Acids And Bases Year 10
Sharyn Requena
In our everyday lives, we use many compounds which scientists call acids. The orange or grapefruit juice you drink for breakfast contains citric acid (also known as Vitamin C). When milk turns sour, it contains lactic acid. The vinegar used in salad dressing contains acetic acid. According to this, a chemical bond is considered as being made up of an acid-base combination. The properties of a molecule, therefore, can be understood by dividing it into acid and base fragments.
The terms acid and base have been defined in different ways, depending on the particular way of looking at the properties of acidity and basicity. Arrhenius first defined acids as compounds which ionize to produce hydrogen ions, and bases as compounds which ionize to produce hydroxide ions. According to the Lowry-Bronsted definition, an acid is a proton donor and a base is a proton acceptor.
According to the Lewis definition, acids are molecules or ions capable of coordinating with unshared electron pairs, and bases are molecules or ions having unshared electron pairs available for sharing with acids. To be acidic in the Lewis sense, a molecule must be electron deficient. This is the most general acid base concept. All Lowery Bronstead acids are Lewis acids but, in addition, the Lewis definition includes many other reagents such as boron trifluoride, aluminium chloride, etc.
Some properties, like a bitter taste, are owned by all bases. The bases feel slippery, too. Dream on what slippery soap looks like. And this is a foundation. Furthermore, when immersed in water, bases conduct electricity because they consist of charged particles in the solution.
The sodium hydroxide, calcium carbonate and potassium oxide are examples of bases. A base is a material that interacts with hydrogen ions and can neutralize the acid. Bases are classified as acceptors of a proton (H+), and ammonium hydroxide are typical examples of the bases.
Acids are ionic compounds that, when dissolved in water, produce positive hydrogen ions ( H+) When dissolved in water, acids are sour in taste, conduct electricity and react with metals to produce hydrogen gas. Certain indicator compounds may be used to detect acids, such as litmus. Acids turn blue litmus red.
Two types of corrosive compounds are the acids and bases. Any material with a pH value between 0 and 7 is known to be acidic while a pH value between 7 and 14 is a base. whereas, bases are ionic compounds that produce hydroxide ions(OH-) when dissolved in water.
Acids play significant roles within the human body. The presence of hydrochloric acid in the stomach helps digestion by breaking down large and complex food molecules. Amino acids are required for protein synthesis which helps to grow and repair body tissues.
The sodium hydroxide, calcium carbonate and potassium oxide are examples of bases. A base is a substance that reacts with hydrogen ions and can neutralize the acid. Most bases are minerals which form water and salts by reacting with acids. Bases include the metal oxides, hydroxides, and carbonates.
To decide whether a substance is an acid or base, count the hydrogens on each substance before and after the reaction. If the number has decreased that substance is the acid (which donates hydrogen ions) . If the number of hydrogens has increased that substance is the base (accepts hydrogen ions).
Weak acids partially dissociate/ionise because their conjugate bases are able to re-gain protons to reform the weak acids. In other words, the ionisation of a weak acid is reversible. The same concept applies to weak bases and their conjugate acids.
Most weak bases are organic bases which are Brnsted-Lowry bases (proton acceptor). Since the ionisation of a weak base is incomplete (partial), its chemical equation is written with a reversible arrow. Similar to weak acid ionisation, weak base ionisation can also reach a dynamic equilibium.
As a result, some undissociated solid Ca(OH)2 will be found undissolved in the water or lying at the bottom of the solution as precipitate. Thus, sometimes the dissociation of a base can be limited by its solubility in water.
In this chapter learners will explore acid-base reactions and redox reactions. Redox reactions were briefly introduced in gr10. The concepts of acids, bases, reduction, oxidation and oxidation numbers are all introduced here. The following list provides a summary of the topics covered in this chapter.
This chapter begins by revising all the concepts done on acids and bases up to this point. Learners are reminded what an acid and a base are (in particular the Bronsted-Lowry definition) and how the definition and concept have changed over time. Although the most recent definition of an acid and a base is the Lowry definition this is not covered at school level and the Bronsted-Lowry definition serves as a good working model for the acids and bases that learners encounter at school.
