level l chemistry bq

1

Chapter 4: Chemical Reactions

List ALL

Basic Questions

Basic Question 1

4.1.1 Classify and justify changes as chemical or physical _

Which of the following is a physical change? Which is a chemical change?

1. Heating wax until it melts Physical change, no new

substance is formed

2. Crush some salt crystals into a powder Physical change, no new

substance is formed

3. Boiling water until it evaporates, then condensing the

steam. Physical change, no new

substance is formed

4. Boiling an egg in boiling water for two minutes, then

cooling the water. Chemical change, a new

substance is formed

5. Heating a piece of paper until it turns black, then cooling

it. Chemical change, a new

substance is formed

6. Heating a mixture of hydrogen and oxygen until it

“pops”, and then cooling the gases to a liquid. Chemical change, a new

substance is formed

7. Dissolving salt into water to make a solution Physical change, no new

substance is formed

8. Cooking a steak until it is well done Chemical change, a new

substance is formed

9. Burning magnesium ribbon in air Chemical change, a new

substance is formed

2

Basic Question 2

4.2.1 Classify changes as exothermic or endothermic -

Which of the following is an endothermic process?

1. Heating water from 20°C to 80

°C. Endothermic, raising the

temperature requires

absorption of energy

2. Heating water from 20°C through to boiling continuously at

100°C.

Endothermic

3. The burning of a magnesium ribbon in air Exothermic, all burnings release

energy

4. The reaction that takes place when a spark is introduced to a

mixture of H2 and O2 gas. Exothermic

5. Any reaction or process that consumes heat energy. Endothermic

6. The process of cooling liquid paradichlorobenzene until it

solidifies. Exothermic, solidification

involves a temperature drop,

which requires release of

energy,

7. The decomposition of water into H2 and O2 gas. Endothermic

8. Any reaction or process that releases heat energy Exothermic

9. Condensation of steam Exothermic

10. Sublimation of iodine. Endothermic

11. Vaporization of ethanol Endothermic

Basic Question 3

4.2.3 Know that mass and # of atoms are conserved in chemical reactions G

In chemical reactions what can you say about?

a) Conservation of mass?

Mass is always conserved in all chemical reactions. The sum of the masses of all reactants is equal

to the sum of the masses of all products.

b) Conservation of atoms?

The number of atoms is always conserved in any chemical reaction. The number of atoms on the

left (reactants) is equal to the number of atoms on the right (products).

3

c) Conservation of molecules?

Molecules are not necessarily conserved in chemical reactions.

Basic Question 4

4.3.1 Read a given chemical reaction in terms of molecules and atoms G

Consider the equation

a) 2C2H2 + 5O2 →2H2O + 4CO2

How do you read it in terms of molecules? Show that atoms are conserved?

We can read the equation as:

Two molecules of C2H2 react with five molecules of O2 (oxygen gas) to give two molecules of water

and 4 molecules of carbon dioxide gas.

The number of H atoms on the left is: 2×2=4 and the number of H atoms on the right is 2×2=4

The number of C atoms on the left is 2×2=4 and the number of C atoms on the right is 4×1=4

The number of O atoms on the left is 5×2=10 and the number of O atoms on the right is

(2×1) +(4×2)=10

The total number of atoms on the left is 18 and on the right is also 18

b) C + O2 → CO2

How do you read it in terms of molecules? Show that atoms are conserved?

This equation cannot be read in terms of molecules

The equation can be read as follows:

1 atom of carbon reacts with 1 molecule of oxygen to produce one molecule of CO2

Basic Question 5

4.3.1 Identify coefficients and subscripts G

Given: 4CO2.

a) Which number represents the coefficient in 4CO2? 4 is a coefficient

b) Which number represents the subscript? 2 is a subscript

4

Basic Question 6

4.3.1 Represent a reaction by a balanced chemical equation G

Write the balanced equation for the reaction between methane (CH4) and oxygen to give water vapor and

carbon dioxide.

CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)

Basic Question 7

4.3.2 Balance simple chemical equations _

Balance the equations for each of the following reactions

a) 2Na + Cl2 → 2NaCl

b) 2Na + O2 → Na2O2

c) 2Al + 3Cl2→2AlCl3

d) P4 + 3O2→ P4O6

e) 2NH3 → 3H2 + N2

Basic Question 8

4.4.1 Balance a given chemical equation and give the reacting ratios

in moles, molecules and mass

G

a) Balance the equation: NH3 + O2 → H2O + NO2

4 NH3 + 7 O2 → 6 H2O + 4 NO2

b) Give the reacting ratios in molecules for the above equation.

Four molecules of ammonia react with seven molecules of oxygen gas to give six molecules of

water and four molecules of nitrogen dioxide gas.

c) Give the reacting ratios in moles for the above equation.

Four moles of ammonia react with seven moles of oxygen gas to give six moles of water and four

moles of nitrogen dioxide gas.

d) Give the mass reacting ratios for the above equation.

4NH3 + 7O2 → 6H2O + 4NO2

4×(14+3)= 68g 7×32=224g 6×(2+16)=108g 4×(14+32)=184g

68 grams of ammonia react with 224g of oxygen to give 108g of water and 184g of nitrogen

dioxide.

e) Verify that mass is conserved.

Total mass of reactants = 68+224=292g.

Total mass of products = 108 + 184=292g, so mass is conserved in chemical reactions.

5

Basic Question 9

4.4.2 Read a given balanced equation with fractional coefficients _

How would you read the equation: H2 + ½ O2 → H2O.

Explain.

Since half molecules do not exist, we can only read this equation in terms of moles:

One mole of hydrogen gas reacts with half a mole of oxygen gas to give one mole of water.

4.4.3 Stoichiometric calculations G

Basic Question 10

Consider the following equation. N2H4 + O2 → H2O + N2.

4.4.3 Balance a given chemical equation

a) Balance the equation.

N2H4 + O2 → 2 H2O + N2

4.4.3 Find # molecules of product formed from # molecules of reactant given

b) How many molecules of nitrogen does one molecule of N2H4 give?

Given

One molecule of N2H4

R.T.F

Number of molecules of N2 formed

1 molecule of N2H4 gives 1 molecule of nitrogen gas.

c) How many molecules of water does one molecule of N2H4 give?

Given

One molecule of N2H4

R.T.F

Number of molecules of H2O formed

1 molecule of N2H4 gives 2 molecules of water

6

4.4.3 Find # moles of product formed from # moles of reactant given

d) How many moles of nitrogen does one mole of N2H4 give?

Given

One mole of N2H4

R.T.F

Number of moles of N2 formed

1 mole of N2H4 gives 1 mole of nitrogen gas.

e) How many moles of water does one mole of N2H4 give?

Given

One mole of N2H4

R.T.F

Number of moles of H2O formed

1 mole of N2H4 gives 2 moles of water.

4.4.3 Find # moles of reactant needed to from given # moles of product formed

f) In the above balanced equation, how many moles of N2H4 are required to form 16 moles of H2O?

Given

16 moles of H2O

R.T.F

Number of moles of N2H4

N2H4 + O2 → 2H2O + N2

1 mole 2 moles

?? moles 16 moles

Number of moles of N2H4 needed is (16×1)/2 = 8 moles

4.4.3 Find m of given # moles of substance

g) How many grams are there in one mole of N2H4?

Given

One mole of N2H4

R.T.F

mass of N2H4

m = n x M = 1 ×[(2×14) + (4×1)] = 32g

h) How many grams are there in three moles of H2?

7

Given

3 moles of H2

R.T.F

m of H2

m = n × M = 3 × (1×2) = 6g

4.4.3 Write mass ratio of a given reaction

i) Write the ratios of reactants and products by mass. Verify that these ratios conserve mass.

N2H4 + O2 → 2H2O + N2

32g 32g 2×18 = 36g 2×14=28g

Total mass of reactants = 32 + 32 = 64g

Total mass of products = 36 + 28 = 64g so mass is conserved.


