What makes an acid strong or weak

Determine if a salt produces an acidic or a basic solution. Strong and Weak Acids Except for their names and formulas, so far we have treated all acids as equals, especially in a chemical reaction. The nitrogen in C 5 H 5 N would act as a proton acceptor and therefore can be considered a base, but because it does not contain an OH compound, it cannot be considered a strong base; it is a weak base. Salts of weak acids or bases can affect the acidity or basicity of their aqueous solutions.

Recall from Section NH 4 Cl is an example of an acid salt. The molecule NH 3 is a weak base, and it will form when it can, just like a weak acid will form when it can. So there are two general rules: 1 If an ion derives from a strong acid or base, it will not affect the acidity of the solution. Some salts are composed of ions that come from both weak acids and weak bases.

The overall effect on an aqueous solution depends on which ion exerts more influence on the overall acidity. We will not consider such salts here. Identify each as a strong acid or a weak acid. Assume aqueous solutions. Identify each as a strong base or a weak base. Write the balanced chemical equation for the reaction of each acid and base pair. Write the hydrolysis reaction that occurs, if any, when each salt dissolves in water. Write chemical equations for both reactions.

Can you tell if the solution will be acidic or basic overall? A lab technician mixes a solution of 0. Explain your answer. Privacy Policy. Skip to main content. Chapter Acids and Bases. Search for:. Find out! To take a specific common example, the equilibrium for the dissociation of ethanoic acid is properly written as:. Note: Because you are likely to come across both of these versions depending on where you read about K a , you would be wise to get used to using either.

For exam purposes, though, use whichever your examiners seem to prefer. These are all weak acids because the values for K a are very small. They are listed in order of decreasing acid strength - the K a values get smaller as you go down the table. However, if you aren't very happy with numbers, that isn't immediately obvious.

Because the numbers are in two parts, there is too much to think about quickly! If you use your calculator on all the K a values in the table above and convert them into pK a values, you get:. Note: Notice that unlike K a , pK a doesn't have any units. Notice that the weaker the acid, the larger the value of pK a. It is now easy to see the trend towards weaker acids as you go down the table. Note: If you need to know about K a and pK a , you are quite likely to need to be able to do calculations with them.

You will probably need to be able to calculate the pH of a weak acid from its concentration and K a or pK a. You may need to reverse this and calculate a value for pK a from pH and concentration. I can't help you with these calculations on this site, but they are all covered in detail in my chemistry calculations book. If this is the first set of questions you have done, please read the introductory page before you start.

Strong acids Explaining the term "strong acid" We are going to use the Bronsted-Lowry definition of an acid. In the general case. Other common strong acids include sulphuric acid and nitric acid.

You may find the equation for the ionisation written in a simplified form: This shows the hydrogen chloride dissolved in the water splitting to give hydrogen ions in solution and chloride ions in solution. Strong acids and pH pH is a measure of the concentration of hydrogen ions in a solution. Defining pH. Working out the pH of a strong acid Suppose you had to work out the pH of 0. With strong acids this is easy. Each mole of HCl reacts with the water to give 1 mole of hydrogen ions and 1 mole of chloride ions That means that if the concentration of the acid is 0.

Weak acids Explaining the term "weak acid" A weak acid is one which doesn't ionise fully when it is dissolved in water. Comparing the strengths of weak acids The position of equilibrium of the reaction between the acid and water varies from one weak acid to another. The acid dissociation constant, K a You can get a measure of the position of an equilibrium by writing an equilibrium constant for the reaction.

Here is the equilibrium again: You might expect the equilibrium constant to be written as: However, if you think about this carefully, there is something odd about it. In 1 dm 3 of solution, there are going to be about 55 moles of water.

You may find the K a expression written differently if you work from the simplified version of the equilibrium reaction: This may be written with or without state symbols. To take a specific common example, the equilibrium for the dissociation of ethanoic acid is properly written as: The K a expression is: If you are using the simpler version of the equilibrium.

The table shows some values of K a for some simple acids: acid K a mol dm -3 hydrofluoric acid. Remember this: The lower the value for pK a , the stronger the acid.