Material Changes
Curriculum:
- Use a pH scale.
- Understand neutralisation and some of its applications.
- Use indicators to distinguish acid and alkaline solutions.
- Use a word equation to describe a common reaction. Secondary sources can be used.
- Describe chemical reactions which are not useful, e.g. rusting.
- Describe the reactivity of metals with oxygen, water and dilute acids.
- Give examples of displacement reactions.
- Explain how to prepare some common salts by the reactions of metals and metal carbonates and be able to write word equations for these reactions.
- Explore and understand the reactivity series.
- Explore and explain the idea of endothermic processes, e.g. melting of ice, and exothermic reactions, e.g. burning, oxidation.
- Give an explanation of the effects of concentration, particle size, temperature and catalysts on the rate of a reaction.
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pH
A pH scale is usually measured from 1 to 14, with 1 being the most acidic and 14 the most basic (or alkaline) sides of the scale. The middle, pH 7, is called neutral. When acids and bases react the reaction is called a neutralisation reaction. The pH of the solution after the reaction depends on the acid and base strength and lies somewhere between both pH values and water is usually a product of this reaction. In other words, the reaction product is closer to a neutral pH than the reactants were, hence the name, neutralisation reaction. Some of its applications are quantitative analysis of compounds, neutralisation of stomach acid and it is used in wastewater treatment. Indicators Using indicators such as red and blue litmus can identify acidic and basic solutions. Blue litmus will turn red when it comes in contact with an acidic solution and red litmus will turn blue when it comes in contact with a basic solution. There are other indicators around which all have specific pH values at which their colour changes. It is important that clear and specific colour changes can be observed when picking an indicator. |
The pH scale and a few examples of acids and bases.
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Chemical Reactions
Because new compounds with different properties are formed in chemical reactions, chemical reactions can be identified by changes in physical properties. These reactions can be described by using word equations.
Reaction:
Hydrogen and oxygen react to produce water.
Word equation:
Hydrogen + Oxygen --> Water
Reactants are placed on the left-hand side of the equation, separated by a plus sign, and on the right-hand side of the equation the reaction products are placed, also separated by a plus sign. But sides of the equation are separated by and arrow, indicating that a reaction has occurred.
There are many different types of reactions and some of them are unwanted. Rusting, for example, is a chemical reaction of iron with oxygen catalyzed by water, resulting in iron oxide or more commonly named, rust.
Iron is not the only metal that reacts with oxygen. A lot of other metals react as well, resulting in various metal oxides, but there are some who do not, e.g. gold. Reactions with oxygen are called oxidation reactions.
Metal + oxygen --> metal oxide (some metals)
Alkali and alkaline earth metals react with water to form a metal hydroxide and hydrogen gas. Both types of metals become more reactive when going down in the periodic table.
Alkali metal + water --> metal hydroxide + hydrogen (also alkaline earth metals & other metals)
Metals can also react with acids to form salts and hydrogen gas. When a metal reacts with hydrochloric acid, its reaction product will be a metal chloride for example.
Metal + acid --> salt + hydrogen
During this reaction, the metal displaces one or more hydrogen atoms of the acid. This type of reaction is called a displacement reaction. Displacement reactions also occur when a more reactive metal displaces a less reactive metal from a salt solution of the metal. Copper for instance, can displace silver in silver sulfate solution.
Copper + silver sulfate --> copper sulfate + silver
Another method of making salts is letting acids and carbonates react. A salt, water and carbon dioxide will be formed in the process.
Carbonate + acid --> Salt + carbon dioxide + water
You will have to be able to make word equations of all these types of reactions.
The reactivity series is a short summary of the reactivity of metals, listed by reactivity with other substances from high to low. An example is listed below.
Because new compounds with different properties are formed in chemical reactions, chemical reactions can be identified by changes in physical properties. These reactions can be described by using word equations.
Reaction:
Hydrogen and oxygen react to produce water.
Word equation:
Hydrogen + Oxygen --> Water
Reactants are placed on the left-hand side of the equation, separated by a plus sign, and on the right-hand side of the equation the reaction products are placed, also separated by a plus sign. But sides of the equation are separated by and arrow, indicating that a reaction has occurred.
There are many different types of reactions and some of them are unwanted. Rusting, for example, is a chemical reaction of iron with oxygen catalyzed by water, resulting in iron oxide or more commonly named, rust.
Iron is not the only metal that reacts with oxygen. A lot of other metals react as well, resulting in various metal oxides, but there are some who do not, e.g. gold. Reactions with oxygen are called oxidation reactions.
Metal + oxygen --> metal oxide (some metals)
Alkali and alkaline earth metals react with water to form a metal hydroxide and hydrogen gas. Both types of metals become more reactive when going down in the periodic table.
Alkali metal + water --> metal hydroxide + hydrogen (also alkaline earth metals & other metals)
Metals can also react with acids to form salts and hydrogen gas. When a metal reacts with hydrochloric acid, its reaction product will be a metal chloride for example.
