Chemistry_Grade_10-12 (1).pdf

Definition acids and bases according to the bronsted

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Definition: Acids and bases According to the Bronsted-Lowry theory of acids and bases, an acid is a substance that gives away protons (H + ), and is therefore called a proton donor . A base is a substance that takes up protons, and is therefore called a proton acceptor . Below are some examples: 1. HCl(g) + NH 3 (g) NH 4+ + Cl In order to decide which substance is a proton donor and which is a proton acceptor, we need to look at what happens to each reactant. The reaction can be broken down as follows: HCl Cl + H + and NH 3 + H + NH + 4 From these reactions, it is clear that HCl is a proton donor and is therefore an acid , and that NH 3 is a proton acceptor and is therefore a base . 2. CH 3 COOH + H 2 O H 3 O + + CH 3 COO The reaction can be broken down as follows: CH 3 COOH CH 3 COO + H + and H 2 O + H + H 3 O + In this reaction, CH 3 COOH (acetic acid) is a proton donor and is therefore the acid . In this case, water acts as a base because it accepts a proton to form H 3 O + . 3. NH 3 + H 2 O NH + 4 + OH The reaction can be broken down as follows: 268
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CHAPTER 15. TYPES OF REACTIONS - GRADE 11 15.1 H 2 O OH + H + and NH 3 + H + NH + 4 In this reaction, water donates a proton and is therefore an acid in this reaction. Ammonia accepts the proton and is therefore the base . Notice that in the previous equation, water acted as a base and that in this equation it acts as an acid. Water can act as both an acid and a base depending on the reaction. This is also true of other substances. These substances are called ampholytes and are said to be amphoteric . Definition: Amphoteric An amphoteric substance is one that can react as either an acid or base. Examples of amphoteric substances include water, zinc oxide and beryllium hydroxide. 15.1.3 Conjugate acid-base pairs Look at the reaction between hydrochloric acid and ammonia to form ammonium and chloride ions: HCl + NH 3 NH + 4 + Cl Looking firstly at the forward reaction (i.e. the reaction that proceeds from left to right ), the changes that take place can be shown as follows: HCl Cl + H + and NH 3 + H + NH + 4 Looking at the reverse reaction (i.e. the reaction that proceeds from right to left ), the changes that take place are as follows: NH + 4 NH 3 + H + and Cl + H + HCl In the forward reaction , HCl is a proton donor (acid) and NH 3 is a proton acceptor (base). In the reverse reaction , the chloride ion is the proton acceptor (base) and NH + 4 is the proton donor (acid). A conjugate acid-base pair is two compounds in a reaction that change into each other through the loss or gain of a proton. The conjugate acid-base pairs for the above reaction are shown below. HCl + NH 3 NH + 4 + Cl acid 1 base 2 base 1 acid 2 conjugate pair conjugate pair The reaction between ammonia and water can also be used as an example: 269
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15.1 CHAPTER 15. TYPES OF REACTIONS - GRADE 11 H 2 O + NH 3 NH + 4 + OH acid 1 base 2 base 1 acid 2 conjugate pair conjugate pair Definition: Conjugate acid-base pair The term refers to two compounds that transform into each other by the gain or loss of a proton.
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