Why does decreasing concentration decrease the rate of a chemical reaction?

Ethanol (C$_2$H$_6$O), a gasoline additive, is formed by the reaction of ethylene (CH$_2$ CH$_2$) with water. This reaction releases 9.0 kcal/mol of energy.
a Is $\Delta$H for this reaction positive or negative?
b. Is the reaction endothermic or exothermic?
c. Are the reactants or products lower in energy?

The metabolism of glucose with oxygen forms CO$_2$ and H$_2$O and releases 678 kcal/mol of energy.

$$\text{C$_6$H$_{12}$O$_6$(aq) + 6 O$_2$(g) $\rightarrow$ 6 CO$_2$(g) + 6 H$_2$O(l)}$$

a. Is $\Delta$H for this reaction positive or negative?
b. Is the reaction endothermic or exothermic?
c. Are the reactants or products lower in energy?

Ammonia (NH3) is formed from hydrogen and nitrogen according to the given equation.\

$$\text{3 H$_2$(g) + N$_2$(g) $\rightarrow$ 2 NH$_3$(g) $\Delta$H = –22.0 kcal/mol}$$

a. Is heat absorbed or released?
b. Are the reactants or products lower in energy?
c. Is the reaction endothermic or exothermic?

The combustion of coal with oxygen forms CO$_2$ according to the given equation.

$$\text{C(s) + O$_2$(g) $\rightarrow$ CO$_2$(g) $\Delta$H = $– 94 $kcal/mol}$$

a. Is heat absorbed or released?
b. Are the reactants or products lower in energy?
c. Is the reaction endothermic or exothermic?

Do each of the following statements describe an endothermic or exothermic reaction?
a. $\Delta$H is a positive value.
b. The energy of the products is lower than the energy of the reactants.
c. Energy is released in the reaction.