The concept of a polyprotic acid is introduced although it is not in CAPS. This is done to help learners understand how to handle acids such as sulfuric acid in reactions. You should try to use polyprotic acids sparingly in your examples.
The concept of conjugate acids and bases requires learners to think about reactions going in reverse. By writing the equation in reverse, learners can see how the acid becomes a base. This base is said to be the conjugate base of the acid since it is conjugated (linked) to the acid.
This topic is placed after redox reactions in CAPS but must be taught before redox reactions and so is placed before redox reactions in this book. This topic provides the tools needed to understand redox reactions.
In grade 10 learners learnt how to balance chemical equations by inspection. In this topic they will learn how to balance redox reactions which often cannot be balanced by inspection. The simpler examples can be balanced by inspection and this can be used as a comparison for the two techniques. Learners need to be able to break a reaction up into two parts and follow different chemical species through an equation. This skill starts with conjugate acids and bases and carries over into this topic.
Coloured text has been used as a tool to highlight different parts of reactions. Ensure that learners understand that the coloured text does not mean there is anything special about that part of the reaction, this is simply a teaching tool to help them identify the important parts of the reaction.
All around you there are chemical reactions taking place. Green plants are photosynthesising, car engines are relying on the reaction between petrol and air and your body is performing many complex reactions. In this chapter we will look at two common types of reactions that can occur in the world around you and in the chemistry laboratory. These two types of reactions are acid-base reactions and redox reactions.
Most acids share certain characteristics, and most bases also share similar characteristics. It is important to be able to have a definition for acids and bases so that they can be correctly identified in reactions.
One of the first things that was noted about acids is that they have a sour taste. Bases were noted to have a soapy feel and a bitter taste. However you cannot go around tasting and feeling unknown substances since they may be harmful. Also when chemists started to write down chemical reactions more practical definitions were needed.
A number of definitions for acids and bases have developed over the years. One of the earliest was the Arrhenius definition. Arrhenius (1887) noticed that water dissociates (splits up) into hydronium \((\textH_3\textO^+)\) and hydroxide \((\textOH^-)\) ions according to the following equation:
Arrhenius described an acid as a compound that increases the concentration of \(\textH_3\textO^+\) ions in solution and a base as a compound that increases the concentration of \(\textOH^-\) ions in solution.
However, this definition could only be used for acids and bases in water. Since there are many reactions which do not occur in water it was important to come up with a much broader definition for acids and bases.
In 1923, Lowry and Bronsted took the work of Arrhenius further to develop a broader definition for acids and bases. The Bronsted-Lowry model defines acids and bases in terms of their ability to donate or accept protons.
We highlight the chlorine and the nitrogen so that we can follow what happens to these two elements as they react. We do not highlight the hydrogen atoms as we are interested in how these change. This colour coding is simply to help you identify the parts of the reaction and does not represent any specific property of these elements.
Notice in these examples how we looked at the common elements to break the reaction into two parts. So in the first example we followed what happened to chlorine to see if it was part of the acid or the base. And we also followed nitrogen to see if it was part of the acid or the base. You should also notice how in the reaction for the acid there is one less hydrogen on the right hand side and in the reaction for the base there is an extra hydrogen on the right hand side.
Depending on what water is reacting with it can either react as a base or as an acid. Water is said to be amphoteric. Water is not unique in this respect, several other substances are also amphoteric.
An amphiprotic substance is one that can react as either a proton donor (Bronsted-Lowry acid) or as a proton acceptor (Bronsted-Lowry base). Examples of amphiprotic substances include water, hydrogen carbonate ion (\(\textHCO_3^-\)) and hydrogen sulfate ion (\(\textHSO_4^-\)).
In this chapter we will mostly consider monoprotic acids (acids with only one proton to donate). If you do see a polyprotic acid in a reaction then write the resulting reaction equation with the acid donating all its protons.
Up to now you have looked at reactions as starting with the reactants and going to the products. For acids and bases we also need to consider what happens if we swop the reactants and the products around. This will help you understand conjugate acid-base pairs.
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