Unit 3 – Chemical Reactions

Part 2

Science 1206

Chemical changes involve breaking old bonds and forming new chemical bonds

During a chemical reaction, the atoms are neither created nor destroyed, instead they are just rearranged

The old substances whose bonds are broken are called reactants, and the new substances that are formed as a result of the reaction are called products

Chemical changes can be written as an equation

Conservation of Mass

4 main observations to indicate a chemical change:1. Energy change: the absorption or the release

of heat and/or light, sound, electrical energy2. Colour change3. Precipitate formation (solid product forms)4. Gas formation

in a chemical reaction the mass of the reactants before a chemical reaction equals the mass of the products after the reaction is complete

Mass of reactants = mass of products

Conservation of Mass

In a chemical reaction the mass of the reactants before a chemical reaction equals the mass of the products after the reaction is complete.

Note: this is only in the ideal world. In the real world, the masses are hardly ever the same. Why do you think that is?

Law of Conservation of Mass

Since mass is constant, the number of atoms of each element must be the same on both sides of the equation

If a chemical equation is not balanced, then coefficients have to be placed in front of the chemical formulas in order to balance the equation.

Ex. HgO(s) Hg(l) + O2(g)

Balancing Chemical Equations

Take each element in turn:Element LHS RHSHg 1 1O 1 2So we need 2O on the LHS.\ 2HgO(s) Hg(l) + O2(g)

\ Check each element again.

Balancing Chemical Equations

Element LHS RHSHg 2 1O 2 2So now we need 2 Hg on the RHS2HgO(s) 2 Hg(l) + O2(g)

Check each element again. Element LHS RHSHg 2 2O 2 2the equation is now balanced.

Balancing Chemical Equations

Ex 2. Fe(s) + O2(g) Fe2O3(s)

*See other board for solution

Polyatomic ions are a little trickyIn order to balance polyatomic ions treat them

as a whole unit.

Balancing Chemical Equations

Ex 3. KI + Pb(NO3)2 KNO3 + PbI2

Balancing Chemical Equations

Your turn. Balance the following equations.1. Mg(s) + O2(g) MgO(s)

2. Cu(s) + AgNO3(aq) Ag(s) + Cu(NO3)2(aq)

3. KI(aq) + Pb(OH)2(aq) PbI2(s) + KOH(aq)

4. P4(s) + F2(g) PF3(l)

Worksheet #8

Balancing Chemical Equations

When two or more reactants combine to form a single product.

Ex 1. Sodium metal reacts with chlorine gas.2 Na(s) + Cl2(g) → 2 NaCl(s)

Ex 2. hydrogen gas reacts with chlorine gasEx 3. carbon reacts with sulfur to produce

carbon disulfide.Note: You will only be told the product when it is

a molecular compound. In the case of ionic compounds you will have to determine the product.

Formation (Combination) Reactions

Your turn. Write a balanced chemical equation for the following.

1. Magnesium + oxygen2. Hydrogen reacts with oxygen to form water3. Nitrogen reacts with hydrogen to produce

ammonia4. Aluminum reacts with oxygen5. Sulfur reacts with oxygen to produce sulfur

dioxide

Formation (Combination) Reactions

Separating compounds into simpler substance, most often as elements

Opposite of formation reactionsWhen a compound is decomposed into its

elements, the reaction is a simple decomposition.

Ex 1. Water is decomposed into its elements.2 H2O(ℓ) → 2 H2(g) + O2(g)

Decomposition Reactions

Ex 2. Nitrogen trichloride decomposes into its elements

Ex 3. Iron (III) oxide is broken down into its elements

Note: It is important to indicate the state (i.e. solid, liquid or gas) of the species as many substances can exist easily as two or three of the states. If the state is not told, then use the state at room temperature. (Consult your periodic table if you are unsure about the state of an element).

Decomposition Reactions

Your turn. Write a balanced chemical equation for the following.

1. Chromium (III) oxide is decomposed into its elements.

2. Nitrogen is produced from the breakdown of ammonia.

3. Mercury (II) sulfide breaks down into its elements

4. Sodium hydroxide is decomposed into its elements (tricky!!)

Worksheet #9

Decomposition Reactions

In order to determine whether or not a substance dissolves in water we use a solubility table. (See data handout)

Ex 1 Will barium hydroxide, Ba(OH)2, dissolve in water?

Sol’n: Find the anion OH in the table (top row)Figure out which row Ba2+ is in, (aq) or

(s)?According to this table Ba(OH)2 is

soluble in water.DataSheet.pdf

Solubility

Ex 2. Will silver acetate, AgCH3COO, dissolve in water?

Sol’n: Using the solubility table, we determine that silver acetate _______________ dissolve in water.

DataSheet.pdf

Solubility

Your turn. Use the solubility table to determine whether each of the following is soluble or insoluble in water.

1. PbI2

2. K2CO3

3. H2SO4

4. Barium sulfate5. Zinc nitrate

Solubility

When chemical reactions occur, bonds are broken and reformed to form new compounds.

