Refer to Section 4.2: Chemical Reaction Rates (1).

Change in concentration = [latex]\Delta{\text{[reactant or product]}={[\;]\mathrm{_f} – [\;]\mathrm{_i}}}[/latex]

[latex]\begin{equation} \text { time interval }=\Delta \mathrm{t=t_f-t_i} \end{equation}[/latex]

The stoichiometry in the reaction is used as coefficients in your rates. However, they are used in their reciprocal form.

Reaction rates are positive. In the reaction rate expression, reacting compounds/molecules will have a negative sign in front of their fraction to indicate that they are decreasing in concentration over reaction time, while product terms will be positive to indicate they are increasing in concentration over reaction time.

[latex]\begin{aligned} \text{For a reaction } \mathrm{A}\rightarrow\mathrm{B}\\ \end{aligned}[/latex]

[latex]\begin{aligned} \text{rate} & =-\dfrac{\Delta[\mathrm{A}]}{\Delta \mathrm{t}}\\ \text{rate} & =\dfrac{\Delta[\mathrm{B}]}{\Delta \mathrm{t}} \end{aligned}[/latex]

Make sure the reactant has a negative sign in front of the concentration, and you use the ‘Δ’ symbol to represent change in both the numerator and denominator. Square brackets in the numerator represent concentration in M (moles/litre).

The number of products formed will always equal the number of reactants used. This is because reactants break to turn into products.

In a reaction rate, the reactants will need a negative sign. This is because the number of reactants will always decrease (negative) over time.

When a negative is multiplied by a negative, it becomes a positive, making the two rates equal.