Basic Question 11

If iron is burned in chlorine, it produces Iron (III) chloride:

Fe + Cl2 → FeCl3.



2Fe + 3Cl2 → 2FeCl3

4.4.3 Write mass ratio of a given reaction

b) Write the reacting ratios in moles and in grams.


2 moles 3 moles 2 moles

2×56 = 112g 3×(35.5×2) = 213g 2× ( 56 + 3×35.5) = 325g

4.4.3 Find # moles of reactant needed to from given # moles of product formed

c) How many moles of iron are required to form 4 moles of FeCl3?

Given

4 moles of FeCl3

R.T.F

n of Fe

8


2 moles 2 moles

?? moles 4 moles

Number of moles of Fe needed = (4×2)/2 = 4 moles

4.4.3 Find mass of product formed from given mass of reactant

d) What mass of FeCl3 is produced when 35.5 g of chlorine are consumed?

What mass of Fe is consumed in the process?

Given

m of Cl2 = 35.5 g

R.T.F

m of FeCl3

m of Fe

3Cl2 → 2FeCl3

3 moles 2 moles

3×71=213g 325g

35.5g ?? g

Mass of FeCl3 needed is (35.5 ×325) / 213 = 54.2g

2Fe + 3Cl2

2 moles 3 moles

2×56=112g 3×71=213g

?? g 35.5g

Mass of Fe consumed in the reaction = (35.5 ×112) / 213 = 18.7g

Basic Question 12

4.4.4 Write the reacting ratio in terms of volume for a given reaction G

In the balanced equation below, at STP, which of the following is/are correct ratio by volume?

2 C2H6 (g) + 7 O2 (g) → 4 CO2 (g) + 6 H2O (l)

a) 2 x 22.4 dm3 7 x 22.4 dm

3 4 x 22.4 dm

3 _____

b) 2 x 22.4 dm3 7 x 22.4 dm

3 4 x 22.4 dm

3 negligible

c) 2 cm3 7 cm

3 4 cm

3 _____

d) 44.8 dm3 156.8 dm

3 89.6 cm

3 _____

9


Basic Question 13

Ethane has the formula C2H6. Its combustion products are carbon dioxide and water.


a) Write the balanced equation for the combustion of ethane:

C2H6 (g) + O2 (g) →H2O (l) + CO2 (g).

2C2H6 (g) + 7O2 (g) → 6H2O (l) + 4CO2 (g)

4.4.4 Give the reacting ratios in moles, mass and volume G

b) Write the ratios of reactants and products in moles.

2C2H6 (g) + 7O2 (g) → 6H2O (l) + 4CO2 (g)

2 moles 7 moles → 6 moles + 4 moles

c) Write the ratios of reactants and products in grams.

2C2H6 (g) + 7O2 (g) → 6H2O (l) + 4CO2 (g)

2×[(2×12)+(1×6)]=60g 7×(16×2)=224g 6×[(1×2)+16)]=108g [(4×12)+16×2)]=176g

d) Write the ratios of reactants and products by volume at STP.

2C2H6 (g) + 7O2 (g) → 6H2O (l) + 4CO2 (g)

2×22.4 = 44.8dm3 7×22.4=156.8dm

3 negligible 4×22.4=89.6 dm

3

4.4.4 Find # moles of products formed from given # moles of reactants

e) One mole of ethane produces how many moles of water?

Given

One mole of C2H6

R.T.F

Number of moles of H2O

2C2H6 (g) + 7O2 (g) → 6H2O (l) + 4CO2 (g)

2 moles 6 moles

1 mole ??? moles

Number of moles of water formed = 6/2=3 moles

10

4.4.4 Find volume of one reactant needed to react with a given # moles of other reactant

f) 4.00 moles of ethane combine with what volume of oxygen at STP?