Single replacement reactions have an element reacting with a compound. In this case, the bond in the compound breaks and the elements in the compound form a new compound by joining with the other reacting element. The products will also be an element and a compound but different from that of the reactants.

Single Replacement Reactions

In order to determine which element in the compound the reacting element replaces you look at their ions. Replace the element with the same charge (i.e. Positive with positive)

Another way to look at is to replace metal with metal and non-metal with non-metal.

For example, Zn(s) + HCl(aq) → H2(g) + ZnCl2(aq)

Note: When writing water in chemical equations, write as HOH (hydrogen and hydroxide) as this is how it reacts in reactions.

Single Replacement Reactions

Ex 1. Cl2(g) + NaBr(aq)

Ex 2. calcium reacts with water

Ex 3. copper reacts with silver nitrate

Single Replacement Reactions

Your turn. Write balanced chemical equations for each of the following.1. Zn(s) + CuSO4(aq)

2. Mg(s) + HCl(aq)

3. Sodium reacts with water4. Hydrogen sulfide reacts with silver5. Hydrochloric acid reacts with fluorine

Worksheet #10

Single Replacement Reactions

Involves the replacement of an element in one compound by another element in another compound.

AB + CD → AD + CBSwitch the first element in each compound. Remember that opposite charges attract, i.e. you need a positive and a negatively charge element/polyatomic ion to form a compound.

Double Replacement Reactions

Ex 1. Silver nitrate reacts with a solution of sodium chloride.

Ex 2. Write a balanced chemical equation for the double replacement reaction involving calcium chloride and sodium carbonate.

Double Replacement Reactions

Your turn. Write a balanced chemical equation for the following:

1. HCl + NaOH 2. Al(OH)3 + HCl

3. Cd(NO3)2 + (NH4)2S

4. Solutions of barium chloride and silver nitrate are reacted.

5. A solution of potassium phosphate reacts with cobalt (III) chloride.

Worksheet #11

Double Replacement Reactions

Require oxygen gas as a reactantEnergy in the form of heat and light is often released

Any reaction which releases energy is said to be exothermic while reactions that absorb energy are said to be endothermic

Hydrocarbons consist of hydrogen and carbon, for example, propane, C3H8.We will be looking at the combustion of hydrocarbons

only and you will be given the formula for these compounds.

Two types of combustion reactions are complete and incomplete.

Combustion Reactions

Complete combustion results in carbon dioxide (CO2) being produced. The evidence for complete combustion is a blue flame.

Incomplete combustion results in carbon monoxide (CO) being produced. The evidence fro incomplete combustion is an orange, smoky flame.

Whether a combustion is complete or incomplete depends on the amount of oxygen that is available. If there is not enough, then there will be an incomplete combustion.

Combustion Reactions

When balancing combustion reactions balance the C’s first, then H’s, and then the O’s, i.e. work left to right.

Ex 1. Write a balanced equation for the complete combustion of propane, C3H8.

Ex 2. Write a balanced equation for the complete combustion of butane, C4H10.

Combustion Reactions

Your turn. Write a balanced equation for the following.

1. CH4 + O2 → CO2 + H2O2. C2H12 + O2 → CO2 + H2O3. C25H52 + O2 → CO2 + H2O4. Octane, C8H18, is completely burned.

Combustion Reactions

Always make sure you have written the chemical formulas correctly

Never balance an equation by changing the numerical subscripts in chemical formulas

Whenever possible treat polyatomic ions as a whole unit

If you place a coefficient in front of a formula to balance one of its elements then check to see if you have to change the number of other formulas containing the same element

Review

If you are having trouble balancing an equation, check the chemical formulas to make sure they are correct.

When an element occurs in two or more formulas on the same side of an equation, balance it last

Make sure coefficients are in lowest ratio, i.e. make sure that there is no number that will divide each coefficient evenly.

Review

Chemical reactions occur all around us. These chemical reactions result in various substances being produced.

There are tests that we can perform on the products to determine what they are.

We are going to look at 7 common tests for commonly produced substances.

Chemical Tests

1. Oxygen gas (glowing splint test)To test for the presence of oxygen, collect a

sample of the gas in a small container such as a test tube. Light a wooden splint and blow out the flame, leaving the tip reddish and glowing. Insert the tip of the splint into the mouth of the test tube. If the splint reignites, then the gas is oxygen.

This test works because oxygen supports burning. Since the wooden splint is a form of fuel and the tip is red hot, it will began to burn in the presence of the oxygen.

Chemical Tests

2. Hydrogen gas (burning splint test)To test for the presence of hydrogen gas,

collect a sample of the gas in a small container. Light a wooden splint and insert the burning splint into the mouth of the container. If you hear a slight explosive “pop” sound, then the gas is hydrogen.

This test works because hydrogen will tend to mix slightly with the oxygen in the air creating an explosive mixture. The burning splint will ignite the hydrogen gas and you will hear a slight “pop” sound from the tiny explosion.

Chemical Tests

3. Carbon dioxide (lime water test)To test the presence of carbon dioxide, collect

a sample of the gas in a small container. Add a small amount (2 – 3 mL) of lime water to the container and shake. If the lime water solution turns milky white, then the gas is carbon dioxide.

