your image you can generate with the following small document:
\documentclass[tikz,border=3mm]{standalone}
\usetikzlibrary{arrows.meta,
chains,
decorations.markings,
positioning,
quotes,
}
\begin{document}
\begin{tikzpicture}[
> = Triangle,
decoration = {markings,% switch on markings
mark=at position 0.5 with {\arrow[thick]{>}}
},
N/.style = {on chain},
node distance = 7mm and 21 mm,
start chain = going right
]
\node (a) [N] {A};
\node (b) [N] {B};
\node (c) [N] {C};
%
\coordinate[below=of a] (aux);
\draw[postaction={decorate}] (a) to ["$k_1$"] (b);
\draw[postaction={decorate}] (b) to ["$k_2$"] (c);
\draw[postaction={decorate}] (a) -- (aux) to ["$k_3$" '] (aux -| c) -- (c);
\end{tikzpicture}
\end{document}

or include its code directly to your document:
\documentclass[11pt]{article}
\usepackage[margin=1in]{geometry}
\usepackage{graphicx}
\usepackage{setspace}\onehalfspacing
\usepackage{tikz} % <--- added
\usetikzlibrary{arrows.meta, % <--- added
chains, % <--- added
decorations.markings,% <--- added
positioning, % <--- added
quotes, % <--- added
}
\begin{document}
\title{Experiment 3}
\author{Sophie Parsons%
\thanks{CRSid: \texttt{bb70}}}
%\affil{university}
\date{Dated: \today}
\maketitle
\section{Introduction}
In the reactor, the benzene and maleic anhydride can also degrade into other useless products, as shown in figure \ref{fig:7}.
\begin{figure}[ht]
\centering% <--- changed
\begin{tikzpicture}[ % <--- added, instead of includegraphics
> = Triangle,
decoration = {markings,% switch on markings
mark=at position 0.5 with {\arrow[thick]{>}}
},
N/.style = {on chain},
node distance = 7mm and 21 mm,
start chain = going right
]
\node (a) [N] {A};
\node (b) [N] {B};
\node (c) [N] {C};
%
\coordinate[below=of a] (aux);
\draw[postaction={decorate}] (a) to ["$k_1$"] (b);
\draw[postaction={decorate}] (b) to ["$k_2$"] (c);
\draw[postaction={decorate}] (a) -- (aux) to ["$k_3$" '] (aux -| c) -- (c);
\end{tikzpicture}
\caption{default}
\label{fig:7}
\end{figure}
\end{document}
and obtain:

note:
- instead of
tikz
package, you can use pstric
(with latter i'm not familiar)
- image generated in separate document (its name is name of image, if you like to include as
.pdf
image) or with code by which you replace \includegraphics{...}
hasn't spurious space around it. consequently it should be on the same page as they are inserted in text (with exception, that it is inserted till before bottom of page).
- i drown this image for fun and exercise. if you have question about
tikz
, don't hesitate to ask new questions ...
H
position specifier, as it may result in unwanted white spaces. Use!htb
instead. This being said, try replacing thecenter
environment (which adds a supplementary vertical spacing to the one added by thefigure
environment) with the\centering
directive.\fbox{\includegraphics[scale=0.8]{scheme.pdf}
.\begin{figure}[!htb] {\centering \fbox{\includegraphics[scale=0.8]{scheme.pdf}} %\includegraphics[scale=0.8]{scheme.pdf} \caption{default} \label{fig:7} } \end{figure}