Download
1 / 36
480 likes | 871 Vues
What are acids and alkalis?. Naming acids and alkalis. What are acids?. Acids can be defined as substances that release H + ions when they dissolve in water. When hydrogen chloride dissolves in water, it releases H + ions and Cl – ions:. +. HCl. H +. Cl –.
E N D
What are acids? Acids can be defined as substances that release H+ ions when they dissolve in water. When hydrogen chloride dissolves in water, it releases H+ ions and Cl– ions: + HCl H+ Cl– When ethanoic acid dissolves in water, it releases H+ ions and CH3COO–ions: + CH3COO– CH3COOH H+ Since acids release H+ ions, we call them H+ donors.
What are H+ ions? Hydrogen atoms contain one proton and one electron. A H+ ion is a hydrogen atom that has lost its single electron. This leaves a lone, positively charged proton. Because acids donate protons, they are sometimes called proton donors. When dissolved in water, the H+ ion is hydrated. This is represented in formulae by adding a state symbol, H+ (aq).
What are bases and alkalis? Bases are substances that react with acids. They absorb hydrogen ions (H+). Bases are therefore H+ acceptors, or proton acceptors. The oxides, hydroxides and carbonates of metals, such as sodium carbonate, are bases. Some bases are soluble in water: these bases are called alkalis. bases: react with acids Alkalis release hydroxide ions (OH–) in solution. alkalis: soluble bases All alkalis are bases, but not all bases are alkalis.
Examples of bases and alkalis Sodium hydroxide is an alkali. It dissolves in water to release Na+ ions and OH– ions: + NaOH Na+ OH– Ammonia is a base that does not contain a metal. It can accept a proton to form the NH4+ ion. Ammonia is also an alkali. It dissolves in water to release NH4+ ions and OH– ions: + + NH3 H2O NH4+ OH–
What happens in neutralization? In aneutralization reaction, an acid reacts with an alkali to produce a salt and water. When the acid reacts with the alkali, the acid’s H+ ions react with the alkali’s OH– ions, forming water. H+ + OH– H2O This process takes place in all neutralization reactions. + acid alkali salt + water
Theories about acids and bases When the 25-year-old Svante Arrhenius submitted his dissertation to Uppsala University in 1884, he was disappointed to receive only a fourth-class degree. His theories relied on the idea that compounds disassociate in water to form ions. This was a new idea that had not been proposed before. In contrast Brønsted and Lowry’s ideas were immediately accepted and embraced by other scientists. Their theory was based on Arrhenius’s ideas. Both were among many respected scientists working on acids and bases at the time. Why do you think the scientific community responded differently to the two theories?
Strong acids Acids can be strong or weak. Strong acids dissociate fully into solution, so that all their H+ ions are released into the mixture. + HA H+ A– Hydrochloric acid is a typical strong acid, so the dissociation reaction is complete: + HCl H+ Cl–
Weak acids Weak acids do not dissociate fully, some of their H+ ions stay attached to the acid molecule. The dissociation of a weak acid in water is a reversible reaction: ⇌ + HA H+ A– Ethanoic acid is a typical weak acid, with its ions in dynamicequilibrium with the un-dissociated acid. The reaction is moving both directions at the same rate. ⇌ + CH3COOH H+ CH3COO–
Sulfuric acid Sulfuric acid (H2SO4) contains two hydrogen ions per sulphate ion. Both hydrogen ions can dissociate in solution. It is therefore called a diprotic acid. In general the first H+ ion of a diprotic acid will dissociate more readily than the second. H2SO4 is a strong acid, so it fully dissociates to release its first H+ ion: H+ + HSO4– H2SO4 However, HSO4– is a weak acid, so it only dissociates partially to release the second H+ ion: ⇌ HSO4– H+ + SO42–
pH of strong and weak acids pH is a measure of the number of H+ ions in solution, with a lower pH meaning more H+ ions. Because strong acids dissociate fully in solution, they contain more H+ ions per molecule of acid, producing a lower pH. Compared to weak acids of the same concentration, strong acids: • have lower pH values • are better conductors of electricity • react more quickly. high H+ low H+
Properties of strong and weak alkalis Alkalis can be classified as strong and weak in the same way as acids. A strong alkali, such as sodium hydroxide, fully dissociates in solution. + NaOH Na+ OH– A weak alkali, such as ammonia, does not fully dissociate, and some of the OH– ions are not released into solution. ⇌ + + NH3 H2O NH4+ OH– Comparable levels of ion dissociation mean that strong and weak alkalis have similar properties to strong and weak acids.
Uses of sulfuric acid Sulfuric acid has a role in the manufacture of many products. explosives steel fertilizers nylon paper oil
Dangers of sulfuric acid Concentrated sulfuric acid is a very powerful dehydrating agent. Dehydrating agents remove water from a compound, in a vigorous exothermic reaction. An example is the reaction of sulfuric acid with glucose. conc. H2SO4 glucose carbon + water The sugar is rapidly broken down, producing an expanding mass of steaming carbon. Sulfuric acid will remove water from your skin, leading to bad burns.
Conditions for the contact process The contact process involves a reversible reaction which reaches an equilibrium: ⇌ + 2SO2 O2 2SO3 This reaction is exothermic in the forward direction. What conditions would you suggest to produce the highest and most economical yield of sulfur trioxide? Remember: Whenever a change is made to a reversible reaction in dynamic equilibrium, the equilibrium position will shift to try and oppose that change.
Compromise conditions The conditions used during this stage of the contact process are: ⇌ + 2SO2 O2 2SO3 • 400 to 450°C • 10atmospheres • vanadium(V)oxide (V2O5)catalyst These provide a compromise in which the cost of making the sulfuric acid is balanced by the yield and speed of production. What costs are involved in the manufacture of sulfuric acid? • raw materials • equipment • energy • wages
