## Graph of Product/Reactant Change Against Time

1. In a chemical reaction,
1. the reactants will decrease over time
2. the product will increase over time.
2. the rate of reaction will decrease over time owing to the decrease in concentration and total surface area of reactants.
3. In a graph of quantity of product/reactant over time, the rate of reaction is equal to the gradient of the graph.

Example
The reaction between dilute hydrochloric acid and excess marble will produce calcium chloride and gas of carbon dioxide. Sketch the graph of

1. the mass of the marble against time.
2. the volume of carbon dioxide against time.
3. the concentration of hydrochloric acid against time.
4. the concentration of calcium chloride against time.

a.

b.

c.

d.

## Finding Average Rate of Reaction

Rate of reaction is a measure of how fast a reaction occur, or how much the reactant/product change in a period of time.
Rates of reaction = Quantity change of reactants/products Total time for the reaction

Example
In a chemical reaction, 2.5g of calcium carbonate react completely with excess hydrochloric acid to produce 600cm³ of carbon dioxide gas in 1.5 minutes. Find the rate of reaction in term of
a. decreasing mass of calcium carbonate
b. increasing volume of carbon dioxide gas produced

a.
Change of the amount of reactant=-2.5g Tima taken for the change=1.5minute=90s Rate of Reaction = -2.5g / 90s = 0.027gs-1

b.
Change of the amount of product
=600cm3 Tima taken for the change
= 1.5minute = 90s

Rate of Reaction
= 600cm3/90s
=6.7cm3s-1

## Understanding Rate of Reaction

1. Rate of reaction is defined as the change in the amount of reactants or products per unit time.
2. It is a measure of how fast a reaction occurs.
1. Fast Reaction = Rate of reaction is high.
2. Slow Reaction = Rate of reaction is low
3. A fast reaction taken shorter time for the reaction to complete.
Example of fast reaction
 Type of Reaction Example Combustion Combustion of magnesium in oxygen $2Mg + {O_2} \to 2MgO$ Combustion of ethane (C2H6) ${C_2}{H_6} + \frac{7}{2}{O_2} \to 2C{O_2} + 3{H_2}O$ Reaction between reactive metal and water Reaction between potassium and water $2K + 2{H_2}O \to 2KOH + {H_2}$ Reaction between metal carbonate and acid Reaction between limestone/ marble and sulphuric acid $\begin{gathered} CaC{O_3} + {H_2}S{O_4} \hfill \\ \to CaS{O_4} + C{O_2} + {H_2}O \hfill \\ \end{gathered}$ Ionic precipitation (Double decomposition) Precipitation of silver(I) chloride $AgN{O_3} + HCl \to AgCl + HN{O_3}$
Example of slow reaction
 Type of Reaction Example Photosynthesis $6C{O_2} + 6{H_2}O \to {C_6}{H_{12}}{O_6} + 6{O_2}$ Rusting $4Fe + 3{O_2} + 2{H_2}O \to 2F{e_2}{O_3} \bullet 2{H_2}O$ Fermentation ${C_6}{H_{12}}{O_6} \to 2{C_2}{H_5}OH + 2C{O_2}$