy to the surroundings usually by heating

Question 6

does an endothermic reaction take energy form the surroundings or transfer energy to the surroundings

Answer 6

the reaction takes energy from the surrounding

this is shown by a fall in temperature

Question 7

give an example of an exothermic reaction

Answer 7

hand warmers

self heating cans

Question 8

give an example of an endothermic reaction

Answer 8

sports injury packs

Question 9

explain a exothermic reaction

Answer 9

this reaction is exothermic because energy needed to break the bonds is less than energy released in bond formation

Question 10

explain a endothermic reaction

Answer 10

energy released in bond formation
is less than
energy needed to break the bonds

Question 11

are endothermic reaction energy changes positive or negative

Answer 11

positive +

Question 12

are exothermic reaction energy changes positive or negative

Answer 12

negative -

Question 13

explain bond energies

Answer 13

when we want to break a chemical bond, it requires energy normally endothermic

this then releases energy normally exothermic

every chemical bond has an energy value

this tells us the energy needed to break the bond

Question 14

is breaking bonds endo or exo themic

Answer 14

endothermic +

Question 15

why is it not always possible for a 100% yield

Answer 15

product is lost in transfer

Question 16

give the equation of percentage yield

Answer 16

product that was actually produced / product that should of been produced x 100

Question 17

why can we not achieve more than 100% yield

Answer 17

because it means you would have created atoms and that is not possible

Question 18

state what is meant by an exothermic reaction

Answer 18

an reaction that releases energy

Question 19

in energy transfer diagrams, exothermic,

do the products have more or less energy than the reactants

Answer 19

less

because energy is being transferred from the reaction to the surroundings

Question 20

in energy transfer diagrams, endothermic

do the products have more or less energy than the reactants

Answer 20

more

because energy has been taken in from the surroundings

Question 21

if the products have more energy than the reactants what does this mean

Answer 21

energy has been taken in from the surroundings

energy increases

endothermic
positive

Question 22

if the products have less energy than the reactants what does this mean

Answer 22

energy is being transferred from the reaction to the surroundings,

energy decreases

• exothermic
  negative

Question 23

if a reversible reaction is exothermic in one direction what happens to the other direction

Answer 23

it is endothermic in the opposite direction

the same amount of energy is transferred in each case

Question 24

what is a dynamic equilibrum

Answer 24

rate of forward reaction is equal to rate of backward reaction in a closed system

Question 25

what happens if temperature is increased in an reversible reaction

Answer 25

if temperature increases, endothermic reaction is favoured

Question 26

what happens if temperature is increased in an exothermic reaction

Answer 26

if temperature increases, exothermic reaction is favoured

Question 27

what is the effect of pressure

Answer 27

pressure favours the reaction which produces fewer molecules meaning the reaction will produce more of the favoured reactions

Question 28

what happens in a dynamic equilibrium

Answer 28

concentration of reactants and products stay constant

Question 29

what must happen in a dynamic equilibrium

Answer 29

reaction must be reversible reaction must undergo in a closed system so no change of matter, but energy can still be exchanged

Question 30

what is the calculation of concentration.

Answer 30

mass (g) / volume (dm3)

Question 31

if we increase the mass of solute and keep the volume the same what happens to the concentration

Answer 31

it stays the same

Question 32

if we increase the volume of solution and keep the mass of solute the same what will happen to the concentration

Answer 32

it will decrease

Question 33

what is atom economy

Answer 33

is the measure of the amount of starting materials that end up as useful products

Question 34

what is the equation of atom economy

Answer 34

mr of desired products / mr of all reactants (from equation) x 100

Question 35

what is the equations for atomic mass

Answer 35

(Ar x Adundance) + (Ar x abundance) // 100

Question 36

what is the equation for moles in solids

Answer 36

mass / Mr

Question 37

what is the equation for moles in gases

Answer 37

if in dm3 volume dm3 / 24dm3 if in cm3 volume cm3 / 24000

Question 38

what is the equation for concentration?

Answer 38

mols or mass / volume dm3

Question 39

what is a reversible reaction

Answer 39

when the products can react together to make the reactants

Question 40

what is a closed system

Answer 40

when no exchange of matter can occur only energy can be transferred

Question 41

effect of increasing temperature in a dynamic equilibrium

Answer 41

a increase of temperature favours endothermic reaction

Question 42

effect of increasing pressure in a dynamic equilibrium

Answer 42

an increase in pressure favours the side with the least moles (molecules)

Question 43

effect of concentration in a dynamic equilibrium

Answer 43

an increase in concentration of reactants move the equilibrium forwards direction to make more products

Question 44

explain in terms of bond making and breaking why a reaction is endothermic

Answer 44

energy released in bond formation is less than energy needed to break the bonds

Question 45

explain in terms of bond making and breaking why a reaction is exothermic

Answer 45

energy released in bond formation is more than energy needed to break the bonds

Question 46

explain why a non metal has a low boiling point

Answer 46

simple molecular structure weak intermolecular forces little energy needed to overcome these