Given

n of C2H6 = 4 moles

R.T.F

V of O2 at STP

2C2H6 (g) + 7O2 (g) → 6H2O (l) + 4CO2 (g)

2 moles 7 moles

2 moles 156.8dm3 (7x22.4)

4 moles ???dm3

Volume of O2 needed = (156.8×4)/2 = 313.6 dm3 = 314 dm

3

4.4.4 Find volume of air needed to react with a given # moles of reactant

g) 4.00 moles of ethane combine with what volume of air at STP, knowing that air is 20% oxygen?

Given

n of C2H6 = 4.00 moles

air is 20% O2

R.T.F

V of air

Volume of air needed = volume of O2 needed × (100/20)

= 314 x (100/20) = 1,570dm3 = 1.57 x 10

3 dm

3

11


Basic Question 14

Consider the reaction between hydrogen and oxygen to produce water, which releases 284 kJ/mol of H2.

4.4.5 Find amount of heat released when given mass of product is formed from molar heat of

reaction

a) How much heat is released when 180 g of water are produced?

Given

m of H2O = 180 g

molar heat = - 240 kJ

R.T.F

heat released

H2 + ½ O2 → H2O + 284 kJ

1 mole 284 kJ

1 x 18g 284kJ

180g ???kJ

Amount of heat evolved = (284×180)/18 = 2840 kJ = 2.84 x 103 kJ

4.4.5 Find the heat released by given mass of reactant and the heat of reaction

b) How much heat is released when 0.20 mole of hydrogen are used?

Given

n = 0.20 mole of H2

R.T.F

heat released

H2 + ½ O2 → H2O + 284kJ

1 mole 284kJ

0.20 moles ??? kJ

Amount of heat evolved = (284×0.20)/1 = 56.8kJ = 57 kJ

12

Basic Question 15

4.4.5 Balance a given chemical equation using whole-number coefficients -

a) Balance the equations for the decomposition (to elements) of nitrogen trifluoride,NF3, and nitrogen

trichloride,NCl3.Base each equation upon the production of one mole of N2.

NF3 → 1N2 + F2

NCl3 → 1N2 + Cl2

2NF3 → 1N2 + 3F2

2NCl3 → 1N2 + 3Cl2

4.4.5 Write balanced equations including heat change as a term in the equation -

b) Rewrite the equations to include the information that the decomposition of NF3 is endothermic,

absorbing 232 kJ/mol N2, and the decomposition of NCl3 is exothermic, releasing 457 kJ/mol N2.

2NF3 + 232kJ → 1N2 + 3F2

2NCl3 → 1N2 + 3Cl2 + 457kJ

4.4.5 Select a dangerously explosive reaction based on its heat change

c) One of the two compounds NF3 and NCl3 is dangerously explosive. Which would you expect to be the

explosive substance? Explain.

NCl3 is the dangerously explosive substance because it is highly exothermic, it releases heat when

decomposed. The heat released can be used to further decompose it.

4.4.6 Stoichiometric Calculations G

Basic Question 16

Consider the following equation:

3H2 + N2 → 2NH3.

If 6.00 g of H2 react completely, determine the needed volume of N2 measured at STP conditions.

4.4.6 Find the V of a reactant (or product) needed to react (or is produced) given

mass of other reactant

a) Write the most convenient reaction ratio for this problem.

3×2=6g 22.4dm3

13

b) Solve the problem.

Given

m of H2 = 6.00 g

R.T.F

V of N2= ?

3H2 + N2 → 2NH3

3 moles 1 mole

3×2=6g 1 x 22.4dm3

6.00g ??? dm3

Volume of N2 needed = (22.4 ×6.00)/6 = 22.4 dm3

Basic Question 17

Consider the reaction between hydrogen and oxygen to form water. If 4.00g of H2 react completely,

determine the quantity of heat produced

H2 + O2 → H2O + 284kJ



H2 + ½ O2 → H2O + 284kJ

4.4.6 Find the heat released by given mass of reactant and the heat of the reaction

b) Write the most convenient reaction ratio for this problem.

2g 284kJ

c) Solve the problem.

Given

m = 4.00 g of H2

R.T.F

Amount of heat produced = ?

