Acids are soluble in water and have a pH of less than seven. They react with metals and metal compounds. A common example of such a reaction is between sodium hydroxide and hydrochloric acid. To understand how reactions work, it is necessary to understand what concentration is and how and why acids react. How increasing concentration affects reactions will then become clearer.
The concentration of a substance is the amount (mass or volume) of a substance divided by the total volume of the mixture. It is commonly applied to solutions. In chemistry, it is measured as the number of moles per liter. A mole is the number of elementary particles (protons and neutrons) in the atom in grams.
How and Why Acid Reacts
When dissolved in water, acids form positive and negative ions, or positively and negatively charged particles. When an alkali such as sodium hydroxide is dissolved in water, it also does this. Particles in liquids are constantly moving. When a positively charged metal ion of sodium comes into contact with a negatively charged chloride particle, they react, forming a salt, sodium chloride. However, it is necessary to have a minimum level of energy for the reaction, known as the activation energy. There are therefore two basic factors necessary for a reaction to take place: collisions and energy. Increasing either of these will increase the rate of reaction.
The Effect of Concentration on The Rate of Reaction
Increasing the concentration of the solution means that there are more particles in a given area, more collisions are likely to take place, and therefore, reactions are more likely. An analogy that is helpful in understanding this is in comparison to gases: as pressure increases, gases are compressed into a smaller volume. This increases the rate of reaction.
Concentration and the Stage of Reaction
As the reaction progresses and more of the acid and metal is "used up" and forms a different molecule, the concentration of the original substances decreases. Therefore, the concentration of acid and the rate of reaction is initially highest, and decreases as the reaction progresses.