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Topic 1 National Chemistry Summary Notes

Rates of Reaction

Chemical Reactions

We know a chemical reaction has taken place when we notice one or more of the

following things happening:

Colour change

Gas is produced (also known as effervescence)

Energy change i.e. energy is taken in or given out, for example, heat, light etc…

Solid forming (also known as precipitation)

Note: all chemical reactions involve the formation of one or more new

substances

LI 1, 2 and 3

Effervescence

Precipitation

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Everyday Chemical Reactions

Our lives are full of chemical reactions. Some examples of everyday chemical

reactions include:

Striking a match

Wool growing on a sheep – whenever something is produced in nature it is an

example of a chemical reaction

Baking a cake

A car rusting

WORD EQUATIONS

A shorthand way of showing what goes on in a chemical reaction is to write a

WORD EQUATION.

Substances you start with are called REACTANTS. These appear on the left

hand side of the equation.

Substances that form are called PRODUCTS. These appear on the right hand

side of the equation.

In a word equation ‗+‘ means ‗and‘ and the means ‗produces‘.

e.g. magnesium burns in oxygen to produce magnesium oxide.

magnesium + oxygen magnesium oxide

REACTANTS PRODUCT(S)

LI 4

LI 5

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When is it Not a Chemical Reaction?

Sometimes a substance can appear to change appearance drastically or even

seem to disappear. These may not be chemical reactions. They could simply be

changes of state or mixtures being formed. If the substance can be easily

brought back (or separated out) it is not a chemical reaction. Here is a reminder

of work you did in first year.

Changes of State

Melting, freezing, boiling, condensing are not chemical changes but physical

changes. They do not produce a new substance, just a new form. Changes in

state are not examples of chemical reactions as no new substances are being

formed.

The four changes of state are:

Melting

Freezing

Evaporating (liquid to gas)

Condensing (gas to liquid)

LI6

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Separation techniques

A mixture is formed when two or more substances come together without

reacting. As no new substances form in a mixture, it is not an example of a

chemical reaction.

For example, air is a mixture of gases, mainly nitrogen and oxygen. Mixtures

can be separated in different ways depending on their properties some of these

ways include filtration, evaporation and chromatography.

Filtration

Filtration is used to separate a mixture of a solid and a liquid e.g. sand and

water. The liquid collected is the filtrate and the solid collected in the filter

paper is called the residue.

Evaporation

Evaporation is used when we want to separate the mixture created when a solid

has dissolved in a liquid e.g. salty water. The salty water is heated until all the

water evaporates and the salt is left behind in the evaporating basin.

LI 7, 8

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Chromatography

Chromatography is used to separate mixtures of substances into their

components. It is particularly useful for separating mixtures of inks into their

separate colours.

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Making Solutions

To make a solution a solute must dissolve in a solvent.

Solute – substance being dissolved

Solvent – substance doing the dissolving

Solution – solute plus solvent

For example, when salt is dissolved in water a salt solution is made.

Solute – salt

Solvent – water

Solution – salt solution

Making a solution is not an example of a chemical reaction. This is because the

solute and the solvent can be easily brought back. e.g. the salt in salty water can

be brought back by evaporation.

Note: when the starting substances can be easily brought back a chemical

reaction has not taken place.

A substance which dissolves in a liquid is said to be soluble; a substance which

does not dissolve is said to be insoluble. e.g. chalk is insoluble in water, salt is

soluble in water.

A dilute solution has less dissolved solute than a concentrated solution e.g. a

glass of dilute orange juice has less orange dissolved in it than a glass of

concentrated orange juice

A solution is diluted by adding more solvent. For example, adding water to

concentrated orange juice makes it more dilute and as a result, it tastes less

‗orangey‘ than before.

LI 9 to 13

A saturated solution is one in which no more solute can be dissolved.

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Factors Affecting the Rate of Reaction

We often want to speed up a chemical reaction. There are 4

main ways we can alter the speed (rate) of a chemical reaction.

The Effect of Particle Size

Solids with a smaller particle size (e.g. powders or small chips) react more

quickly than solids with a larger particle size (e.g. lumps). The diagram explains

why.

The perimeter (solid line) of the large chip is 12 units. The acid particles

can only collide with the edge of the chip. However, if we break up the

large chip into 9 smaller chips:

LI 14

E

acid particle

8

then the perimeter around each chip is 4 units, but there are 9 of them so the

total perimeter is 4 x 9 = 36 units.

This increases the number of collisions.

If a solid reactant is broken into small pieces or ground into a powder:

its surface area increases

more particles are exposed to the other reactant

there are more collisions

the rate of reaction increases

Effect of Temperature and Concentration

The rate of a chemical reaction can be increased by raising the temperature. It

can also be increased by increasing the concentration of a reactant.