This test works because lime water and carbon dioxide react to form a white precipitate.

Chemical Tests

4. Water (cobalt chloride paper test)To test any sample for the presence of water,

touch the sample with a piece of cobalt chloride paper. If the paper turns from blue to pink, water is present.

This test works because cobalt chloride is a chemical (salt) that turns colour from blue when dried, but turns pink when even a slight amount of moisture is present.

Chemical Tests

5. Acid (litmus paper test)An acid turns blue litmus paper red.

6. Base (litmus paper test)A base turns red litmus paper blue.

Chemical Tests

7. Aqueous solution of salt (conductivity apparatus test)

To test the presence of a dissolved salt in a solution, set up a conductivity apparatus, such as the one displayed below. Place the two electrical probes into the solution to be tested. Make sure the two probes are close but not touching. If the light bulb comes on, the solution contains ions.

Chemical Tests

Prelab:Identify and write down the reaction type for

each reaction.Complete the balanced chemical equation,

include predicted states of matter.Complete the word equation.

Core Lab #2

Recall that acidity is measured using a pH scale. What pH values indicate a substance is an acid? How do you tell which acid is stronger than another?

Any precipitation that has a pH value of less than 5.6 us considered to be acid precipitation.

One of the main causes of acid rain is sulfur dioxide.Natural sources, which emit this gas, are

volcanoes, sea spray, rotting vegetation, and plankton.

The burning of fossil fuels are largely to be blamed for about half of the emissions of this gas in the world.

Acid Precipitation

When sulfur dioxide reaches the atmosphere, it oxidizes (i.e. picks up oxygen) to form a sulfate ion. It then becomes sulfuric acid as it joins with hydrogen atoms in the air and falls back down to earth.Ammonia and ozone act as catalysts in the

oxidation of the sulfur dioxide.Not all of the sulfur dioxide is converted to

sulfuric acid. A fair amount of it just goes up into the atmosphere, gets carried away, and falls back to earth as is.

Acid Precipitation

Nitric oxide and nitric dioxide are also components of acid rain.These are emitted from power stations and

exhaust fumes.Like the sulfur dioxide, these nitrogen oxides

rise into the atmosphere, oxidize to nitrate ions, and then react with the hydrogen in the atmosphere to become nitric acid.

These reactions are catalyzed by iron, manganese, ammonia, and hydrogen peroxide.

Acid Precipitation

Sulfur dioxide is generally a by-product of industrial processes ad burning of fossil fuels.

Ore smelting, coal-fired power generators, and natural gas processing are the main contributors.

The main source of nitrogen oxides is the combustion of fuels in motor vehicles, residential and commercial furnaces, industrial and electrical-utility boilers and engines, and other equipment.

It is no good for just Canada to reduce the emissions of these gases. Why not?

Acid Precipitation

Acid rain has been, for a few decades, a major ecological concern. Its full-blown effects have only been realized recently.

Acid rain is a problem in eastern Canada because many of the water and soil systems in this region lack natural alkalinity, such as a limestone base, and therefore cannot neutralize acid naturally.

Much of the bedrock in Canada consists mainly of granite, which acid will wear away.

Canada has predominantly eastwardly moving weather, so all the pollution from western Canada gets pushed to eastern Canada.

Acid Precipitation

Lakes act as “sinks” of the earth in that water drains into these lakes. Therefore, acid rain that falls can affect lakes kilometers away because it is carried by rivers and streams.This is at its worst during the spring. Why do

you think that is?Runoff that contains acid rain can pick up

other harmful materials along its route and these materials accumulate in the lake.

Acid Precipitation

Acids in the water affects the organisms that live there.Acids can cause mucus to build up on fish gills,

interfering with the fishes ability to absorb oxygenAcids react with salts and nutrients present in the

lakes reducing the amount available to the organisms that live there

Calcium reacts easily with acids and exoskeletons and egg shells consist of calcium carbonate, causing the to weaken

Acids can cause heavy metals to be released more quickly so that their concentrations can reach toxic levels.

Acid Precipitation

On land, acid causes nutrients to be leached from the soil as well as heavy metals.

Many plants thrive in soils with a narrow pH range. Acid rain can alter the pH of the soil to be outside of these ranges, causing plants to wilt and die.

In the atmosphere, acid pollution exists as dry particles which creates a haze, making it difficult for pilots to see and reduces the amount of sunlight that passes through.

Acid Precipitation

Acid rain can cause corrosion in buildings, bridges, and statutes, especially those made of limestone.

Acid rain causes respiratory problems in humans such as asthma, dry coughs, headaches, and eye, nose, and throat irritations.

Toxic metals that are dissolved in the acid precipitation are absorbed in fruits, vegetables, and in the tissues of animals. The levels found in individual organism is not high enough to harm them, but it accumulates in humans with the more they eat.

Acid Precipitation

How can we help reduce acid precipitation?Use less electricityUse your vehicle lessMaintain your vehicle so that it is fuel efficientBuy products that are energy efficient or bear

a EcoLogo

Acid Precipitation