Metal + acid --> salt + hydrogen
During this reaction, the metal displaces one or more hydrogen atoms of the acid. This type of reaction is called a displacement reaction. Displacement reactions also occur when a more reactive metal displaces a less reactive metal from a salt solution of the metal. Copper for instance, can displace silver in silver sulfate solution.
Copper + silver sulfate --> copper sulfate + silver
Another method of making salts is letting acids and carbonates react. A salt, water and carbon dioxide will be formed in the process.
Carbonate + acid --> Salt + carbon dioxide + water
You will have to be able to make word equations of all these types of reactions.
The reactivity series is a short summary of the reactivity of metals, listed by reactivity with other substances from high to low. An example is listed below.
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Metal |
Reaction with oxygen |
Reaction with water |
Reaction with acid |
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Potassium |
Oxide forms very vigorously |
Produces hydrogen with cold water |
Violent reaction |
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Sodium |
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Calcium |
Rate of reaction decreases down the table |
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Magnesium |
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Aluminium |
Produces hydrogen with steam |
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Zinc |
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Iron |
Oxide form slowly |
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Tin |
Oxide forms without burning |
No reaction with water or steam |
Very slow reaction |
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Copper |
No reaction |
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Silver |
No reaction |
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Gold |
During these reactions, some energy effects might be observed. These energy effects are not limited to chemical reactions. Every process has an energy effect. Processes that require energy are called endothermic, while processes that release energy are called exothermic. A few examples are listed below
Rate of a reaction
A chemical reaction takes place when two molecules collide with enough kinetic energy to overcome a small energy barrier, which prevents them from reacting. When they overcome this energy barrier during their collision they will react to form a reaction product. Only a small amount of the collisions will overcome the energy barrier so it is important to have as many collisions between the particles as possible. The speed of this process is called the rate of the reaction.
There are five parameters that affect the rate of a reaction.
Temperature
Imagine a bumper kart track where every bumper kart has a speed of 100 km/h. They are more likely to collide than a bumper kart track where every bumper kart has a speed of 10 km/h. When the temperature of a mixture is increased, the particles gain more kinetic energy and start moving around faster. They are therefor more likely to collide and more likely to react.
Concentration
Now imagine putting more karts on the bumper kart track. Again this increases the chances of a collision. If the concentration of the substance is increased, its chances of collision increase and therefor the rate of the reaction increases.
Particle size
Imagine cutting the bumper kart into small pieces to make millions of tiny bumper carts. With that many small carts on the track, a collision between to karts seems imminent and will be much more likely than between the larger (and fewer) bumper karts, since the smaller bumper karts have more relative area than the larger ones. Decreasing particle size increases the area of the particles and therefor the chance of collision and the rate of the reaction.
Catalyst
Adding a catalyst lowers the energy barrier, which the substances have to overcome. This means that the percentage of collision that overcomes the energy barrier increases and therefor the reaction rate increases as well.
Substance
Every substance reacts differently. Some have a much higher reactivity even though the substances are similar, for instance if they are from the same group in the periodic table. Reactivity is related to the energy barrier, which differs for every substance. Higher reactivity and higher reaction rate usually means the energy barrier is lower.
- Melting of ice is endothermic; the change of state requires energy;
- The baking of break is endothermic; the chemical reaction requires heat to take place;
- Photosynthesis is endothermic; the chemical reaction requires uv-light;
- The combustion of coal is exothermic; heat is released from the coal;
- When an increase in temperature is noticed during a chemical reaction the reaction is exothermic; the energy released during the reaction heats the solution in which the reaction takes place.
Rate of a reaction
A chemical reaction takes place when two molecules collide with enough kinetic energy to overcome a small energy barrier, which prevents them from reacting. When they overcome this energy barrier during their collision they will react to form a reaction product. Only a small amount of the collisions will overcome the energy barrier so it is important to have as many collisions between the particles as possible. The speed of this process is called the rate of the reaction.
There are five parameters that affect the rate of a reaction.
Temperature
Imagine a bumper kart track where every bumper kart has a speed of 100 km/h. They are more likely to collide than a bumper kart track where every bumper kart has a speed of 10 km/h. When the temperature of a mixture is increased, the particles gain more kinetic energy and start moving around faster. They are therefor more likely to collide and more likely to react.
Concentration
Now imagine putting more karts on the bumper kart track. Again this increases the chances of a collision. If the concentration of the substance is increased, its chances of collision increase and therefor the rate of the reaction increases.
Particle size
Imagine cutting the bumper kart into small pieces to make millions of tiny bumper carts. With that many small carts on the track, a collision between to karts seems imminent and will be much more likely than between the larger (and fewer) bumper karts, since the smaller bumper karts have more relative area than the larger ones. Decreasing particle size increases the area of the particles and therefor the chance of collision and the rate of the reaction.
Catalyst
Adding a catalyst lowers the energy barrier, which the substances have to overcome. This means that the percentage of collision that overcomes the energy barrier increases and therefor the reaction rate increases as well.
Substance
Every substance reacts differently. Some have a much higher reactivity even though the substances are similar, for instance if they are from the same group in the periodic table. Reactivity is related to the energy barrier, which differs for every substance. Higher reactivity and higher reaction rate usually means the energy barrier is lower.