Temperature

If the temperature is increased:

the reactant particles move more quickly

they have more energy

the particles collide more often, and more of the collisions result in a reaction

the rate of reaction increases

= particle A

= particle B

Reducing the size of particles increases the rate of a reaction because

it increases the surface area available for collisions to take place.

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Concentration

If the concentration of a reactant is increased

the reactant particles become more crowded

there is a greater chance of the particles colliding

the rate of reaction increases

Catalysts

Some reactions have catalysts that can speed them up, but for many reactions

there is no catalyst that works.

Here is an example of a reaction with a catalyst. Hydrogen peroxide decomposes

(falls apart) to form water and oxygen gas:

hydrogen peroxide —> water + oxygen

This reaction only occurs very slowly unless we add the compound manganese

dioxide that acts as a catalyst for this reaction. When the catalyst is added the

reaction speeds up greatly but the manganese dioxide never runs out.

LI 15 and 16

A catalyst is a substance that speeds up a reaction without being used up

itself.

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Note: As a catalyst does not get used up during a reaction, the mass of a

catalyst at the start of a reaction will be the same as at the end.

Different catalysts catalyse different reactions. The table below summarises

some common catalysts used in industry and the reactions they catalyse:

Some common catalysts used in industry and the reactions they catalyse

Catalyst Reaction Catalysed

iron making ammonia from nitrogen and hydrogen

platinum making nitric acid from ammonia

vanadium(V) oxide making sulphuric acid

Transition metals, such as platinum, are used as the catalyst in catalytic

converters. Catalytic converters are devices that fit onto the exhausts of cars.

They change harmful gases into harmless gases, for example, nitrogen oxide is

converted to nitrogen and oxygen.

A gauze or honeycomb structure is a better structure for a catalyst than a lump

because a gauze or honeycomb structure has a bigger surface area.

Gauze

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Measuring the Rate of Reaction

Reactions can be followed by measuring changes in concentration, mass and

volume of reactants and products.

Note: When the line of a graph levels off, this indicates the reaction has

finished i.e. one of the reactants has been completely used up.

LI17

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Time (s)

Calculating the Rate of Reaction

The table shows the volume of hydrogen gas produced over fifty seconds.

Time (s) Volume of gas (cm3)

0 0

10 20

20 40

30 55

40 65

50 72

10. The average rate of a reaction, or stage in a reaction,

can be calculated from initial and final quantities and the

time interval.

Average Rate =

E

Volume

of gas

(cm3)

LI 18

13

The average rate at which gas is produced can be calculated as shown.

= 2 cm3/s

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Tests for Gases

Many reactions produce gases and it is important to be able to identify them:

Oxygen

The test for oxygen is that it relights a glowing splint.

As air is approximately 20% oxygen, there is not enough oxygen in the air for

the test to be positive.

Hydrogen

The test for hydrogen is that it burns with a ‗pop‘.

LI 19 to 22

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Carbon Dioxide

The test for carbon dioxide is that it turns lime water milky.

Summary

Gas Test

Carbon dioxide Turns lime water milky

Hydrogen Burns with a pop

Oxygen Relights a glowing splint

Note: if a burning splint is placed in a test tube of oxygen it will burn more

brightly

Nitrogen gas has no chemical test

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Collecting Gases

The best way to collect a gas depends upon the properties of the gas.

The following 3 set-ups show the best way to collect a gas.

Soluble in water

Less dense than air

Soluble in water

More dense than air

Insoluble in water

LI23

Note: Less dense than air means lighter than air, denser than air means

heavier than air.

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LI24 Naming compounds

COMPOUNDS

• When elements join together during a chemical reaction new substances called

COMPOUNDS are formed.

• A COMPOUND is a substance made up of two or more elements joined

together.

• The name of a compound can tell you the name of the elements in it

• Compounds which end in –IDE USUALLY contain only 2 elements.

• The first element in the name of a compound is usually the furthest left in the

periodic table or if in the same group the furthest up in the table.

• Oxygen is ALWAYS the second element in an oxide compound.

• A MIXTURE of elements is not a compound.

NAME OF COMPOUND ELEMENTS PRESENT

sodium chloride

hydrogen and oxygen

copper bromide

potassium and iodine

rubidium fluoride

COPY & COMPLETE

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EXCEPTIONS TO THE –IDE RULE

• Compounds which end in –IDE usually contain only 2 elements but as usual there

are always exceptions to any rule.

• HYDROXIDES and CYANIDES are two classes of compounds which have more

than two elements in them.

• HYDROXIDES contain HYDROGEN and OXYGEN

CYANIDES contain CARBON and NITROGEN

NAME OF COMPOUND

ELEMENTS PRESENT

calcium hydroxide

sodium, hydrogen and oxygen

hydrogen cyanide

barium, carbon and nitrogen

COPY & COMPLETE

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COMPOUNDS ENDING IN –ATE OR –ITE

• Not all compounds end in –IDE.

• Some other compounds names end in –ATE or –ITE.