H2 + ½ O2 → H2O + 284kJ

1 mole 284 kJ

1 x 2 g 284 kJ

4.00g ??? kJ

Quantity of heat evolved = (284×4.00)/ 2 = 568kJ

14

Basic Question 18

If a piece of potassium metal (a solid) is lowered into a bottle of chlorine gas.

A reaction takes place. A salt, KCl, is formed.


a) Write the balanced equation for the reaction.

2K(s) + Cl2 (g) → 2KCl (s)

4.4.6 Find the # of moles of products formed from given mass of reactant

b) How many moles of KCl could be formed from 3.35 grams of Cl2?

Given

m of Cl2 = 3.35 g

R.T.F

n of KCl

2K(s) + Cl2 (g) → 2KCl (s)

1 mole 2 moles

1 x (2×35.5) = 71g 2 moles

3.35g ??? moles

Number of moles of KCl formed = (2×3.35)/71 = 0.0944 moles

15

Basic Question 19

Consider the following equation:

H2 + N2 → NH3.

If 33.6 dm3 of H2 react completely (measured at STP), determine the number of moles of N2 needed.



3H2 + N2 → 2NH3

4.4.6 Find n of reactant (or product) needed to react (or is produced) given V of

other reactant


67.2dm3 22.4 dm

3

c) Solve the problem.

Given

V of H2 = 33.6 dm3

R.T.F

n of N2 needed

3H2 + N2 → 2NH3

3 moles 1 mole

3 x 22.4 dm3 1 mole

67.2 dm3 1 mole

33.6 dm3 ??? moles

Number of moles of N2 needed = (33.6×1) / 67.2 = 0.500 moles

Basic Question 20

Consider the equation: H2 + O2 → H2O.

It is required to find the number of moles of H2 that will produce 18 g of H2O.



H2 + ½ O2 → H2O

4.4.6 Find the n of reactant needed to produce given mass of product


1 mole 18g

16

c) Solve the problem

Given

m of H2O = 18 g

R.T.F

n of H2 used

H2 + ½ O2 → H2O

1 mole 1 mole

1 mole 1 x 18g

?? moles 18g

Number of moles of H2 needed = 1.0 mole

Basic Question 21

4.4.6 Application on the law of conservation of mass: mass of reactants = mass of products

If 15 grams of substance A combine with 8 grams of substance B to give 20 grams of substance C and

some D, how many grams of D would you expect?

Given

mA = 15g

mB = 8g

mC = 20

R.T.F

mD

Mass is conserved in any chemical reaction. This means that the sum of the masses of the reactants

equals the sum of the masses of the products.

A + B → C + D

15 + 8 = 20 + mD

mD = 3 g

17

Basic Question 22

Propane burns in air according to the following balanced equation:

(C = 12; H = 1; O = 16)

C3H8 (g) + 5 O2 (g) → 3 CO2 (g) + 4 H2O (g).

4.4.6 Find the mass of reactant needed to react with the given mass of second reactant

What mass of C3H8 is required to react with 40 g of oxygen?


C3H8 + 5O2 → 3 CO2 + 4 H2O

(3×12 +8) = 44g 5×(16×2)=160g


Given

m of oxygen = 40 g

R.T.F

m of C3H8

C3H8 + 5O2 → 3 CO2 + 4 H2O

1 mole 5 moles

(3×12 +8) = 44g 5×(16×2)=160g

???g 40g

The mass of C3H8 needed = (44 ×40)/160 = 11g

Basic Question 23

Butane (bottled gas) burns in air according to the following balanced equation: (C = 12; H = 1; O = 16)

2C4H10 (g) + 13 O2 (g) → 8CO2 (g) + 10H2O (g).

What mass of C4H10 reacts with 6.5 moles of oxygen?

4.4.6 Find the mass of reactant needed to react with the given # of moles of other reactant


2C4H10 + 13 O2 → 8CO2 + 10 H2O

116g 13 moles

18


Given

n of oxygen = 6.5 moles

R.T.F

m of C4H10

2C4H10 + 13 O2 → 8CO2 + 10 H2O

2 moles 13 moles

2 x 58=116g 13 moles

??? g 6.5 moles

Mass of C4H10 needed = (6.5 ×116)/13 = 58g

Basic Question 24

Propane burns in air according to the following balanced equation: (C = 12; H = 1; O = 16)

C3H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2O (g).

