Presentation on theme: "2. 14 Rates of Chemical Reactions (Sec 9.4 pg 250-256."— Presentation transcript:
2. 14 Rates of Chemical Reactions (Sec 9.4 pg 250-256
A chemical reaction can be fast, or it can happen very slowly over the course of many years. Figuring out the speed (or ‘rate’) of a reaction is very important. In chemistry, the speed of a reaction is called a reaction rate, and is measured as the amount of a reactant consumed per unit time (or the amount of product formed per unit time).
The speed of chemical reactions can be explained using kinetic molecular theory (KMT). According to the KMT, atoms move around and as they move, they bump into each other.
If two atoms bump each other at just the right speed and at just the right angle, then a reaction takes place (not just any collision will do, it has to be an ‘effective collision’). This is called collision theory, which says ‘changing the frequency of effective collisions changes the reaction rate’.
There are 4 main factors that can affect the rate of effective collisions, and therefore the rate of a chemical reactions:
Concentration – If you increase the amount of a reactant, then you increase the probability of an effective collision taking place with another reactant – this can increase the rate of reaction (Fig.3 p.253). ‘Increasing the concentration of a reactant increases reaction rate by increasing the frequency of effective collisions’.
Surface area - Increasing the area of an exposed solid reactant can allow it to have more surface to interact with another reactant – this too can increase the number of collisions, and as a result the rate of reaction (Fig.4&5 p.253-4). ‘Increasing the surface area of a solid reactant increases the amount of exposed solid, and this increases the reaction rate by increasing the frequency of effective collisions’.
Temperature – Increasing the temperature increase the speed and movement of atoms. As they move more, they can bump into more reactants, and increases the rate of reaction. ‘Increasing the temperature of reactants significantly increases reaction rate by increasing the frequency of effective collisions in two ways: (1) increasing the frequency of all collisions, and (2) increasing the energy of all collisions’.
Catalyst – A catalyst is a substance that increases the reaction rate without being consumed, by increasing the number of effective collisions on the molecular level (Fig.6 p.255). ‘Adding a catalyst increases reaction rate by increasing the percentage of effective collisions’.