• The –ATE or –ITE ending tells you that there is a third element in the

compound which is always OXYGEN.

NAME OF COMPOUND

ELEMENTS PRESENT

calcium carbonate

magnesium, sulphur and oxygen

iron Sulphite

lithium, carbon and oxygen

sodium nitrate

COPY & COMPLETE

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Number Learning Intention Success criteria

1. I will find out chemical reactions

can be identified by changes in

appearance of substance, including

colour change, gas evolved,

precipitate formed.

I can identify a chemical

reaction by

A change of

appearance

A colour change

A gas evolved

A precipitate

formed

2. I will find out chemical reactions

can be identified by energy changes

including temperature change, light

given off.

I can identify a chemical

reaction by energy

changes including

Temperature

increasing or

decreasing

Light being given

off

3. I will find out chemical reactions

involve the formation of one or

more new substances.

I can state that at least

one new substance is

produced during a

chemical reaction.

4. I will find out about some chemical

reactions which occur in our day-to-

day lives.

I can give examples of

chemical reactions such

as

Iron rusting

Cooking food

Substances

burning

5. I am going to find out how to write

word equations in order to

summarise what happens during

chemical reactions.

I can write word

equations for chemical

reactions including :

correctly using the

‗+‘ and ‗→‘ symbols

recognising

reactants and

placing them on

the left of the ‗→‘

recognising

products and

placing them on

the right of the

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‗→‘

6. 1. I will find out the changes of state

melting, freezing, condensing and

evaporating are not chemical

reactions.

I can state that

Melting

Freezing

condensing

evaporating

are not chemical

reactions

7. I will find out mixtures occur when

two or more substances come

together without reacting.

I can state that mixtures

occur when two or more

substances come

together without

reacting.

8. I will find out mixtures can be

separated in different ways depend

on their properties

i.e. filtration, evaporation,

chromatography.

I can state that:

filtration is used

to separate solids

and liquids

evaporation is used

to obtain the solid

which was

dissolved in a liquid

chromatography

can be used to

separate different

liquids.

9. I will find out a solution is a mixture

formed when a solute dissolves in a

solvent.

I can state that a

solution is a mixture

formed when a solute

dissolves in a solvent.

10. I will find out a substance which

dissolves in a liquid is soluble and a

substance which does not dissolve is

insoluble.

I can state that a

substance which

dissolves in a liquid is

soluble and a substance

which does not dissolve is

insoluble.

11. I will find out a saturated solution

is one in which no more substance

can be dissolved.

I can state that a

saturated solution is one

in which no more

substance can be

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dissolved.

12. I will find out a dilute solution has a

lower concentration of dissolved

substance than a concentrated

solution.

I can state that a dilute

solution has a lower

concentration of

dissolved substance than

a concentrated solution

13. I will find out a solution is diluted

by adding more solvent.

I can state that a

solution is diluted by

adding more solvent.

14. I will find out that rates of reaction

are affected by changes in

concentration, particle size and

temperature.

I can state that rates of

reaction can be increased

by

Increasing

concentration

Increasing

temperature

Using smaller particles

15. I will find out catalysts are

substances which speed up some

reactions and are not used up by

the reactions.

I can state that

catalysts are substances

which speed up some

reactions and are not

used up by the reactions

16. I will find out about some everyday

examples of uses of catalysts.

I can state that

transition metals in car

exhaust systems are

examples of uses of

catalysts.

17. I will find out reactions can be

followed by measuring changes in

concentration, mass and volume of

reactants and products.

I can state that

reactions can be followed

by measuring changes in

concentration, mass and

volume of reactants and

products.

18. I will find out the average rate of a

reaction, or stage in a reaction, can

be calculated from initial and final

quantities and the time interval.

I can calculate the

average rate of reaction

using

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19. I will find out the test for oxygen is

that it relights a glowing splint.

I can state that the test

for oxygen is that it

relights a glowing splint

20. I will find out there is not enough

oxygen in the air for the test to be

positive.

I can state that there is

not enough oxygen in the

air for the test to be

positive

21. I will find out the test for hydrogen

is that it burns with a ‗pop‘.

I can state that the test

for hydrogen is that it

burns with a ‗pop‘

22. I will find out the test for carbon

dioxide is that it turns lime water

milky.

I can state that the test

for carbon dioxide is

that it turns lime water

milky.

23. I will find out the best way to

collect a gas depends upon the

properties of the gas.

I can state that the best

way to collect

an insoluble gas is

to bubble through

water.

A soluble gas with

a lower density

than air is to

collect in an

upturned test tube

A soluble gas with

a higher density

than air is to

collect downwards

in a test tube

24. I will find out how to apply the

general rules:

compounds with the name ending in

―ide‖ contain the two elements

indicated (except hydroxides and

cyanides), and that the ending ―ite‖

or ―ate‖ indicates the additional

element oxygen.

I can state the elements

present in a compound

given its name.