How many moles of C3H8 react with 2.0 moles of oxygen?

4.4.6 Find # of moles of reactant needed to react with given # moles of other reactant


1C3H8 + 5O2 → 3CO2 + 4H2O

1 mole 5 moles


Given

n of oxygen = 2.0 moles

R.T.F

n of C3H8

1C3H8 + 5O2 → 3CO2 + 4H2O

1mole 5 moles

?? moles 2.0 moles

Number of moles of propane needed = 2.0/5 = 0.40 moles

19

Basic Question 25

Carbon burns in air according to the following balanced equation: (C = 12; O = 16)

C (g) + O2 (g) → CO2 (g).

What mass of carbon dioxide can be produced by 36.0 g of carbon?

4.4.6 Find mass of product formed from given mass of reactant


C(g) + O2(g) → CO2(g)

12g 44g


Given

m of C = 36.0

R.T.F

m of CO2

C(g) + O2(g) → CO2(g)

1 mole 1 moles

1x12=12g 1x44=44g

36.0g ??g

Mass of carbon dioxide produced = (36 × 44)/12 = 132g

Basic Question 26

Carbon burns in air according to the following balanced equation: (C = 12; O = 16)

C(g) + O2 (g) → CO2 (g).

What volume of O2 at STP will produce 4.00 mole of CO2?

4.4.6 Find V of reactant needed to produce given # of moles of product


C(g) + O2(g) → CO2(g)

22.4dm3 1 mole

20


Given

n of CO2 = 4.00 moles

R.T.F

V of O2

C(g) + O2(g) → CO2(g)

1 mole 1 mole

1x22.4dm3 1 mole

?? dm3 4.00 moles

Volume of O2 needed = 22.4×4 = 89.6 dm3

21

Sample Questions

4.1 Physical and chemical change

4.1.1 Differentiating between physical and chemical change

Sample Question 1

Distinguishing between chemical & physical changes

What is a physical change? What is a chemical change? State some differences between the two types of

changes.

Physical Change Chemical Change

1. Produces no new kind of matter

2. It is generally easily reversible

3. It is not accompanied by great heat change

4. It produces no observable change in mass

1. Always produces a new kind of matter

2. It is generally not easily reversible

3. It is usually accompanied by

considerable heat change

4. It produces no observable change in

mass

Sample Question 2

Know that in chemical (and not in physical) changes new substances are formed

How do we know when a chemical change has taken place?

a) When a new substance is formed, like when burning of wax.

b) When a substance changes form, like when melting wax.

c) When the change can be easily reversed.

d) When there is a change of phase.

e) When there is a change of state.

4.2 Principles of chemical reactions

22

4.2.1 Combustion of hydrogen with oxygen

Sample Question 3

Recognize a combustion reaction

Which of the following is a combustion reaction?

a) The burning of gasoline in air.

b) The reaction between iron and moist air that results in the formation of rust.

c) The burning of a magnesium ribbon in pure chlorine gas (which produces a lot of light and

heat).

d) The reaction that takes place between a cut apple and air that turns the apple brown.

e) A reaction between any substance and any gas, where heat and light are produced.

Sample Question 4

Recognize an exothermic process

What is an exothermic process?

1. A process where there is a net release of heat.

2. An exothermic process is accompanied with an increase in temperature

3. Produces no noticeable change in mass

4.2.2 Decomposition of water

Sample Question 5

Recognize an endothermic process

What is an endothermic process?

1. A process where there is a net absorption of heat.

2. An endothermic process is accompanied with a decrease in temperature

3. Produces no noticeable change in mass

4.2.3 Conservation of mass

Sample Question 6

Application of conservation of atoms & mass in chemical reactions

If 20 molecules of water are decomposed into its elements, how many molecules of each element will be

produced? Are atoms conserved? (Count them) Are molecules conserved? (Count them)

20 molecules of water produce 20 molecules of hydrogen and 10 molecules of oxygen.

Atoms are conserved where the number of hydrogen atoms on the reactants and products side is 40,

and the number of oxygen atoms on either side is 20.

23

Molecules are not conserved: number of water molecules is 20 while the number of hydrogen and

oxygen molecules adds up to 30

4.3 Representing a chemical reaction by a chemical equation

4.3.1 Writing equations

4.3.2 Balancing equations

Sample Question 7

Recognize a balanced equation

Which of the following is a balanced chemical equation?

a) CH4 + ½ O2 → CO2 + 2H2O

b) CaCO3 → CaO + CO2

4.4 Stoichiometry

4.4.1 Equations with smallest whole-number coefficients

Sample Question 8

Read a balanced equation in molecules and moles

Read the following equations in molecules and moles:

a) BaCl2 + Na2SO4 BaSO4 +2NaCl

This equation cannot be read in molecules (ionic compounds)

1 mole of BaCl2 combines with 1 mole of Na2SO4 to produce 1 mole of BaSO4 and 2 moles of NaCl

b) 2 C2H6 + 7 O2 → 4 CO2 + 6 H2O

2 moles of C2H6 react with 7 moles of O2 to form 4 moles of CO2 and 6 moles of H2O

2 molecules of C2H6 react with 7 molecules of O2 to form 4 molecules of CO2 and 6 molecules of

H2O

4.4.2 Equations with fractional coefficient

4.4.3Mass relations in chemical equations

Sample Question 9

Use equations to get mass ratio of reactants & products

Consider the following equation: 3H2 + N2 → 2NH3

a) How can you read the above equation in terms of mass used or produced?

6g of hydrogen react with 28g of nitrogen to produce 34g of ammonia.

24

b) Show that the mass is conserved?

Reactants: 6 g + 28 g = 34 g Products: 34 g

Mass of reactants is equal to mass of products, so the mass is conserved.

4.4.4Volume relations in chemical equations

Sample Question 10

Standard temperature and pressure (STP)

a) What does STP stand for? Standard temperature and pressure.

Know what STP conditions are.

b) What are STP conditions? 0⁰C and 1.00 atm pressure

Sample Question 11

At STP 1.00 mol of gas occupies 22.4 dm3

a) What is the volume of one mole of gas at STP? 22.4 dm3

b) Is this generalization valid for liquids or solids? No

c) Which of the following occupies 22.4 dm3 at STP?

a. 1 mole of alcohol

b. 0.50 moles of steam

c. 1 mole of steam

d. 1 mole of water

e. 1 mole of potassium metal

f. 1 mole of hydrogen gas

Note: 1 mole of any GAS at STP occupies 22.4 dm3

25

Volume reacting ratio

Sample Question 12

Volume relations in balanced chemical equations

In the balanced equation below, at STP, what is the reacting ratio by volume?

(Hint: remember that at STP water is a liquid)

CH4 + 2O2 → CO2 + 2H2O

a) 22.4 dm3 of CH4 react with 44.8dm

3of oxygen to produce 22.4dm

3of carbon dioxide and a

negligible volume of water

b) 1 dm3 of CH4 react with 2 dm

3of oxygen to produce 1 dm

3of carbon dioxide and a negligible

volume of water

c) 22.4 dm3 of CH4 react with 44.8dm

3of oxygen to produce 22.4dm

3of carbon dioxide and 44.8 dm

3 of

water

4.4.5 Energy relations in chemical equations

Sample Question 13

Writing an equation with the energy involved

Consider the equation

2H2 + O2 → 2H2O

Rewrite the equation including the information that the reaction is exothermic evolving 284 kJ/ mol H2.

2H2 + O2 → 2H2O + 568 kJ or H2 + ½ O2 → H2O + 284 kJ

4.4.6 Calculations based upon chemical equations

Making pancakes

Obtaining reaction ratios

Using the most convenient ratio

level l chemistry bq

Documents